Control method for power conversion device and power conversion device

Abstract

A power conversion device (1) comprises a first switch (10) that is connected to input terminals (T1, T2) via an input inductor (L0), a principal resonance circuit (11) that is connected to the first switch (10), a suppression circuit (14) that is connected in parallel to the first switch (10), and a control unit (15). An auxiliary resonance circuit (12) and a second switch (13) are connected in series at the suppression circuit (14). The control unit (15) drives the first switch (10) at a prescribed switching frequency (fsw) and switches the second switch (13) on and off in accordance with an inter-terminal voltage (Vt) at the first switch (10). The resonance frequency (fr2) of the auxiliary resonance circuit (12) is set to a value that is higher than the switching frequency (fsw).

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] A voltage-resonant power conversion circuit including a class E circuit can perform low-loss switching by using zero-voltage switching, which turns on the switch of the class E circuit when the voltage across the switch is zero. This allows the switch's drive frequency to be increased, making it possible to miniaturize passive components such as the inductor and capacitor of the LC series resonant circuit of the class E circuit. However, using voltage resonance increases the voltage between the switch's terminals. This requires the use of switching elements with high voltage resistance, which increases the cost of the switch and increases conduction loss.

[0003] In light of this, a technique is known for suppressing the peak voltage between the terminals of a switch by adding an LC series resonant circuit with a resonant frequency higher than the switching frequency of the switch in parallel with the switch as a peak voltage suppression circuit (see Patent Document 1). This makes it possible to lower the withstand voltage of the switching element, resulting in lower costs and suppressing conduction loss.

[0004] JP 2017-184500 A

[0005] However, in the power conversion circuit of Patent Document 1, when a peak voltage suppression circuit is applied to a voltage-resonant power conversion circuit including a class E circuit in which the input voltage fluctuates, the peak voltage suppression circuit operates even in a region where there is sufficient margin for the device's withstand voltage. As a result, current always flows through the peak voltage suppression circuit, increasing power consumption and reducing the efficiency of the power conversion circuit, resulting in constant losses. Furthermore, the increased power consumption increases heat generation in the switch, requiring an increase in the size of a cooler to cool the switch.

[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 control method for a power conversion device and a power conversion device that can suppress the peak voltage of the voltage between the terminals of a switch and improve efficiency.

[0007] A power conversion device according to one aspect of the present invention includes a first switch connected to an input terminal via an input inductor, a main resonant circuit connected to the first switch, a suppression circuit connected in parallel to the first switch, and a control unit. The suppression circuit has a sub-resonant circuit and a second switch connected in series. The control unit drives the first switch at a predetermined switching frequency and switches the second switch on and off depending on the voltage across the first switch. The resonant frequency of the sub-resonant circuit is set to a value higher than the switching frequency.

[0008] According to the present invention, it is possible to provide a control method for a power conversion device and a power conversion device that can suppress the peak voltage between the terminals of a switch and improve efficiency.

[0009] FIG. 1 is a circuit diagram showing an example of the configuration of a power conversion device according to a first embodiment. FIG. 2A is a waveform diagram schematically showing the relationship between the input voltage V1 of the power conversion device according to the first embodiment and the inter-terminal voltage Vt of the first switch. FIG. 2B is a waveform diagram showing the inter-terminal voltage Vt1 of the first switch of the power conversion device according to the first embodiment and the inter-terminal voltage Vt2 of the first switch of the power conversion device of the comparative example in region B of FIG. 2A. FIG. 2C is a waveform diagram showing the inter-terminal voltage Vt1 of the first switch of the power conversion device according to the first embodiment and the inter-terminal voltage Vt2 of the first switch of the power conversion device of the comparative example in region C of FIG. 2A. FIG. 3 is a circuit diagram showing an example of the configuration of a power conversion device according to a second embodiment. FIG. 4 is a circuit diagram showing an example of 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] First Embodiment The configuration of a power conversion device 1 according to the first embodiment will be described with reference to Fig. 1. The power conversion device 1 according to the first embodiment is a voltage-resonant class E inverter circuit serving as a power factor correction circuit. The power conversion device 1 converts AC power from an AC power supply Vac connected as an external power supply 2 into AC power with a high-frequency switching frequency fsw, and outputs the AC power to an external load 3. The AC power supply Vac is, for example, a commercial power supply with an effective voltage of 100 V and a frequency of 50 Hz or 60 Hz. The switching frequency fsw is, for example, a high frequency of about 100 kHz to several tens of MHz.

[0012] The power conversion device 1 includes a pair of first and second input terminals T1 and T2, an input inductor L0, a first switch 10, a main resonant circuit 11, an output capacitor C0, a suppression circuit 14, a pair of first and second output terminals T3 and T4, and a control unit 15. An external power supply 2 is connected in parallel to the first and second input terminals T1 and T2. An external load 3 is connected in parallel to the first and second output terminals T3 and T4.

[0013] The power conversion device 1 is also provided with a voltmeter 16 for measuring an input voltage V1 input between the first input terminal T1 and the second input terminal T2, and an ammeter 17 for measuring an input current I1 flowing between the external power supply 2 and the power conversion device 1. The values ​​of the input voltage V1 measured by the voltmeter 16 and the input current I1 measured by the ammeter 17 are transmitted to the control unit 15. Note that a configuration may be adopted in which either the voltmeter 16 or the ammeter 17 is provided.

[0014] The first switch 10 is connected to the first input terminal T1 and the second input terminal T2 via an input inductor L0 serving as a choke inductor. In the first embodiment, a first terminal of the input inductor L0 is connected to the first input terminal T1, and the first switch 10 is connected between the second terminal of the input inductor L0 and the second input terminal T2. However, this is not limiting, and the first terminal of the input inductor L0 may be connected to the second input terminal T2, and the first switch 10 may be connected between the first input terminal T1 and the second terminal of the input inductor L0. Alternatively, the first terminal of the input inductor L0 may be connected to the first input terminal T1, the first terminal of another input inductor L0 may be connected to the second input terminal T2, and the first switch 10 may be connected between the second terminal of the input inductor L0 and the second terminal of the other input inductor L0.

[0015] The first switch 10 is a switch group in which a first switching element Q1 and a second switching element Q2 are connected in series. The number of series-connected switching elements in the first switch 10 as a switch group is not limited to two and may be three or more. In the first embodiment, a first terminal of the first switching element Q1 is connected to a second terminal of the input inductor L0, a first terminal of the second switching element Q2 is connected to a second input terminal, and a second terminal of the first switching element Q1 is connected to the second switching element Q2. The control electrodes of the first switching element Q1 and the second switching element Q2 are each connected to a controller 15, and a control signal for switching the first switching element Q1 and the second switching element Q2 on and off is input to the controller 15.

[0016] In the first embodiment, N-channel MOSFETs are used as the first switching element Q1 and the second switching element Q2, but this is not limiting and semiconductor switching elements such as IGBTs can be used. In the first embodiment, the first terminals of the first switching element Q1 and the second switching element Q2 are drains, the second terminals are sources, and the control electrodes are gates.

[0017] An output capacitor C0 serving as a shunt capacitor is connected in parallel to the first terminal and the second terminal of the first switch 10. Note that the output capacitor C0 may be a capacitor that is a stand-alone component, or may not be a capacitor that is a stand-alone component by utilizing the parasitic capacitance of the switching elements Q1 and Q2 used in the first switch 10.

[0018] A main resonant circuit 11 is connected between the first output terminal T3 and the first terminal of the first switching element Q1, which is the first end of the first switch 10. However, without being limited to this, the main resonant circuit 11 may be connected between the second output terminal T4 and the first terminal of the second switching element Q2, which is the second end of the first switch 10. The main resonant circuit 11 is an LC series resonant circuit in which an inductor L1 and a capacitor C1 are connected in series.

[0019] The control unit 15 controls the second switching element Q2 to be always on and drive the first switching element Q1 at the switching frequency fsw when the input voltage V1 is positive on the first input terminal T1 side and negative on the second input terminal T2 side. Furthermore, the control unit 15 controls the first switching element Q1 to be always on and drive the second switching element Q2 at the switching frequency fsw when the input voltage V1 is negative on the first input terminal T1 side and positive on the second input terminal T2 side. The switching frequency fsw is, for example, a frequency of approximately 100 kHz to several tens of MHz. The resonant frequency fr1 of the main resonant circuit 11 is set to a value corresponding to the switching frequency fsw. For example, the resonant frequency fr1 of the main resonant circuit 11 is set to a value approximately equal to the switching frequency fsw. With this configuration, the power conversion device 1 according to the first embodiment can operate as a power factor correction circuit.

[0020] A suppression circuit 14 is connected in parallel to the first switch 10. The breakdown voltage Vr of a typical semiconductor switching element is approximately 650 V, and semiconductor switching elements with a breakdown voltage Vr higher than 650 V are expensive. Furthermore, the inter-terminal voltage Vt of the first switch 10 becomes, for example, approximately four times the absolute value of the input voltage V1 due to voltage resonance of the main resonant circuit 11. Therefore, there is a possibility that the peak voltage of the inter-terminal voltage Vt of the first switch 10 may exceed the breakdown voltage Vr of the semiconductor switching element. The suppression circuit 14 suppresses the peak voltage of the inter-terminal voltage Vt of the first switch 10, for example, to prevent the peak voltage of the inter-terminal voltage Vt from exceeding the breakdown voltage Vr of the semiconductor switching element.

[0021] The suppression circuit 14 has a sub-resonant circuit 12 and a second switch 13 connected in series. The sub-resonant circuit 12 functions as a peak suppression circuit for suppressing the peak voltage of the inter-terminal voltage Vt of the first switch 10. The sub-resonant circuit 12 is an LC series resonant circuit in which an inductor L2 and a capacitor C2 are connected in series. The resonant frequency fr2 of the sub-resonant circuit 12 is set to a value higher than the switching frequency fsw. For example, the resonant frequency fr2 of the sub-resonant circuit 12 is set to a value two or three times the switching frequency fsw. The second switch 13 is a switch for switching the operation of the sub-resonant circuit 12 on and off. When the second switch 13 is off, the parallel connection between the first switch 10 and the sub-resonant circuit 12 is disconnected, and the operation of the sub-resonant circuit 12 is turned off. When the second switch 13 is on, the first switch 10 and the sub-resonant circuit 12 are connected in parallel, and the operation of the sub-resonant circuit 12 is turned on. The second switch 13 is configured by a semiconductor switching element such as a MOSFET or an IGBT. A control electrode of the second switch 13 is connected to the control unit 15. The control unit 15 controls the on / off of the second switch 13.

[0022] The control unit 15 is a general-purpose computer equipped with a CPU (Central Processing Unit), a memory, and an input / output unit. A computer program (control program) is installed in the control unit 15. By executing the computer program, the control unit 15 performs various functions.

[0023] The power conversion device 1 is a voltage-resonant class E inverter circuit in which a first switch 10 repeatedly turns on and off at a high-frequency switching frequency fsw and a high-voltage inter-terminal voltage Vt, thereby generating an AC current having the same frequency as the switching frequency fsw.

[0024] In a circuit configured like the power conversion device 1, the higher the switching frequency fsw, the smaller the inductance required for the inductor L1 and the capacitance required for the capacitor C1, which are passive components, and the smaller the circuit size. Furthermore, during the off-period of the first switch 10, zero voltage switching (ZVS) can be performed by using the current of the main resonant circuit 11 to zero the inter-terminal voltage Vt of the first switch 10 before turning it on, thereby eliminating turn-on loss. Performing ZVS reduces losses even when the switching frequency fsw is increased, potentially enabling the miniaturization of the power conversion device 1. However, voltage resonance increases the inter-terminal voltage Vt of the first switch 10. Therefore, the switching elements Q1 and Q2 of the first switch 10 must have a high breakdown voltage Vr, which increases the cost of the first switch 10 and increases conduction loss.

[0025] Therefore, a sub-resonant circuit 12, which is an LC series resonant circuit, is connected in parallel with the first switch 10 as a peak voltage suppression circuit having a resonant frequency fr2 that is two or three times the switching frequency fsw. This makes it possible to reduce the component of the inter-terminal voltage Vt of the first switch 10 that corresponds to the second or third harmonic of the switching frequency fsw, and to reduce the inter-terminal voltage Vt applied when the first switch 10 is off. This makes it possible to reduce the withstand voltage of the first switch 10, reduce costs, and suppress conduction loss.

[0026] In a voltage-resonant power conversion device 1 including a class E circuit in which the input voltage V1 fluctuates, if the first switch 10 and the sub-resonant circuit 12 are directly connected in parallel, the sub-resonant circuit 12 will operate even in a region where the withstand voltage of the switching elements Q1 and Q2 is sufficient. This will result in constant loss in the sub-resonant circuit 12. This will result in a decrease in the efficiency of the power conversion device 1, an increase in heat generation, and an increase in the size of the cooler.

[0027] Therefore, in the first embodiment, as described above, the suppression circuit 14, in which the sub-resonant circuit 12 and the second switch 13 are connected in series, is connected in parallel with the first switch 10. The control unit 15 determines the value of the inter-terminal voltage Vt of the first switch 10 based on at least one of the input voltage V1 observed by the voltmeter 16 and the input current I1 observed by the ammeter 17, and switches the second switch 13 on and off according to the determined value of the inter-terminal voltage Vt.

[0028] For example, when the control unit 15 determines that the voltage Vt across the first switch 10 exceeds the threshold voltage Vth while the second switch 13 is off, the control unit 15 controls the second switch 13 to switch from off to on. Furthermore, when the control unit 15 determines that the voltage Vt across the first switch 10 falls below the threshold voltage Vth while the second switch 13 is on, the control unit 15 controls the second switch 13 to switch from on to off.

[0029] With this configuration, when the inter-terminal voltage Vt exceeds the threshold voltage Vth, the sub-resonant circuit 12 operates to suppress the peak voltage of the inter-terminal voltage Vt. Furthermore, when the inter-terminal voltage Vt falls below the threshold voltage Vth, the sub-resonant circuit 12 does not operate. In other words, in a region where the withstand voltage of the switching elements Q1 and Q2 of the first switch 10 is sufficiently high, the sub-resonant circuit 12 does not operate, thereby improving the efficiency of the power conversion device 1 and ultimately enabling the size of a cooler used in the power conversion device 1 to be reduced.

[0030] Next, the operation of the power conversion device 1 will be described using FIGS. 2A to 2C. FIG. 2A is a waveform diagram schematically illustrating the relationship between the input voltage V1 of the power conversion device 1 and the inter-terminal voltage Vt of the first switch 10. FIG. 2B is a waveform diagram illustrating the inter-terminal voltage Vt1 of the first switch 10 of the first embodiment and the inter-terminal voltage Vt2 of the first switch 10 of the comparative example in region B of FIG. 2A. FIG. 2C is a waveform diagram illustrating the inter-terminal voltage Vt1 of the first switch 10 of the first embodiment and the inter-terminal voltage Vt2 of the first switch 10 of the comparative example in region C of FIG. 2A. In the first embodiment, when the inter-terminal voltage Vt1 exceeds the threshold voltage Vth, the second switch 13 is turned on to operate the auxiliary resonant circuit 12, and when the inter-terminal voltage Vt1 falls below the threshold voltage Vth, the second switch 13 is turned off to prevent the auxiliary resonant circuit 12 from operating. In contrast to this, in the comparative example, the second switch 13 is always turned on, and the sub-resonant circuit 12 remains in operation, regardless of the value of the inter-terminal voltage Vt2.

[0031] In Fig. 2A, the terminal voltage Vt of the first switch 10 is shown as a waveform that appears to be full-wave rectified with respect to the sinusoidal input voltage V1 with an effective value of 100 V and a frequency of 50 Hz. However, in reality, as shown in Figs. 2B and 2C, the terminal voltages Vt1 and Vt2 increase in proportion to the input voltage V1 when the first switch 10 is driven at the switching frequency fsw due to voltage resonance of the main resonant circuit 11, which is an LC series resonant circuit. In Figs. 2A to 2C, Vr is the element breakdown voltage of the first switch 10 (switching elements Q1 and Q2), and Vth is the threshold voltage at which the operation of the second switch 13 is switched.

[0032] 2B , in region B where the inter-terminal voltage Vt exceeds the threshold voltage Vth, the inter-terminal voltage Vt1 of the first embodiment and the inter-terminal voltage Vt2 of the comparative example have the same waveform, with the peak voltage suppressed by the operation of the sub-resonant circuit 12. In this way, in the region where the inter-terminal voltage Vt exceeds the threshold voltage Vth, the inter-terminal voltage Vt can be reduced in both the first embodiment and the comparative example. Therefore, even when the input voltage V1 is large, the inter-terminal voltage Vt can be made smaller than the element breakdown voltage Vr of the switching elements Q1 and Q2.

[0033] 2C , in region C where the inter-terminal voltage Vt is below the threshold voltage Vth, the inter-terminal voltage Vt1 of the first embodiment has a peak voltage that is not suppressed because the sub-resonant circuit 12 is not operating. In contrast, the inter-terminal voltage Vt2 of the comparative example has a waveform in which the peak voltage is suppressed by the operation of the sub-resonant circuit 12. Thus, in the region where the inter-terminal voltage Vt is below the threshold voltage Vth, the comparative example always generates losses due to the operation of the sub-resonant circuit 12. In contrast, in the first embodiment, the sub-resonant circuit 12 does not operate, so no losses are generated by the sub-resonant circuit 12.

[0034] As in the first embodiment, when the inter-terminal voltage Vt falls below the threshold voltage Vth and the second switch 13 is turned off, the capacitor C2 of the sub-resonant circuit 12 and the parasitic capacitance Cp of the second switch 13 are connected in series to form a composite capacitance Cs. This composite capacitance Cs and the inductor L2 of the sub-resonant circuit 12 form a composite resonant circuit. The resonant frequency fr3 of this composite resonant circuit must be set higher than the resonant frequency fr2 of the sub-resonant circuit 12, which is two or three times the switching frequency fsw, to nullify the peak suppression effect of the composite resonant circuit. To achieve this, the capacitance of the capacitor C2 of the sub-resonant circuit 12 must be set to a value greater than the parasitic capacitance Cp of the second switch 13.

[0035] For this reason, in the first embodiment, the capacitance of the capacitor C2 of the sub-resonant circuit 12 is set to a value larger than the parasitic capacitance Cp of the second switch 13. As a result, when the second switch 13 is turned off, it is possible to nullify the peak suppression effect of the composite resonant circuit formed by the composite capacitance Cs, which is formed by the series connection of the capacitor C2 of the sub-resonant circuit 12 and the parasitic capacitance Cp of the second switch 13, and the inductor L2.

[0036] As described above, in the power conversion device 1 according to the first embodiment, the peak voltage of the inter-terminal voltage Vt of the first switch 10 can be suppressed, efficiency can be improved, and the cooler used in the power conversion device 1 can be made smaller.

[0037] Second Embodiment Next, the configuration of a power conversion device 1A according to a second embodiment will be described with reference to Fig. 3. The power conversion device 1A according to the second embodiment differs from the power conversion device 1 according to the first embodiment shown in Fig. 1 in that a rectifier 18 is provided downstream of the input terminals T1 and T2, and the first switch 10 is configured with only a first switching element Q1. The rest of the configuration of the power conversion device 1A according to the second embodiment is the same as the configuration of the power conversion device 1 according to the first embodiment, and therefore detailed description thereof will be omitted. In the power conversion device 1A, the rectifier 18 full-wave rectifies the input voltage V1, and therefore the first switch 10 is configured with only a single first switching element Q1, and can perform the same operation as the power conversion device 1.

[0038] The second embodiment also provides the same effects as the first embodiment, namely, it is possible to suppress the peak voltage of the inter-terminal voltage Vt of the first switch 10, improve efficiency, and ultimately reduce the size of the cooler.

[0039] Third Embodiment Next, the configuration of a power conversion device 1B according to a third embodiment will be described with reference to FIG. 4 . The power conversion device 1B according to the third embodiment differs from the power conversion device 1 according to the first embodiment in that a DC power supply Vdc is connected as the external power supply 2, and the first switch 10 is configured solely by a first switching element Q1. The DC power supply Vdc is, for example, a rechargeable secondary battery such as a lithium-ion battery. The remaining configuration of the power conversion device 1B according to the third embodiment is the same as the configuration of the power conversion device 1 according to the first embodiment, and therefore detailed description thereof will be omitted. The DC voltage of the DC power supply Vdc fluctuates depending on the charging and discharging state, and therefore the input voltage V1 also fluctuates.

[0040] For this reason, the third embodiment is provided with a voltmeter 16 for measuring the input voltage V1 input between the first input terminal T1 and the second input terminal T2, and an ammeter 17 for measuring the input current I1 flowing between the external power supply 2 and the power conversion device 1. The values ​​of the input voltage V1 measured by the voltmeter 16 and the input current I1 measured by the ammeter 17 are transmitted to the control unit 15. The control unit 15 determines the value of the inter-terminal voltage Vt of the first switch 10 based on the input voltage V1 observed by the voltmeter 16 and the input current I1 observed by the ammeter 17, and switches the second switch 13 on and off according to the determined value of the inter-terminal voltage Vt.

[0041] The third embodiment also provides the same effects as the first embodiment, namely, it is possible to suppress the peak voltage of the inter-terminal voltage Vt of the first switch 10, improve efficiency, and ultimately reduce the size of the cooler.

[0042] Each of the functions described in the above embodiments may be implemented by one or more processing circuits, including programmed processors, electrical circuits, and even devices such as application specific integrated circuits (ASICs), or circuit components arranged to perform the described functions.

[0043] 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.

[0044] 1, 1A, 1B Power conversion device 2 External power supply 3 External load 10 First switch 11 Main resonant circuit 12 Sub-resonant circuit 13 Second switch 14 Suppression circuit 15 Control unit 16 Voltmeter 17 Ammeter C0 Output capacitor C1, C2 Capacitors fr1, fr2 Resonant frequency fsw Switching frequency I1 Input current L0 Input inductor L1, L2 Inductor Q1 First switching element Q2 Second switching element T1 First input terminal T2 Second input terminal T3 First output terminal T4 Second output terminal V1 Input voltage Vac AC power supply Vdc DC power supply Vr Element breakdown voltage Vt Terminal voltage Vth Threshold voltage

Claims

1. A control method for a power conversion device comprising: an input inductor connected to an input terminal to which a voltage from an external power supply is input; a first switch connected to the input terminal via the input inductor; a main resonant circuit connected to the first switch; a suppression circuit in which a sub-resonant circuit and a second switch are connected in series and connected in parallel to the first switch; and a control unit that controls the on and off of the first switch and the second switch, wherein the control unit drives the first switch at a predetermined switching frequency, the control unit switches the second switch on and off according to the voltage between the terminals of the first switch, and the resonant frequency of the sub-resonant circuit is set to a value higher than the switching frequency.

2. The control method for a power conversion device according to claim 1, wherein the control unit switches the second switch from off to on when it determines that the inter-terminal voltage has exceeded a threshold voltage while the second switch is off, and switches the second switch from on to off when it determines that the inter-terminal voltage has fallen below the threshold voltage while the second switch is on.

3. The control method for a power conversion device according to claim 1 or 2, wherein the control unit observes at least one of an input voltage and an input current, and determines the value of the inter-terminal voltage based on the observed value of at least one of the input voltage and the input current.

4. The control method for a power conversion device according to any one of claims 1 to 3, further comprising setting the resonant frequency of the main resonant circuit to a value substantially equal to the switching frequency.

5. The control method for a power conversion device according to any one of claims 1 to 4, wherein the resonant frequency of the sub-resonant circuit is set to two or three times the switching frequency.

6. A power conversion device comprising: an input inductor connected to an input terminal to which a voltage from an external power supply is input; a first switch connected to the input terminal via the input inductor; a main resonant circuit connected to the first switch; a suppression circuit in which a sub-resonant circuit and a second switch are connected in series and which is connected in parallel to the first switch; and a control unit that controls the on and off of the first switch and the second switch, wherein the control unit drives the first switch at a predetermined switching frequency and controls the second switch to switch on and off in accordance with the voltage between the terminals of the first switch, and the resonant frequency of the sub-resonant circuit is set to a value higher than the switching frequency.

7. The power conversion device according to claim 6, further comprising an output capacitor connected in parallel to the first switch, wherein the main resonant circuit is an LC series resonant circuit in which a first inductor and a first capacitor are connected in series, and wherein the resonant frequency of the main resonant circuit is approximately equal to the switching frequency.

8. The power conversion device according to claim 6 or 7, wherein the sub-resonant circuit is an LC resonant circuit in which a second inductor and a second capacitor are connected in series.

9. The power conversion device according to claim 8, wherein the capacitance of the second capacitor of the sub-resonant circuit is set to a value greater than the parasitic capacitance of the second switch.

10. The power conversion device according to any one of claims 6 to 9, wherein the external power supply is an AC power supply, and the first switch is a switch group in which two or more switching elements are connected in series.

11. The power conversion device according to any one of claims 6 to 9, wherein the external power supply is an AC power supply, a rectifier is provided between the input terminal and the input inductor, and the first switch is composed of a single switching element.

12. The power conversion device according to any one of claims 6 to 9, wherein the external power supply is a DC power supply, and the first switch is configured with one switching element.

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