Method for controlling power conversion device, and power conversion device

Abstract

A power conversion device (1) comprises an input rectifier circuit (10), an inverter circuit (20) provided with a switching element (Q1), a capacitor (Cb) connected between the input rectifier circuit (10) and the inverter circuit (20), an output rectifier circuit (30), and a control unit (40). The control unit (40) outputs a control signal (Vg) to cause the switching element (Q1) to perform a switching operation. The control unit (40) sets a switching frequency (fg) of the control signal (Vg) to a starting frequency that corresponds to the voltage (Vb) across the terminals of the capacitor (Cb) at the start of the control signal (Vg). Thereafter, the control unit (40) carries out feedback control of the switching frequency (fg) on the basis of instantaneous values of an input voltage (V1) and an incoming current (I1) and / or values of an output voltage (V2) and an output current (I2).

Description

Control method for power conversion device and power conversion device

[0001] The present invention relates to a control method for a power conversion device and a power conversion device.

[0002] Patent Document 1 describes a power factor correction circuit as a power conversion device. The power conversion device in Patent Document 1 includes an input rectifier circuit and an E-class inverter circuit having a switching element and an LC resonant circuit, and full-wave rectifies an input AC voltage in the input rectifier circuit, and the full-wave rectified voltage is input to the E-class inverter circuit. The E-class inverter circuit controls the input current to a high power factor while performing ZVS (Zero Voltage Switching) by modulating the switching frequency of the switching element.

[0003] JP 2021-145433 A

[0004] In a power conversion device that combines an input rectifier circuit and a class E inverter circuit, a smoothing capacitor may be placed between the input rectifier circuit and the class E inverter circuit to stabilize the input voltage of the class E inverter circuit. In a power conversion device with such a configuration, the smoothing capacitor is not discharged by the class E inverter from the time the input AC voltage is applied until the class E inverter circuit starts switching operation. Therefore, the voltage across the smoothing capacitor maintains the voltage charged by the voltage obtained by rectifying the input AC voltage.

[0005] When the switching elements of the class E inverter circuit start switching operation for the first time after the input AC voltage is applied to the power conversion device, the input voltage of the class E inverter circuit is high because the smoothing capacitor is charged, so the class E inverter circuit starts operating from a state where the input voltage is high and the input current is zero.

[0006] When the input voltage of the class E inverter circuit is high and the input current is small, if the switching elements start operating at a switching frequency determined by control for increasing the power factor, ZVS of the class E inverter circuit cannot be achieved. Research by the inventors has revealed that under such circumstances, problems such as damage to the switching elements due to large power conversion losses and malfunction of the class E inverter circuit due to high-speed changes in the voltage across the switching elements when ZVS is not established can occur.

[0007] The present invention has been made in consideration of the above-mentioned problems, and its purpose is to provide a power conversion device that can suppress heat generation and malfunction of switching elements due to failure of ZVS even when the input voltage of an E-class inverter circuit is high and the input current is small.

[0008] A power conversion device according to one aspect of the present invention includes an input rectifier circuit, an inverter circuit with a switching element, a capacitor connected between the input rectifier circuit and the inverter circuit, an output rectifier circuit, and a control unit. The control unit outputs a control signal that causes the switching element to perform a switching operation. The control unit sets the switching frequency of the control signal to a startup frequency that corresponds to the voltage across the capacitor at startup of the control signal. The control unit then feedback-controls the switching frequency based on at least one of instantaneous values ​​of the input voltage and input current and values ​​of the output voltage and output current.

[0009] According to the power conversion device of one aspect of the present invention, even when the input voltage of the class E inverter circuit is high and the input current is small, heat generation and malfunction of the switching elements due to failure of ZVS can be suppressed.

[0010] FIG. 1 is a circuit diagram showing the configuration of a power conversion device according to each embodiment. FIG. 2A is a time chart showing the operating waveforms of the power conversion device according to the first embodiment. FIG. 2B is a time chart enlarging a portion of region IIB in FIG. 2A. FIG. 3 is a time chart showing the operating waveforms of a power conversion device according to a comparative example of the first embodiment. FIG. 4 is a time chart showing the operating waveforms of a power conversion device according to the second embodiment. FIG. 5 is a time chart showing the operating waveforms of a power conversion device according to a comparative example of the second embodiment.

[0011] The embodiments will be described with reference to the drawings. In the description of the drawings, the same parts are designated by the same reference numerals and the description thereof will be omitted.

[0012] 1 is a circuit diagram showing the configuration of a power conversion device 1 according to each embodiment. The power conversion device 1 is a power factor correction circuit that converts low-frequency AC power from an AC power source 2 having an input AC voltage Vac (e.g., an effective voltage of 200 V and a frequency of 50 Hz) into DC power with a high power factor and supplies the DC power to a load 3.

[0013] The power conversion device 1 includes a pair of input power supply terminals Ti1 and Ti2, an input rectifier circuit 10, a smoothing capacitor Cb, a class E inverter circuit 20, an output rectifier circuit 30, a control unit 40, and a pair of output power supply terminals To1 and To2. An AC power supply 2 is connected in parallel to the first input power supply terminal Ti1 and the second input power supply terminal Ti2. A load 3 is connected in parallel to the first output power supply terminal To1 and the second output power supply terminal To2.

[0014] The power conversion device 1 also includes a first voltmeter 4 that measures the input voltage V1, a first ammeter 5 that measures the input current I1, a second voltmeter 6 that measures the output voltage V2, and a second ammeter 7 that measures the output current I2. The measurement results of the first voltmeter 4, the first ammeter 5, the second voltmeter 6, and the second ammeter 7 are input to the control unit 40. The control unit 40 controls the class E inverter circuit 20 based on the measurement results of the first voltmeter 4, the first ammeter 5, the second voltmeter 6, and the second ammeter 7.

[0015] In this embodiment, the input rectifier circuit 10 is a rectifier bridge circuit in which diodes are arranged in a full bridge configuration. The input rectifier circuit 10 full-wave rectifies a first AC power having a low frequency of, for example, 50 Hz, input from the AC power source 2 to a pair of input terminals (first input terminal, second input terminal) via a pair of input power source terminals Ti1 and Ti2, into a first DC power and outputs the first DC power to a pair of output terminals. The first input power source terminal Ti1 is connected to the first input terminal of the input rectifier circuit 10, and the second input power source terminal Ti2 is connected to the second input terminal of the input rectifier circuit 10. Note that the input rectifier circuit 10 is not limited to a rectifier bridge circuit, and any rectifier circuit capable of full-wave or half-wave rectifying the first AC power from the AC power source 2 can be used.

[0016] A smoothing capacitor Cb is connected in parallel to a pair of output terminals of the input rectifier circuit 10. The pair of output terminals (first output terminal, second output terminal) of the input rectifier circuit 10 are connected to a pair of input terminals (first input terminal, second input terminal) of the class E inverter circuit 20 via the smoothing capacitor Cb. The smoothing capacitor Cb serves to stabilize the voltage input to the pair of input terminals of the class E inverter circuit 20.

[0017] The class E inverter circuit 20 converts the first DC power from the input rectifier circuit 10 into a second AC power with a high frequency of, for example, several MHz and outputs the second AC power to a pair of output terminals (a first output terminal and a second output terminal). The class E inverter circuit 20 includes a choke inductor Lc, a switching element Q1, a shunt capacitor Cs, and an LC resonant circuit 21.

[0018] A first terminal of the choke inductor Lc serves as a first input terminal of the class E inverter circuit 20 and is connected to a first output terminal of the input rectifier circuit 10. A second terminal of the choke inductor Lc is connected to a first main electrode D of the switching element Q1. A second main electrode S of the switching element Q1 serves as a second input terminal of the class E inverter circuit 20 and is connected to a second output terminal of the input rectifier circuit 10.

[0019] However, the connection point of the choke inductor Lc is not limited to this. The first main electrode D of the switching element Q1 may serve as the first input terminal of the class E inverter circuit 20 and be connected to the first output terminal of the input rectifier circuit 10. The first terminal of the choke inductor Lc may serve as the second input terminal of the class E inverter circuit 20 and be connected to the second output terminal of the input rectifier circuit 10, and the second terminal of the choke inductor Lc may be connected to the second main electrode S of the switching element Q1.

[0020] In addition, the choke inductor Lc may be connected both between the first output terminal of the input rectifier circuit 10 and the first main electrode D of the switching element Q1, and between the second output terminal of the input rectifier circuit 10 and the second main electrode S of the switching element Q1.

[0021] In this embodiment, an N-channel MOSFET is used as the switching element Q1, with the first main electrode D serving as the drain, the second main electrode S serving as the source, and the control electrode G serving as the gate. The switching element Q1 is not limited to an N-channel MOSFET, and any semiconductor switching element can be selected depending on the usage situation.

[0022] A shunt capacitor Cs is connected in parallel to the first main electrode D and second main electrode S of the switching element Q1. A first terminal of an LC resonant circuit 21 is connected to the connection point between the first main electrode D of the switching element Q1 and the first terminal of the shunt capacitor Cs. A second terminal of the LC resonant circuit 21 serves as a first output terminal of the class E inverter circuit 20. A connection point between the second main electrode S of the switching element Q1 and the second terminal of the shunt capacitor Cs serves as a second output terminal of the class E inverter circuit 20.

[0023] In this embodiment, the LC resonant circuit 21 is a series circuit of a resonant inductor Lr and a resonant capacitor Cr. The resonant frequency fr of the LC resonant circuit 21 is set to be close to the switching frequency fg of the control signal Vg input to the control electrode G of the switching element Q1.

[0024] A pair of input terminals (first input terminal and second input terminal) of an output rectifier circuit 30 is connected to a pair of output terminals of the class E inverter circuit 20. In this embodiment, the output rectifier circuit 30 uses a class E rectifier circuit, which converts high-frequency second AC power output from the class E inverter circuit 20 into second DC power and outputs it to a load 3 connected via a pair of output power supply terminals To1 and To2. Note that the output rectifier circuit 30 is not limited to a class E rectifier circuit, and a rectifier circuit capable of converting high-frequency AC power to DC power, such as a class D rectifier circuit, can be used. The output rectifier circuit 30 includes a first diode D1, a first capacitor C1, a second capacitor C2, and a first inductor L1.

[0025] The cathode K of the first diode D1 serves as a first input terminal of the output rectifier circuit 30 and is connected to the second terminal of the LC resonant circuit 21, which is the first output terminal of the class E inverter circuit 20. The anode A of the first diode D1 serves as a second input terminal of the output rectifier circuit 30 and is connected to the connection point of the second main electrode S of the switching element Q1 and the second terminal of the shunt capacitor Cs, which is the second output terminal of the class E inverter circuit 20.

[0026] A first capacitor C1 is connected in parallel to the first diode D1. More specifically, the cathode K of the first diode D1 is connected to a first terminal of the first capacitor C1, and the anode A of the first diode D1 is connected to a second terminal of the first capacitor C1. The connection point between the cathode K of the first diode D1 and the first terminal of the first capacitor C1 is connected to a first terminal of a first inductor L1. The second terminal of the first inductor L1 is connected to a first terminal of a second capacitor C2. The connection point between the anode A of the first diode D1 and the second terminal of the first capacitor C1 is connected to a second terminal of the second capacitor C2. The first terminal of the second capacitor C2 is connected to the first output power supply terminal To1, and the second terminal of the second capacitor C2 is connected to the second output power supply terminal To2.

[0027] A first voltmeter 4 is connected between the first input power supply terminal Ti1 and the second input power supply terminal Ti2. The first voltmeter 4 measures an input voltage V1, which is the instantaneous value voltage of the input AC voltage Vac of the AC power supply 2, and inputs the measured value of the input voltage V1 to the control unit 40. A first ammeter 5 is connected between the second input power supply terminal Ti2 and the second input terminal of the input rectifier circuit 10. The first ammeter 5 measures an input current I1, which is the instantaneous value of the AC current flowing between the AC power supply 2 and the input rectifier circuit 10, and inputs the measured value of the input current I1 to the control unit 40. A second voltmeter 6 is connected between the first output power supply terminal To1 and the second output power supply terminal To2. The second voltmeter 6 measures an output voltage V2 of the output rectifier circuit 30 and inputs the measured value of the output voltage V2 to the control unit 40. A second ammeter 7 is connected between the second output power supply terminal To2 and the second output terminal of the output rectifier circuit 30. The second ammeter 7 measures the output current I2 flowing between the output rectifier circuit 30 and the load 3 , and inputs the value of the measured output current I2 to the control unit 40 .

[0028] The control unit 40 outputs a control signal Vg that causes the switching element Q1 to perform a switching operation. The control signal Vg is applied to the control electrode G of the switching element Q1. The control unit 40 performs feedback control based on one or both of the measurement results of the first voltmeter 4 and the first ammeter 5 and the measurement results of the second voltmeter 6 and the second ammeter 7 so that the control signal Vg becomes a signal suitable for power factor correction. The control signal Vg is a pulse-wave voltage signal with a high-frequency switching frequency fg of, for example, several MHz, and is input to the control electrode G of the switching element Q1.

[0029] The power conversion device 1, which functions as a power factor correction circuit, is expected to control the input current I1 to a waveform closer to a sine wave with a small phase difference and less harmonic distortion relative to the input voltage V1. The class E inverter circuit 20 has one switching element Q1, and can change the input current I1 of the power conversion device 1 by changing the switching frequency fg of the control signal Vg that turns the switching element Q1 on and off. Therefore, the input voltage V1, input current I1, output voltage V2, and output current I2 of the power conversion device 1 are detected, and the control unit 40, which forms a feedback loop, modulates the switching frequency fg of the control signal Vg, thereby controlling the input current I1 to a high power factor.

[0030] As long as the input voltage supplied to the class E inverter circuit 20 is the full-wave rectified waveform of the input voltage V1, the switching frequency fg is controlled by this feedback loop. This makes it possible to realize ZVS, in which the switching element Q1 is turned on in a state in which the drain-source voltage Vds of the switching element Q1 is zero, while controlling the input current I1 at a high power factor.

[0031] In contrast, in the power conversion device 1 shown in Fig. 1, a smoothing capacitor Cb is disposed between the input rectifier circuit 10 and the class E inverter circuit 20 to stabilize the input voltage of the class E inverter circuit 20. In this case, the charge stored in the smoothing capacitor Cb is not released from the time the input voltage V1 is applied until the class E inverter circuit 20 is started by the control signal Vg, and the terminal voltage Vb of the smoothing capacitor Cb maintains the maximum value of the applied input voltage V1. Therefore, when the class E inverter circuit 20 is started by the control signal Vg, the terminal voltage Vb equal to the maximum value of the input voltage V1 is applied to the pair of input terminals of the class E inverter circuit 20, regardless of the instantaneous value of the input voltage V1 at that time.

[0032] First Embodiment In the power conversion device 1 according to the first embodiment, the control unit 40 selects a first mode at the start of the control signal Vg, in which the switching frequency fg of the control signal Vg is set to a startup frequency corresponding to the inter-terminal voltage Vb of the smoothing capacitor Cb at the start of the control signal Vg. The inter-terminal voltage Vb of the smoothing capacitor Cb at the start of the control signal Vg is a voltage equal to the maximum value of the input voltage V1. In other words, the control unit 40 controls the switching frequency fg to start at a startup frequency determined by the inter-terminal voltage Vb of the smoothing capacitor Cb at the start of the control signal Vg, which is different from the frequency determined by feedback control of the class E inverter circuit 20. An appropriate range of values ​​for the startup frequency of the switching frequency fg can be determined in advance through experiments, circuit simulations, or the like. Furthermore, when the control unit 40 determines that a predetermined time has elapsed since the start of the control signal Vg, it selects a second mode in which the switching frequency fg is feedback-controlled based on one or both of the input voltage V1 and input current I1 and the output voltage V2 and output current I2.

[0033] As a result, the class E inverter circuit 20 can be started at a switching frequency fg that is different from the frequency of feedback control for power factor correction and is a suitable starting frequency at which ZVS can be easily achieved for the terminal voltage Vb applied to the class E inverter circuit 20. This makes it possible to suppress the turn-on voltage during non-ZVS operation in which the switching element Q1 is turned on with the voltage Vds applied across the switching element Q1 after the start of the class E inverter circuit 20, and to suppress the duration of non-ZVS turn-on. This also makes it possible to suppress heat generation from the switching element Q1 during non-ZVS operation, as well as malfunctions and noise of the class E inverter circuit 20 that are generated due to non-ZVS.

[0034] In the first mode, the control unit 40 starts the control signal Vg when the input voltage V1, which is the instantaneous voltage of the first AC power, is equal to or lower than a predetermined voltage value. For example, when the value of the input voltage V1 measured by the first voltmeter 4 becomes substantially zero, the control unit 40 starts the control signal Vg, thereby starting the switching element Q1.

[0035] As a result, when the inter-terminal voltage Vb of the smoothing capacitor Cb drops due to the start of the class E inverter circuit 20, the power factor correction operation can be started from a state in which both the input current I1 and the input voltage V1 of the power conversion device 1 are small. This makes it possible to achieve a high power factor operation in which the phases of the input voltage V1 and the input current I1 are aligned, and also to suppress heat generation and malfunction of the switching element Q1 due to ZVS failure that occurs when the input voltage V1 is high and the input current I1 is low.

[0036] The control unit 40 may switch from the first mode to the second mode after an operating time determined based on the capacitance value of the smoothing capacitor Cb and the inter-terminal voltage Vb of the smoothing capacitor Cb at the time of starting the control signal Vg has elapsed after starting the control signal Vg in the first mode. Alternatively, the control unit 40 may switch from the first mode to the second mode after starting the control signal Vg in the first mode when it determines that the inter-terminal voltage Vb of the smoothing capacitor Cb has decreased to a value substantially equal to the input voltage V1, which is the instantaneous voltage of the first AC power. This allows the control unit 40 to stably transition from the first mode, which is started in a non-ZVS mode, to the second mode, which feedback-controls the switching frequency fg, after the smoothing capacitor Cb is discharged.

[0037] The power conversion device 1 may also include a third voltmeter (not shown) that measures the terminal voltage Vb of the smoothing capacitor Cb, and the control unit 40 may reduce the switching frequency fg in the first mode in accordance with a decrease in the terminal voltage Vb of the smoothing capacitor Cb. This reduces the switching frequency fg to an appropriate value in accordance with the terminal voltage Vb of the smoothing capacitor Cb that decreases due to the switching operation of the class E inverter circuit 20, thereby suppressing heat generation and malfunction of the switching element Q1 due to failure to establish ZVS.

[0038] Next, the operation of the power conversion device 1 according to the first embodiment will be described with reference to Figures 2A and 2B. Figure 2A is a time chart showing the operating waveforms of the power conversion device 1 according to the first embodiment, and Figure 2B is a time chart showing an enlarged view of region IIB in Figure 2A. In Figures 2A and 2B, (1) shows the operating waveforms of the input voltage V1 and the terminal voltage Vb of the smoothing capacitor Cb. (2) shows the operating waveform of the input current I1. (3) shows the operating waveform of the control signal Vg. (4) shows the operating waveform of the drain-source voltage Vds, which is the voltage across the switching element Q1. (5) shows the operating waveform of the drain-source voltage instantaneous value Vds(ON), which is the instantaneous value of the voltage across the switching element Q1 when it is turned on. (6) shows the operating waveform of the switching frequency fg.

[0039] In the initial state before startup of the power conversion device 1, the smoothing capacitor Cb is charged to approximately 282 V, which is a peak voltage of the effective value of 200 V, by the input voltage V1, which is an AC voltage having an effective value of 200 V and a frequency of 50 Hz applied as the input AC voltage Vac of the AC power supply 2. Therefore, the initial state of the terminal voltage Vb of the smoothing capacitor Cb is a constant value of approximately 282 V. Thereafter, at time t0, when the instantaneous value of the input voltage V1 has decreased to approximately zero, the control unit 40 starts operation of the switching element Q1 in the first mode with a switching frequency fg of 3.6 MHz, which is a startup frequency different from the frequency determined by feedback control.

[0040] When the switching element Q1 is first turned on after startup, it is always turned on with the terminal voltage Vb applied, causing the class E inverter circuit 20 to start in a non-ZVS state. Therefore, the drain-source voltage Vds(ON), which is the instantaneous value of the voltage across the switching element Q1 when it is turned on, is approximately equal to the maximum value of the input voltage V1, which is the initial state of the terminal voltage Vb. However, immediately thereafter, the drain-source voltage Vds(ON), which is the instantaneous value of the voltage across the switching element Q1 when it is turned on, drops to near zero. Furthermore, when the switching element Q1 starts switching operation in response to the control signal Vg, the terminal voltage Vb drops and approaches the input voltage V1 supplied to the power conversion device 1, i.e., the full-wave rectified waveform output by the input rectifier circuit 10.

[0041] Then, at time t1, when a certain time has elapsed since startup, the control unit 40 switches the switching frequency fg from the startup frequency of 3.6 MHz to a frequency determined by feedback control and operates in the second mode, thereby making it possible to control the input current I1 to a high power factor.

[0042] Here, time t1, which is the time required for the control unit 40 to transition the switching frequency fg from the startup frequency to the frequency determined by feedback control, may be determined taking into consideration the rate at which the inter-terminal voltage Vb decreases. The rate at which the inter-terminal voltage Vb decreases depends on the inter-terminal voltage Vb of the switching element Q1 at startup, the capacitance value of the smoothing capacitor Cb, and the switching frequency fg at startup. The higher the inter-terminal voltage Vb at startup, the larger the capacitance value of the smoothing capacitor Cb, and the higher the switching frequency fg at startup, the longer the time until the smoothing capacitor Cb is discharged.

[0043] (Comparative Example of the First Embodiment) Fig. 3 is a comparative example of the first embodiment, and shows operational waveforms when the control unit 40 starts the switching element Q1 with a control signal Vg having a switching frequency fg determined by feedback control in the power conversion device 1 shown in Fig. 1. The operational waveforms shown in (1) to (6) of Fig. 3 correspond to the operational waveforms shown in (1) to (6) of Fig. 2A and 2B, and therefore a description of each operational waveform will be omitted.

[0044] 2A and 2B, in the comparative example of the first embodiment, after the switching element Q1 starts at time t0, Vds(ON) of (5) occurs for approximately 30 μs, indicating a long period of non-ZVS operation. Therefore, if the control unit 40 starts the operation of the switching element Q1 with the control signal Vg having a switching frequency fg determined by feedback control, the switching element Q1 is likely to generate heat or malfunction due to non-ZVS at startup.

[0045] In this way, in the first embodiment, a startup frequency different from the frequency determined by feedback control is selected as the switching frequency fg, and the operation of the switching element Q1 of the E-class inverter circuit 20 is started, thereby making it possible to suppress non-ZVS operation.

[0046] In the following description, the inverter circuit that generates a high-frequency current from the full-wave rectified waveform output from the input rectifier circuit 10 is referred to as the class E inverter circuit 20. However, this does not guarantee that the class E inverter circuit 20 achieves the so-called class E condition of simultaneously satisfying ZVS and ZDS (Zero Derivative Switching). It should be noted that the term "class E inverter circuit" is merely used as a general name for this circuit configuration.

[0047] Also, the example described here is the first operation of the class E inverter circuit 20 at time t0 from a state in which the input voltage V1 is initially applied. For example, when the class E inverter circuit 20 stops operating, the smoothing capacitor Cb is charged by the input voltage V1, and the terminal voltage Vb is applied, and the operation is started again, similar control can be performed to suppress non-ZVS operation.

[0048] As described above, according to the power conversion device 1 of the first embodiment, even when the input voltage V1 of the E-class inverter circuit 20 is high and the input current I1 is small, heat generation and malfunction of the switching element Q1 due to failure of ZVS can be suppressed.

[0049] Second Embodiment In the first embodiment, the control unit 40 fixed the on-duty ratio Pon of the control signal Vg to 0.5 and did not control the on-duty ratio Pon. In contrast, in the second embodiment, the control unit 40, in the first mode, sets the on-duty ratio Pon of the control signal Vg to a start-up on-duty ratio corresponding to the voltage Vb across the smoothing capacitor Cb at the start of the control signal Vg, which is a voltage equal to the maximum value of the input voltage V1. In addition, in the second mode, the control unit 40 feedback-controls the on-duty ratio based on one or both of the instantaneous values ​​of the input voltage V1 and the input current I1 and the values ​​of the output voltage V2 and the output current I2. The other configurations of the second embodiment are the same as those of the first embodiment, and therefore will not be described again.

[0050] In the second embodiment, the control unit 40 sets the on-duty ratio Pon in addition to the switching frequency fg to values ​​appropriate for startup in accordance with the voltage Vb across the smoothing capacitor Cb at startup of the control signal Vg, which is a voltage equal to the maximum value of the input voltage V1. The appropriate ranges for the switching frequency fg and on-duty ratio Pon at startup can be determined in advance through experiments, circuit simulations, etc. Therefore, in the second embodiment, the turn-on voltage during non-ZVS operation and the duration of non-ZVS turn-on can be further suppressed than in the first embodiment.

[0051] In addition, the power conversion device 1 may be provided with a third voltmeter (not shown) that measures the terminal voltage Vb of the smoothing capacitor Cb, and the control unit 40 may increase the on-duty ratio Pon in the first mode in accordance with a decrease in the terminal voltage Vb of the smoothing capacitor Cb.

[0052] This allows the on-duty ratio Pon to be increased to an appropriate value in accordance with the terminal voltage Vb of the smoothing capacitor Cb, which decreases due to the switching operation of the E-class inverter circuit 20, thereby suppressing heat generation and malfunction of the switching element Q1 due to ZVS not being established.

[0053] Next, the operation of the power conversion device 1 according to the second embodiment will be described with reference to FIG. 4. FIG. 4 is a time chart showing the operating waveforms of the power conversion device 1 according to the second embodiment. The operating waveforms shown in (1) to (6) of FIG. 4 correspond to the operating waveforms shown in (1) to (6) of FIGS. 2A and 2B, and therefore a description of each of the operating waveforms (1) to (6) will be omitted. (7) of FIG. 4 shows the operating waveform of the on-duty ratio Pon of the control signal Vg.

[0054] In the second embodiment, the control unit 40 manipulates the switching frequency fg and the on-duty ratio Pon by feedback control in order to control the input current I1 and output power of the class E inverter circuit 20 at a high power factor. As a result, in the second embodiment, it becomes possible to achieve ZVS over a wider range of operating current (power) than in the first embodiment.

[0055] In the initial state, the input voltage V1 is applied and the smoothing capacitor Cb is charged, and at time t0, the control unit 40 starts the switching operation of the switching element Q1 of the class E inverter circuit 20. At this time, the control unit 40 sets the switching frequency fg of the control signal Vg to the switching frequency at start-up (3.6 MHz in FIG. 4 ), as in the first embodiment. Furthermore, in the second embodiment, the control unit 40 also starts up the inverter by setting the on-duty ratio Pon of the control signal Vg to an on-duty ratio (0.40 in FIG. 4 ) different from the on-duty ratio determined by feedback control (0.55 in FIG. 4 ).

[0056] Then, at time t1, a certain time after the start of the switching element Q1 (time t0), the control unit 40 switches the switching frequency fg and the on-duty ratio Pon from the values ​​at the time of start to the frequency and on-duty ratio determined by feedback control.

[0057] Thus, in the second embodiment, when the E-class inverter circuit 20 is started, the on-duty ratio Pon as well as the switching frequency fg are switched from a value determined by feedback control to a value determined by the inter-terminal voltage Vb of the smoothing capacitor Cb at the time of start-up of the control signal Vg.

[0058] This makes it possible to suppress the turn-on voltage and the duration of non-ZVS turn-on during non-ZVS operation in which the switching element Q1 is turned on with a voltage applied across the switching element Q1 after the start of operation of the class E inverter circuit 20. This also makes it possible to suppress heat generation from the switching element Q1 during non-ZVS operation, and malfunctions and noise of the class E inverter circuit 20 that are generated due to non-ZVS.

[0059] (Comparative Example of Second Embodiment) Fig. 5 is a comparative example of the second embodiment, and shows operational waveforms when the control unit 40 starts operation of the switching element Q1 with a control signal Vg having an on-duty ratio Pon (0.55 in Fig. 5) determined by feedback control, as compared to the second embodiment. Note that the switching frequency fg of the control signal Vg is set to the switching frequency (3.6 MHz in Fig. 5) at start-up (time t0) as in the second embodiment, and at time t1, the switching frequency fg is switched to a frequency determined by feedback control. The operational waveforms shown in (1) to (7) of Fig. 5 correspond to the operational waveforms shown in (1) to (7) of Fig. 4, and therefore description of each operational waveform will be omitted.

[0060] In the comparative example of the second embodiment shown in Fig. 5, compared to the second embodiment shown in Fig. 4, the operating waveform of Vds(ON) in (5) shows that the non-ZVS operation period continues from the start at time t0, and the turn-on voltage of the switching element Q1 is high during non-ZVS operation. Therefore, in the comparative example of the second embodiment, the risk of heat generation, noise, and malfunction of the switching element Q1 is greater than in the second embodiment.

[0061] As described above, according to the power conversion device 1 of the second embodiment, even when the input voltage V1 of the E-class inverter circuit 20 is high and the input current I1 is small, heat generation and malfunction of the switching element Q1 due to failure of ZVS can be suppressed.

[0062] Although the embodiments of the present invention have been described above, the descriptions and drawings that form part of this disclosure should not be understood to limit the present invention. Various alternative embodiments, examples, and operating techniques will become apparent to those skilled in the art from this disclosure.

[0063] REFERENCE SIGNS LIST 1 Power conversion device 2 AC power supply 3 Load 4 First voltmeter 5 First ammeter 6 Second voltmeter 7 Second ammeter 10 Input rectifier circuit 20 Class E inverter circuit 21 LC resonant circuit 30 Output rectifier circuit 40 Control unit C1 First capacitor C2 Second capacitor Cb Smoothing capacitor Cr Resonant capacitor D1 First diode fg Switching frequency I1 Input current I2 Output current L1 First inductor Lc Choke inductor Lr Resonant inductor Pon On-duty ratio Q1 Switching element Ti1, Ti2 Input power supply terminals To1, To2 Output power supply terminals V1 Input voltage V2 Output voltage Vac Input AC voltage Vb Terminal voltage Vds Drain-source voltage Vg Control signal

Claims

an input rectifier circuit that rectifies the first AC power into a first DC power; an inverter circuit including a choke inductor, a switching element, and an LC resonant circuit, and configured to convert the first DC power into second AC power having a frequency higher than that of the first AC power; a capacitor connected between the input rectifier circuit and the inverter circuit; an output rectifier circuit that converts the second AC power into second DC power; a control unit that outputs a control signal for causing the switching element to perform a switching operation; A control method for a power conversion device comprising: The control unit Selecting a first mode in which the switching frequency of the control signal is set to a startup frequency corresponding to the voltage across the capacitor at startup of the control signal; after selecting the first mode, selecting a second mode in which the switching frequency is feedback-controlled based on at least one of instantaneous values ​​of the voltage and current of the first AC power and values ​​of the voltage and current of the second DC power; A method for controlling a power conversion device. 　 The method for controlling a power conversion device according to claim 1 , wherein the control unit initiates the control signal when, in the first mode, an instantaneous voltage of the first AC power is equal to or lower than a predetermined voltage value. 　 The control unit In the first mode, an on-duty ratio of the control signal is set to a starting on-duty ratio corresponding to the voltage between the terminals of the capacitor at the time of starting the control signal; In the second mode, the on-duty ratio is feedback-controlled based on at least one of instantaneous values ​​of the voltage and current of the first AC power and values ​​of the voltage and current of the second DC power. A method for controlling a power conversion device according to claim 1 or 2.

3. The control method for a power conversion device according to claim 1, wherein the control unit switches from the first mode to the second mode when an operating time determined from a capacitance value of the capacitor and a voltage between the terminals of the capacitor at the time of starting the control signal has elapsed after starting the control signal in the first mode.

3. The control method for a power conversion device according to claim 1, wherein the control unit switches from the first mode to the second mode when it determines that the inter-terminal voltage of the capacitor has decreased to a value substantially equal to an instantaneous voltage of the first AC power after starting the control signal in the first mode. 　 The method for controlling a power conversion device according to claim 1 or 2, wherein the control unit reduces the switching frequency in the first mode in response to a decrease in the inter-terminal voltage of the capacitor. 　 The method for controlling a power conversion device according to claim 3 , wherein the control unit increases the on-duty ratio in the first mode in response to a decrease in the inter-terminal voltage of the capacitor. 　 an input rectifier circuit that rectifies the first AC power into a first DC power; an inverter circuit including a choke inductor, a switching element, and an LC resonant circuit, and configured to convert the first DC power into second AC power having a frequency higher than that of the first AC power; a capacitor connected between the input rectifier circuit and the inverter circuit; an output rectifier circuit that converts the second AC power into second DC power; a control unit that outputs a control signal for causing the switching element to perform a switching operation; Equipped with The control unit Selecting a first mode in which the switching frequency of the control signal is set to a startup frequency corresponding to the voltage across the capacitor at startup of the control signal; after selecting the first mode, selecting a second mode in which the switching frequency is feedback-controlled based on at least one of instantaneous values ​​of the voltage and current of the first AC power and values ​​of the voltage and current of the second DC power; Power conversion device.

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