Power conversion device

Abstract

In this power conversion device (10), a bidirectional switch (21) of an inverter circuit (20) having an LC resonance circuit (22) switches the direction and on / off of a current flowing between a pair of input terminals (IN1, IN2). The inverter circuit (20) generates a high-frequency AC current from the AC power inputted to the pair of input terminals (IN1, IN2). The inverter circuit (20) has a shunt capacitor (C2) connected between the pair of input terminals (IN1, IN2) in parallel with the bidirectional switch (21), and a choke inductor connected between at least one input terminal among the pair of input terminals (IN1, IN2) and the bidirectional switch (21). The control unit (40) changes the on-duty of the switching operation of the bidirectional switch (21) according to the polarity of the voltage of the AC power inputted to the pair of input terminals (IN1, IN2).

Description

Power Conversion Device

[0001] The present invention relates to a power conversion device.

[0002] Patent Document 1 describes a power conversion circuit that converts AC input power into high-frequency AC current using a class E inverter circuit having an LC resonant circuit, rectifies the AC current using a rectifier circuit, and outputs the rectified current to a load. In the power conversion circuit of Patent Document 1, the switching frequency of the switch elements of the class E inverter circuit is changed based on the input voltage of the power conversion circuit, thereby achieving zero voltage switching (ZVS).

[0003] JP 2021-145433 A

[0004] An object of the present invention is to further reduce switching loss in an inverter circuit having an LC resonant circuit, such as the power conversion circuit of Patent Document 1.

[0005] In order to solve the above-mentioned problems, in a power conversion device according to one aspect of the present invention, a bidirectional switch of an inverter circuit having an LC resonant circuit switches the direction and on / off of a current flowing between a pair of input terminals. The inverter circuit generates a high-frequency AC current from AC power input to the pair of input terminals. The inverter circuit has a shunt capacitor connected in parallel with the bidirectional switch between the pair of input terminals, and a choke inductor connected between at least one of the pair of input terminals and the bidirectional switch. A control unit changes the on-duty of the switching operation of the bidirectional switch depending on the polarity of the voltage of the AC power input to the pair of input terminals.

[0006] According to the present invention, it is possible to further reduce switching loss in an inverter circuit having an LC resonant circuit.

[0007] Fig. 1 is a diagram showing the configuration of a power conversion device according to a first embodiment of the present invention. Fig. 2 is a diagram showing the relationship between a control signal for switching a switch element of an inverter circuit and a waveform of a resonant current generated in a resonant circuit of the inverter circuit by switching the switch element when the input voltage of the power conversion device of Fig. 1 is positive. Fig. 3 is a diagram showing the relationship between a control signal for switching a switch element of an inverter circuit and a waveform of a resonant current generated in a resonant circuit of the inverter circuit by switching the switch element when the input voltage of the power conversion device of Fig. 1 is negative. Fig. 4 is a diagram showing the relationship between the input current and input power of the power conversion device, which are controlled to have a waveform with a high power factor and low total harmonic distortion relative to the waveform of the input voltage of the power conversion device, when the power conversion device of Fig. 1 is operating at an average power of 3.5 kW, which is the maximum rated condition. Fig. 5 is a diagram showing the combination of switching frequency and on-duty to be selected by the switch element of the inverter circuit when the instantaneous value of the input voltage of the power conversion device of Fig. 1 is +50 V. Fig. 6 is a diagram showing combinations of switching frequencies and on-duties to be selected by the switch elements of the inverter circuit when the instantaneous value of the input voltage of the power conversion device of Fig. 1 is +100 V. Fig. 7 is a diagram showing combinations of switching frequencies and on-duties to be selected by the switch elements of the inverter circuit when the instantaneous value of the input voltage of the power conversion device of Fig. 1 is +150 V. Fig. 8 is a diagram showing combinations of switching frequencies and on-duties to be selected by the switch elements of the inverter circuit when the instantaneous value of the input voltage of the power conversion device of Fig. 1 is +200 V. Fig. 9 is a diagram showing combinations of switching frequencies and on-duties to be selected by the switch elements of the inverter circuit when the instantaneous value of the input voltage of the power conversion device of Fig. 1 is +250 V. Fig. 10 is a diagram showing combinations of switching frequencies and on-duties to be selected by the switch elements of the inverter circuit when the instantaneous value of the input voltage of the power conversion device of Fig. 1 is +283 V. FIG. 11 is a diagram comparing the ZVS establishment range of the on-duty of the switching element between instantaneous values ​​of the positive input voltage of the power conversion device when the average input power of the power conversion device of FIG. 1 is 3.5 kW.Fig. 12 is a diagram comparing the ZVS establishment range of the on-duty of the switch elements between instantaneous values ​​of positive input voltage of the power conversion device when the average input power of the power conversion device of Fig. 1 is 1.0 kW. Fig. 13 is a diagram showing combinations of switching frequency and on-duty to be selected by the switch elements of the inverter circuit when the instantaneous value of the input voltage of the power conversion device of Fig. 1 is -50 V. Fig. 14 is a diagram showing combinations of switching frequency and on-duty to be selected by the switch elements of the inverter circuit when the instantaneous value of the input voltage of the power conversion device of Fig. 1 is -100 V. Fig. 15 is a diagram showing combinations of switching frequency and on-duty to be selected by the switch elements of the inverter circuit when the instantaneous value of the input voltage of the power conversion device of Fig. 1 is -150 V. Fig. 16 is a diagram showing combinations of switching frequency and on-duty to be selected by the switch elements of the inverter circuit when the instantaneous value of the input voltage of the power conversion device of Fig. 1 is -200 V. FIG. 17 is a diagram showing combinations of switching frequencies and on-duties to be selected by the switch elements of the inverter circuit when the instantaneous value of the input voltage of the power conversion device of FIG. 1 is −250 V. FIG. 18 is a diagram showing combinations of switching frequencies and on-duties to be selected by the switch elements of the inverter circuit when the instantaneous value of the input voltage of the power conversion device of FIG. 1 is −283 V. FIG. 19 is a diagram comparing the ZVS achieving range of the on-duties of the switch elements between each instantaneous value of the negative input voltage of the power conversion device when the average input power of the power conversion device of FIG. 1 is 3.5 kW. FIG. 20 is a diagram comparing the ZVS achieving range of the on-duties of the switch elements between each instantaneous value of the negative input voltage of the power conversion device when the average input power of the power conversion device of FIG. 1 is 1.0 kW. FIG. 21 is a flowchart showing an example of a processing procedure performed by the on-duty control unit of FIG. 1. FIG. 22 is a diagram showing the configuration of a power conversion device according to a second embodiment of the present invention. FIG. 23 is a flowchart showing an example of a processing procedure performed by the on-duty control unit of FIG. 22. FIG. 24 is a diagram showing the configuration of a power conversion device according to the third embodiment of the present invention.FIG. 25 is a diagram showing the configuration of a power conversion device according to a fourth embodiment of the present invention.

[0008] Hereinafter, an embodiment of the present invention and its modifications will be described with reference to the drawings. In the description of the drawings, the same parts are given the same reference numerals and the description thereof will be omitted.

[0009] (First Embodiment) (Configuration of Power Conversion Device According to First Embodiment) FIG. 1 is a diagram illustrating the configuration of a power conversion device according to a first embodiment of the present invention. The power conversion device 10 illustrated in FIG. 1 includes a pair of first and second input terminals IN1 and IN2, an inverter circuit 20, a rectifier circuit 30, a pair of first and second output terminals OUT1 and OUT2, a control unit 40, and an input voltmeter 60. An AC power supply AC is connected between the first and second input terminals IN1 and IN2, and a DC load RLoad is connected between the first and second output terminals OUT1 and OUT2. In this embodiment, an AC power supply AC having an effective value of 200 V and 50 Hz is connected between the first and second input terminals IN1 and IN2. The polarity of the input voltage of the power conversion device 10 is switched between positive and negative depending on the AC power of the AC power supply AC input between the first and second input terminals IN1 and IN2. The input voltmeter 60 is connected between the first and second input terminals IN1 and IN2. The input voltmeter 60 measures the input voltage of the power conversion device 10 .

[0010] The inverter circuit 20 includes a smoothing capacitor C1, a choke inductor L1, a switch element 21, a shunt capacitor C2, and an LC resonant circuit 22. The smoothing capacitor C1 is connected between the first input terminal IN1 and the second input terminal IN2. A series circuit of the choke inductor L1 and the switch element 21 is connected in parallel to the connection point between the first input terminal IN1 and the smoothing capacitor C1. The shunt capacitor C2 is connected in parallel to the switch element 21. Considering the symmetry of the inverter circuit 20, in addition to the choke inductor L1 between the first input terminal IN1 and the shunt capacitor C2, a choke inductor L1 may be further provided between the second input terminal IN2 and the shunt capacitor C2. In the following description, it is assumed that the choke inductor L1 is provided only between the first input terminal IN1 and the shunt capacitor C2, as shown in FIG. 1 .

[0011] The LC resonant circuit 22 is a series circuit of a resonant inductor Lr and a resonant capacitor Cr. The resonant inductor Lr is connected to the connection point between the choke inductor L1 and the switch element 21. The inverter circuit 20 rectifies a high-frequency resonant current generated in the LC resonant circuit 22 by switching the switch element 21 using a rectifier circuit 30, and supplies the rectified current to a DC load RLoad.

[0012] The switch element 21 includes two semiconductor switches. The semiconductor switches are so-called power transistors. For example, the semiconductor switches may be a unipolar transistor, such as a metal oxide semiconductor field effect transistor (MOSFET), or a bipolar transistor, such as an insulated gate bipolar transistor (IGBT). In this embodiment, the two semiconductor switches of the switch element 21 are two MOSFETs Q1 and Q2 whose source terminals are connected to each other.

[0013] The MOSFETs Q1 and Q2 have parasitic capacitance. The parasitic capacitance of the MOSFETs Q1 and Q2 may be used as a shunt capacitor C2. The shunt capacitor C2 may be configured with a component that is also used for other purposes, like the parasitic capacitance of the MOSFETs Q1 and Q2. Of course, the shunt capacitor C2 may be configured with a component that is dedicated to shunting.

[0014] In the switch element 21, while one of the MOSFETs Q1 and Q2 is turned on with a 100% duty, the other element is repeatedly turned on and off with a duty of less than 100%. By interchanging the element of the switch element 21 that is turned on with a 100% duty with the element that is turned on and off with a duty of less than 100%, the switch element 21 can function as a bidirectional switch.

[0015] The switch element 21 functions as a bidirectional switch, thereby switching the direction of current flowing through the switch element 21 between a direction from MOSFET Q1 to MOSFET Q2 and a direction from MOSFET Q2 to MOSFET Q1. The switch element 21, which is a bidirectional switch, corresponds to the positive and negative input voltages of the power conversion device 10, so that the number of elements through which the input current passes in the power conversion device 10 is reduced compared to the number of elements through which the input current passes when the input voltage of the power conversion device 10 is full-wave rectified in the upstream stage of the inverter circuit 20. Reducing the number of elements through which the input current passes in the power conversion device 10 allows the power conversion device 10 to achieve highly efficient power conversion.

[0016] The rectifier circuit 30 includes a diode D1, a rectifier-side capacitor C3, an inductor L2, and an output-side capacitor C4. The cathode of the diode D1 is connected to the resonant capacitor Cr. The anode of the diode D1 is connected to the shunt capacitor C2. The rectifier-side capacitor C3 is connected in parallel with the diode D1. The inductor L2 is connected to the cathode of the diode D1, the rectifier-side capacitor C3, and the first output terminal OUT1. The output-side capacitor C4 is connected between the first output terminal OUT1 and the second output terminal OUT2. The rectifier circuit 30 rectifies the resonant current of the LC resonant circuit 22 and outputs the resulting DC voltage to a DC load RLoad from the first output terminal OUT1 and the second output terminal OUT2.

[0017] The control unit 40 controls the switching of the switch element 21 of the inverter circuit 20. When the switch element 21 performs zero voltage switching (ZVS), switching is performed while the voltage across the switch element 21 is zero, and loss when the switch element 21 is turned on is zero. When the switch element 21 performs switching that satisfies the ZVS condition, loss due to switching is reduced, and high-frequency switching of the switch element 21 becomes possible. Switching the switch element 21 at high frequency allows the inductor and capacitor of the inverter circuit 20 to be smaller, and the input current of the power conversion device 10 can be converted into high-frequency AC current by the inverter circuit 20 with high efficiency.

[0018] When the switch element 21 is turned on and off at the resonant frequency of the resonant circuit of the inverter circuit 20, it can perform switching that satisfies the ZVS condition. The control unit 40 controls the switch element 21 so that it performs switching that satisfies at least the ZVS condition. In general, an inverter in which the switch element performs switching operation that satisfies the ZVS condition is called a quasi-class E inverter. In general, an inverter in which the switch element performs switching operation that satisfies both the ZVS and zero derivative switching (ZDS) conditions is called a class E inverter. By controlling the switch element 21 by the control unit 40, the inverter circuit 20 can operate as a quasi-class E inverter or a class E inverter. In this embodiment, a case will be described in which the control unit 40 controls the switching of the switch element 21 so that the inverter circuit 20 operates as a quasi-class E inverter. A detailed configuration of the control unit 40 and details of the control performed by the control unit 40 will be described later.

[0019] (Resonant Circuit and Resonant Current of Inverter Circuit 20) In the power conversion device 10 of this embodiment, the path through which the resonant current of the inverter circuit 20 flows differs between the period when the switch element 21 is on and the period when it is off. During the period when the switch element 21 is on, the path of the resonant current is the switch element 21-LC resonant circuit 22-rectifier circuit 30. During the period when the switch element 21 is off, the path of the resonant current is the shunt capacitor C2 in parallel with the switch element 21-LC resonant circuit 22-rectifier circuit 30.

[0020] During the period when the switch element 21 is off, the number of series-connected capacitors on the path of the resonant current increases compared to the period when the switch element 21 is on. When the number of series-connected capacitors on the path of the resonant current increases, the capacitance components that constitute the LC resonance of the inverter circuit 20 decrease, and the resonant frequency of the resonant circuit, expressed as 1 / (2π√(LC)), decreases. The resonant frequency of the resonant circuit of the inverter circuit 20 differs between the period when the switch element 21 is on and the period when it is off.

[0021] 2 is a diagram showing the relationship between a control signal for switching the switch element 21 of the inverter circuit 20 and the waveform of a resonant current generated in the resonant circuit of the inverter circuit 20 by the switching of the switch element 21 when the input voltage of the power conversion device 10 is positive. When one of the MOSFETs Q1 and Q2 of the switch element 21 is repeatedly turned on and off in response to the control signal in Fig. 2, the switch element 21 turns on and off the input current in the direction corresponding to the positive input voltage of the power conversion device 10. In principle, the resonant current generated in the resonant circuit of the inverter circuit 20 during the period when one of the switch elements 21 is repeatedly turned on and off is not an ideal sine wave, but rather has an asymmetric waveform between positive and negative.

[0022] Since a bidirectional switch is used for the switch element 21, the inverter circuit 20 can switch on and off the input current in both directions corresponding to the positive and negative input voltages of the power conversion device 10, thereby generating a high-frequency resonant current whether the input voltage is positive or negative.

[0023] 3 is a diagram showing the relationship between a control signal for switching the switch element 21 of the inverter circuit 20 and the waveform of a resonant current generated in the resonant circuit of the inverter circuit 20 by the switching of the switch element 21 when the input voltage of the power conversion device 10 is negative. When the other of the MOSFETs Q1 and Q2 of the switch element 21 is repeatedly turned on and off by the control signal of Fig. 3, the switch element 21 turns on and off the input current in the direction corresponding to the negative input voltage of the power conversion device 10. During the period when the other element of the switch element 21 is repeatedly turned on and off, the resonant current generated in the resonant circuit of the inverter circuit 20 has a waveform whose positive and negative polarities are inverted from the waveform of the resonant current when the input voltage of the power conversion device 10 is positive.

[0024] The rectifier circuit 30 is supplied with the resonant current of the inverter circuit 20, which has an asymmetrical waveform with positive and negative inverted in accordance with the polarity of the input voltage of the power conversion device 10. The rectifier circuit 30 is a circuit that obtains a positive DC voltage from a high-frequency AC current, and operates asymmetrically with respect to the positive and negative polarities of the current.

[0025] When the polarity of the input voltage of the power conversion device 10 changes, the operating state of the power conversion device 10 including the inverter circuit 20 and the rectifier circuit 30 changes due to the asymmetry of the resonant current of the inverter circuit 20 and the asymmetry of the operation of the rectifier circuit 30. Simply turning the switch element 21 on and off at the resonant frequency of the resonant circuit of the inverter circuit 20 cannot cause the switch element 21 to perform switching that satisfies the ZVS condition whether the input voltage of the power conversion device 10 is positive or negative.

[0026] In general, in order to suppress adverse effects on downstream transformers and connected devices, it is desirable for a power conversion device connected to an AC power source to control the input current of the power conversion device to a waveform with a high power factor and low total harmonic distortion (THD). To control the input current of the power conversion device to a waveform with a high power factor and low total harmonic distortion, the input current of the power conversion device can be controlled to a waveform similar to the AC input voltage of the power conversion device. If the power conversion device operates with a high power factor and low total harmonic distortion, the input current of the power conversion device will have a sinusoidal waveform that is in phase with the input voltage of the power conversion device, and the input power of the power conversion device, which is the product of the input voltage and input current of the power conversion device, will have a squared sine wave waveform.

[0027] 4 is a diagram showing the relationship between the input current and input power of the power conversion device 10, which is controlled to have a high power factor and a low total harmonic distortion waveform relative to the input voltage waveform of the power conversion device 10 when the power conversion device 10 is operating at an average power of 3.5 kW, which is the maximum rated condition. As shown in FIG. 4, for example, when the power conversion device 10 is operating at an average input power of 3.5 kW, the value that the input current of the power conversion device 10 should take can be uniquely determined in accordance with changes in the AC 200 V that is the input voltage of the power conversion device 10.

[0028] In the power conversion device 10, when the instantaneous value of the input voltage and the output voltage are given as external conditions, the parameters that the control unit 40 can manipulate to change the operating power of the circuit are the switching frequency and on-duty of the switch element 21. If the control unit 40 appropriately manipulates the switching frequency and on-duty in response to changes in the instantaneous value of the input voltage of the power conversion device 10, it is possible to obtain a desired current through power conversion by the inverter circuit 20 and realize turn-on of the switch element 21 that satisfies the ZVS condition.

[0029] 5 to 10 are diagrams showing combinations of switching frequencies and on-duties to be selected by the switch elements 21 of the inverter circuit 20 when the instantaneous value of the input voltage to the power conversion device 10 is positive. Figures 5 to 10 show the maximum instantaneous values ​​of the input voltage to the power conversion device 10 from an AC power source AC with an effective value of 200 V, 50 Hz, of +50 V, +100 V, +150 V, +200 V, +250 V, and +283 V, respectively.

[0030] 5 to 10 are loci of points indicating combinations of switching frequency and on-duty that the switch element 21 should adopt when switching at each instantaneous value of input voltage. The solid line locus ZVS(3.5) is a locus of combinations that provide the instantaneous power that should be obtained by power conversion by the power conversion device 10 corresponding to the instantaneous value of input voltage when the power conversion device 10 operates with an average input power of 3.5 kW. The dashed line locus ZVS(1.0) is a locus of combinations that provide the instantaneous power that should be obtained by power conversion by the power conversion device 10 when the power conversion device 10 operates with an average input power of 1.0 kW.

[0031] For example, when the instantaneous value of the input voltage of the power conversion device 10 operating at an average input power of 3.5 kW is +50 V, the switch element 21 is switched at a switching frequency and on-duty on the solid line locus ZVS(3.5) in FIG. 5 . By switching the switch element 21 at this switching frequency and on-duty, the instantaneous power of 218.75 W to be obtained through power conversion by the power conversion device 10 can be obtained. Even when the instantaneous value of the input voltage is other than +50 V, the switch element 21 is switched at a switching frequency and on-duty on the locus ZVS(3.5) in FIGS. 6 to 10 . By switching the switch element 21 at each combination of switching frequency and on-duty, the instantaneous power to be obtained through power conversion by the power conversion device 10 can be obtained. As shown in FIG. 8 , for example, when the instantaneous value of the input power of the power conversion device 10 is +200 V, the instantaneous power to be obtained through power conversion by the power conversion device 10 is 3.5 kW.

[0032] When the average input power of the power conversion device 10 is 1.0 kW, the switching frequency and on-duty of the switch element 21 are set to a combination of points on the locus ZVS(1.0) in Figures 5 to 10 at each instantaneous value of the input voltage. By switching at each combination of switching frequency and on-duty, it is possible to obtain the instantaneous power that should be obtained by power conversion of the power conversion device 10, even when the average input power of the power conversion device 10 is 1.0 kW.

[0033] From Figures 5 to 10, it can be seen that in order to change the instantaneous power obtained by the power conversion device 10 in accordance with changes in the instantaneous value of the input voltage during the period when the input voltage of the power conversion device 10 is positive, it is necessary to set the switching frequency and on-duty of the switch element 21 to appropriate values.

[0034] 5 to 10 , the solid areas excluding the hatched areas indicate the ZVS region Azvs in which the switch element 21 can be turned on in a ZVS manner at instantaneous input voltage values ​​of +50 V, +100 V, +150 V, +200 V, +250 V, and +283 V. The ZVS turn-on of the switch element 21 means that the switch element 21 is turned on in a manner that satisfies the ZVS conditions. Within the on-duty range of the locus ZVS(3.5) or locus ZVS(1.0), which overlaps with the ZVS region Azvs, the switch element 21 can be turned on in a ZVS manner while still obtaining the instantaneous power required by the power conversion device 10.

[0035] 5 to 10, the ZVS achieving range Rzvs (3.5) indicates the range of on-duty in which the switching element 21 can be turned on in a ZVS manner while obtaining the instantaneous power that the power conversion device 10 should obtain, when the average input power of the power conversion device 10 is 3.5 kW and the input voltage is positive. The ZVS achieving range Rzvs (1.0) indicates the range of on-duty in which the switching element 21 can be turned on in a ZVS manner while obtaining the instantaneous power that the power conversion device 10 should obtain, when the average input power of the power conversion device 10 is 1.0 kW and the input voltage is positive.

[0036] For example, when the average input power of the power conversion device 10 is 3.5 kW and the instantaneous value of the input voltage is +200 V, the on-duty of the switch element 21 is set to a value between 0.53 and 0.57 within the ZVS achieving range Rzvs (3.5) in Fig. 8. When the average input power of the power conversion device 10 is 3.5 kW and the instantaneous value of the input voltage is +283 V, the on-duty of the switch element 21 is set to a value between 0.54 and 0.59 within the ZVS achieving range Rzvs (3.5) in Fig. 10. By selecting a combination of the on-duty and switching frequency of the switch element 21 within the ZVS achieving range Rzvs (3.5) for each instantaneous value of the input voltage, it is possible to obtain a desired instantaneous value of power while ZVS-turning on the switch element 21.

[0037] For example, when the average input power of the power conversion device 10 is 1.0 kW and the instantaneous value of the input voltage is +200 V, the on-duty of the switch element 21 is set to a value between 0.36 and 0.52 within the ZVS achieving range Rzvs (1.0) in Fig. 8. By selecting a combination of the on-duty and switching frequency within the ZVS achieving range Rzvs (1.0), the switch element 21 can be turned on in a ZVS manner while obtaining a desired instantaneous value of power.

[0038] Fig. 11 is a diagram comparing the ZVS achieving range Rzvs (3.5) of the on-duty of the switch element 21 between instantaneous positive input voltage values ​​of the power conversion device 10 when the average input power of the power conversion device 10 is 3.5 kW. Fig. 12 is a diagram comparing the ZVS achieving range Rzvs (1.0) of the on-duty of the switch element 21 between instantaneous positive input voltage values ​​of the power conversion device 10 when the average input power of the power conversion device 10 is 1.0 kW.

[0039] 11, when the input voltage of the power conversion device 10 is positive and the average input power is 3.5 kW, if the on-duty of the switch element 21 is within the range of 0.54 to 0.57, ZVS switching of the switch element 21 can be achieved for all instantaneous values ​​of the input voltage. Considering a margin for reliably achieving ZVS switching of the switch element 21, it is considered that a value near 0.555, which is the median value Tp (3.5) of the range of 0.54 to 0.57, can be selected as the on-duty of the switch element 21, for example.

[0040] 12, when the input voltage of the power conversion device 10 is positive and the average input power is 1.0 kW, ZVS switching of the switch element 21 can be achieved for all instantaneous values ​​of the input voltage if the on-duty of the switch element 21 is within the range of 0.38 to 0.50. Taking a margin into consideration, it is considered acceptable to select a value near 0.44, which is the median value Tp (1.0) of the range of 0.38 to 0.50, as the on-duty of the switch element 21, for example.

[0041] 13 to 18 are diagrams showing combinations of switching frequencies and on-duties to be selected by the switch elements 21 of the inverter circuit 20 when the instantaneous value of the input voltage to the power conversion device 10 is negative. Figures 13 to 18 show the maximum instantaneous values ​​of the input voltage to the power conversion device 10 of -50 V, -100 V, -150 V, -200 V, -250 V, and -283 V, respectively.

[0042] For example, when the instantaneous value of the input voltage of the power conversion device 10 operating at an average input power of 3.5 kW is −200 V, the switch element 21 is switched at a switching frequency and on-duty on the solid line locus ZVS(3.5) in FIG. 16 . By switching the switch element 21 at this switching frequency and on-duty, the instantaneous power that should be obtained through power conversion by the power conversion device 10 can be obtained. Even when the instantaneous value of the input voltage is other than −200 V, the switch element 21 is switched at a switching frequency and on-duty on the locus ZVS(3.5) in FIGS. 13 to 15 , 17 , and 18 . By switching the switch element 21 at each combination of switching frequency and on-duty, the instantaneous power that should be obtained through power conversion by the power conversion device 10 can be obtained.

[0043] When the average input power of the power conversion device 10 is 1.0 kW, the switching frequency and on-duty of the switch element 21 are set to a combination of points on the locus ZVS(1.0) in Figures 13 to 18 at each instantaneous value of the input voltage. By switching at each combination of switching frequency and on-duty, it is possible to obtain the instantaneous power that should be obtained by power conversion of the power conversion device 10, even when the average input power of the power conversion device 10 is 1.0 kW.

[0044] 13 to 18, it can be seen that in order to change the instantaneous power obtained by the power conversion device 10 in accordance with changes in the instantaneous value of the input voltage during the period when the input voltage of the power conversion device 10 is negative, the switching frequency and on-duty of the switch element 21 must be set to appropriate values.

[0045] Even when the input voltage of the power conversion device 10 is negative, there exists a ZVS region Azvs in which the switch element 21 can be ZVS turned on, as shown in Figures 13 to 18. When the switch element 21 is switched at a switching frequency and on-duty on the locus ZVS(3.5) or the locus ZVS(1.0) in the ZVS region Azvs in Figures 13 to 18, the ZVS turn-on of the switch element 21 can be achieved while obtaining the desired instantaneous power.

[0046] 13 to 18, the ZVS achieving range Rzvs (3.5) indicates the range of on-duty in which the switching element 21 can be turned on in a ZVS manner while obtaining the instantaneous power that the power conversion device 10 should obtain, when the average input power of the power conversion device 10 is 3.5 kW and the input voltage is negative. The ZVS achieving range Rzvs (1.0) indicates the range of on-duty in which the switching element 21 can be turned on in a ZVS manner while obtaining the instantaneous power that the power conversion device 10 should obtain, when the average input power of the power conversion device 10 is 1.0 kW and the input voltage is negative.

[0047] For example, when the average input power of the power conversion device 10 is 3.5 kW and the instantaneous value of the input voltage is −200 V, the on-duty of the switch element 21 is set to a value between 0.52 and 0.55 within the ZVS achieving range Rzvs (3.5) in Fig. 16. When the average input power of the power conversion device 10 is 3.5 kW and the instantaneous value of the input voltage is −283 V, the on-duty of the switch element 21 is set to a value between 0.53 and 0.57 within the ZVS achieving range Rzvs (3.5) in Fig. 18. By selecting a combination of the on-duty and the switching frequency within the ZVS achieving range Rzvs (3.5) for each instantaneous value of the input voltage, the switch element 21 can be turned on in a ZVS manner while obtaining a desired instantaneous value of power.

[0048] For example, when the average input power of the power conversion device 10 is 1.0 kW and the instantaneous value of the input voltage is −200 V, the on-duty of the switch element 21 is set to a value between 0.36 and 0.55 within the ZVS achieving range Rzvs (1.0) in Fig. 16. By selecting a combination of the on-duty and switching frequency within the ZVS achieving range Rzvs (1.0), the switch element 21 can be turned on in a ZVS manner while obtaining a desired instantaneous value of power.

[0049] Fig. 19 is a diagram comparing the ZVS achieving range Rzvs (3.5) of the on-duty of the switch element 21 between instantaneous values ​​of the negative input voltage of the power conversion device 10 when the average input power of the power conversion device 10 is 3.5 kW. Fig. 20 is a diagram comparing the ZVS achieving range Rzvs (1.0) of the on-duty of the switch element 21 between instantaneous values ​​of the negative input voltage of the power conversion device 10 when the average input power of the power conversion device 10 is 1.0 kW.

[0050] 19, when the input voltage of the power conversion device 10 is negative and the average input power is 3.5 kW, ZVS switching of the switch element 21 can be achieved for all instantaneous values ​​of the input voltage if the on-duty of the switch element 21 is within the range of 0.54 to 0.545. Considering a margin for reliably achieving ZVS switching of the switch element 21, it is considered acceptable to select a value near 0.542, which is the median value Tn (3.5) of the range of 0.54 to 0.545, as the on-duty of the switch element 21, for example.

[0051] 20, when the input voltage of the power conversion device 10 is negative and the average input power is 1.0 kW, ZVS switching of the switch element 21 can be achieved for all instantaneous values ​​of the input voltage if the on-duty of the switch element 21 is within the range of 0.38 to 0.52. Taking a margin into consideration, it is considered acceptable to select a value near 0.45, which is the median value Tn (1.0) of the range of 0.38 to 0.52, as the on-duty of the switch element 21, for example.

[0052] 11 with FIG. 19 , it can be seen that the on-duty range in which the switch element 21 can perform ZVS switching at all instantaneous values ​​of the input voltage of the power conversion device 10 having an average input power of 3.5 kW differs depending on whether the input voltage of the power conversion device 10 is positive or negative. A comparison of FIG. 12 with FIG. 20 reveals that the on-duty range in which the switch element 21 can perform ZVS switching at all instantaneous values ​​of the input voltage of the power conversion device 10 having an average input power of 1.0 kW also differs depending on whether the input voltage of the power conversion device 10 is positive or negative. These comparisons reveal that the on-duty range in which the switch element 21 can perform ZVS switching differs depending on whether the input voltage of the power conversion device 10 is positive or negative, regardless of whether the average input power of the power conversion device 10 is high or low.

[0053] In this embodiment, the control unit 40 sets the on-duty of the switch element 21 separately for periods when the input voltage of the power conversion device 10 is positive and periods when it is negative, and changes the on-duty of the switch element 21 depending on whether the input voltage of the power conversion device 10 is positive or negative. By changing the on-duty of the switch element 21 depending on whether the input voltage of the power conversion device 10 is positive or negative, the control unit 40 can cause the switch element 21 to perform ZVS switching throughout periods when the input voltage of the power conversion device 10 is positive and negative.

[0054] The control unit 40 shown in FIG. 1 includes, for example, a general-purpose microcontroller. The microcontroller includes a central processing unit (CPU) and a memory. The memory includes a read-only memory (ROM) and a random access memory (RAM). The microcontroller can virtually configure multiple information processing circuits by having the CPU execute programs stored in the memory. The multiple information processing circuits can configure the polarity detection unit 41, on-duty operation unit 42, and control signal generation unit 43 of the control unit 40.

[0055] In this embodiment, an example is shown in which multiple information processing circuits built in a microcontroller are realized by software. Of course, it is also possible to configure the information processing circuits by preparing dedicated hardware for executing the information processing of each of the units 41 to 43 described below. Alternatively, the multiple information processing circuits may be configured by individual hardware. The dedicated hardware includes devices such as application specific integrated circuits (ASICs) and conventional circuit components arranged to execute the functions of each of the units 41 to 43.

[0056] The polarity detection unit 41 detects whether the polarity of the input voltage of the power conversion device 10 measured by the input voltmeter 60 connected between the first input terminal IN1 and the second input terminal IN2 is positive or negative. The on-duty control unit 42 controls the on-duty for switching of the switch element 21 in accordance with the polarity of the input voltage of the power conversion device 10 detected by the polarity detection unit 41.

[0057] FIG. 21 is a flowchart showing an example of a processing procedure performed by the on-duty control unit 42. The on-duty control unit 42 checks the polarity of the input voltage VAC of the power conversion device 10 based on whether the input voltage VAC detected by the polarity detection unit 41 is higher than zero (step S101). If the input voltage VAC is higher than zero (YES in step S101), the on-duty control unit 42 determines that the input voltage VAC is positive and adjusts the on-duty of the switch element 21 to a value obtained by applying a positive polarity duty ratio (step S103). The positive polarity duty ratio may be a value selected from an on-duty range that allows ZVS switching of the switch element 21 to be achieved for all instantaneous values ​​of the input voltage of the power conversion device 10 during a period when the input voltage VAC is positive. This value may be, for example, the median of the on-duty range that allows ZVS switching of the switch element 21 to be achieved, as described with reference to FIGS. 11 and 12 .

[0058] If the input voltage VAC is equal to or less than zero (NO in step S101), the input voltage VAC is considered to be negative, and the on-duty control unit 42 controls the on-duty of the switch element 21 to a value obtained by applying a negative duty ratio (step S105). The negative duty ratio can be a value selected from a range of on-duty values ​​that can achieve ZVS switching of the switch element 21 at all instantaneous values ​​of the input voltage of the power conversion device 10 during a period when the input voltage VAC is negative. This value can be, for example, the median or a value near the median of the range of on-duty values ​​that can achieve ZVS switching of the switch element 21, as described with reference to FIGS. 19 and 20 . After controlling the on-duty of the switch element 21 in step S103 or step S105, the on-duty control unit 42 terminates the process.

[0059] The control signal generating unit 43 generates a switching control signal for the switch element 21 corresponding to the on-duty operated by the on-duty operating unit 42, and outputs the signal to the switch element 21. The control signal generating unit 43 generates a control signal for the MOSFET Q1 of the switch element 21 and a control signal for the MOSFET Q2. Each control signal is input to the gate of the MOSFET Q1, Q2, respectively. The control signal generating unit 43 generates a control signal for turning on the MOSFET Q1 with a 100% duty during a period when the input voltage VAC is a positive value, and a control signal for turning on the MOSFET Q2 with a duty that is a value obtained by applying the duty ratio for positive polarity. The control signal generating unit 43 generates a control signal for turning on the MOSFET Q1 with a duty that is a value obtained by applying the duty ratio for negative polarity during a period when the input voltage VAC is a negative value, and a control signal for turning on the MOSFET Q2 with a 100% duty.

[0060] (Action and effect of the power conversion device 10 of the first embodiment) In the power conversion device 10 of Fig. 1 , whether the resonant circuit of the inverter circuit 20 includes a shunt capacitor C2 changes depending on whether the switch element 21, which is a bidirectional switch, is turned on or off. As shown in Figs. 2 and 3 , the waveform of the high-frequency AC current generated by the resonant circuit of the inverter circuit 20 becomes a waveform with asymmetrical positive and negative polarities because the configuration of the resonant circuit changes depending on whether the switch element 21 is turned on or off. The waveform of the high-frequency AC current generated by the inverter circuit 20 is inverted in positive and negative polarities depending on the polarity of the input voltage VAC of the power conversion device 10. Because the asymmetrical AC current is inverted in positive and negative polarities, the waveform of the high-frequency AC current generated by the inverter circuit 20 becomes a waveform that differs depending on the polarity of the input voltage VAC.

[0061] The control unit 40 varies the on-duty of the switching operation of the switch element 21 depending on the polarity of the input voltage VAC of the power conversion device 10. Specifically, the control unit 40 sets the on-duty of the switching operation of the switch element 21, corresponding to the polarity of the input voltage VAC, within the ZVS establishment ranges Rzvs (3.5) and Rzvs (1.0) for each instantaneous value of the input voltage. By setting and varying the on-duty of the switching operation of the switch element 21 within different ranges depending on the polarity of the input voltage VAC, the parameters of the resonant circuit of the inverter circuit 20 are less likely to change between polarities while the switch element 21 is on. Because the parameters of the resonant circuit are less likely to change between polarities, the waveform of the high-frequency AC current generated by the inverter circuit 20 is less likely to differ depending on the polarity of the input voltage VAC, making it easier for the switch element 21 to achieve low-loss switching through ZVS switching. Achieving low-loss switching of the switch element 21 promotes low-loss switching, thereby further reducing switching loss in the inverter circuit 20.

[0062] (Variation of the First Embodiment) When the average input power of the power conversion device 10 is 3.5 kW on average, which is the maximum rated condition, the ZVS achieving range Rzvs(3.5) is distributed in a lower range when the input voltage VAC of the power conversion device 10 is negative than when it is positive. When the average input power of the power conversion device 10 is 3.5 kW on average, the control unit 40 may reduce the on-duty of the switching operation of the switch element 21 when the input voltage VAC is negative more than when it is positive. When the average input power of the power conversion device 10 is 3.5 kW on average, reducing the on-duty when the input voltage VAC is negative more than when it is positive can more appropriately reduce switching loss in the inverter circuit 20. For example, the control unit 40 may reduce the on-duty of the switching operation of the switch element 21 when the input voltage VAC is negative more than when it is positive, regardless of the value of the average input power of the power conversion device 10. Even if the on-duty when the input voltage VAC is negative is reduced compared to when it is positive, the same effect as in the first embodiment can be obtained by selecting the on-duty for each polarity from the ZVS establishment range.

[0063] (Second embodiment) (Configuration of power conversion device according to second embodiment) Fig. 22 is a diagram showing the configuration of a power conversion device according to a second embodiment of the present invention. The power conversion device 10 of the second embodiment shown in Fig. 22 has a configuration in which an input ammeter 70 and a multiplier 44 of a control unit 40 are added to the power conversion device 10 of the first embodiment shown in Fig. 1. Of the configuration of the power conversion device 10 of the second embodiment, descriptions of the same parts as those of the power conversion device 10 of the first embodiment will be omitted to avoid duplication.

[0064] The input ammeter 70 is connected between the connection point of the second input terminal IN2 and the input voltmeter 60 and the connection point of the second input terminal IN2 and the smoothing capacitor C1. The input ammeter 70 measures the input current of the power conversion device 10. The multiplier 44 multiplies the value of the input voltage VAC of the power conversion device 10 measured by the input voltmeter 60 by the value of the input current of the power conversion device 10 measured by the input ammeter 70, and outputs the value of the input power of the power conversion device 10 to the on-duty control unit 42. The on-duty control unit 42 of this embodiment controls the on-duty for switching of the switch element 21 in accordance with the polarity of the input voltage VAC detected by the polarity detection unit 41 and the average value of the input power value of the multiplier 44. The on-duty control unit 42 functions as an average detection unit that detects the average value of the AC power input to the first input terminal IN1 and the second input terminal IN2.

[0065] 11 and 12 , when the input voltage VAC of the power conversion device 10 is positive, the ZVS achieving range Rzvs (3.5) is narrower than the ZVS achieving range Rzvs (1.0). Comparing FIGS. 19 and 20 , when the input voltage VAC is negative, the ZVS achieving range Rzvs (3.5) is also narrower than the ZVS achieving range Rzvs (1.0). Regardless of the polarity of the input voltage VAC, it is considered that the ZVS achieving range tends to become narrower as the average input power of the power conversion device 10 increases. When the average input power of the power conversion device 10 is 3.5 kW, which is the maximum rated condition, the ZVS achieving range Rzvs (3.5) is distributed in a lower range when the input voltage VAC is negative than when the input voltage VAC is positive. It is considered that as the average input power of the power conversion device 10 increases, if the on-duty of the switch element 21 is made lower when the input voltage VAC is negative than when it is positive, the switch element 21 can perform ZVS switching at all instantaneous values ​​of the input voltage.

[0066] The control unit 40 may reduce the on-duty of the switching operation of the switch element 21 when the input voltage VAC of the power conversion device 10 is negative compared to when it is positive, as the average input power of the power conversion device 10 increases. In the present embodiment, when the input power of the power conversion device 10 is equal to or greater than a predetermined percentage of the input power under the maximum rated conditions, the control unit 40 reduces the on-duty of the switching of the switch element 21 when the input voltage VAC is negative compared to when it is positive.

[0067] FIG. 23 is a flowchart showing an example of a processing procedure performed by the on-duty ratio control unit 42 of the power conversion device 10 of the second embodiment. The on-duty ratio control unit 42 checks whether the average input power value of the multiplier 44 is equal to or greater than a predetermined percentage of the average 3.5 kW, which is the maximum rated condition of the power conversion device 10 (step S201). The predetermined percentage may be, for example, 3.15 kW, which is 90% of the maximum rated condition. If the average input power value of the multiplier 44 is less than the predetermined percentage of the maximum rated condition (NO in step S201), the on-duty ratio control unit 42 proceeds to another processing (step S203). The other processing may be, for example, the processing shown in FIG. 21 performed by the on-duty ratio control unit 42 in the power conversion device 10 of the first embodiment. If the average input power value of the multiplier 44 is equal to or greater than the predetermined percentage of the maximum rated condition (YES in step S201), the on-duty ratio control unit 42 checks the polarity of the input voltage VAC of the power conversion device 10 detected by the polarity detection unit 41. The polarity can be confirmed by checking whether the input voltage VAC is higher than zero (step S205).

[0068] If the input voltage VAC is higher than zero (YES in step S205), the on-duty control unit 42 determines that the input voltage VAC is positive and adjusts the on-duty of the switch element 21 to a value obtained by applying the positive duty ratio (step S207). If the input voltage VAC is equal to or lower than zero (NO in step S205), the on-duty control unit 42 determines that the input voltage VAC is negative and adjusts the on-duty of the switch element 21 to a value lower than that when the input voltage VAC is positive (step S209). This value can be selected from a range of on-duties that can achieve ZVS switching of the switch element 21 for all instantaneous values ​​of the input voltage of the power conversion device 10 while the input voltage VAC of the power conversion device 10 is negative. After adjusting the on-duty of the switch element 21 in step S207 or step S209, the on-duty control unit 42 terminates the process.

[0069] (Operation and Effect of the Power Conversion Device 10 of the Second Embodiment) In this embodiment, when the average value of the input power value of the power conversion device 10 is equal to or greater than a predetermined percentage of the input power under the maximum rated conditions, the control unit 40 reduces the on-duty of the switch element 21 when the input voltage VAC is negative compared to when it is positive. The power conversion device 10 of this embodiment can also achieve even lower-loss switching operation.

[0070] (Variation of the Second Embodiment) The on-duty control unit 42 may perform an alternative process of checking whether the average value of the input power value of the multiplier 44 is equal to or greater than a predetermined percentage of the input power under the maximum rated condition of the power conversion device 10. For example, the alternative process may be a process of checking whether the average value of the input current value of the input ammeter 70 is equal to or greater than a predetermined percentage of the input current under the maximum rated condition of the power conversion device 10. The input current under the maximum rated condition of the power conversion device 10 can be calculated, for example, from the input power under the maximum rated condition of the power conversion device 10 and the value of the input voltage VAC of the input voltmeter 60. The on-duty control unit 42 functions as an average detection unit that detects the average value of the current flowing between the first input terminal IN1 and the second input terminal IN2. When the average value of the input current value of the input ammeter 70 is equal to or greater than a predetermined percentage of the input current under the maximum rated condition, the on-duty control unit 42 may reduce the on-duty of the switching operation of the switch element 21 when the input voltage VAC is negative compared to when it is positive. In this case, a similar effect can be obtained.

[0071] The same effect can be obtained even if the control unit 40 reduces the on-duty of the switching operation of the switch element 21 when the input voltage VAC is negative compared to when it is positive, as the average input power or average input current of the power conversion device 10 increases.

[0072] (Third embodiment) (Configuration of power conversion device according to third embodiment) Fig. 24 is a diagram showing the configuration of a power conversion device according to a third embodiment of the present invention. The power conversion device 10 of the third embodiment shown in Fig. 24 has a configuration in which a look-up table (LUT) 45 is added to the power conversion device 10 of the second embodiment shown in Fig. 22. Of the configuration of the power conversion device 10 of the third embodiment, descriptions of the same parts as those of the power conversion device 10 of the second embodiment will be omitted to avoid duplication.

[0073] The lookup table 45 can be stored in, for example, the memory of the microcontroller of the control unit 40. The lookup table 45 is a table that associates, for example, the polarity and average input power value of the input voltage VAC of the power conversion device 10 with on-duty values ​​that achieve ZVS switching of the switch element 21 at all instantaneous values ​​of the input voltage of the power conversion device 10. The lookup table 45 can be configured for each polarity of the input voltage VAC. The on-duty operation unit 42 of the control unit 40 of the power conversion device 10 according to the third embodiment extracts, from the lookup table 45, an on-duty value that corresponds to the polarity confirmed from the value of the input voltage VAC of the polarity detection unit 41 and the input power value of the multiplier 44. The on-duty operation unit 42 sets the extracted on-duty value as the on-duty of the switch element 21 that corresponds to the polarity and average input power value of the input voltage VAC of the power conversion device 10, for example, in the process of step S205 or step S207 of FIG. 23 .

[0074] (Operation and Effect of the Power Conversion Device 10 of the Third Embodiment) In the present embodiment, the control unit 40 can easily extract, from the lookup table 45, the value of the on-duty of the switch element 21 that corresponds to the polarity of the input voltage VAC and the average input power value. In the power conversion device 10 of the present embodiment, depending on the polarity of the input voltage VAC, it is possible to easily select, from the contents defined in the lookup table 45, an on-duty suitable for the switching operation that enables the switch element 21 to perform low-loss switching. The lookup table 45 of the present embodiment can also be used, for example, when the on-duty operation unit 42 of the first embodiment operates the on-duty of the switch element 21 in accordance with the polarity of the input voltage VAC.

[0075] (Fourth embodiment) (Configuration of power conversion device according to fourth embodiment) Fig. 25 is a diagram showing the configuration of a power conversion device according to a fourth embodiment of the present invention. The power conversion device 10 of the fourth embodiment shown in Fig. 25 is obtained by adding an output voltage meter 80 and an output current meter 90 to the power conversion device 10 of the second embodiment shown in Fig. 22. The power conversion device 10 of the fourth embodiment is obtained by adding multipliers 46 and 50, adders 47 and 51, an output power command value output unit 48, and PI control units 49 and 52 to a control unit 40 to the power conversion device 10 of the second embodiment. Of the configuration of the power conversion device 10 of the fourth embodiment, descriptions of the same parts as those of the power conversion device 10 of the second embodiment will be omitted to avoid duplication.

[0076] The output voltage meter 80 is connected between the first output terminal OUT1 and the second output terminal OUT2. The output voltage meter 80 measures the output voltage of the power conversion device 10. The output ammeter 90 is connected between the connection point of the second output terminal OUT2 and the output voltage meter 80 and the connection point of the second input terminal IN2 and the output-side capacitor C4. The output ammeter 90 measures the output current of the power conversion device 10. The multiplier 46 multiplies the value of the output voltage of the power conversion device 10 measured by the output voltage meter 80 by the output current value of the power conversion device 10 measured by the output ammeter 90, and outputs the value of the output power of the power conversion device 10 to the adder 47.

[0077] The adder 47 outputs the difference between the command value for output power to be obtained from the power conversion device 10 output by the output power command value output unit 48 and the output power value of the multiplier 46 to the PI control unit 49. The PI control unit 49 outputs a power control value for reducing the difference of the adder 47 to the multiplier 50. The multiplier 50 outputs a current control value obtained by dividing the power control value of the PI control unit 49 by the value of the input voltage VAC of the input voltmeter 60 to the adder 51. The adder 51 outputs the difference between the current control value of the multiplier 50 and the input current value of the input ammeter 70 to the PI control unit 52. The PI control unit 52 outputs the current control value for reducing the difference of the adder 51 to the control signal generation unit 43.

[0078] The control signal generating unit 43 generates a switching frequency of the switch element 21 corresponding to the current control value of the PI control unit 52 and a switching control signal of the switch element 21 corresponding to the on-duty operated by the on-duty operating unit 42, and outputs these to the switch element 21. The control signal generating unit 43 generates a control signal for the MOSFET Q1 and a control signal for the MOSFET Q2 of the switch element 21, similar to the control signal generating unit 43 in the power conversion device 10 of the second embodiment. However, the control signals for the MOSFET Q1 and the MOSFET Q2 generated by the control signal generating unit 43 are control signals of a switching frequency corresponding to the current control value of the PI control unit 52.

[0079] 5 to 10 and 13 to 18 show that within the ZVS region Azvs, the locus ZVS(3.5) and the locus ZVS(1.0) extend substantially parallel to the horizontal axis. From the locus ZVS(3.5) and the locus ZVS(1.0) within the ZVS region Azvs, it can be seen that within the ZVS region Azvs, the switching on-duty of the switch element 21 has almost no effect on the average input power of the power conversion device 10. FIGS. 5 to 10 and 13 to 18 show that within the ZVS region Azvs, the instantaneous value of the input power of the power conversion device 10 is determined almost entirely by the switching frequency of the switch element 21. This is because a change in the switching frequency of the switch element 21 changes the input current of the power conversion device 10, which in turn changes the input power of the power conversion device 10. The change in the gain of the input power of the power conversion device 10 due to the change in the switching frequency of the switch element 21 is much larger than the change in the gain of the input power of the power conversion device 10 due to the change in the on-duty of the switch element 21 .

[0080] (Operation and Effect of the Power Conversion Apparatus 10 of the Fourth Embodiment) As described in the first embodiment, the controllable parameters of the inverter circuit 20 are the switching frequency and on-duty of the switch element 21. As shown in FIGS. 5 to 10 and 13 to 18, when the switching frequency of the switch element 21 is manipulated, the input current of the power conversion apparatus 10 changes with high responsiveness, and the input power VAC of the power conversion apparatus 10 also changes with high responsiveness. It can be seen that the power conversion efficiency of the power conversion apparatus 10 changes with high responsiveness when the switching frequency of the switch element 21 is manipulated. Because the power conversion efficiency is more responsive when the switching frequency is controlled than when the on-duty of the switch element 21 is controlled, the gain of the inverter circuit 20 can be significantly changed in response to parameter manipulation by controlling the switching frequency. By appropriately manipulating the switching frequency while setting the on-duty within the ZVS establishment range Rzvs using the control unit 40, ZVS switching of the switch element 21 can be achieved, and output power can be obtained from the input power of the power conversion apparatus 10 with high responsiveness.

[0081] (Modification of Fourth Embodiment) In this embodiment, the output power of the power conversion device 10 is controlled by manipulating the switching frequency of the switch element 21. However, either the output voltage or the output current of the power conversion device 10 may be controlled by manipulating the switching frequency of the switch element 21. Even when either the output voltage or the output current is controlled, the output power of the power conversion device 10 can be controlled as a result.

[0082] (Modifications of All Embodiments) The bidirectional switch of the inverter circuit 20 is not limited to the switch element 21 having two unipolar transistors, MOSFETs O1 and O2, whose source terminals are connected to each other, and may be configured to include two semiconductor switches. When the bidirectional switch includes two semiconductor switches, the control unit 40 changes the on-duty of the switching operation of one of the two semiconductor switches, which switches on and off corresponding to the polarity of the input voltage of the power conversion device 10. The control unit 40 may also change the switching frequency of the two semiconductor switches. The bidirectional switch of the inverter circuit 20 does not necessarily have to include two semiconductor switches. Even in a bidirectional switch that does not include two semiconductor switches, the control unit 40 may change the on-duty of the switching operation of the bidirectional switch depending on the polarity of the input voltage of the power conversion device 10. The control unit 40 may also change the switching frequency of the bidirectional switch as necessary.

[0083] The above-described embodiment is merely an example of the present invention, and therefore the present invention is not limited to the above-described embodiment, and various modifications can be made to the design and other aspects of the present invention without departing from the technical concept of the present invention.

[0084] REFERENCE SIGNS LIST 10 Power conversion device 20 Inverter circuit 21 Switch element (bidirectional switch) 22 LC resonant circuit 30 Rectifier circuit 40 Control unit 41 Polarity detection unit 42 On-duty operation unit (average detection unit) 45 Look-up table (table) C2 Shunt capacitor IN1 First input terminal IN2 Second input terminal L1 Choke inductor Q1, Q2 MOSFET (semiconductor switch, unipolar transistor) RLoad DC load (load)

Claims

A pair of input terminals to which AC power is input; an inverter circuit having an LC resonant circuit and configured to generate a high-frequency AC current from the AC power input to the pair of input terminals; a rectifier circuit that rectifies the high-frequency AC current generated by the inverter circuit and outputs the rectified current to a load; a control unit for controlling a switching operation of the inverter circuit, The inverter circuit includes: a bidirectional switch that switches the direction and on / off of a current flowing between the pair of input terminals; a shunt capacitor connected in parallel with the bidirectional switch between the pair of input terminals; a choke inductor connected between at least one of the pair of input terminals and the bidirectional switch, The control unit changes an on-duty of a switching operation of the bidirectional switch according to a polarity of an input voltage of the pair of input terminals. Power conversion equipment.

2. The power conversion device according to claim 1, wherein the bidirectional switch includes two semiconductor switches, and the control unit changes an on-duty of a switching operation of one of the two semiconductor switches that switches on and off corresponding to a polarity of the input voltage. 　 The power conversion device according to claim 2 , wherein the two semiconductor switches are two unipolar transistors having source terminals connected to each other.

2. The power conversion device according to claim 1, further comprising a polarity detection unit that detects a polarity of the input voltage, wherein the control unit changes an on-duty of a switching operation of the bidirectional switch depending on the polarity detected by the polarity detection unit. 　 The power conversion device according to claim 1 , wherein the control unit reduces an on-duty of the switching operation of the bidirectional switch when the polarity of the input voltage is negative compared to when the polarity of the input voltage is positive.

6. The power conversion device according to claim 5, further comprising an average detection unit that detects an average value of the AC power input to the pair of input terminals or the current flowing between the pair of input terminals, wherein the control unit reduces an on-duty of the switching operation of the bidirectional switch when the polarity of the input voltage is negative as the average value increases, compared to an on-duty of the switching operation of the bidirectional switch when the polarity of the input voltage is positive.

6. The power conversion device according to claim 5, further comprising an average detection unit that detects an average value of the AC power input to the pair of input terminals or the current flowing between the pair of input terminals, wherein when the average value is equal to or greater than a predetermined percentage of a maximum rated condition of the average value, the control unit reduces an on-duty of the switching operation of the bidirectional switch when the polarity of the input voltage is negative to be lower than an on-duty of the switching operation of the bidirectional switch when the polarity of the input voltage is positive. 　 The power conversion device according to claim 1 , wherein the control unit controls the switching operation of the bidirectional switch at an on-duty corresponding to a polarity of the input voltage among on-duties for different polarities defined in a table. 　 The power conversion device according to claim 1 , wherein the control unit changes a frequency of a switching operation of the bidirectional switch to change the current flowing between the pair of input terminals.