Power conversion device and method for controlling same

Abstract

Provided is a method for controlling a power conversion device (1, 1A, 1B) that comprises a first conversion circuit (11) provided with a switch (S1) and connected to DC terminals (T1, T2), a second conversion circuit (12) connected to AC terminals (T3, T4), and an LC resonance circuit (13) connected between the first conversion circuit (11) and the second conversion circuit (12). The power conversion device (1, 1A, 1B) converts DC power supplied from the DC terminals (T1, T2) into AC power, and outputs an AC voltage (V2) to the AC terminals (T3, T4). The method for controlling the power conversion device (1, 1A, 1B) involves: detecting the instantaneous value of the AC voltage (V2) outputted to the AC terminals (T3, T4); and operating the switching frequency (fs) of the switch (S1) on the basis of the instantaneous value of the AC voltage (V2) so as to increase the on-duty ratio (Ron) of the switch (S1) as the instantaneous value of the AC voltage (V2) increases, and decrease the on-duty ratio (Ron) of the switch (S1) as the instantaneous value of the AC voltage (V2) decreases.

Description

Power conversion device and control method thereof

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

[0002] Patent Document 1 describes a technology that uses a so-called class E inverter circuit, which performs zero voltage switching by voltage resonance operation using an LC resonance circuit, to perform highly efficient power conversion as a power conversion device that converts AC power supplied from an AC power source into DC power and supplies the DC power to a DC load.

[0003] Japanese Patent Application Laid-Open No. 2021-145433

[0004] In a power conversion device connected to an electric vehicle charging facility or an AC power system, in addition to supplying power from an AC power source to a DC load, there is a need to use the same power conversion device to supply power from a battery or the like connected to the DC side to a power system or AC load connected to the AC side.

[0005] The power conversion device of Patent Document 1 is configured to control the operation of a switch so as to be suitable for converting input AC power into DC power, but when it is desired to convert input DC power into AC power, there is a problem in that the efficiency of power conversion cannot be improved even if the operation of the switch is similarly controlled.

[0006] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a power conversion device that can realize low-loss turn-on operation of a switch and operate with high efficiency when converting DC power to AC power.

[0007] A control method for a power conversion device according to one aspect of the present invention controls a switch of a power conversion device including a first conversion circuit having a switch. The first conversion circuit is connected to a DC terminal. The second conversion circuit is connected to an AC terminal. An LC resonant circuit is connected between the first conversion circuit and the second conversion circuit. The power conversion device converts DC power supplied from the DC terminal into AC power and outputs an AC voltage to the AC terminal. The control method for the power conversion device detects an instantaneous value of the AC voltage output to the AC terminal, and operates a switching frequency of the switch based on the instantaneous value of the AC voltage, increasing the on-duty ratio of the switch as the instantaneous value of the AC voltage increases and decreasing the on-duty ratio of the switch as the instantaneous value of the AC voltage decreases.

[0008] According to the present invention, it is possible to provide a power conversion device that can realize a low-loss turn-on operation of a switch when converting DC power to AC power, and can operate with high efficiency.

[0009] Fig. 1 is a circuit diagram showing the configuration of a power conversion device according to a first embodiment. Fig. 2 is a contour diagram showing the loss at turn-on when the on-duty ratio and switching frequency of the first switch are changed, assuming that the instantaneous value of the AC voltage output is 141 V, in the power conversion device according to the first embodiment. Fig. 3 is a contour diagram showing the loss at turn-on when the on-duty ratio and switching frequency of the first switch are changed, assuming that the instantaneous value of the AC voltage output is 100 V, in the power conversion device according to the first embodiment. Fig. 4 is a contour diagram showing the loss at turn-on when the on-duty ratio and switching frequency of the first switch are changed, assuming that the instantaneous value of the AC voltage output is 100 V, in the power conversion device according to the first embodiment. Fig. 5 is a contour diagram showing the loss at turn-on when the on-duty ratio and switching frequency of the first switch are changed, assuming that the instantaneous value of the AC voltage output is 100 V, in the power conversion device according to the first embodiment. FIG. 4 is a contour diagram showing the loss at turn-on when the on-duty ratio and switching frequency of the first switch are changed, assuming that the instantaneous value of the AC voltage output in the power conversion device according to the first embodiment is 50 V, and is a diagram superimposed with the locus of the on-duty ratio and switching frequency when the average power of the AC power is 250 W. FIG. 5 is a graph showing the relationship between the instantaneous value of the AC voltage and a suitable on-duty ratio when the average power of the AC power is 250 W. FIG. 6 is a graph showing the relationship between the instantaneous value of the AC voltage and a suitable on-duty ratio when the average power of the AC power is 50 W. FIG. 7 is a circuit diagram showing the configuration of a power conversion device according to a second embodiment. FIG. 8 is a circuit diagram showing the configuration of a power conversion device according to a third embodiment.

[0010] 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.

[0011] 1 is a circuit diagram showing the configuration of a power conversion device 1 according to a first embodiment. The power conversion device 1 receives DC power from a DC power supply 2 connected to DC terminals T1 and T2, and supplies AC power to a load 3 connected to AC terminals T3 and T4. The power conversion device 1 includes a first conversion circuit 11 connected to the DC terminals T1 and T2, a second conversion circuit 12 connected to the AC terminals T3 and T4, an LC resonant circuit 13 connected between the first conversion circuit 11 and the second conversion circuit 12, and a control unit 20 that controls the first conversion circuit 11 and the second conversion circuit 12.

[0012] The first conversion circuit 11 is a class E inverter including a first bypass capacitor Cb1, a first reactor L1, a first parallel capacitor Cp1, and a first switch S1. The first bypass capacitor Cb1 is connected in parallel to the DC terminals T1 and T2. One end of the first reactor L1 is connected to one of the DC terminals T1. One end of the first parallel capacitor Cp1 is connected to the other end of the first reactor L1, and the other end is connected to the other DC terminal T2. The first switch S1 is connected in parallel to the first parallel capacitor Cp1. The first switch S1 is, for example, a semiconductor switching element such as an N-channel MOSFET.

[0013] The second conversion circuit 12 includes an output polarity switching circuit 14, a second bypass capacitor Cb2, a second reactor L2, a second parallel capacitor Cp2, and a second diode D2. The second bypass capacitor Cb2 is connected in parallel between one end and the other end of the output polarity switching circuit 14. One end of the second reactor L2 is connected to one end of the output polarity switching circuit 14. One end of the second parallel capacitor Cp2 is connected to the other end of the second reactor L2, and the other end is connected to the other end of the output polarity switching circuit 14. The second diode D2 is connected in parallel with the second parallel capacitor Cp2.

[0014] The output polarity switching circuit 14 includes a first changeover switch Sa, a second changeover switch Sb, a third changeover switch Sc, and a fourth changeover switch Sd. The first changeover switch Sa, the second changeover switch Sb, the third changeover switch Sc, and the fourth changeover switch Sd are implemented using semiconductor switching elements such as N-channel MOSFETs. One end of the first changeover switch Sa and one end of the third changeover switch Sc are connected to one end of the output polarity switching circuit 14. The other end of the first changeover switch Sa and one end of the second changeover switch Sb are connected in series. The other end of the third changeover switch Sc and one end of the fourth changeover switch Sd are connected in series. The other end of the second changeover switch Sb and the other end of the fourth changeover switch Sd are connected to the other end of the output polarity switching circuit 14. The connection point between the first changeover switch Sa and the second changeover switch Sb is connected to one AC terminal T3. The connection point between the third changeover switch Sc and the fourth changeover switch Sd is connected to the other AC terminal T4.

[0015] The LC resonant circuit 13 is a series circuit of a resonant capacitor Cr and a resonant reactor Lr.

[0016] An AC voltage detection circuit 15 is connected between the AC terminals T3 and T4 to detect an AC voltage V2 output to the load 3.

[0017] The control unit 20 monitors the instantaneous value of the AC voltage V2 detected by the AC voltage detection circuit 15, and controls the on / off of the first switch S1 of the first conversion circuit 11 and the first to fourth changeover switches Sa-Sd of the output polarity switching circuit 14.

[0018] A DC current I1 input via a first reactor L1 of a first conversion circuit 11, which is a class E inverter, is applied to a first switch S1, a first parallel capacitor Cp1, and an LC resonant circuit 13. At this time, the first switch S1 repeatedly turns on and off at a high-frequency switching frequency fs, thereby generating a high-frequency resonant current Ir1 in the LC resonant circuit 13. The resonant current Ir1 then flows through a second conversion circuit 12, which is a rectifier circuit, whereby the high-frequency component is rectified to become a rectified current Ir2 with a full-wave rectified waveform of, for example, 100 Hz. By switching the on / off states of the first to fourth changeover switches Sa-Sd of an output polarity switching circuit 14 based on the polarity of the AC power to be output as the rectified current Ir2, AC power of, for example, 50 Hz is supplied to AC terminals T3 and T4. That is, by turning on the first changeover switch Sa and the fourth changeover switch Sd and turning off the second changeover switch Sb and the third changeover switch Sc in accordance with the rectified current Ir2, positive AC power is supplied to the AC terminals T3 and T4. On the other hand, by turning off the first changeover switch Sa and the fourth changeover switch Sd and turning on the second changeover switch Sb and the third changeover switch Sc in accordance with the rectified current Ir2, negative AC power is supplied to the AC terminals T3 and T4. At this time, the AC voltage V2 detected by the AC voltage detection circuit 15 is input to the control unit 20.

[0019] The control unit 20 compares the detected instantaneous value of the AC voltage V2 with a target value for the instantaneous value of the AC voltage V2 and controls the switching frequency fs of the first switch S1 so as to reduce the difference between the two, thereby supplying the desired AC power to the load 3. When the first switch S1 is in the off state, a resonant voltage Vr1, which is the voltage across the first switch S1 and the first parallel capacitor Cp1, rises and falls due to changes in the voltage and current of the LC resonant circuit 13. Then, at the moment when the resonant voltage Vr1 becomes zero or drops sufficiently, the first switch S1 is turned on, i.e., switched from the off state to the on state, thereby reducing losses that occur when the first switch S1 is turned on. However, because the resonant voltage Vr1 constantly fluctuates at the high-frequency switching frequency fs, it is important to turn on the first switch S1 at the appropriate timing.

[0020] 2 is a contour diagram showing the turn-on losses when the on-duty ratio Ron and switching frequency fs of the first switch S1 are varied under the condition that the AC voltage V2 has a frequency of 50 Hz and an effective value of 100 V in the power conversion device 1, where the AC voltage V2 has a maximum absolute value of 141 V. The diagram also shows the loci of the on-duty ratio Ron and switching frequency fs at which the average AC power is 250 W (i.e., 500 W as the instantaneous power when the instantaneous value of the AC voltage V2 is 141 V) superimposed on the contour diagram. Here, the on-duty ratio Ron is the ratio expressed as on-duty time / (on-duty time + off-duty time). In other words, in the portion of the contour diagram where the locus of the average power of 250 W passes through the region where the turn-on losses are zero, it is possible to output the instantaneous power of 500 W required to output an average power of 250 W, and to achieve low-loss switching with zero turn-on losses. In this case, since the switching frequency fs is manipulated to control AC power, the parameter that can be manipulated to suppress loss during turn-on is the on-duty ratio Ron. Therefore, under the condition that the AC voltage V2 outputs an instantaneous voltage of 141 V and an instantaneous power of 500 W, it can be said that it is preferable to set the switching frequency fs to approximately 0.95 MHz and the on-duty ratio Ron between 0.43 and 0.51.

[0021] 3 is a contour diagram showing the loss at turn-on when the on-duty ratio Ron and switching frequency fs of the first switch S1 are changed, assuming that the instantaneous value of the AC voltage V2 output in the power conversion device 1 is 100 V, and is a diagram superimposed with the locus of the on-duty ratio Ron and switching frequency fs when the average power of the AC power is 250 W (250 W as the instantaneous power when the instantaneous value of the AC voltage V2 is 100 V). Under these conditions, the value of the on-duty ratio Ron that can turn on the first switch S1 at zero voltage with a switching frequency fs of approximately 1.02 MHz is between 0.35 and 0.42.

[0022] 4 is a contour diagram showing the loss at turn-on when the on-duty ratio Ron and switching frequency fs of the first switch S1 are changed, assuming that the instantaneous value of the AC voltage V2 output in the power conversion device 1 is 50 V, and is a diagram superimposed with the locus of the on-duty ratio Ron and switching frequency fs when the average power of the AC power is 250 W (62.5 W as the instantaneous power when the instantaneous value of the AC voltage V2 is 50 V). Under these conditions, complete zero-voltage switching of the first switch S1 cannot be achieved, but under the condition of an instantaneous power of 62.5 W, the optimum value of the on-duty ratio Ron that enables turn-on with the lowest loss is between 0.15 and 0.21 when the switching frequency fs is approximately 1.15 MHz.

[0023] As described above, the inventors' research has revealed that the value of the on-duty ratio Ron suitable for realizing low-loss turn-on of the first switch S1 changes dynamically in accordance with changes in the instantaneous value of the AC voltage V2.

[0024] 5 is a graph showing the relationship between the instantaneous value of the AC voltage V2 and the desirable on-duty ratio Ron when the average power of the AC power is 250 W. As shown in FIG. 5 , the desirable on-duty ratio Ron increases or decreases as the instantaneous value of the AC voltage V2 increases or decreases. That is, the control unit 20 controls the first switch S1 so that the on-duty ratio Ron increases or decreases in response to an increase in the instantaneous value of the AC voltage V2 and decreases in response to a decrease in the instantaneous value of the AC voltage V2.

[0025] Fig. 6 is a graph showing the relationship between the instantaneous value of AC voltage V2 and the suitable on-duty ratio Ron when power conversion device 1 outputs an AC voltage V2 with an effective value of 100 V and operates with a smaller average AC power of 50 W. Comparing Fig. 5 with Fig. 6, when the AC power flowing to load 3 is large, it is possible to suppress loss at turn-on by setting the value of on-duty ratio Ron to a high value.

[0026] The value of the on-duty ratio Ron corresponding to the instantaneous value of the AC voltage V2 of the power conversion device 1 can be calculated, for example, as follows. For example, when the AC power is 250 W, the relational expression between the instantaneous value of the AC voltage V2 and the on-duty ratio Ron, obtained by fitting the graph of FIG. 5 , is stored in the control unit 20. Then, the control unit 20 sets the on-duty ratio Ron to a value calculated from the relational expression, in accordance with the instantaneous value of the AC voltage V2. Similarly, when the AC power is 50 W, the relational expression between the instantaneous value of the AC voltage V2 and the on-duty ratio Ron, obtained by fitting the graph of FIG. 6 , is stored in the control unit 20. Then, the control unit 20 sets the on-duty ratio Ron to a value calculated from the relational expression, in accordance with the instantaneous value of the AC voltage V2. Alternatively, a table showing the relationship between the instantaneous value of the AC voltage V2 and the on-duty ratio Ron calculated from the relational expression may be stored in the control unit 20, and the on-duty ratio Ron corresponding to the instantaneous value of the AC voltage V2 of the power conversion device 1 may be set based on this table. By storing such a relational expression or table created according to the value of AC power in the control unit 20, it is possible to set the value of the on-duty ratio Ron according to the instantaneous value of the AC voltage V2 by the control unit 20. When the on-duty ratio Ron is calculated using the relational expression between the instantaneous value of the AC voltage V2 and the on-duty ratio Ron, if the instantaneous value is small, the on-duty ratio Ron may become a value of 0 or less. In such cases, the on-duty ratio Ron can be set to zero.

[0027] In the power conversion device 1, when converting DC power to AC power, the first switch S1 is controlled to change the on-duty ratio Ron in accordance with the instantaneous value of the AC voltage V2, thereby achieving a low-loss turn-on operation of the first switch S1 and enabling highly efficient operation of the power conversion device 1. Note that the same effect can be obtained by providing an AC current detection circuit that detects the AC current I2 instead of the AC voltage detection circuit 15, and controlling the control unit 20 to increase or decrease the on-duty ratio Ron of the first switch S1 in accordance with an increase or decrease in the instantaneous value of the AC current I2.

[0028] 7 is a circuit diagram showing the configuration of a power conversion device 1A according to a second embodiment. The power conversion device 1A differs from the power conversion device 1 according to the first embodiment in that it further includes an AC current detection circuit 16 that detects an AC current I2 supplied to a load 3 from AC terminals T3 and T4.

[0029] In the power conversion device 1A, the control unit 20 controls the on-duty ratio Ron of the first switch S1 based on the instantaneous value of the AC voltage V2 detected by the AC voltage detection circuit 15 and the instantaneous value of the AC current I2 detected by the AC current detection circuit 16.

[0030] When converting DC power to AC power, the power conversion device 1A detects the instantaneous value of the AC current I2 in addition to the change in the instantaneous value of the AC voltage V2. When the value of the AC power obtained by integrating the instantaneous values ​​of the AC voltage V2 and the AC current I2 or the instantaneous values ​​of the AC voltage V2 and the AC current I2 increases or decreases, the on-duty ratio Ron is increased or decreased.

[0031] As a result, even in the power conversion device 1A, low-loss turn-on operation of the first switch S1 can be achieved, and the power conversion device 1A can be operated with high efficiency even under conditions in which the AC power supplied to the load 3 changes.

[0032] 8 is a circuit diagram showing the configuration of a power conversion device 1B according to a third embodiment. The power conversion device 1B differs from the power conversion device 1A according to the second embodiment in that the power conversion device 1B has a first diode D1 connected in parallel to the first switch S1 of the first conversion circuit 11 and a second switch S2 connected in parallel to the second diode D2 of the second conversion circuit 12. The second switch S2 is, for example, a semiconductor switching element such as an N-channel MOSFET.

[0033] When the power conversion device 1B converts DC power from the DC power source 2 and supplies AC power to the load 3, the power conversion device 1B performs a switching operation on the first switch S1 of the first conversion circuit 11, and performs power conversion operation with the first conversion circuit 11 as a class E inverter, similar to the power conversion devices 1 and 1A of the first and second embodiments. At this time, the second switch S2 is turned off.

[0034] On the other hand, when AC power supplied from the AC power supply 4 to the AC terminals T3 and T4 is converted into DC power and supplied from the DC terminals T1 and T2 to the DC power supply 2, the switch S2 of the second conversion circuit performs a high-frequency on-off switching operation to operate as a class E inverter. At this time, a high-frequency current is generated in the LC resonant circuit 13, and the first conversion circuit 11 including the first diode D1 operates as a rectifier to rectify the high-frequency current, thereby supplying DC power to the DC power supply 2. More specifically, the control unit 20 turns on the first switch Sa and the fourth switch Sd of the output polarity switching circuit 14, turns off the second switch Sb and the third switch Sc, and turns off the first switch S1. Then, the control unit 20 operates the switching frequency fs2 of the second switch S2 based on the AC voltage V2 detected by the AC voltage detection circuit 15 and the AC current I2 detected by the AC current detection circuit 16, and controls the DC voltage, DC current, or DC power supplied to the DC terminals T1 and T2.

[0035] The power conversion device 1B can also realize a low-loss turn-on operation of the first switch S1 when converting DC power to AC power, and can operate with high efficiency. Furthermore, the power conversion device 1B can function as a bidirectional power conversion device that can convert not only the DC power of the DC power supply 2 to AC power to be supplied to the load 3, but also the AC power of the AC power supply 4 to DC power to be supplied to the DC power supply 2.

[0036] 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.

[0037] REFERENCE SIGNS LIST 1, 1A, 1B Power conversion device 2 DC power supply 3 Load 4 AC power supply 11 First conversion circuit 12 Second conversion circuit 13 LC resonant circuit 15 AC voltage detection circuit 16 AC current detection circuit 20 Control unit Cp1 First parallel capacitor Cp2 Second parallel capacitor D1 First diode D2 Second diode fs Switching frequency I1 DC current I2 AC current L1 First reactor L2 Second reactor Ron On-duty ratio S1 First switch S2 Second switch T1, T2 DC terminals T3, T4 AC terminals V2 AC voltage

Claims

1. A control method for a power conversion device comprising: a first conversion circuit having a switch connected to a DC terminal; a second conversion circuit connected to an AC terminal; and an LC resonant circuit connected between the first conversion circuit and the second conversion circuit, which converts DC power supplied from the DC terminal into AC power and outputs an AC voltage to the AC terminal, the control method comprising the steps of: detecting an instantaneous value of the AC voltage output to the AC terminal; manipulating a switching frequency of the switch based on the instantaneous value of the AC voltage; increasing an on-duty ratio of the switch as the instantaneous value of the AC voltage rises; and decreasing the on-duty ratio of the switch as the instantaneous value of the AC voltage falls.

2. A control method for a power conversion device as described in claim 1, comprising the steps of: detecting an instantaneous value of an AC current output to the AC terminal; manipulating the switching frequency of the switch based on the instantaneous value of the AC voltage and the instantaneous value of the AC current; increasing the on-duty ratio of the switch as the instantaneous value of the AC voltage and the instantaneous value of the AC current rises; and decreasing the on-duty ratio of the switch as the instantaneous value of the AC voltage and the instantaneous value of the AC current fall.

3. The method for controlling a power conversion device according to claim 1, further comprising: storing a relational expression between the instantaneous value of the AC voltage and the on-duty ratio; and setting the on-duty ratio based on the relational expression between the instantaneous value of the AC voltage and the on-duty ratio.

4. The method for controlling a power conversion device according to claim 1, further comprising: storing a table of the relationship between the instantaneous value of the AC voltage and the on-duty ratio; and setting the on-duty ratio based on the table of the relationship between the instantaneous value of the AC voltage and the on-duty ratio.

5. A power conversion device comprising: a first conversion circuit connected to a DC terminal and including a first switch; a second conversion circuit connected to an AC terminal; an LC resonant circuit arranged between the first conversion circuit and the second conversion circuit; an AC voltage detection circuit connected to the AC terminal; and a control unit for controlling the first conversion circuit and the second conversion circuit, wherein the control unit converts DC power supplied from the DC terminal into AC power, and when outputting an AC voltage to the AC terminal, detects an instantaneous value of the AC voltage output to the AC terminal by the AC voltage detection circuit, operates a switching frequency of the first switch based on the instantaneous value of the AC voltage, increases an on-duty ratio of the first switch as the instantaneous value of the AC voltage rises, and decreases the on-duty ratio of the first switch as the instantaneous value of the AC voltage falls.

6. The power conversion device according to claim 5, wherein the first conversion circuit further comprises a first parallel capacitor connected in parallel to the first switch, and a first reactor connected between the first switch and the DC terminal, and the second conversion circuit comprises a second diode, a second parallel capacitor connected in parallel to the second diode, and a second reactor connected between the second diode and the AC terminal.

7. The power conversion device according to claim 6, wherein the first conversion circuit further comprises a first diode connected in parallel with the first switch, the second conversion circuit further comprises a second switch connected in parallel with the second diode, and the control unit converts AC power supplied from the AC terminal into DC power, and, when outputting a DC voltage to the DC terminal, operates a switching frequency of the second switch to control the DC voltage, DC current, or DC power supplied to the DC terminal.

Images