Power conversion apparatus and control method therefor

Abstract

Provided are a power conversion apparatus and a control method therefor. The power conversion apparatus comprises a direct current end, a direct current side bridge circuit, a transformer, an alternating current side bridge circuit, an alternating current end, and a controller. The transformer comprises a primary winding and a secondary winding, and the direct current side bridge circuit and the alternating current side bridge circuit comprise switch transistors. The direct current side bridge circuit is connected between the direct current end and the primary winding, and the alternating current side bridge circuit is connected between the secondary winding and the alternating current end. As the alternating current at the alternating current end converges to 0, the controller is used for first controlling the switching frequency of the switch transistor to increase to the maximum switching frequency value of the switch transistor, and then used for, when the alternating current is at 0, controlling the switching frequency of the switch transistor to be smaller than the maximum switching frequency value. As the alternating current at the alternating current end diverges from 0, the controller controls the switching frequency of the switch transistor to increase to the maximum switching frequency value of the switch transistor, and then controls the switching frequency to decrease. The present application is conducive to improving the control effect and the output performance of the power conversion apparatus.

Description

Power conversion device and control method thereof

[0001] This application claims priority to the Chinese patent application filed with the State Intellectual Property Office of China on January 18, 2024, with application number 202410074361.4, and priority to the Chinese patent application entitled “Power conversion device and control method thereof”, all contents of which are incorporated by reference into this application. Technical Field

[0002] The present application relates to the field of power supply technology, and in particular to a power conversion device and a control method thereof. Background Art

[0003] The dual active bridge (DAB) converter can achieve electrical isolation between input and output, has the advantages of high power density and easy soft switching, and can meet the needs of various applications of DC voltage to AC voltage conversion. Therefore, the DAB converter has been widely used.

[0004] Traditional DAB converters primarily utilize the circuit structure shown in Figure 1a. As shown in Figure 1a, the DAB converter primarily comprises a DC-side full bridge, a transformer T, an inductor Lr, and an AC-side full bridge. The DC-side full bridge comprises a first and second parallel-connected bridge arm, while the AC-side full bridge comprises a third and fourth parallel-connected bridge arm. The first bridge arm consists of switches S1 and S2 connected in series, the second bridge arm consists of switches S3 and S4 connected in series, the third bridge arm consists of switches S5 and S6 connected in series, and the fourth bridge arm consists of switches S7 and S8 connected in series. The DAB converter shown in Figure 1a can be controlled by adjusting the switching frequency, the phase difference between switches S1 and S4, the phase difference between switches S5 and S8, and the phase difference between switches S1 and S5. In practical applications, to achieve lower inductor current peaks and lower current losses, a specific optimal switching frequency fs is typically sought for a given DC voltage vdc, AC voltage vac, and AC current iac, based on the fundamental principles of a DAB converter. For example, when the AC voltage vac and AC current iac are shown in Figure 1b, the optimal switching frequency fs sought is the gray dashed line shown in Figure 1b. As shown in Figure 1b, when the AC current iac is zero, the optimal switching frequency fs sought is very high. However, in actual DAB converters, due to limitations of the controller, switches, and transformer, the switching frequency of the switches has an upper limit. Therefore, the actual switching frequency curve of the switches is the solid black line shown in Figure 1b.

[0005] As shown in Figure 1b, when the AC current iac is low or the AC voltage vac is low, the switching frequency fs of the switches in the DAB converter reaches the upper limit fs_limit. When the switching frequency fs reaches the upper limit fs_limit, closed-loop control of the switching frequency fs is easily limited by the limiter, thereby affecting the control effect of the DAB converter. Furthermore, at the zero-crossing moments of the AC current iac and the AC voltage vac, the DC-side full-bridge and / or AC-side full-bridge of the DAB converter undergo operating mode switching. However, at this time, the switching frequency fs operates at the upper limit fs_limit, which can easily distort the voltage and current waveforms generated by the DAB converter during the operating mode switching, reducing the output performance of the DAB converter. Summary of the Invention

[0006] The present application provides a power conversion device and a control method thereof, which are beneficial to improving the control effect and output performance of the power conversion device.

[0007] In a first aspect, the present application provides a power conversion device, which includes a DC terminal, a DC side bridge circuit, a transformer, an AC side bridge circuit, an AC terminal, and a controller. The transformer includes a primary winding and a secondary winding. The DC side bridge circuit and the AC side bridge circuit include a switching tube. The DC side bridge circuit is connected between the DC terminal and the primary winding, and the AC side bridge circuit is connected between the secondary winding and the AC terminal. The controller is used to control the switching frequency of the switching tube to increase to the maximum switching frequency of the switching tube when the AC current at the AC terminal approaches 0, and then control the switching frequency of the switching tube to be less than the maximum switching frequency when the AC current is 0. And when the AC current at the AC terminal is far away from 0, the switching frequency of the switching tube is first controlled to increase to the maximum switching frequency of the switching tube, and then control the switching frequency of the switching tube to decrease.

[0008] In this embodiment, as the AC current at the AC end approaches or moves away from zero, in the current range where the AC current at the AC end is close to zero (including zero), the power converter controls the switching frequency of the switches in the DC-side bridge circuit and the AC-side bridge circuit to be less than the maximum switching frequency. This prevents the switching frequency of the switches from being limited by the limiter during closed-loop control, thereby improving the control effect of the power converter. Furthermore, because the switching frequency of the switches is less than the maximum switching frequency when the AC current at the AC end is zero, the voltage and current waveform distortion caused by the power converter switching when the AC current at the AC end is zero can be reduced, thereby improving the output performance of the power converter. Furthermore, as the AC current at the AC end approaches or moves away from zero, in the current range where the AC current at the AC end is far from zero, the power converter controls the switching frequency of the switches based on the principle that the absolute value of the AC current is negatively correlated with the optimal switching frequency of the switches, thereby reducing the current peak and current loss of the inductor in the power converter, thereby improving the efficiency of the power converter.

[0009] In combination with the first aspect, in a first possible implementation, the controller is used to, when the AC current at the AC end approaches 0, control the switching frequency of the switching tube to increase to the maximum switching frequency of the switching tube, first control the switching frequency of the switching tube to decrease to the first switching frequency, and then control the switching frequency of the switching tube to decrease to the second switching frequency.

[0010] In this embodiment, the switching frequency control method of the power conversion device is applicable to application scenarios in which the switching frequency increases to the maximum switching frequency before the absolute value of the AC voltage drops to 0. As the AC current approaches or moves away from 0, the AC voltage decreases or increases to 0 in the current range where the AC current is close to 0 (including 0). The power conversion device controls the switching frequency of the switch tube to first decrease to a first switching frequency, then decrease to a second switching frequency, and finally increase in the current range where the AC current is close to 0. This ensures that the switching frequency of the switch tube is less than the maximum switching frequency in the current range where the AC current is close to 0 and the voltage range where the AC voltage vac is close to 0 (including 0). This prevents the switching frequency of the switch tube from being limited by the limiter during closed-loop control, thereby improving the control effect of the power conversion device. In addition, because the switching frequency of the switch tube is less than the maximum switching frequency when the AC current is 0 and the AC voltage is 0, the voltage and current waveform distortion caused by the power conversion device switching between operating modes when the AC current is 0 and the AC voltage is 0 can be reduced, thereby improving the output performance of the power conversion device.

[0011] In combination with the first aspect, in a second possible implementation, the controller is used to, when the AC current at the AC end approaches 0, control the switching frequency of the switching tube to increase to the maximum switching frequency of the switching tube, first control the switching frequency of the switching tube to decrease to a first switching frequency, and then control the switching frequency of the switching tube to increase to a second switching frequency, wherein the second switching frequency is less than the maximum switching frequency.

[0012] In this embodiment, the switching frequency control method of the power conversion device is applicable to application scenarios in which the switching frequency increases to the maximum switching frequency before the absolute value of the AC voltage drops to 0. As the AC current approaches or moves away from 0, the AC voltage drops or increases to 0 in the current range where the AC current is close to 0 (including 0). The power conversion device controls the switching frequency of the switch tube to first decrease to a first switching frequency, then increase to a second switching frequency, and finally continue to increase in the current range where the AC current is close to 0. This ensures that the switching frequency of the switch tube is less than the maximum switching frequency in the current range where the AC current is close to 0 and the voltage range where the AC voltage vac is close to 0 (including 0). This prevents the switching frequency of the switch tube from being limited by the limiter during closed-loop control, thereby improving the control effect of the power conversion device. In addition, since the switching frequency of the switch tube is less than the maximum switching frequency when the AC current is 0 and the AC voltage is 0, the voltage and current waveform distortion caused by the power conversion device switching between operating modes when the AC current is 0 and the AC voltage is 0 can be reduced, thereby improving the output performance of the power conversion device. Furthermore, since the first switching frequency is lower than the second switching frequency in this embodiment, the power conversion device can output a larger AC current when the AC voltage is low, thereby lowering the power factor and improving applicability.

[0013] In combination with the first aspect, in a third possible implementation, the controller is used to control the switching frequency of the switching tube to decrease to the first switching frequency, and then control the switching frequency of the switching tube to remain at the first switching frequency, after controlling the switching frequency of the switching tube to increase to the maximum switching frequency of the switching tube when the AC current at the AC end approaches 0.

[0014] In this embodiment, the switching frequency control method of the power conversion device is applicable to application scenarios in which the switching frequency increases to the maximum switching frequency before the absolute value of the AC voltage drops to 0. As the AC current approaches or moves away from 0, the AC voltage drops or increases to 0 in the current range where the AC current is close to 0 (including 0). The power conversion device controls the switching frequency of the switch tube to first decrease to a first switching frequency, then maintain the first switching frequency, and finally continue to increase in the current range where the AC current is close to 0. This ensures that the switching frequency of the switch tube is less than the maximum switching frequency in the current range where the AC current is close to 0 and the voltage range where the AC voltage vac is close to 0 (including 0). This prevents the switching frequency of the switch tube from being limited by the limiter during closed-loop control, thereby improving the control effect of the power conversion device. In addition, because the switching frequency of the switch tube is less than the maximum switching frequency when the AC current is 0 and the AC voltage is 0, the voltage and current waveform distortion caused by the power conversion device switching between operating modes when the AC current is 0 and the AC voltage is 0 can be reduced, thereby improving the output performance of the power conversion device. Furthermore, in this embodiment, the first switching frequency is the same as the second switching frequency, which makes the switching frequency control method of the power conversion device simple and easy to implement.

[0015] In combination with the first aspect, in a fourth possible implementation, the controller is used to control the switching frequency of the switching tube to decrease to a second switching frequency after controlling the switching frequency of the switching tube to increase to the maximum switching frequency of the switching tube when the AC current at the AC end approaches 0.

[0016] In this embodiment, the switching frequency control method of the power conversion device is applicable to application scenarios in which the switching frequency increases to the maximum switching frequency after the absolute value of the AC voltage drops to 0. During the process of the AC current approaching or moving away from 0, the AC voltage has already dropped or increased to 0 before the AC current reaches a current range close to 0 (including 0). By controlling the switching frequency of the switch tube to first reduce to a second switching frequency and then increase it in the current range close to 0, the power conversion device ensures that the switching frequency of the switch tube is less than the maximum switching frequency in the current range close to 0. This prevents the switching frequency of the switch tube from being limited by the limiter during closed-loop control, thereby improving the control effect of the power conversion device. In addition, because the switching frequency of the switch tube is less than the maximum switching frequency when the AC current is 0 and the AC voltage is 0, the voltage and current waveform distortion caused by the power conversion device switching between operating modes when the AC current is 0 and the AC voltage is 0 can be reduced, thereby improving the output performance of the power conversion device.

[0017] In combination with any one of the first possible implementation manner of the first aspect to the fourth possible implementation manner of the first aspect, in a fifth possible implementation manner, the first switching frequency is the switching frequency of the switching tube when the AC voltage at the AC end is 0, and the first switching frequency is less than or equal to a first switching frequency threshold. The second switching frequency is the switching frequency of the switching tube when the AC current is 0, and the second switching frequency is less than or equal to a second switching frequency threshold.

[0018] In combination with the fifth possible implementation of the first aspect, in a sixth possible implementation, the first switching frequency threshold or the second switching frequency threshold is positively correlated with the DC current at the DC end or the effective value of the AC current at the AC end.

[0019] In this embodiment, the power conversion device can also adjust the size of the first switching frequency threshold or the second switching frequency threshold based on the current DC current at the DC end or the current effective value of the AC current at the AC end, thereby adjusting the first switching frequency or the second switching frequency, thereby ensuring that while improving the control effect and output performance of the power conversion device, the current peak and current loss of the inductor are reduced to improve the efficiency of the power conversion device.

[0020] In combination with the fifth possible implementation of the first aspect, in a seventh possible implementation, the first switching frequency threshold or the second switching frequency is positively correlated with the power factor of the AC end, and the power factor is the cosine value of the phase difference between the AC voltage and the AC current at the AC end.

[0021] In this embodiment, the power conversion device can also adjust the size of the first switching frequency threshold or the second switching frequency threshold based on the current power factor of the AC end, thereby achieving adjustment of the first switching frequency or the second switching frequency, thereby ensuring that while improving the control effect and output performance of the power conversion device, the current peak and current loss of the inductor are reduced to improve the efficiency of the power conversion device.

[0022] In combination with the fifth possible implementation of the first aspect, in an eighth possible implementation, the first switching frequency threshold or the second switching frequency threshold is negatively correlated with the DC voltage at the DC end, or the first switching frequency threshold or the second switching frequency threshold is positively correlated with the effective value of the AC voltage at the AC end.

[0023] In this embodiment, the power conversion device can also adjust the size of the first switching frequency threshold or the second switching frequency threshold based on the current DC voltage at the DC end or the current effective value of the AC voltage at the AC end, thereby adjusting the first switching frequency or the second switching frequency, thereby ensuring that while improving the control effect and output performance of the power conversion device, the current peak and current loss of the inductor are reduced to improve the efficiency of the power conversion device.

[0024] In combination with any one of the first aspect to the eighth possible implementation manner of the first aspect, in a ninth possible implementation manner, the power conversion device further includes an inductor, and the input end of the DC-side bridge circuit is connected to the DC end. The inductor is connected in series with the primary winding and then connected to the output end of the DC-side bridge circuit, and the secondary winding is connected to the input end of the AC-side bridge circuit. Alternatively, the primary winding is connected to the output end of the DC-side bridge circuit, and the inductor is connected in series with the secondary winding and then connected to the input end of the AC-side bridge circuit. The output end of the AC-side bridge circuit is connected to the AC end.

[0025] In this embodiment, the inductor can be connected in series not only with the primary winding but also with the secondary winding, thereby making the structure of the power conversion device diverse and highly flexible.

[0026] In combination with the ninth possible implementation of the first aspect, in a tenth possible implementation, the DC side bridge circuit includes a half-bridge circuit or a full-bridge circuit, and the AC side bridge circuit includes a half-bridge circuit or a full-bridge circuit.

[0027] In this embodiment, any bridge circuit of the DC side bridge circuit and the AC side bridge circuit can be a full-bridge circuit or a half-bridge circuit, and the power conversion device has diverse structures and high flexibility.

[0028] In combination with the tenth possible implementation manner of the first aspect, in an eleventh possible implementation manner, the DC side bridge circuit is a half-bridge circuit, and the power conversion device further includes a first resonant capacitor and a second resonant capacitor, wherein the first resonant capacitor and the second resonant capacitor are connected in series and then in parallel at the input end of the DC side bridge circuit. The primary winding is respectively connected to the output end of the DC side bridge circuit and the midpoint of the capacitor, or the primary winding and the inductor are connected in series between the output end of the DC side bridge circuit and the midpoint of the capacitor, and the midpoint of the capacitor is the connection point of the first resonant capacitor and the second resonant capacitor.

[0029] In this embodiment, the first resonant capacitor, the second resonant capacitor and the inductor form a resonant circuit, so that the power conversion device is a resonant power conversion device, so that the current waveform of the inductor tends to a sine wave, which is beneficial to reducing the peak value of the current flowing through the switching tube in the power conversion device, and is beneficial to the switching tube in the power conversion device to achieve soft switching, so as to reduce the loss of the switching tube, thereby improving the efficiency of the power conversion device.

[0030] In conjunction with the tenth possible implementation of the first aspect, in a twelfth possible implementation, the AC side bridge circuit is a half-bridge circuit, and the power conversion device further includes a first resonant capacitor and a second resonant capacitor, wherein the first resonant capacitor and the second resonant capacitor are connected in series and then in parallel at the output end of the AC side bridge circuit. The secondary winding is respectively connected to the input end of the AC side bridge circuit and the midpoint of the capacitor, or the secondary winding and the inductor are connected in series between the input end of the AC side bridge circuit and the midpoint of the capacitor, and the midpoint of the capacitor is the connection point of the first resonant capacitor and the second resonant capacitor.

[0031] In this embodiment, the first resonant capacitor, the second resonant capacitor and the inductor form a resonant circuit, so that the power conversion device is a resonant power conversion device, so that the current waveform of the inductor tends to a sine wave, which is beneficial to reducing the peak value of the current flowing through the switching tube in the power conversion device, and is beneficial to the switching tube in the power conversion device to achieve soft switching, so as to reduce the loss of the switching tube, thereby improving the efficiency of the power conversion device.

[0032] In combination with the tenth possible implementation manner of the first aspect, in the thirteenth possible implementation manner, the DC side bridge circuit and the AC side bridge circuit are both full-bridge circuits, the power conversion device also includes a first resonant capacitor, the primary winding and the first resonant capacitor are connected in series at the output end of the DC side bridge circuit, or, the secondary winding, the inductor and the first resonant capacitor are connected in series at the input end of the AC side bridge circuit, or, the primary winding, the inductor and the first resonant capacitor are connected in series at the output end of the DC side bridge circuit, or, the secondary winding and the first resonant capacitor are connected in series at the input end of the AC side bridge circuit.

[0033] In this embodiment, the first resonant capacitor and the inductor form a resonant circuit, so that the power conversion device is a resonant power conversion device, so that the current waveform of the inductor tends to a sine wave, which is beneficial to reducing the peak value of the current flowing through the switching tube in the power conversion device, and is beneficial to the switching tube in the power conversion device to achieve soft switching, so as to reduce the loss of the switching tube, thereby improving the efficiency of the power conversion device.

[0034] In a second aspect, the present application provides a control method for a power conversion device, the method comprising: when the AC current at the AC end approaches 0, first controlling the switching frequency of the switch tube to increase to the maximum switching frequency of the switch tube, and then controlling the switching frequency of the switch tube to be less than the maximum switching frequency when the AC current is 0. And when the AC current at the AC end moves away from 0, first controlling the switching frequency of the switch tube to increase to the maximum switching frequency of the switch tube, and then controlling the switching frequency of the switch tube to decrease. The method is applicable to a power conversion device, which includes a DC end, a DC side bridge circuit, a transformer, an AC side bridge circuit, and an AC end, the transformer including a primary winding and a secondary winding, wherein the DC side bridge circuit is connected between the DC end and the primary winding, and the AC side bridge circuit is connected between the secondary winding and the AC end.

[0035] In combination with the second aspect, in a first possible implementation, in the process of the AC current at the AC end approaching 0, the power conversion device controls the switching frequency of the switching tube to increase to the maximum switching frequency of the switching tube, first controls the switching frequency of the switching tube to decrease to the first switching frequency, and then controls the switching frequency of the switching tube to decrease to the second switching frequency.

[0036] In combination with the second aspect, in a second possible implementation, in the process of the AC current at the AC end approaching 0, the power conversion device controls the switching frequency of the switching tube to increase to the maximum switching frequency of the switching tube, first controls the switching frequency of the switching tube to decrease to the first switching frequency, and then controls the switching frequency of the switching tube to increase to the second switching frequency, wherein the second switching frequency is less than the maximum switching frequency.

[0037] In combination with the second aspect, in a third possible implementation, in the process of the AC current at the AC end approaching 0, the power conversion device controls the switching frequency of the switching tube to increase to the maximum switching frequency of the switching tube, first controls the switching frequency of the switching tube to decrease to the first switching frequency, and then controls the switching frequency of the switching tube to remain at the first switching frequency.

[0038] In combination with the second aspect, in a fourth possible implementation, when the AC current at the AC end approaches 0, the power conversion device controls the switching frequency of the switching tube to increase to the maximum switching frequency of the switching tube, and then controls the switching frequency of the switching tube to decrease to the second switching frequency.

[0039] In combination with any one of the first possible implementation manner of the second aspect to the fourth possible implementation manner of the second aspect, in a fifth possible implementation manner, the first switching frequency is the switching frequency of the switching tube when the AC voltage at the AC end is 0, and the first switching frequency is less than or equal to a first switching frequency threshold. The second switching frequency is the switching frequency of the switching tube when the AC current is 0, and the second switching frequency is less than or equal to a second switching frequency threshold.

[0040] In combination with the fifth possible implementation of the second aspect, in a sixth possible implementation, the first switching frequency threshold or the second switching frequency threshold is positively correlated with the DC current at the DC end or the effective value of the AC current at the AC end.

[0041] In combination with the fifth possible implementation of the second aspect, in a seventh possible implementation, the first switching frequency threshold or the second switching frequency is positively correlated with the power factor of the AC end, and the power factor is the cosine value of the phase difference between the AC voltage and the AC current at the AC end.

[0042] In combination with the fifth possible implementation of the second aspect, in an eighth possible implementation, the first switching frequency threshold or the second switching frequency threshold is negatively correlated with the DC voltage at the DC end, or the first switching frequency threshold or the second switching frequency threshold is positively correlated with the effective value of the AC voltage at the AC end.

[0043] In combination with any one of the second aspect to the eighth possible implementation manner of the second aspect, in a ninth possible implementation manner, the power conversion device further includes an inductor, and the input end of the DC-side bridge circuit is connected to the DC end. The inductor is connected in series with the primary winding and then connected to the output end of the DC-side bridge circuit, and the secondary winding is connected to the input end of the AC-side bridge circuit. Alternatively, the primary winding is connected to the output end of the DC-side bridge circuit, and the inductor is connected in series with the secondary winding and then connected to the input end of the AC-side bridge circuit. The output end of the AC-side bridge circuit is connected to the AC end.

[0044] In combination with the ninth possible implementation of the second aspect, in a tenth possible implementation, the DC side bridge circuit includes a half-bridge circuit or a full-bridge circuit, and the AC side bridge circuit includes a half-bridge circuit or a full-bridge circuit.

[0045] In conjunction with the tenth possible implementation of the second aspect, in an eleventh possible implementation, the DC side bridge circuit is a half-bridge circuit, and the power conversion device further includes a first resonant capacitor and a second resonant capacitor, wherein the first resonant capacitor and the second resonant capacitor are connected in series and then in parallel at the input end of the DC side bridge circuit. The primary winding is respectively connected to the output end of the DC side bridge circuit and the midpoint of the capacitor, or the primary winding and the inductor are connected in series between the output end of the DC side bridge circuit and the midpoint of the capacitor, and the midpoint of the capacitor is the connection point of the first resonant capacitor and the second resonant capacitor.

[0046] In conjunction with the tenth possible implementation of the second aspect, in a twelfth possible implementation, the AC side bridge circuit is a half-bridge circuit, and the power conversion device further includes a first resonant capacitor and a second resonant capacitor, wherein the first resonant capacitor and the second resonant capacitor are connected in series and then in parallel at the output end of the AC side bridge circuit. The secondary winding is respectively connected to the input end of the AC side bridge circuit and the midpoint of the capacitor, or the secondary winding and the inductor are connected in series between the input end of the AC side bridge circuit and the midpoint of the capacitor, and the midpoint of the capacitor is the connection point of the first resonant capacitor and the second resonant capacitor.

[0047] In combination with the tenth possible implementation manner of the second aspect, in the thirteenth possible implementation manner, the DC side bridge circuit and the AC side bridge circuit are both full-bridge circuits, the power conversion device also includes a first resonant capacitor, the primary winding and the first resonant capacitor are connected in series at the output end of the DC side bridge circuit, or, the secondary winding, the inductor and the first resonant capacitor are connected in series at the input end of the AC side bridge circuit, or, the primary winding, the inductor and the first resonant capacitor are connected in series at the output end of the DC side bridge circuit, or, the secondary winding and the first resonant capacitor are connected in series at the input end of the AC side bridge circuit.

[0048] It should be understood that the implementation and beneficial effects of the above-mentioned aspects of the present application can be referenced to each other. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] FIG1a is a schematic structural diagram of a DAB converter provided by the prior art;

[0050] FIG1b is a schematic diagram of a switching frequency waveform of a switch tube in a DAB converter provided by the prior art;

[0051] FIG2 is a schematic diagram of an application scenario of the power conversion device provided by the present application;

[0052] FIG3a is a schematic structural diagram of a power conversion device provided by the present application;

[0053] FIG3 b is another schematic structural diagram of the power conversion device provided by the present application;

[0054] FIG4 is another schematic structural diagram of the power conversion device provided by the present application;

[0055] FIG5 is a schematic diagram of a switching frequency waveform of a switch tube in the power conversion device provided by the present application;

[0056] FIG6 is a schematic diagram of another switching frequency waveform of a switch tube in the power conversion device provided by the present application;

[0057] FIG7 is a schematic diagram of another switching frequency waveform of a switch tube in the power conversion device provided by the present application;

[0058] FIG8 is another schematic diagram of a switching frequency waveform of a switch tube in the power conversion device provided by the present application;

[0059] FIG9a is another schematic structural diagram of the power conversion device provided by the present application;

[0060] FIG9b is another schematic structural diagram of the power conversion device provided by the present application;

[0061] FIG9c is another schematic structural diagram of the power conversion device provided by the present application;

[0062] FIG9d is another structural schematic diagram of the power conversion device provided by the present application

[0063] FIG10a is another schematic structural diagram of the power conversion device provided by the present application;

[0064] FIG10b is another schematic structural diagram of the power conversion device provided by the present application;

[0065] FIG11 is another structural schematic diagram of a power conversion device provided by the present application;

[0066] FIG12 is a flow chart of a control method for a power conversion device provided in the present application. DETAILED DESCRIPTION

[0067] The power conversion device provided in this application can be applied to various application fields such as energy storage power generation, photovoltaic power generation, new energy smart microgrid, power transmission and distribution, etc. The power conversion device provided in this application can be an inverter, a power storage converter (Power Conversion System, PCS), an uninterruptible power supply (Uninterrupted Power Supply, UPS), etc., and is applicable to different application scenarios, such as photovoltaic power supply scenarios, energy storage power supply scenarios, photovoltaic and storage hybrid power supply scenarios, UPS power supply scenarios, etc. The photovoltaic power supply scenario is used as an example for explanation below.

[0068] Refer to Figure 2, which is a schematic diagram of an application scenario of the power conversion device provided by the present application. In the photovoltaic power supply scenario, the power conversion device provided by the present application is the DAB converter shown in Figure 2, which includes a DC end, a DC side bridge circuit, a transformer T, an inductor Lr, an AC side bridge circuit, a controller and an AC end, and the transformer T includes a primary winding Lp and a secondary winding Ls. Among them, the DC side bridge circuit and the AC side bridge circuit include a full-bridge circuit or a half-bridge circuit. Here, the DC side bridge circuit and the AC side bridge circuit are both full-bridge circuits for description. For example, the DC side bridge circuit includes a first bridge arm and a second bridge arm connected in parallel, and the AC side bridge circuit includes a third bridge arm and a fourth bridge arm connected in parallel. The DC end of the DAB converter is connected to the photovoltaic module, and the AC end is connected to the AC power grid or household appliances. The first bridge arm consists of switches S1 and S2 connected in series, the second bridge arm consists of switches S3 and S4 connected in series, the third bridge arm consists of switches S5 and S6 connected in series, and the fourth bridge arm consists of switches S7 and S8 connected in series. Switches S1 through S8 can be controllable switches. The first and second bridge arms are connected in parallel at the DC end of the DAB converter, while the third and fourth bridge arms are connected in parallel at the AC end of the DAB converter. The two ends of the primary winding Lp are connected to the midpoint of the first and second bridge arms, respectively. The inductor Lr and the secondary winding Ls are connected in series between the midpoints of the third and fourth bridge arms.

[0069] After the DAB converter begins operation, the controller inverts the DC power at the DC end of the DAB converter into AC power by adjusting the switching frequencies of the switches in the four bridge arms, the phase difference between switch S1 and switch S4, the phase difference between switch S2 and switch S3, the phase difference between switch S5 and switch S8, the phase difference between switch S6 and switch S5, and the phase difference between any switch in the first bridge arm and any switch in the third bridge arm, thereby supplying power to various types of electrical devices, such as an AC grid or a load. Simultaneously, as the AC current at the AC end of the DAB converter approaches zero, the controller first increases the switching frequency of the switches in the four bridge arms to a maximum switching frequency, and then, when the AC current is zero, controls the switching frequency of the switches in the four bridge arms to be less than the maximum switching frequency. The controller also controls the switching frequency of the switches in the four bridge arms to increase to the maximum switching frequency of the switches, and then controls the switching frequency of the switches in the four bridge arms to decrease when the AC current at the AC end of the DAB converter is far away from zero.

[0070] It is understood that as the AC current at the AC end of the DAB converter approaches or moves away from zero, in the current range where the AC current at the AC end is close to zero (including zero), the DAB converter controls the switching frequency of the switch tube to be less than the maximum switching frequency, thereby preventing the switching frequency of the switch tube from being limited by the limiter during closed-loop control, thereby facilitating improved control of the DAB converter. Furthermore, because the switching frequency of the switch tube is less than the maximum switching frequency when the AC current at the AC end of the DAB converter is zero, voltage and current waveform distortion caused by the DAB converter switching operating modes when the AC current at the AC end is zero can be reduced, thereby facilitating improved output performance of the DAB converter. Furthermore, as the AC current at the AC end of the DAB converter approaches or moves away from zero, in the current range where the AC current at the AC end is far from zero, the DAB converter controls the switching frequency of the switch tubes in the four bridge arms according to the principle that the absolute value of the AC current is negatively correlated with the optimal switching frequency of the switch tube, thereby reducing the current peak and current loss of the inductor Lr, thereby improving the efficiency of the DAB converter.

[0071] The above is only an example of the application scenarios of the power conversion device provided by this application, and is not an exhaustive list. This application does not limit the application scenarios.

[0072] The working principle of the power conversion device provided in this application is illustrated below with reference to FIG. 3 a to FIG. 11 .

[0073] Refer to Figure 3a, which is a structural schematic diagram of the power conversion device provided by the present application. As shown in Figure 3a, the power conversion device 1 includes a DC end, a DC side bridge circuit 11, a transformer 12, an AC side bridge circuit 13, a controller 14 and an AC end. The transformer 12 includes a primary winding Lp and a secondary winding Ls. The DC side bridge circuit 11 and the AC side bridge circuit 13 include switching tubes. Among them, the DC end of the power conversion device 1 includes a first DC end i11 and a second DC end i12, and the first DC end i11 and the second DC end i12 are used to connect to the positive and negative poles of the DC power supply, respectively. The DC power supply includes a photovoltaic module or a battery cluster. The AC end of the power conversion device 1 includes a first AC end o11 and a second AC end o12, and the first AC end o11 and the second AC end o12 are used to connect to the AC power grid. The DC-side bridge circuit 11 is connected between the DC terminal and the primary winding Lp. Specifically, the input terminals i111 and i112 of the DC-side bridge circuit 11 are connected to the first DC terminal i11 and the second DC terminal i12, respectively. The output terminal o111 of the DC-side bridge circuit 11 is connected to the primary winding Lp. The AC-side bridge circuit 13 is connected between the secondary winding Ls and the AC terminal. Specifically, the input terminal i131 of the AC-side bridge circuit 13 is connected to the secondary winding Ls, and the output terminals o131 and o132 of the AC-side bridge circuit 13 are connected to the first AC terminal o11 and the second AC terminal o12, respectively, of the power conversion device 1. The DC-side bridge circuit 11 comprises a full-bridge circuit or a half-bridge circuit, and the AC-side bridge circuit 13 comprises a full-bridge circuit or a half-bridge circuit. The switching tubes in the DC side bridge circuit 11 and the AC side bridge circuit 13 can be controllable switching tubes, such as Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET), Bipolar Junction Transistor (BJT), Insulated Gate Bipolar Transistor (IGBT), etc.

[0074] The power conversion device 1 also includes an inductor Lr, which is connected in series with the secondary winding Ls and then connected to the input terminal i131 of the AC-side bridge circuit 13. Optionally, the inductor Lr can also be connected in series with the primary winding Lp. For details, see the power conversion device 1 shown in Figure 3b. As shown in Figure 3b, the inductor Lr is connected in series with the primary winding Lp and then connected to the output terminal o111 of the DC-side bridge circuit 11. The secondary winding Ls is connected to the input terminal i131 of the AC-side bridge circuit 13.

[0075] In one embodiment, after the power converter 1 is in operation, the controller 14 begins detecting the AC current at the AC end of the power converter 1. As the AC current at the AC end of the power converter 1 approaches zero, the controller 14 first controls the switching frequency of the switches in the DC-side bridge circuit 11 and the AC-side bridge circuit 13 to increase to the maximum switching frequency of the switches. Then, when the AC current at the AC end of the power converter 1 is zero, the controller 14 controls the switching frequency of the switches in the DC-side bridge circuit 11 and the AC-side bridge circuit 13 to be less than the maximum switching frequency. Furthermore, as the AC current at the AC end of the power converter 1 moves away from zero, the controller 14 first controls the switching frequency of the switches in the DC-side bridge circuit 11 and the AC-side bridge circuit 13 to increase to the maximum switching frequency of the switches. Then, the controller 14 controls the switching frequency of the switches in the DC-side bridge circuit 11 and the AC-side bridge circuit 13 to decrease. The maximum switching frequency may be the maximum operating frequency of the transformer 12, and all switches in the DC-side bridge circuit 11 and the AC-side bridge circuit 13 have the same switching frequency.

[0076] In the embodiment of the present application, as the AC current at the AC end of the power conversion device 1 approaches or moves away from zero, in the current range where the AC current at the AC end is close to zero (including zero), the power conversion device 1 controls the switching frequency of the switching tubes in the DC-side bridge circuit 11 and the AC-side bridge circuit 13 to be less than the maximum switching frequency, thereby preventing the switching frequency of the switching tubes from being limited by the limiter during closed-loop control, thereby facilitating improved control effects of the power conversion device 1. In addition, since the switching frequency of the switching tubes is less than the maximum switching frequency when the AC current at the AC end of the power conversion device 1 is zero, the voltage and current waveform distortions caused by the switching of the operating mode of the power conversion device 1 when the AC current at the AC end is zero can be reduced, thereby facilitating improved output performance of the power conversion device 1. Furthermore, in the process of the AC current at the AC end of the power conversion device 1 approaching or moving away from 0, in the current range where the AC current at the AC end is far from 0, the power conversion device 1 controls the switching frequency of the switching tube according to the principle that the absolute value of the AC current is negatively correlated with the optimal switching frequency of the switching tube, so as to reduce the current peak and current loss of the inductor Lr, thereby improving the efficiency of the power conversion device 1.

[0077] Since the power conversion device 1 controls the switching frequency of the switching tube in the same manner regardless of whether the inductor Lr in the power conversion device 1 is connected in series with the primary winding Lp or the secondary winding Ls, for the sake of convenience, the working principle of the power conversion device 1 is introduced below by taking the inductor Lr connected in series with the secondary winding Ls as an example.

[0078] For example, refer to Figure 4, which is another structural schematic diagram of the power conversion device provided by the present application. As shown in Figure 4, compared with the power conversion device 1 shown in Figure 3a, the DC side bridge circuit 11 and the AC side bridge circuit 13 in the power conversion device 1 shown in Figure 4 are both full-bridge circuits. Specifically, as shown in Figure 4, the DC side bridge circuit 11 includes a bridge arm 111 and a bridge arm 112 connected in parallel, and the AC side bridge circuit 13 includes a bridge arm 131 and a bridge arm 132 connected in parallel. The bridge arm 111 includes a switch tube S1 and a switch tube S2 connected in series, the bridge arm 112 includes a switch tube S3 and a switch tube S4 connected in series, the bridge arm 131 includes a switch tube S5 and a switch tube S6 connected in series, and the bridge arm 132 includes a switch tube S7 and a switch tube S8 connected in series. The input terminals i111 and i112 of the DC-side bridge circuit 11 are the two ends of bridge arm 111, respectively. The output terminals o111 (i.e., the midpoint of bridge arm 111) and o112 (i.e., the midpoint of bridge arm 112) of the DC-side bridge circuit 11 are connected to the same-name terminal and opposite-name terminal of the primary winding Lp, respectively. The secondary winding Ls is connected in series with the inductor Lr and then connected to the input terminal of the AC-side bridge circuit 13. Specifically, the secondary winding Ls and the inductor Lr are connected in series between the input terminal i131 (i.e., the midpoint of bridge arm 131) and the input terminal i132 (i.e., the midpoint of bridge arm 132) of the AC-side bridge circuit 13. The output terminals o131 and o132 of the AC-side bridge circuit 13 are the two ends of bridge arm 131, respectively. The midpoint of any of the four bridge arms is the junction of the two switching transistors within that arm. Optionally, the power conversion device 1 further includes an input capacitor Cdc and an output capacitor Cac, the input capacitor Cdc is connected between the first DC terminal i11 and the second DC terminal i12 of the power conversion device 1, and the output capacitor Cac is connected between the first AC terminal o11 and the second AC terminal o12 of the power conversion device 1.

[0079] After the power conversion device 1 is in operation, the controller 14 controls the output power of the power conversion device 1 by adjusting the switching frequency of the switches in each bridge arm, the phase difference between the phases of switch S1 and switch S4, the phase difference between the phases of switch S2 and switch S3, the phase difference between the phases of switch S5 and switch S8, the phase difference between the phases of switch S6 and switch S7, and the phase difference between the phases of any switch in bridge arm 111 and any switch in bridge arm 131. Simultaneously, the controller 14 detects the AC current iac at the AC end of the power conversion device 1. As the AC current iac approaches zero, the controller 14 first increases the switching frequency of each switch in the four bridge arms to its maximum switching frequency. Then, when the AC current iac is zero, the controller 14 controls the switching frequency of each switch in the four bridge arms to be less than the maximum switching frequency. When the AC current iac is far away from 0, the controller 14 first controls the switching frequency of each switch in the four bridge arms to increase to the maximum switching frequency of the switch, and then controls the switching frequency of each switch in the four bridge arms to decrease.

[0080] In one embodiment, before the absolute value of the AC voltage vac and the absolute value of the AC current iac at the AC end of the power conversion device 1 drop to 0, the switching frequency of any switch tube in the four bridge arms increases to the maximum switching frequency:

[0081] In an optional embodiment, before controlling the power conversion device 1 to output power, the controller 14 determines the magnitude of the first switching frequency threshold and the second switching frequency threshold when the power factor PF at the AC end of the power conversion device 1 is the preset power factor based on the preset mapping relationship between the power factor PF at the AC end and the first switching frequency threshold and the second switching frequency threshold. The power factor PF at the AC end is the cosine of the phase difference between the AC voltage vac and the AC current iac. Since the smaller the absolute value of the power factor PF at the AC end, the larger the instantaneous absolute value of the AC voltage vac when the AC current iac crosses zero, and the larger the instantaneous absolute value of the AC current iac when the AC voltage vac crosses zero, the smaller the switching frequency limit value (i.e., the first switching frequency threshold) when the AC current iac crosses zero, and the smaller the switching frequency limit value (i.e., the second switching frequency threshold) when the AC voltage vac crosses zero. Based on this, the preset mapping relationship between the power factor PF at the AC end and the first switching frequency threshold and the second switching frequency threshold is that both the first switching frequency threshold and the second switching frequency threshold are positively correlated with the power factor PF at the AC end.

[0082] The controller 14 then adjusts the phase difference between switch S1 and switch S4, the phase difference between switch S2 and switch S3, the phase difference between switch S5 and switch S8, the phase difference between switch S6 and switch S7, and the phase difference between any switch in bridge arm 111 and any switch in bridge arm 131, and adjusts the switching frequency of the switches in each bridge arm to control the output power of the power conversion device 1 and ensure that the power factor PF at the AC end is a preset power factor. Simultaneously, the controller 14 detects the AC current iac. As the AC current iac approaches zero, the controller 14 first increases the switching frequency of each switch in the four bridge arms to its maximum switching frequency. After increasing the switching frequency of each switch to its maximum switching frequency, the controller 14 first decreases the switching frequency of each switch to the first switching frequency, and then decreases the switching frequency of each switch to the second switching frequency. When the AC current iac is far away from 0, the controller 14 first controls the switching frequency of each switch tube to increase from the second switching frequency to the maximum switching frequency of the switch tube, and then controls the switching frequency of each switch tube to decrease.

[0083] Among them, the first switching frequency is the switching frequency of each switching tube when the AC voltage vac is 0, and the first switching frequency is less than or equal to the first switching frequency threshold. The second switching frequency is the switching frequency of each switching tube when the AC current iac is 0, and the second switching frequency is less than or equal to the second switching frequency threshold. For ease of understanding, the following is a detailed introduction to this embodiment with the first switching frequency as the first switching frequency threshold and the second switching frequency as the second switching frequency threshold as an example in conjunction with the switching frequency waveform diagram of the switching tube shown in Figure 5. As shown in Figure 5, in the t0 to t1 period within a working cycle T of the AC current iac (i.e., the t0 to t7 period), the AC current iac continues to increase from 0, that is, the AC current iac begins to gradually move away from 0, then the controller 14 controls the switching frequency fs of each switching tube to increase from the second switching frequency threshold fs_i_limit.

[0084] At time t1 , the controller 14 controls the switching frequency fs of each switch to increase to the maximum switching frequency fs_limit of the switch.

[0085] During the period from t1 to t2, the AC current iac continues to increase, that is, the AC current iac continues to gradually move away from 0. The controller 14 controls the switching frequency fs of each switching tube to decrease from the maximum switching frequency fs_limit according to the principle that the absolute value of the AC current iac is negatively correlated with the optimal switching frequency of the switching tube sought by the power conversion device 1.

[0086] At time t2, the AC current iac reaches its maximum value. The controller 14 controls the switching frequency fs of each switching tube to decrease to the switching frequency fs_1 according to the principle that the absolute value of the AC current iac is negatively correlated with the optimal switching frequency of the switching tube sought by the power conversion device 1.

[0087] During the period from t2 to t3, the AC current iac begins to decrease continuously, that is, the AC current iac begins to approach 0. Then the controller 14 controls the switching frequency fs of each switching tube to gradually increase from the switching frequency fs_1 according to the principle that the absolute value of the AC current iac is negatively correlated with the optimal switching frequency of the switching tube sought by the power conversion device 1.

[0088] At time t3 , the controller 14 controls the switching frequency fs of each switch to increase to the maximum switching frequency fs_limit of the switch according to the principle that the absolute value of the AC current iac is negatively correlated with the optimal switching frequency of the switch sought by the power conversion device 1 .

[0089] During the period from t3 to t4, the AC current iac continues to decrease, that is, the AC current iac continues to approach 0. The controller 14 no longer follows the principle that the absolute value of the AC current iac is negatively correlated with the optimal switching frequency of the switching tube sought by the power conversion device 1, but instead controls the switching frequency fs of each switching tube to decrease from the maximum switching frequency fs_limit.

[0090] At time t4 , the AC voltage vac decreases to 0, and the controller 14 controls the switching frequency fs of each switch tube to decrease to the first switching frequency threshold fs_v_limit.

[0091] During the period from t4 to t5 , the AC current iac continues to decrease, that is, the AC current iac continues to approach 0, and the controller 14 controls the switching frequency fs of each switch tube to continue to decrease from the first switching frequency threshold fs_v_limit.

[0092] At time t5 , the AC current iac drops to 0, and the controller 14 controls the switching frequency fs of each switch tube to decrease to the second switching frequency threshold fs_i_limit.

[0093] During the period from t5 to t6 , the AC current iac decreases continuously from 0, that is, the AC current iac gradually moves away from 0, and the controller 14 controls the switching frequency fs of each switch tube to increase from the second switching frequency threshold fs_i_limit.

[0094] At time t6 , the controller 14 controls the switching frequency fs of each switch to increase to a maximum switching frequency fs_limit.

[0095] During the period from t6 to t7, the AC current iac continues to decrease, that is, the AC current iac continues to gradually move away from 0. The controller 14 controls the switching frequency fs of each switching tube to decrease from the maximum switching frequency fs_limit according to the principle that the absolute value of the AC current iac is negatively correlated with the optimal switching frequency of the switching tube sought by the power conversion device 1.

[0096] At time t7 , the controller 14 controls the switching frequency fs of each switching tube to decrease to the switching frequency fs_1 according to the principle that the absolute value of the AC current iac is negatively correlated with the optimal switching frequency of the switching tube sought by the power conversion device 1 .

[0097] During the period from t7 to t8, the AC current iac continues to increase from 0, that is, the AC current iac begins to approach 0. Then the controller 14 controls the switching frequency fs of each switching tube to increase from the switching frequency fs_1 according to the principle that the absolute value of the AC current iac is negatively correlated with the optimal switching frequency of the switching tube sought by the power conversion device 1.

[0098] At time t8 , the controller 14 controls the switching frequency fs of each switch to increase to a maximum switching frequency fs_limit.

[0099] During the period from t8 to t9, the AC current iac continues to increase, that is, the AC current iac continues to approach 0. The controller 14 no longer follows the principle that the absolute value of the AC current iac is negatively correlated with the optimal switching frequency of the switching tube sought by the power conversion device 1, but instead controls the switching frequency fs of each switching tube to start decreasing from the maximum switching frequency fs_limit.

[0100] At time t9 , the AC voltage vac increases to 0, and the controller 14 controls the switching frequency fs of each switch tube to decrease to the first switching frequency threshold fs_v_limit.

[0101] During the period from t9 to t10 , the AC current iac continues to increase, that is, the AC current iac continues to approach 0, and the controller 14 controls the switching frequency fs of each switch tube to continue to decrease from the first switching frequency threshold fs_v_limit.

[0102] At time t10 , the AC current iac increases to 0, and the controller 14 controls the switching frequency fs of each switch tube to decrease to the second switching frequency threshold fs_i_limit.

[0103] It should be noted that in order to ensure that the efficiency of the power conversion device 1 is improved while improving the control effect of the power conversion device 1 and enhancing the output performance of the power conversion device 1, the controller 14 can also adjust the size of the first switching frequency threshold fs_v_limit and the second switching frequency threshold fs_i_limit based on the change of the power factor PF of the AC end. Specifically, the controller 14 can detect the current power factor PF of the AC end according to a preset frequency (such as an integer multiple of the AC frequency of the AC current iac), and adjust the first switching frequency threshold fs_v_limit and the second switching frequency threshold fs_i_limit based on the current power factor PF of the AC end, so that the first switching frequency threshold fs_v_limit and the second switching frequency threshold fs_i_limit are both positively correlated with the power factor PF of the AC end. In other words, the first switching frequency threshold fs_v_limit and the second switching frequency threshold fs_i_limit both increase with the increase of the power factor PF of the AC end, and decrease with the decrease of the power factor PF of the AC end.

[0104] In addition, for this embodiment, the controller 14 can still make the switching frequency of each switching tube less than the maximum switching frequency within the current range where the AC current iac is close to 0 (corresponding to the current range (iac_th2, iac_th1) in Figure 5) without limiting the first switching frequency to be less than or equal to the first switching frequency threshold and the second switching frequency to be less than or equal to the second switching frequency threshold.

[0105] It can be understood that, in the process of the AC current iac approaching or moving away from 0, in the current range where the AC current iac is closer to 0, the power conversion device 1 controls the switching frequency fs of each switching tube to first reduce to the first switching frequency, then reduce to the second switching frequency, and finally increase, thereby ensuring that in the current range where the AC current iac is closer to 0 and the voltage range where the AC voltage vac is closer to 0, the switching frequency fs of each switching tube is less than the maximum switching frequency, so that the switching frequency of the switching tube will not be limited by the limiter during closed-loop control, thereby facilitating the improvement of the control effect of the power conversion device 1. In addition, since the switching frequency fs of each switching tube is less than the maximum switching frequency when the AC current iac is 0 and the AC voltage vac is 0, the voltage and current waveform distortion caused by the power conversion device 1 switching the working mode when the AC current iac is 0 and the AC voltage vac is 0 can be reduced, thereby facilitating the improvement of the output performance of the power conversion device 1. Furthermore, in the process of the AC current iac approaching or moving away from 0, in the current interval where the AC current iac is far from 0 (corresponding to the current interval where the AC current iac is greater than or equal to iac_th1 and the current interval where the AC current iac is less than or equal to iac_th2 in Figure 5), the power conversion device 1 controls the switching frequency fs of each switching tube according to the principle that the absolute value of the AC current is negatively correlated with the optimal switching frequency of the switching tube, so as to reduce the current peak and current loss of the inductor Lr, thereby improving the efficiency of the power conversion device 1.

[0106] In another optional embodiment, before controlling the power conversion device 1 to output power, the controller 14 determines the magnitudes of the first switching frequency threshold and the second switching frequency threshold when the power factor PF at the AC end of the power conversion device 1 is the preset power factor, based on a preset mapping relationship between the power factor PF at the AC end and the first switching frequency threshold and the second switching frequency threshold. The controller 14 then controls the output power of the power conversion device 1 and sets the power factor PF at the AC end to the preset power factor. Simultaneously, the controller 14 detects the AC current iac. As the AC current iac approaches zero, the controller 14 first controls the switching frequency of each switch in the four bridge arms to increase to the maximum switching frequency of the switch. After controlling the switching frequency of each switch to increase to the maximum switching frequency of the switch, the controller 14 first controls the switching frequency of each switch to decrease to the first switching frequency, and then controls the switching frequency of each switch to increase to the second switching frequency, where the second switching frequency is less than the maximum switching frequency. When the AC current iac is far away from 0, the controller 14 first controls the switching frequency of each switch tube to increase from the second switching frequency to the maximum switching frequency of the switch tube, and then controls the switching frequency of each switch tube to decrease.

[0107] The first switching frequency is the switching frequency of each switch when the AC voltage vac is 0, and the first switching frequency is less than or equal to the first switching frequency threshold. The second switching frequency is the switching frequency of each switch when the AC current iac is 0, and the second switching frequency is less than or equal to the second switching frequency threshold.

[0108] For ease of understanding, the present embodiment is described in detail below with reference to the schematic diagram of the switching frequency waveform of the switching tube shown in FIG6 , taking the first switching frequency as the first switching frequency threshold and the second switching frequency as the second switching frequency threshold as an example. As shown in FIG6 , the specific control method by which the controller 14 controls the switching frequency fs of each switching tube during the period t0 to t4 within a duty cycle T of the AC current iac can be found in the description corresponding to the period t0 to t4 in FIG5 , and will not be repeated here.

[0109] At time t4 , the AC voltage vac decreases to 0, and the controller 14 controls the switching frequency fs of each switch tube to decrease to the first switching frequency threshold fs_v_limit.

[0110] During the period from t4 to t5 , the AC current iac continues to decrease, that is, the AC current iac continues to approach 0, and the controller 14 controls the switching frequency fs of each switch tube to increase from the first switching frequency threshold fs_v_limit.

[0111] At time t5 , the AC current iac drops to 0, and the controller 14 controls the switching frequency fs of each switch tube to increase to a second switching frequency threshold fs_i_limit, where the second switching frequency threshold fs_i_limit is less than the maximum switching frequency fs_limit.

[0112] During the period from t5 to t6 , the AC current iac decreases continuously from 0, that is, the AC current iac gradually moves away from 0, and the controller 14 controls the switching frequency fs of each switch tube to increase from the second switching frequency threshold fs_i_limit.

[0113] At time t6 , the controller 14 controls the switching frequency fs of each switch to increase to a maximum switching frequency fs_limit.

[0114] For the specific control method of the controller 14 controlling the switching frequency fs of each switch tube during the period from t6 to t9, please refer to the description corresponding to the period from t6 to t9 in FIG. 5 , which will not be repeated here.

[0115] At time t9 , the AC voltage vac increases to 0, and the controller 14 controls the switching frequency fs of each switch tube to decrease to the first switching frequency threshold fs_v_limit.

[0116] During the period from t9 to t10 , the AC current iac continues to increase, that is, the AC current iac continues to approach 0, and the controller 14 controls the switching frequency fs of each switch tube to increase from the first switching frequency threshold fs_v_limit.

[0117] At time t10 , the AC current iac increases to 0, and the controller 14 controls the switching frequency fs of each switch tube to increase to the second switching frequency threshold fs_i_limit.

[0118] It should be noted that in order to ensure that the efficiency of the power conversion device 1 is improved while improving the control effect of the power conversion device 1 and enhancing the output performance of the power conversion device 1, the controller 14 can also adjust the size of the first switching frequency threshold fs_v_limit and the second switching frequency threshold fs_i_limit based on the change of the power factor PF of the AC end. Specifically, the controller 14 can detect the current power factor PF of the AC end according to a preset frequency (such as an integer multiple of the AC frequency of the AC current iac), and adjust the first switching frequency threshold fs_v_limit and the second switching frequency threshold fs_i_limit based on the current power factor PF of the AC end, so that the first switching frequency threshold fs_v_limit and the second switching frequency threshold fs_i_limit are both positively correlated with the power factor PF of the AC end.

[0119] In addition, for this embodiment, the controller 14 does not limit the second switching frequency to be less than or equal to the second switching frequency threshold, but only limits the second switching frequency to be less than the maximum switching frequency. In this case, the switching frequency of each switching tube can still be less than the maximum switching frequency within the current range where the AC current iac is close to 0 (corresponding to the current range (iac_th2, iac_th1) in Figure 6).

[0120] It can be understood that, in the process of the AC current iac approaching or moving away from 0, in the current range where the AC current iac is closer to 0, the power conversion device 1 controls the switching frequency fs of each switch tube to first reduce to the first switching frequency, then increase to the second switching frequency, and finally continue to increase, thereby ensuring that in the current range where the AC current iac is closer to 0 and the voltage range where the AC voltage vac is closer to 0, the switching frequency fs of each switch tube is less than the maximum switching frequency, so that the switching frequency of the switch tube will not be limited by the limiter during closed-loop control, thereby facilitating the improvement of the control effect of the power conversion device 1. In addition, since the switching frequency fs of each switch tube is less than the maximum switching frequency when the AC current iac is 0 and the AC voltage vac is 0, the voltage and current waveform distortion caused by the power conversion device 1 switching the working mode when the AC current iac is 0 and the AC voltage vac is 0 can be reduced, thereby facilitating the improvement of the output performance of the power conversion device 1. In addition, as the AC current iac approaches or moves away from zero, in the current interval where the AC current iac is far from zero (corresponding to the current interval where the AC current iac is greater than or equal to iac_th1 and the current interval where the AC current iac is less than or equal to iac_th2 in FIG6 ), the power conversion device 1 controls the switching frequency fs of each switching tube in accordance with the principle that the absolute value of the AC current is negatively correlated with the optimal switching frequency of the switching tube, so as to reduce the current peak and current loss of the inductor Lr, thereby improving the efficiency of the power conversion device 1. Furthermore, since the first switching frequency is lower than the second switching frequency in this embodiment, when the AC voltage vac is low, the power conversion device 1 can output a larger AC current iac, thereby lowering the power factor PF and enhancing applicability.

[0121] In another optional embodiment, before controlling the power conversion device 1 to output power, the controller 14 determines the magnitudes of the first switching frequency threshold and the second switching frequency threshold when the power factor PF at the AC end of the power conversion device 1 is at the preset power factor, based on a preset mapping relationship between the power factor PF at the AC end and the first switching frequency threshold and the second switching frequency threshold. The controller 14 then controls the output power of the power conversion device 1 to ensure that the power factor PF at the AC end is at the preset power factor. Simultaneously, the controller 14 detects the AC current iac. As the AC current iac approaches zero, the controller 14 first controls the switching frequency of each switch in the four bridge arms to increase to its maximum switching frequency. After controlling the switching frequency of each switch to increase to its maximum switching frequency, the controller 14 first controls the switching frequency of each switch to decrease to the first switching frequency, and then controls the switching frequency of each switch to remain at the first switching frequency. Furthermore, as the AC current iac moves away from zero, the controller 14 first controls the switching frequency of each switch to increase from the first switching frequency to its maximum switching frequency, and then controls the switching frequency of each switch to decrease.

[0122] The first switching frequency is the switching frequency of each switch when the AC voltage vac is 0, and the first switching frequency is less than or equal to the first switching frequency threshold. The second switching frequency is the switching frequency of each switch when the AC current iac is 0, and the second switching frequency is less than or equal to the second switching frequency threshold.

[0123] For ease of understanding, the present embodiment is described in detail below with reference to the schematic diagram of the switching frequency waveform of the switching tube shown in FIG7 , taking the first switching frequency as the first switching frequency threshold and the second switching frequency as the second switching frequency threshold as an example. As shown in FIG7 , the specific control method by which the controller 14 controls the switching frequency fs of each switching tube during the period t0 to t4 within a duty cycle T of the AC current iac can be found in the description corresponding to the period t0 to t4 in FIG5 , and will not be repeated here.

[0124] At time t4 , the AC voltage vac decreases to 0, and the controller 14 controls the switching frequency fs of each switch tube to decrease to the first switching frequency threshold fs_v_limit.

[0125] During the period from t4 to t5 , the AC current iac continues to decrease, that is, the AC current iac continues to approach 0, and the controller 14 controls the switching frequency fs of each switch to be maintained at the first switching frequency threshold fs_v_limit.

[0126] At time t5 , the AC current iac drops to 0, and the controller 14 controls the switching frequency of each switch to remain at the first switching frequency threshold fs_v_limit. The first switching frequency threshold fs_v_limit is the same as the second switching frequency threshold fs_i_limit.

[0127] During the period from t5 to t6 , the AC current iac decreases continuously from 0, that is, the AC current iac gradually moves away from 0, and the controller 14 controls the switching frequency fs of each switch tube to increase from the first switching frequency threshold fs_v_limit.

[0128] At time t6 , the controller 14 controls the switching frequency fs of each switch to increase to a maximum switching frequency fs_limit.

[0129] For the specific control method of the controller 14 controlling the switching frequency fs of each switch tube during the period from t6 to t9, please refer to the description corresponding to the period from t6 to t9 in FIG. 5 , which will not be repeated here.

[0130] At time t9 , the AC voltage vac increases to 0, and the controller 14 controls the switching frequency fs of each switch tube to decrease to the first switching frequency threshold fs_v_limit.

[0131] During the period from t9 to t10 , the AC current iac continues to increase, that is, the AC current iac continues to approach 0, and the controller 14 controls the switching frequency fs of each switch to be maintained at the first switching frequency threshold fs_v_limit.

[0132] At time t10 , the AC current iac increases to 0, and the controller 14 controls the switching frequency fs of each switch to remain at the first switching frequency threshold fs_v_limit.

[0133] It should be noted that in order to ensure that the efficiency of the power conversion device 1 is improved while improving the control effect of the power conversion device 1 and the output performance of the power conversion device 1, the controller 14 can also adjust the first switching frequency threshold value fs_v_limit and the second switching frequency threshold value fs_i_limit based on the change of the power factor PF at the AC end. Specifically, the controller 14 can detect the current power factor PF at the AC end according to a preset frequency, and adjust the first switching frequency threshold value fs_v_limit and the second switching frequency threshold value fs_i_limit based on the current power factor PF at the AC end, so that the first switching frequency threshold value fs_v_limit and the second switching frequency threshold value fs_i_limit are both positively correlated with the power factor PF at the AC end.

[0134] In addition, for this embodiment, the controller 14 can still make the switching frequency of each switching tube less than the maximum switching frequency within the current range where the AC current iac is close to 0 (corresponding to the current range (iac_th2, iac_th1) in Figure 7) without limiting the first switching frequency to be less than or equal to the first switching frequency threshold.

[0135] It can be understood that, in the process of the AC current iac approaching or moving away from 0, in the current range where the AC current iac is closer to 0, the power conversion device 1 controls the switching frequency fs of each switch tube to first reduce to the first switching frequency, then increase to the second switching frequency, and finally continue to increase, thereby ensuring that in the current range where the AC current iac is closer to 0 and the voltage range where the AC voltage vac is closer to 0, the switching frequency fs of each switch tube is less than the maximum switching frequency, so that the switching frequency of the switch tube will not be limited by the limiter during closed-loop control, thereby facilitating the improvement of the control effect of the power conversion device 1. In addition, since the switching frequency fs of each switch tube is less than the maximum switching frequency when the AC current iac is 0 and the AC voltage vac is 0, the voltage and current waveform distortion caused by the power conversion device 1 switching the working mode when the AC current iac is 0 and the AC voltage vac is 0 can be reduced, thereby facilitating the improvement of the output performance of the power conversion device 1. In addition, as the AC current iac approaches or moves away from zero, in the current interval where the AC current iac is far from zero (corresponding to the current interval where the AC current iac is greater than or equal to iac_th1 and the current interval where the AC current iac is less than or equal to iac_th2 in FIG7 ), the power conversion device 1 controls the switching frequency fs of each switching tube in accordance with the principle that the absolute value of the AC current is negatively correlated with the optimal switching frequency of the switching tube, thereby reducing the current peak and current loss of the inductor Lr, thereby improving the efficiency of the power conversion device 1. Furthermore, in this embodiment, the first switching frequency is the same as the second switching frequency, which makes the switching frequency control method of the power conversion device 1 simple and easy to implement.

[0136] In another embodiment, after the absolute value of the AC voltage vac drops to 0, the switching frequency of any switch in the four bridge arms increases to the maximum switching frequency:

[0137] In one embodiment, before controlling the power output of the power converter 1, the controller 14 determines the magnitude of the first switching frequency threshold when the power factor PF at the AC end of the power converter 1 is at the preset power factor based on a preset mapping relationship between the power factor PF at the AC end and the first switching frequency threshold. The controller 14 then controls the output power of the power converter 1 to ensure that the power factor PF at the AC end is at the preset power factor. Simultaneously, the controller 14 detects the AC current iac. As the AC current iac approaches zero, the controller 14 first controls the switching frequency of each switch in the four bridge arms to increase to its maximum switching frequency. After controlling the switching frequency of each switch to increase to its maximum switching frequency, the controller 14 controls the switching frequency of each switch to decrease to a second switching frequency. Furthermore, as the AC current iac moves away from zero, the controller 14 first controls the switching frequency of each switch to increase from the second switching frequency to its maximum switching frequency, and then controls the switching frequency of each switch to decrease.

[0138] The second switching frequency is the switching frequency of each switch tube when the AC current iac is 0, and the second switching frequency is less than or equal to the second switching frequency threshold.

[0139] For ease of understanding, the present embodiment is described in detail below with reference to the schematic diagram of the switching frequency waveform of the switching tube shown in FIG8 , taking the second switching frequency as the second switching frequency threshold as an example. As shown in FIG8 , during the period t0 to t1 within a duty cycle T of the AC current iac (i.e., the period t0 to t10 ), the AC current iac continuously increases from 0, i.e., the AC current iac gradually begins to move away from 0. The controller 14 then controls the switching frequency fs of each switching tube to increase from the second switching frequency threshold fs_i_limit.

[0140] At time t1 , the controller 14 controls the switching frequency fs of each switch to increase to the maximum switching frequency fs_limit of the switch.

[0141] During the period from t1 to t2, the AC current iac continues to increase, that is, the AC current iac continues to gradually move away from 0. The controller 14 controls the switching frequency fs of each switching tube to decrease from the maximum switching frequency fs_limit according to the principle that the absolute value of the AC current iac is negatively correlated with the optimal switching frequency of the switching tube sought by the power conversion device 1.

[0142] At time t2, the AC current iac reaches its maximum value. The controller 14 controls the switching frequency fs of each switching tube to decrease to the switching frequency fs_1 according to the principle that the absolute value of the AC current iac is negatively correlated with the optimal switching frequency of the switching tube sought by the power conversion device 1.

[0143] During the period from t2 to t3, the AC current iac begins to decrease continuously, that is, the AC current iac begins to approach 0. Then the controller 14 controls the switching frequency fs of each switching tube to gradually increase from the switching frequency fs_1 according to the principle that the absolute value of the AC current iac is negatively correlated with the optimal switching frequency of the switching tube sought by the power conversion device 1.

[0144] At time t3, the AC voltage vac decreases to 0. The controller 14 continues to increase the switching frequency fs of each switch, based on the principle that the absolute value of the AC current iac is negatively correlated with the optimal switching frequency sought by the power converter 1. At this point, the switching frequency fs of each switch has not reached the maximum switching frequency fs_limit.

[0145] During the period from t3 to t4, the AC current iac continues to decrease, that is, the AC current iac continues to approach 0. The controller 14 continues to control the switching frequency fs of each switching tube to increase according to the principle that the absolute value of the AC current iac is negatively correlated with the optimal switching frequency of the switching tube sought by the power conversion device 1.

[0146] At time t4, the controller 14 controls the switching frequency fs of each switch to increase to the maximum switching frequency fs_limit of the switch according to the principle that the absolute value of the AC current iac is negatively correlated with the optimal switching frequency of the switch sought by the power conversion device 1.

[0147] During the period from t4 to t5, the AC current iac continues to decrease, that is, the AC current iac continues to approach 0. The controller 14 no longer follows the principle that the absolute value of the AC current iac is negatively correlated with the optimal switching frequency of the switching tube sought by the power conversion device 1, but instead controls the switching frequency fs of each switching tube to decrease from the maximum switching frequency fs_limit.

[0148] At time t5 , the AC current iac drops to 0, and the controller 14 controls the switching frequency fs of each switch tube to decrease to the second switching frequency threshold fs_i_limit.

[0149] During the period from t5 to t6 , the AC current iac decreases continuously from 0, that is, the AC current iac gradually moves away from 0, and the controller 14 controls the switching frequency fs of each switch tube to increase from the second switching frequency threshold fs_i_limit.

[0150] At time t6 , the controller 14 controls the switching frequency fs of each switch to increase to a maximum switching frequency fs_limit.

[0151] During the period from t6 to t7, the AC current iac continues to decrease, that is, the AC current iac continues to gradually move away from 0. The controller 14 controls the switching frequency fs of each switching tube to decrease from the maximum switching frequency fs_limit according to the principle that the absolute value of the AC current iac is negatively correlated with the optimal switching frequency of the switching tube sought by the power conversion device 1.

[0152] At time t7 , the controller 14 controls the switching frequency fs of each switch to decrease to the switching frequency fs_1 .

[0153] During the period t7 to t9, the AC current iac continuously increases from zero, i.e., the AC current iac begins to approach zero. Then, the controller 14 controls the switching frequency fs of each switch to increase from the switching frequency fs_1, based on the principle that the absolute value of the AC current iac is negatively correlated with the optimal switching frequency sought by the power converter 1. Furthermore, at time t8 during this period, the AC voltage vac increases to zero. At this point, the switching frequency fs of each switch has not yet reached the maximum switching frequency fs_limit.

[0154] At time t9 , the controller 14 controls the switching frequency fs of each switch to increase to a maximum switching frequency fs_limit.

[0155] During the period from t9 to t10 , the AC current iac continues to increase, that is, the AC current iac continues to approach 0, and the controller 14 controls the switching frequency fs of each switch tube to decrease from the maximum switching frequency fs_limit.

[0156] At time t10 , the AC current iac increases to 0, and the controller 14 controls the switching frequency fs of each switch tube to decrease to the second switching frequency threshold fs_i_limit.

[0157] It should be noted that in order to ensure that the efficiency of the power conversion device 1 is improved while improving the control effect of the power conversion device 1 and the output performance of the power conversion device 1, the controller 14 can also adjust the size of the first switching frequency threshold value fs_v_limit based on the change of the power factor PF of the AC end. Specifically, the controller 14 can detect the current power factor PF of the AC end according to the preset frequency, and adjust the first switching frequency threshold value fs_v_limit based on the current power factor PF of the AC end, so that the first switching frequency threshold value fs_v_limit is positively correlated with the power factor PF of the AC end. In other words, the first switching frequency threshold value fs_v_limit increases as the power factor PF of the AC end increases, and decreases as the power factor PF of the AC end decreases.

[0158] In addition, for this embodiment, the controller 14 can still make the switching frequency of each switching tube less than the maximum switching frequency within the current range where the AC current iac is close to 0 (corresponding to the current range (iac_th2, iac_th1) in Figure 8) without limiting the second switching frequency to be less than or equal to the second switching frequency threshold.

[0159] It is understandable that, in the process of the AC current iac approaching or moving away from 0, in the current range where the AC current iac is closer to 0, the power conversion device 1 controls the switching frequency fs of each switching tube to first reduce to the second switching frequency and then increase, thereby ensuring that in the current range where the AC current iac is closer to 0, the switching frequency fs of each switching tube is less than the maximum switching frequency, so that the switching frequency of the switching tube will not be limited by the limiter during closed-loop control, thereby facilitating the improvement of the control effect of the power conversion device 1. In addition, since the switching frequency fs of each switching tube is less than the maximum switching frequency when the AC current iac is 0 and the AC voltage vac is 0, the voltage and current waveform distortion caused by the power conversion device 1 switching the working mode when the AC current iac is 0 and the AC voltage vac is 0 can be reduced, thereby facilitating the improvement of the output performance of the power conversion device 1. Furthermore, in the process of the AC current iac approaching or moving away from 0, in the current interval where the AC current iac is far from 0 (corresponding to the current interval where the AC current iac is greater than or equal to iac_th1 and the current interval where the AC current iac is less than or equal to iac_th2 in Figure 8), the power conversion device 1 controls the switching frequency fs of each switching tube according to the principle that the absolute value of the AC current is negatively correlated with the optimal switching frequency of the switching tube, so as to reduce the current peak and current loss of the inductor Lr, thereby improving the efficiency of the power conversion device 1.

[0160] In the present application, when the AC current at the AC end approaches or moves away from 0, in the current range (including 0) where the AC current at the AC end is closer to 0, the power conversion device 1 controls the switching frequency of the switch tubes in the DC side bridge circuit 11 and the AC side bridge circuit 13 to be less than the maximum switching frequency, so that when the AC current iac is 0 and the AC voltage vac is 0, the switching frequency of each switch tube is less than the maximum switching frequency, so that the switching frequency of the switch tube will not be limited by the limiter during closed-loop control, thereby facilitating the improvement of the control effect of the power conversion device 1. In addition, since the switching frequency of each switch tube is less than the maximum switching frequency when the AC current iac is 0 and the AC voltage vac is 0, the voltage and current waveform distortion caused by the power conversion device 1 switching the working mode when the AC current iac is 0 and / or the AC voltage vac is 0 can also be reduced, thereby facilitating the improvement of the output performance of the power conversion device 1. Furthermore, in the process of the AC current at the AC end approaching or moving away from 0, in the current range where the AC current at the AC end is far from 0, the power conversion device 1 controls the switching frequency of each switching tube according to the principle that the absolute value of the AC current is negatively correlated with the optimal switching frequency of the switching tube, so as to reduce the current peak and current loss of the inductor Lr, thereby improving the efficiency of the power conversion device 1.

[0161] It should be noted that in the present application, the controller 14 only needs to ensure that the switching frequency of the switching tube is monotonic when controlling the switching frequency of the switching tube, and there is no restriction on whether the rate of change of the switching frequency of the switching tube changes. In the present application, the inductor Lr can be either the leakage inductance of the transformer 12 or an inductor independent of the transformer 12, and the present application does not impose any restrictions on this. In addition, in addition to being able to adjust the first switching frequency threshold or the second switching frequency based on the power factor PF at the AC end, the power conversion device 1 can also adjust the first switching frequency threshold or the second switching frequency threshold based on the DC current idc at the DC end of the power conversion device 1, the effective value of the AC current at the AC end, the DC voltage vdc at the DC end, or the effective value of the AC voltage at the AC end.

[0162] Exemplarily, before controlling the output power of the power conversion device 1, the controller 14 determines the magnitude of the first switching frequency threshold or the second switching frequency threshold when the DC current IDC is a preset DC current based on a preset mapping relationship between the DC current IDC and the first switching frequency threshold or the second switching frequency threshold. The preset mapping relationship between the DC current IDC and the first switching frequency threshold or the second switching frequency threshold is that the first switching frequency threshold or the second switching frequency threshold is positively correlated with the DC current IDC. Accordingly, in order to ensure that the efficiency of the power conversion device 1 is improved while improving the control effect and output performance of the power conversion device 1, the controller 14 may further adjust the magnitude of the first switching frequency threshold or the second switching frequency threshold based on changes in the DC current IDC. Specifically, the controller 14 may detect the DC current IDC at a preset frequency and adjust the first switching frequency threshold or the second switching frequency threshold based on the current DC current so that the first switching frequency threshold or the second switching frequency threshold is positively correlated with the DC current IDC.

[0163] Exemplarily, before controlling the power conversion device 1 to output power, the controller 14 determines the magnitude of the first switching frequency threshold or the second switching frequency threshold when the AC current RMS value at the AC end is the preset AC current RMS value based on a preset mapping relationship between the AC current RMS value at the AC end and the first switching frequency threshold or the second switching frequency threshold. The preset mapping relationship between the AC current RMS value at the AC end and the first switching frequency threshold or the second switching frequency threshold is that the first switching frequency threshold or the second switching frequency threshold is positively correlated with the AC current RMS value at the AC end. Accordingly, to ensure that the efficiency of the power conversion device 1 is improved while enhancing the control effect and output performance of the power conversion device 1, the controller 14 may further adjust the magnitude of the first switching frequency threshold or the second switching frequency threshold based on changes in the AC current RMS value at the AC end. Specifically, the controller 14 may detect the AC current RMS value at the AC end at a preset frequency and adjust the first switching frequency threshold or the second switching frequency threshold based on the current AC current RMS value at the AC end so that the first switching frequency threshold or the second switching frequency threshold is positively correlated with the AC current RMS value at the AC end.

[0164] Exemplarily, before controlling the output power of the power conversion device 1, the controller 14 determines the magnitude of the first switching frequency threshold or the second switching frequency threshold when the DC voltage VDC is the preset DC voltage based on a preset mapping relationship between the DC voltage VDC and the first switching frequency threshold or the second switching frequency threshold. Because the smaller the DC voltage VDC, the lower the optimal switching frequency is to achieve the same AC current IAC under the same AC voltage VAC, and thus the switching frequency can only reach the maximum switching frequency when the AC current IAC drops to a smaller value, the larger the first switching frequency threshold is, the larger the second switching frequency threshold is. Based on this, the preset mapping relationship between the DC voltage VDC and the first switching frequency threshold or the second switching frequency threshold is that the first switching frequency threshold or the second switching frequency threshold is negatively correlated with the DC voltage VDC. Accordingly, to ensure that the efficiency of the power conversion device 1 is improved while improving the control effect and output performance of the power conversion device 1, the controller 14 can also adjust the magnitude of the first switching frequency threshold or the second switching frequency threshold based on changes in the DC voltage VDC. Specifically, the controller 14 may detect the DC voltage VDC at a preset frequency and adjust the first switching frequency threshold or the second switching frequency threshold based on the current DC voltage, so that the first switching frequency threshold or the second switching frequency threshold is negatively correlated with the DC voltage VDC. In other words, the first switching frequency threshold or the second switching frequency threshold decreases as the DC voltage VDC increases, and increases as the DC voltage VDC decreases.

[0165] Exemplarily, before controlling the power output of the power conversion device 1, the controller 14 determines the magnitude of the first switching frequency threshold or the second switching frequency threshold when the AC voltage RMS value at the AC end is the preset AC voltage RMS value based on a preset mapping relationship between the AC voltage RMS value at the AC end and the first switching frequency threshold or the second switching frequency threshold. Because the larger the AC voltage RMS value at the AC end, the lower the optimal switching frequency is to achieve the same AC current iac at the same AC voltage vac, and thus the switching frequency can only reach the maximum switching frequency when the AC current iac drops to a smaller value, the larger the first switching frequency threshold is, the larger the second switching frequency threshold is. Based on this, the preset mapping relationship between the AC voltage RMS value at the AC end and the first switching frequency threshold or the second switching frequency threshold is that the first switching frequency threshold or the second switching frequency threshold is positively correlated with the AC voltage RMS value at the AC end. Accordingly, to ensure that the efficiency of the power conversion device 1 is improved while improving the control effect and output performance of the power conversion device 1, the controller 14 can also adjust the magnitude of the first switching frequency threshold or the second switching frequency threshold based on changes in the AC voltage RMS value at the AC end. Specifically, the controller 14 can detect the effective value of the AC voltage at the AC end according to a preset frequency, and adjust the first switching frequency threshold or the second switching frequency threshold based on the current effective value of the AC voltage at the AC end, so that the first switching frequency threshold or the second switching frequency threshold is positively correlated with the effective value of the AC voltage at the AC end.

[0166] It is understandable that the power conversion device 1 can also adjust the first switching frequency threshold or the second switching frequency threshold based on the DC current idc, the effective value of the AC current at the AC end, the DC voltage vdc at the DC end or the effective value of the AC voltage at the AC end, thereby adjusting the first switching frequency or the second switching frequency, thereby ensuring that while improving the control effect and output performance of the power conversion device 1, the current peak and current loss of the inductor Lr are reduced to improve the efficiency of the power conversion device 1.

[0167] Furthermore, the method for controlling the switching frequency of the switching transistors in the power conversion device 1 shown in FIG4 is also applicable to the power conversion devices 1 shown in FIG9a through FIG11. As shown in FIG9a, compared to the power conversion device 1 shown in FIG4, the power conversion device 1 shown in FIG9a further includes a first resonant capacitor Cr1. The first resonant capacitor Cr1, the inductor Lr, and the secondary winding Ls are connected in series at the input end of the AC-side bridge circuit 13. Specifically, the first resonant capacitor Cr1, the inductor Lr, and the secondary winding Ls are connected in series between the input end i131 and the input end i132 of the AC-side bridge circuit 13. Optionally, the first resonant capacitor Cr1 can also be connected in series with the primary winding Lp. For details, see the power conversion device 1 shown in FIG9b. As shown in FIG9b, the first resonant capacitor Cr1 and the primary winding Lp are connected in series at the output end of the DC-side bridge circuit 11. Specifically, the first resonant capacitor Cr1 and the primary winding Lp are connected in series between the output end o111 and the output end o112 of the DC-side bridge circuit 11.

[0168] As shown in FIG9c , compared to the power conversion device 1 shown in FIG4 , the power conversion device 1 shown in FIG9c further includes a first resonant capacitor Cr1. The inductor Lr, the primary winding Lp, and the first resonant capacitor Cr1 are connected in series at the output end of the DC side bridge circuit 11. Specifically, the inductor Lr, the primary winding Lp, and the first resonant capacitor Cr1 are connected in series between the output end o111 and the output end o112 of the DC side bridge circuit 11. Optionally, the first resonant capacitor Cr1 may also be connected in series with the secondary winding Ls. For details, see the power conversion device 1 shown in FIG9d . As shown in FIG9d , the secondary winding Ls and the first resonant capacitor Cr1 are connected in series at the input end of the AC side bridge circuit 13. Specifically, the secondary winding Ls and the first resonant capacitor Cr1 are connected in series between the input end i131 and the input end i132 of the AC side bridge circuit 13.

[0169] As shown in FIG10a , compared to the power conversion device 1 shown in FIG4 , the AC-side bridge circuit 13 in the power conversion device 1 shown in FIG10a is a half-bridge circuit, and the power conversion device 1 further includes a first resonant capacitor Cr1 and a second resonant capacitor Cr2. Specifically, the AC-side bridge circuit 13 includes a bridge arm 131 formed by a series-connected switch S5 and a switch S6. Each switch S5 and switch S6 is formed by two MOSFETs connected in reverse series. The two ends of the bridge arm 131 (i.e., the output terminals o131 and o132 of the AC-side bridge circuit 13) are respectively connected to the first AC terminal o11 and the second AC terminal o12 of the power conversion device 1. The first resonant capacitor Cr1 and the second resonant capacitor Cr2 are connected in series between the first AC terminal o11 and the second AC terminal o12 of the power conversion device 1. The inductor Lr is connected in series with the secondary winding Ls between the input terminal i131 (i.e., the midpoint of the bridge arm 131) of the AC-side bridge circuit 13 and the midpoint of the capacitor, which is the connection point between the first resonant capacitor Cr1 and the second resonant capacitor Cr2. Optionally, the inductor Lr in the power conversion device 1 shown in FIG10a can also be connected in series with the primary winding Lp between the output terminals o111 and o112 of the DC-side bridge circuit 11.

[0170] As shown in FIG10b , compared to the power conversion device 1 shown in FIG4 , the DC side bridge circuit 11 in the power conversion device 1 shown in FIG10b is a half-bridge circuit, and the power conversion device 1 further includes a first resonant capacitor Cr1 and a second resonant capacitor Cr2. Specifically, the DC side bridge circuit 11 includes a bridge arm 111 composed of a series-connected switch S5 and a switch S6, each of which is composed of two MOSFETs connected in reverse series. The two ends of the bridge arm 131 (i.e., the input terminal i111 and the input terminal i112 of the DC side bridge circuit 11) are respectively connected to the first DC terminal i11 and the second DC terminal i12 of the power conversion device 1. The first resonant capacitor Cr1 and the second resonant capacitor Cr2 are connected in series between the first DC terminal i11 and the second DC terminal i12 of the power conversion device 1. The inductor Lr is connected in series with the primary winding Lp between the output terminal o111 (i.e., the midpoint of the bridge arm 111) of the DC-side bridge circuit 11 and the midpoint of the capacitor, which is the connection point between the first resonant capacitor Cr1 and the second resonant capacitor Cr2. Optionally, the inductor Lr in the power conversion device 1 shown in FIG10b can also be connected in series with the secondary winding Ls between the input terminals i131 and i132 of the AC-side bridge circuit 13.

[0171] As shown in FIG11 , compared to the power conversion device 1 shown in FIG4 , the DC-side bridge circuit 11 and the AC-side bridge circuit 13 in the power conversion device 1 shown in FIG11 are both half-bridge circuits, and the power conversion device 1 further includes a first resonant capacitor Cr1, a second resonant capacitor Cr2, a third resonant capacitor Cr3, and a fourth resonant capacitor Cr4. Specifically, the DC-side bridge circuit 11 includes a bridge arm 111 formed by a series connection of a switch transistor S1 and a switch transistor S2, and the AC-side bridge circuit 13 includes a bridge arm 131 formed by a series connection of a switch transistor S3 and a switch transistor S4. Each of the switches S1 to S4 is formed by two MOSFETs connected in reverse series. The two ends of the bridge arm 111 (i.e., the input terminals i111 and i112 of the DC-side bridge circuit 11) are respectively connected to the first DC terminal i11 and the second DC terminal i12 of the power conversion device 1. The first resonant capacitor Cr1 and the second resonant capacitor Cr2 are connected in series between the first DC terminal i11 and the second DC terminal i12 of the power conversion device 1. Inductor Lr and primary winding Lp are connected in series between the output terminal o111 (i.e., the midpoint of bridge arm 111) of DC-side bridge circuit 11 and the midpoint of the first capacitor, which is the junction of first resonant capacitor Cr1 and second resonant capacitor Cr2. The two ends of bridge arm 131 (i.e., the output terminal o131 and the output terminal o132 of AC-side bridge circuit 13) are connected to the first AC terminal o11 and the second AC terminal o12 of power converter 1, respectively. Third resonant capacitor Cr3 and fourth resonant capacitor Cr4 are connected in series between the first AC terminal o11 and the second AC terminal o12 of power converter 1. The same-name and opposite-name ends of secondary winding Ls are connected to the input terminal i131 (i.e., the midpoint of bridge arm 131) of AC-side bridge circuit 13 and the midpoint of the second capacitor, which is the junction of third resonant capacitor Cr3 and fourth resonant capacitor Cr4, respectively. Optionally, the inductor Lr in the power conversion device 1 shown in FIG11 may also be connected in series with the secondary winding Ls between the input terminal i131 of the AC side bridge circuit 13 and the midpoint of the second capacitor.

[0172] See Figure 12, which is a flow chart of a control method for a power conversion device provided by the present application. The control method for a power conversion device provided by an embodiment of the present application is applicable to the power conversion device 1 shown in Figures 3a to 11. The control method for a power conversion device may include the following steps:

[0173] S101 , when the AC current at the AC end approaches 0, first control the switching frequency of the switch tube to increase to the maximum switching frequency of the switch tube, and then control the switching frequency of the switch tube to be less than the maximum switching frequency when the AC current is 0.

[0174] In an optional embodiment, while the AC current at the AC end approaches zero, the power conversion device first controls the switching frequencies of the switches in the DC-side bridge circuit and the AC-side bridge circuit to increase to the maximum switching frequencies of the switches. After the switching frequencies of the switches increase to the maximum switching frequencies, the power conversion device first controls the switching frequencies of the switches to decrease to the first switching frequency, and then controls the switching frequencies of the switches to decrease to the second switching frequency.

[0175] In another optional embodiment, while the AC current at the AC end approaches zero, the power conversion device first controls the switching frequency to increase to a maximum switching frequency. After the switching frequency of the switching tube increases to the maximum switching frequency, the power conversion device first controls the switching frequency of the switching tube to decrease to a first switching frequency, and then controls the switching frequency of the switching tube to increase to a second switching frequency, wherein the second switching frequency is less than the maximum switching frequency.

[0176] In another optional embodiment, while the AC current at the AC end approaches zero, the power conversion device first controls the switching frequency to increase to a maximum switching frequency. After the switching frequency of the switching tube increases to the maximum switching frequency, the power conversion device first controls the switching frequency of the switching tube to decrease to a first switching frequency, and then controls the switching frequency of the switching tube to maintain at the first switching frequency.

[0177] In another optional embodiment, while the AC current at the AC end approaches zero, the power conversion device first controls the switching frequency of the switch tube to increase to a maximum switching frequency. After the switching frequency of the switch tube increases to the maximum switching frequency, the power conversion device controls the switching frequency of the switch tube to decrease to a second switching frequency.

[0178] The first switching frequency is the switching frequency of the switch tube when the AC voltage at the AC end is 0, and the first switching frequency is less than or equal to the first switching frequency threshold. The second switching frequency is the switching frequency of the switch tube when the AC current is 0, and the second switching frequency is less than or equal to the second switching frequency threshold.

[0179] It should be noted that in order to ensure that the efficiency of the power conversion device is improved while improving the control effect of the power conversion device and enhancing the output performance of the power conversion device, the power conversion device can also adjust the size of the first switching frequency threshold or the second switching frequency threshold based on the changes in the DC current or DC voltage at the DC end, the AC current effective value, the AC voltage effective value or the power factor at the AC end, thereby achieving adjustment of the first switching frequency or the second switching frequency.

[0180] Exemplarily, the power conversion device further adjusts the first switching frequency threshold or the second switching frequency threshold based on the effective value of the DC current or AC current at the DC terminal, so that the first switching frequency threshold or the second switching frequency threshold is positively correlated with the effective value of the DC current or AC current. Exemplarily, the power conversion device further adjusts the first switching frequency threshold or the second switching frequency threshold based on the power factor at the AC terminal, so that the first switching frequency threshold or the second switching frequency is positively correlated with the power factor. Exemplarily, the power conversion device further adjusts the first switching frequency threshold or the second switching frequency threshold based on the effective value of the DC voltage at the DC terminal or the AC voltage at the AC terminal, so that the first switching frequency threshold or the second switching frequency threshold is negatively correlated with the DC voltage, and the first switching frequency threshold or the second switching frequency threshold is positively correlated with the effective value of the AC voltage.

[0181] S102 , when the AC current at the AC end is far away from zero, first control the switching frequency of the switch tube to increase to the maximum switching frequency of the switch tube, and then control the switching frequency of the switch tube to decrease.

[0182] In one embodiment, as the AC current at the AC end moves away from zero, the power conversion device first controls the switching frequency of the switching tubes in the DC-side bridge circuit and the AC-side bridge circuit to increase from the first switching frequency or the second switching frequency to the maximum switching frequency of the switching tubes. After the switching frequency of the switching tubes increases to the maximum switching frequency, the power conversion device controls the switching frequency of the switching tubes to decrease.

[0183] In a specific implementation, more operations performed by the power conversion device in the control method of the power conversion device provided in this application can be referred to the implementation method performed by the power conversion device 1 shown in Figures 3a to 11, and will not be repeated here.

[0184] In the present application, when the AC current at the AC end approaches or moves away from 0, in the current range (including 0) where the AC current at the AC end is closer to 0, the power conversion device controls the switching frequency of the switch tubes in the DC side bridge circuit and the AC side bridge circuit to be less than the maximum switching frequency, so that when the AC current is 0 and when the AC voltage is 0, the switching frequency of each switch tube is less than the maximum switching frequency, thereby preventing the switching frequency of the switch tube from being limited by the limiter during closed-loop control, thereby facilitating improved control effects of the power conversion device. In addition, since the switching frequency of each switch tube is less than the maximum switching frequency when the AC current is 0 and when the AC voltage is 0, the voltage and current waveform distortion caused by the power conversion device switching the operating mode when the AC current is 0 and / or the AC voltage is 0 can also be reduced, thereby facilitating improved output performance of the power conversion device. Furthermore, in the process of the AC current at the AC end approaching or moving away from 0, in the current range where the AC current at the AC end is far from 0, the power conversion device controls the switching frequency of the switching tube according to the principle that the absolute value of the AC current is negatively correlated with the optimal switching frequency of the switching tube, so as to reduce the current peak and current loss of the inductor, thereby improving the efficiency of the power conversion device.

[0185] The above are only specific embodiments of the present application, but the scope of protection of this application is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in this application should be included in the scope of protection of this application. Therefore, the scope of protection of this application should be based on the scope of protection of the claims.

Claims

1. A power conversion device, characterized in that, The power conversion device includes a DC terminal, a DC-side bridge circuit, a transformer, an AC-side bridge circuit, an AC terminal, and a controller. The transformer includes a primary winding and a secondary winding. The DC-side bridge circuit and the AC-side bridge circuit include switching tubes, where: The DC-side bridge circuit is connected between the DC terminal and the primary winding, and the AC-side bridge circuit is connected between the secondary winding and the AC terminal; The controller is configured to, during the process that the AC current at the AC terminal approaches 0, first control the switching frequency of the switching tubes to increase to the maximum value of the switching frequency of the switching tubes; and then, when the AC current is 0, control the switching frequency of the switching tubes to be less than the maximum value of the switching frequency; During the process that the AC current at the AC terminal moves away from 0, first control the switching frequency of the switching tubes to increase to the maximum value of the switching frequency of the switching tubes, and then control the switching frequency to decrease.

2. The power conversion device according to claim 1, wherein The controller is configured to, during the process that the AC current at the AC terminal approaches 0, after controlling the switching frequency of the switching tubes to increase to the maximum value of the switching frequency of the switching tubes, first control the switching frequency to decrease to a first switching frequency, and then control the switching frequency to decrease to a second switching frequency.

3. The power conversion device according to claim 1, wherein The controller is configured to, during the process that the AC current at the AC terminal approaches 0, after controlling the switching frequency of the switching tubes to increase to the maximum value of the switching frequency of the switching tubes, first control the switching frequency to decrease to a first switching frequency, and then control the switching frequency to increase to a second switching frequency, where the second switching frequency is less than the maximum value of the switching frequency.

4. The power conversion device according to claim 1, characterized in that, The controller is configured to, during the process that the AC current at the AC terminal approaches 0, after controlling the switching frequency of the switching tubes to increase to the maximum value of the switching frequency of the switching tubes, first control the switching frequency to decrease to a first switching frequency, and then control the switching frequency to remain at the first switching frequency.

5. The power conversion device according to claim 1, characterized in that The controller is configured to, during the process that the AC current at the AC terminal approaches 0, after controlling the switching frequency of the switching tubes to increase to the maximum value of the switching frequency of the switching tubes, control the switching frequency to decrease to a second switching frequency.

6. The power conversion device according to any one of claims 2-5, characterized in that, The first switching frequency is the switching frequency of the switching tubes when the AC voltage at the AC terminal is 0, and the first switching frequency is less than or equal to a first switching frequency threshold; the second switching frequency is the switching frequency of the switching tubes when the AC current is 0, and the second switching frequency is less than or equal to a second switching frequency threshold.

7. The power conversion device according to claim 6, wherein The first switching frequency threshold or the second switching frequency threshold is positively correlated with the DC current at the DC terminal or the effective value of the AC current at the AC terminal.

8. The power conversion device according to claim 6, characterized in that, The first switching frequency threshold or the second switching frequency threshold is positively correlated with the power factor at the AC terminal, and the power factor is the cosine value of the phase difference between the AC voltage and the AC current at the AC terminal.

9. The power conversion device according to claim 6, wherein, The first switching frequency threshold or the second switching frequency threshold is negatively correlated with the DC voltage at the DC terminal, or the first switching frequency threshold or the second switching frequency threshold is positively correlated with the effective value of the AC voltage at the AC terminal.

10. The power conversion device according to any one of claims 1-9, characterized in that, The power conversion device further includes an inductor. The input end of the DC-side bridge circuit is connected to the DC terminal; the inductor is connected in series with the primary winding and then connected to the output end of the DC-side bridge circuit, and the secondary winding is connected to the input end of the AC-side bridge circuit. Alternatively, the primary winding is connected to the output end of the DC-side bridge circuit, the inductor is connected in series with the secondary winding and then connected to the input end of the AC-side bridge circuit; the output end of the AC-side bridge circuit is connected to the AC terminal.

11. The power conversion device according to claim 10, characterized in that, The DC-side bridge circuit includes a half-bridge circuit or a full-bridge circuit, and the AC-side bridge circuit includes a half-bridge circuit or a full-bridge circuit.

12. The power conversion device according to claim 11, characterized in that, The DC-side bridge circuit is a half-bridge circuit, and the power conversion device further includes a first resonant capacitor and a second resonant capacitor, where: The first resonant capacitor and the second resonant capacitor are connected in series and then connected in parallel to the input end of the DC-side bridge circuit; the two ends of the primary winding are respectively connected to the output end of the DC-side bridge circuit and the capacitor midpoint. Alternatively, the primary winding and the inductor are connected in series between the output end of the DC-side bridge circuit and the capacitor midpoint, and the capacitor midpoint is the connection point of the first resonant capacitor and the second resonant capacitor.

13. The power conversion device according to claim 11, characterized in that, The AC-side bridge circuit is a half-bridge circuit, and the power conversion device further includes a first resonant capacitor and a second resonant capacitor, where: The first resonant capacitor and the second resonant capacitor are connected in series and then connected in parallel to the output end of the AC-side bridge circuit; the two ends of the secondary winding are respectively connected to the input end of the AC-side bridge circuit and the capacitor midpoint. Alternatively, the secondary winding and the inductor are connected in series between the input end of the AC-side bridge circuit and the capacitor midpoint, and the capacitor midpoint is the connection point of the first resonant capacitor and the second resonant capacitor.

14. The power conversion device according to claim 11, wherein, Both the DC-side bridge circuit and the AC-side bridge circuit are full-bridge circuits, and the power conversion device further includes a first resonant capacitor. The primary winding is connected in series with the first resonant capacitor at the output end of the DC-side bridge circuit. Alternatively, the secondary winding, the inductor, and the first resonant capacitor are connected in series at the input end of the AC-side bridge circuit. Alternatively, the primary winding, the inductor, and the first resonant capacitor are connected in series at the output end of the DC-side bridge circuit. Alternatively, the secondary winding is connected in series with the first resonant capacitor at the input end of the AC-side bridge circuit.

15. A control method for a power conversion device, characterized in that, The method includes: During the process that the AC current at the AC terminal approaches 0, first control the switching frequency of the switching device to increase to the maximum switching frequency of the switching device; then when the AC current is 0, control the switching frequency of the switching device to be less than the maximum switching frequency; during the process that the AC current at the AC terminal moves away from 0, first control the switching frequency of the switching device to increase to the maximum switching frequency of the switching device, and then control the switching frequency to decrease; the method is applicable to the power conversion device, and the power conversion device includes a DC terminal, a DC-side bridge circuit, a transformer, an AC-side bridge circuit and the AC terminal, and the transformer includes a primary winding and a secondary winding, wherein the DC-side bridge circuit is connected between the DC terminal and the primary winding, and the AC-side bridge circuit is connected between the secondary winding and the AC terminal.

16. The method according to claim 15, characterized in that, When the AC current is 0, controlling the switching frequency of the switching device to be less than the maximum switching frequency includes: After controlling the switching frequency of the switching device to increase to the maximum switching frequency of the switching device, first control the switching frequency to decrease to a first switching frequency, and then control the switching frequency to decrease to a second switching frequency.

17. The method according to claim 15, wherein When the AC current is 0, controlling the switching frequency of the switching device to be less than the maximum switching frequency includes: After controlling the switching frequency of the switching device to increase to the maximum switching frequency of the switching device, first control the switching frequency to decrease to a first switching frequency, and then control the switching frequency to increase to a second switching frequency, wherein the second switching frequency is less than the maximum switching frequency.

18. The method according to claim 15, characterized in that, When the AC current is 0, controlling the switching frequency of the switching device to be less than the maximum switching frequency includes: After controlling the switching frequency of the switching device to increase to the maximum switching frequency of the switching device, first control the switching frequency to decrease to a first switching frequency, and then control the switching frequency to be maintained at the first switching frequency.

19. The method according to claim 15, characterized in that When the AC current is 0, controlling the switching frequency of the switching device to be less than the maximum switching frequency includes: After controlling the switching frequency of the switching device to increase to the maximum switching frequency of the switching device, control the switching frequency to decrease to a second switching frequency.

20. The method according to any one of claims 16-19, characterized in that, The first switching frequency is the switching frequency of the switching device when the AC voltage at the AC terminal is 0, and the first switching frequency is less than or equal to a first switching frequency threshold; the second switching frequency is the switching frequency of the switching device when the AC current is 0, and the second switching frequency is less than or equal to a second switching frequency threshold.

21. The method according to claim 20, wherein The first switching frequency threshold or the second switching frequency threshold is positively correlated with the DC current at the DC terminal or the effective value of the AC current at the AC terminal.

22. The method according to claim 20, characterized in that, The first switching frequency threshold or the second switching frequency is positively correlated with the power factor at the AC terminal, and the power factor is the cosine value of the phase difference between the AC voltage and the AC current at the AC terminal.

23. The method according to claim 20, wherein The first switching frequency threshold or the second switching frequency threshold is negatively correlated with the DC voltage at the DC terminal, or the first switching frequency threshold or the second switching frequency threshold is positively correlated with the effective value of the AC voltage at the AC terminal.

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