WO2020070850A1

WO2020070850A1 - Power conversion circuit and air conditioner

Info

Publication number: WO2020070850A1
Application number: PCT/JP2018/037149
Authority: WO
Inventors: 勇紀江幡; 橋本　浩之
Original assignee: 日立ジョンソンコントロールズ空調株式会社
Priority date: 2018-10-04
Filing date: 2018-10-04
Publication date: 2020-04-09
Also published as: TWI785198B; TW202015324A

Abstract

In order to suppress power loss at a time of conversion of AC voltage to DC voltage, the present invention is provided with: a plurality of main circuit switching elements (QD1-QD4) for converting AC power supplied from an AC system (162) to DC power and supplying the DC power to a load device (164); a reactor (174) connected between the main circuit switching elements (QD1-QD4) and the AC system (162); a plurality of reflux diodes (DD1-DD4) that are connected in reverse parallel respectively to the plurality of main circuit switching elements (QD1-QD4); reverse voltage application circuits (71-74) that are each connected to at least one of the reflux diodes (DD1-DD4), and that apply reverse voltage lower than DC voltage (VE) to be outputted to the load device (164), to the corresponding reflux diodes (DD1-DD4) when the corresponding reflux diodes (DD1-DD4) are cut off; and a control unit (180) that controls the reverse voltage application circuits (71-74).

Description

Power conversion circuit and air conditioner

The present invention relates to a power conversion circuit and an air conditioner.

As background art of the present technical field, claim 1 of Patent Document 1 below describes “a pair of main circuit switching elements connected in series to a DC voltage source and supplying power to a load, and each of these main circuit switching elements. A freewheel diode connected in anti-parallel to each of the freewheeling diodes, and a reverse voltage applying circuit for applying a reverse voltage smaller than the DC voltage source to each freewheeling diode when each of the freewheeling diodes shuts off. An auxiliary power supply having a lower voltage value than the DC voltage source; a reverse voltage applying switching element which is turned on at the time of reverse recovery of the return diode and has a withstand voltage lower than that of the main circuit switching element; When the main circuit switching elements are switched on and off with respect to each other, A main circuit switching control circuit for switching the main circuit switching element having a short pause period for turning off both of the circuit switching elements, and the reverse voltage application switching during a pause period starting from a time point when the main circuit switching element is turned off. And a reverse voltage application switching control circuit that turns on the element and turns off after the elapse of the pause period. "

Japanese Patent No. 4204534

However, Patent Literature 1 does not particularly mention that power loss is suppressed when converting an AC voltage to a DC voltage.
The present invention has been made in view of the above circumstances, and has as its object to provide a power conversion circuit and an air conditioner that can suppress power loss when converting an AC voltage to a DC voltage.

In order to solve the above problems, a power conversion circuit according to the present invention includes a plurality of main circuit switching elements that convert AC power supplied from an AC system into DC power and supply the DC power to a load device; A reactor connected to a power system, a plurality of freewheeling diodes connected in anti-parallel to each of the plurality of main circuit switching elements, and a corresponding freewheeling diode connected to at least a part of the freewheeling diodes and shut off A reverse voltage lower than a DC voltage output to the load device, a reverse voltage application circuit that applies a reverse voltage to the corresponding freewheeling diode, and a control unit that controls the reverse voltage application circuit. And

According to the present invention, power loss can be suppressed when converting an AC voltage to a DC voltage.

FIG. 2 is a circuit diagram of the AC / DC converter according to the first embodiment of the present invention. It is a wave form diagram of each part in a 1st embodiment. FIG. 5 is a circuit diagram of an AC / DC converter according to a second embodiment of the present invention. It is a wave form diagram of each part in a 1st embodiment. It is a schematic diagram of an air conditioner according to a third embodiment of the present invention.

[First Embodiment]
<Configuration of First Embodiment>
FIG. 1 is a circuit diagram of an AC / DC converter 100 (power conversion circuit) according to a first embodiment of the present invention.
The AC / DC converter 100 converts AC power supplied from an AC power supply 162 (AC system) into DC power and supplies the DC power to the load device 164. The AC / DC converter 100 includes a pair of

AC terminals

60A and 60B,

DC terminals

62P and 62N, four switching elements QD1 to QD4 (first to fourth main circuit switching elements), and four return diodes. DD1 to DD4, four reverse voltage application circuits 71 to 74, a converter control circuit 180 (control unit), a driver circuit 172, a reactor 174, a smoothing capacitor 176, a current detection unit 177, and a voltage detection unit 178.

In the present embodiment, the switching elements QD1 to QD4 and other switching elements to be described later are all MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors). The gate-source voltages of the switching elements QD1 and QD2 are referred to as VgsQD1 and VgsQD2, respectively. The AC power supply 162 is, for example, a commercial power supply, and an AC voltage output from the AC power supply 162 is applied between the

AC terminals

60A and 60B. Smoothing capacitor 176 is connected between

DC terminals

62P and 62N, and smoothes output voltages of switching elements QD1 to QD4. The terminal voltage of the smoothing capacitor 176 is called a main DC voltage VE (DC voltage). The load device 164 is connected to the

DC terminals

62P and 62N.

The switching elements QD1 and QD2 are connected in series between the

DC terminals

62P and 62N. Similarly, switching elements QD3 and QD4 are also connected in series between

DC terminals

62P and 62N. One end of reactor 174 is connected to AC terminal 60A, and the other end of reactor 174 is connected to a connection point of switching elements QD1 and QD2. The connection point between the switching elements QD3 and QD4 is connected to the AC terminal 60B. The voltage instantaneous value at the other end of reactor 174 is referred to as AC voltage instantaneous value vs. AC terminal 60B as a reference. The instantaneous value of the current flowing from the AC power supply 162 to the AC / DC converter 100 is referred to as the instantaneous value of the alternating current is.

The freewheel diodes DD1 to DD4 are connected in anti-parallel to the switching elements QD1 to QD4, respectively. The reverse voltage applying circuits 71 to 74 are connected in parallel to the freewheel diodes DD1 to DD4, respectively. The reverse voltage applying circuits 71 to 74 apply a reverse voltage lower than the main DC voltage VE to the freewheel diodes DD1 to DD4 when the corresponding freewheel diodes DD1 to DD4 perform reverse recovery, thereby suppressing the reverse recovery loss. .

Converter control circuit 180 includes main circuit control signals SD1 to SD4 for controlling ON / OFF states of switching elements QD1 to QD4, and reverse voltage control signals SE1 to SE4 for controlling operations of reverse voltage application circuits 71 to 74. Output. In particular, converter control circuit 180 stores the target value of main DC voltage VE, and sets the duty ratio of switching elements QD1 to QD4 such that main DC voltage VE approaches the target value. The driver circuit 172 buffers the main circuit control signals SD1 to SD4 and the reverse voltage control signals SE1 to SE4, and outputs the results as voltage signals VD1 to VD4 and VE1 to VE4. The current detector 177 detects the AC instantaneous value is, and the voltage detector 178 detects the AC voltage instantaneous value vs.

Next, the reverse voltage application circuit 71 will be described.
In FIG. 1, it is assumed that the switching element QD1 is in the off state and a forward return current flows through the diode DD1. Here, when the switching element QD1 is turned on, a reverse voltage is applied to the diode DD1. At this time, a reverse current flows due to the residual charge of the diode DD1. This reverse current is called a reverse recovery current, and the loss due to the reverse recovery current is called a reverse recovery loss. When the main DC voltage VE is directly applied to the diode DD1 when the reverse recovery current flows through the diode DD1, the reverse recovery current and the reverse recovery loss increase. Therefore, the reverse voltage application circuit 71 is provided to cope with this problem. That is, when the reverse recovery current flows through the diode DD1, the reverse voltage application circuit 71 applies a reverse voltage lower than the main DC voltage VE to the diode DD1 to reduce the reverse recovery current and the reverse recovery loss. Is what you do.

(4) Inside the reverse voltage application circuit 71, the DC power supply 22 outputs a DC voltage VG lower than the main DC voltage VE at the

DC terminals

62P and 62N described above. For example, when the AC / DC converter 100 is applied to an air conditioner, the effective value of the main DC voltage VE is about 300 [V], and the DC voltage VG is 1/10 or less of the voltage, for example, 5V to 24V. [V] or so. The resistor 24 and the capacitor 26 constitute a filter circuit for removing a noise component from the DC voltage VG. Switching element 28 (reverse voltage application switching element) is turned on / off by voltage signal VE1 supplied from converter control circuit 180 via driver circuit 172. The diode 30 is a freewheel diode of the switching element 28.

The diode 34 (reverse voltage application diode) is connected between the source terminal of the switching element 28 and the drain terminal of the switching element QD1 to prevent backflow.
The switching element 28 has a lower withstand voltage than the switching elements QD1 to QD4 in order to switch the DC voltage VG output from the DC power supply 22. Further, since it is preferable to operate the diode 34 at high speed, a diode having a shorter reverse recovery time than the freewheeling diodes DD1 to DD4 is employed. Specifically, a wide band gap semiconductor (silicon carbide, gallium nitride, gallium oxide, or the like) is preferably used as the diode 34. Although the other reverse voltage applying circuits 72 to 74 are not shown, they are configured similarly to the reverse voltage applying circuit 71.

<Operation of First Embodiment>
Converter control circuit 180 can select any one of a plurality of operation modes based on the magnitude of the load (for example, the amplitude value of AC instantaneous value is). Here, the selectable operation modes include a “synchronous rectification mode” and a “switching mode”.

(Synchronous rectification mode)
The synchronous rectification mode is an operation mode in which the on / off state of the switching elements QD1 to QD4 is switched every half cycle of the instantaneous AC voltage value vs according to the polarity of the instantaneous AC voltage value vs. That is, when instantaneous AC voltage value vs is a positive value, converter control circuit 180 turns on switching elements QD1 and QD4 and turns off switching elements QD2 and QD3. Thus, when a current is supplied from the AC power supply 162, the current flows through the reactor 174, the switching element QD1, the load device 164, and the switching element QD4 sequentially, and returns to the AC power supply 162.

Conversely, when AC instantaneous value vs is a negative value, converter control circuit 180 turns on switching elements QD2 and QD3 and turns off switching elements QD1 and QD4. Thus, when a current is supplied from the AC power supply 162, the current flows through the switching element QD3, the load device 164, the switching element QD2, and the reactor 174 sequentially, and returns to the AC power supply 162. The synchronous rectification mode may be used when the delay of the current phase due to the reactor 174 is not significant, for example, when the amplitude value of the AC current instantaneous value is is less than a predetermined threshold.

(Switching mode)
By the way, since the AC / DC converter 100 includes the reactor 174, in the synchronous rectification mode described above, the instantaneous value of the AC current is delayed with respect to the voltage of the AC power supply 162. The switching mode is an operation mode in which the power factor of the AC / DC converter 100 is to be improved. In other words, in the switching mode, converter control circuit 180 complementarily switches on / off states of switching elements QD1 and QD2 a plurality of times within a half cycle of AC voltage instantaneous value vs. When the ON / OFF states of the switching elements QD1 and QD2 are complementarily switched, a dead time period occurs in which both the switching elements QD1 and QD2 are in the OFF state. Further, converter control circuit 180 controls the on / off states of switching elements QD3 and QD4 so as to be the same as those in the synchronous rectification mode described above.

For example, when AC voltage instantaneous value vs is a positive value, converter control circuit 180 keeps switching element QD3 in the off state, keeps switching element QD4 in the on state, and complements the on / off state of switching elements QD1 and QD2. Switch multiple times. Here, assuming that the frequency for switching the on / off state of the switching elements QD1 and QD2 is fw and the frequency of the AC voltage instantaneous value vs is fs, the frequency fw may be about 2 to 3 times the frequency fs, It may be about several hundred times. For example, when the frequency fs is 50 Hz or 60 Hz, the frequency fw may be set to 10 kHz to 20 kHz.

Here, if the switching elements QD1 and QD4 are both on and the switching elements QD2 and QD3 are both off, current flows as in the case of the synchronous rectification mode described above. On the other hand, when switching elements QD2 and QD4 are both on and switching elements QD1 and QD3 are both off, the current supplied from AC power supply 162 causes reactor 174, switching element QD2, and switching element QD4 to pass through. The sequence returns to the AC power supply 162 in sequence.

In other words, in this case, the reactor 174 is directly connected to the AC power supply 162 without passing through the load device 164, and a large current can flow through the reactor 174. The energy supplied to the reactor 174 as a current is stored in the reactor 174 as a magnetic flux, and then supplied to the load device 164 as a current when both the switching elements QD1 and QD4 are turned on. The synchronous rectification mode may be employed when the delay of the current phase due to the reactor 174 is conspicuous as deterioration of the power factor, for example, when the amplitude value of the instantaneous AC current value is is equal to or larger than the above-described threshold value.

(4) In the synchronous rectification mode, since the forward current flowing through the freewheel diodes DD1 to DD4 is small, the reverse recovery current is also small. Then, the power consumption of the reverse voltage application circuits 71 to 74 may be larger than the reverse recovery loss. In such a case, it is preferable to make the reverse voltage application circuits 71 to 74 inactive. Therefore, in the synchronous rectification mode, converter control circuit 180 maintains the reverse voltage control signals SE1 to SE4 at low level, and turns off reverse voltage application circuits 71 to 74.

On the other hand, in the switching mode, since the forward current flowing through the freewheel diodes DD1 to DD4 is large, the reverse recovery current is also large. Therefore, in the switching mode, converter control circuit 180 sets reverse voltage control signals SE1 to SE4 to high level in synchronization with the timing when freewheeling diodes DD1 to DD4 perform reverse recovery. Thus, the reverse voltage applying circuits 71 to 74 apply a reverse voltage lower than the main DC voltage VE to the freewheel diodes DD1 to DD4 when the freewheel diodes DD1 to DD4 perform reverse recovery. Suppress.

FIG. 2 is a waveform diagram of each part of the AC / DC converter 100 in the switching mode.
In FIG. 2, main circuit control signals SD1 and SD2 are signals output from converter control circuit 180 to control switching elements QD1 and QD2, as described above. The state before time t0 in FIG. 2 assumes that the polarity of the instantaneous alternating current value is is “positive” (the direction shown in FIG. 1) and the switching elements QD2 and QD4 are on. That is, in FIG. 1, it is assumed that the current flowing from the AC terminal 60A flows out of the AC terminal 60B via the reactor 174 and the switching elements QD2 and QD4 in sequence.

以前 Before time t0 in FIG. 2, since the main circuit control signal SD2 is at a high level, the gate-source voltage VgsQD2 is also at a high level. Therefore, the switching element QD2 (see FIG. 1) is in the ON state as described above. Next, at time t0 in FIG. 2, the main circuit control signal SD2 falls from a high level to a low level, and the gate-source voltage VgsQD2 of the switching element QD2 gradually falls from time t0. As described above, when the gate-source voltage VgsQD2 falls, the switching element QD2 is turned off. Then, the current flowing from AC terminal 60A via reactor 174 in FIG. 1 is returned to AC power supply 162 via diode DD1, load device 164, and switching element QD4.

Next, when the main circuit control signal SD1 rises from a low level to a high level at time t2 in FIG. 2, the gate-source voltage VgsQD1 of the switching element QD1 gradually rises from time t4 to time t10. After the main circuit control signal SD1 rises at time t2, the gate-source voltage VgsQD1 does not change during a period until time t4, but this period is a delay time in the driver circuit 172. Next, during the period from time t4 to time t6, voltage VgsQD1 gradually increases. This period is a period during which the gate-source capacitance of the switching element QD1 is charged. Next, during a period from time t6 to t8, the voltage VgsQD1 is substantially constant. This is because the mirror effect appears in the switching element QD1. Thus, the period from time t6 to t8 is called a mirror period.

As described above, when the switching element QD1 is turned on while the forward return current flows through the diode DD1 (see FIG. 1), a reverse recovery current flows through the diode DD1. The timing at which the reverse recovery current can actually occur varies depending on conditions such as the ambient temperature. Here, at time t10 when the gate-source voltage VgsQA1 of the switching element QA1 is stabilized, it is considered that the drain-source voltage is also stable, so the reverse recovery occurs within the period from time t0 to t10. it is conceivable that. However, timings such as times t4, t6, t8, and t10 in the drawing vary depending on conditions such as ambient temperature and noise. Therefore, in the present embodiment, constants and the like of the respective components are set so that reverse recovery can occur until time t12, which is a time margin obtained by adding a margin to time t10.

(2) As shown in FIG. 2, the reverse voltage control signal SE1 is a signal that rises at time t1 and falls at time t12. The driver circuit 172 (see FIG. 1) outputs a voltage signal VE1 (not shown) having substantially the same waveform as the reverse voltage control signal SE1, and the switching element 28 (see FIG. 1) is turned on / off by the voltage signal VE1. You. The current flowing through the diode 34 in the reverse voltage application circuit 71 is called a current ig. In the example shown in FIG. 2, the current ig has a positive value during the period from time t1 to time t2. The current ig during this period is a reverse recovery current that flows through the diode DD1 via the diode 34 when the diode DD1 enters the reverse recovery state.

{Circle around (4)} The current ig has a negative value during the period from time t6 to time t8. The current ig during this period is a reverse recovery current generated by the reverse recovery of the diode 34 itself. The current ih indicated by a broken line in FIG. 2 is an example of a reverse recovery current flowing through the diode DD1 by the main DC voltage VE when the reverse voltage application circuit 71 is not provided. According to the present embodiment, the reverse recovery current (current ig during the period from time t1 to t2) generated in the diode DD1 and the reverse recovery current (current ig during the period from time t6 to t8) generated in the diode 34 are determined. Even if they are combined, the current value can be greatly reduced as compared with the reverse recovery current ih indicated by the broken line, whereby the reverse recovery loss can be significantly suppressed.

<Effect of First Embodiment>
As described above, the AC / DC converter 100 according to the present embodiment includes the plurality of freewheel diodes (DD1 to DD4) connected in anti-parallel to each of the plurality of main circuit switching elements (QD1 to QD4), and at least a part thereof. When the corresponding freewheeling diodes (DD1 to DD4) are cut off and connected to the freewheeling diodes (DD1 to DD4), a reverse voltage lower than the DC voltage (VE) output to the load device (164) is supplied to the corresponding freewheeling diode. (DD1 to DD4), and a control unit (180) for controlling the reverse voltage application circuits (71 to 74).
Thereby, when converting the AC voltage to the DC voltage, the reverse recovery current and the reverse recovery loss can be reduced, and the power loss can be suppressed.

Further, according to the present embodiment, the plurality of main circuit switching elements (QD1 to QD4) switch the reactor (174) to the AC system by switching on / off at a frequency higher than the frequency of the AC system (162). 162), the first and second main circuit switching elements (QD1, QD2) intermittently and directly, and the third and fourth main circuit switching elements (ON / OFF) which switch on / off at the frequency of the AC system (162). QD3, QD4), and the reverse voltage application circuits (71, 72) each include a reverse voltage application switching element (28) for applying a reverse voltage to the freewheeling diodes (DD1, DD2), and It is connected in parallel to the second main circuit switching element (QD1, QD2).
This makes it possible to reduce the reverse recovery current in the first and second main circuit switching elements (QD1, QD2) having a particularly large number of switching times.

In addition, the control unit (180) in the present embodiment selects either the synchronous rectification mode or the switching mode as the operation mode. In the synchronous rectification mode, the first to the first switching are performed at the frequency of the AC system (162). This is an operation mode for switching the ON / OFF state of the fourth main circuit switching elements (QD1 to QD4), and the switching mode includes the first and second main circuit switching at a frequency higher than the frequency of the AC system (162). This is an operation mode in which the on / off states of the elements (QD1, QD2) are switched, and the on / off states of the third and fourth main circuit switching elements (QD3, QD4) are switched at the frequency of the AC system (162).
Thereby, an operation mode according to the situation can be selected, and the power loss can be further reduced according to the situation.

Further, according to the present embodiment, the control unit (180) applies the reverse voltage during the dead time period (t0 to t2) in which both the first and second main circuit switching elements (QD1, QD2) are turned off. The switching element (28) is set to the ON state, and after the dead time period (t0 to t2) ends, the reverse voltage applying switching element (28) is set to the OFF state.
Thereby, the possibility that a reverse voltage lower than the DC voltage (VE) can be applied to the freewheeling diodes (DD1, DD2) during the reverse recovery of the freewheeling diodes (DD1, DD2) can be increased.

Further, according to the present embodiment, since the reverse voltage application diode (34) is made of a wide band gap semiconductor, the reverse recovery time of the reverse voltage application diode (34) is longer than that of the reflux diodes (DA1 to DA6). The length can be shortened, and the reverse voltage application circuits (11 to 16) can be operated at high speed.

[Second embodiment]
<Configuration of Second Embodiment>
FIG. 3 is a circuit diagram of an AC / DC converter 170 (power conversion circuit) according to the second embodiment of the present invention. In the following description, portions corresponding to the respective portions of the above-described first embodiment are denoted by the same reference numerals, and description thereof may be omitted.
Similarly to the AC / DC converter 100 of the first embodiment (see FIG. 1), the AC / DC converter 170 converts AC power supplied from an AC power supply 162 (AC system) into DC power, and loads the load device. 164. The AC / DC converter 170 includes reverse voltage applying circuits 51 to 54 instead of the reverse voltage applying circuits 71 to 74 in the first embodiment.

(4) The reverse voltage application circuit 51 includes the capacitor 32. The capacitor 32 is connected between the source terminal of the switching element 28 and the source terminal of the switching element QD1. The other reverse voltage application circuits 52 to 54 have the same configuration as the reverse voltage application circuit 51. The configuration of the AC / DC converter 170 other than that described above is the same as that of the AC / DC converter 100 of the first embodiment.

<Operation of Second Embodiment>
Also in the present embodiment, the converter control circuit 180 can select any one of a plurality of operation modes based on the size of the load (for example, the amplitude value of the instantaneous AC current value is). The selectable operation modes include a “synchronous rectification mode” and a “switching mode”. The contents of these operation modes are the same as those of the first embodiment.

FIG. 4 is a waveform diagram of each part in the switching mode of the AC / DC converter 170. The waveforms of the main circuit control signals SD1 and SD2 and the gate-source voltages VgsQD1 and VgsQD2 are those of the first embodiment (FIG. Reference). However, the waveform of the reverse voltage control signal SE1 in the present embodiment is different from that of the first embodiment. That is, in the present embodiment, the reverse voltage control signal SE1 rises from a low level to a high level at time t0, and falls from a high level to a low level at time t2.

{However, the rising timing of the reverse voltage control signal SE1 may be after time t0, and the falling timing may be before time t2. That is, the reverse voltage control signal SE1 is a signal whose rising timing and falling timing appear in the dead time period t0 to t2. The driver circuit 172 (see FIG. 3) outputs a voltage signal VE1 (not shown) having substantially the same waveform as the reverse voltage control signal SE1, and the switching element 28 (see FIG. 3) is turned on / off by the voltage signal VE1. You. Thus, the waveform of the terminal voltage VC1 of the capacitor 32 becomes as shown in FIG.

That is, the terminal voltage VC1 rises from time t0 and is stable at a constant value before time t2. While the terminal voltage VC1 is rising, the capacitor 32 is charged by the current supplied from the DC power supply 22 (see FIG. 1). Then, the terminal voltage VC1 gradually falls after the reverse voltage control signal SE1 falls at time t2. While the terminal voltage VC1 is falling, the capacitor 32 is discharged.

However, during the period from time t2 to time t12 when a reverse recovery current can be generated, the terminal voltage VC1 is also maintained at a high value in the forward voltage drop VF (for example, 0.6 V) of the diode. In other words, the capacitance of the capacitor 32 is determined so that the terminal voltage VC1 is maintained at a value higher than the forward voltage drop VF during the period from time t2 to time t12. As described above, according to the present embodiment, the terminal voltage VC1 lower than the main DC voltage VE is applied as a reverse voltage to the diode DD1 during the period from the time t2 to the time t12, whereby the reverse recovery current and the reverse recovery loss Can be reduced.

In the above-described first embodiment, the reverse voltage control signal SE1 is maintained at the high level during the period from the time t1 to the time t12 when the diode DD1 is likely to be in the reverse recovery state (see FIG. 2). However, when noise is superimposed on the reverse voltage control signal SE1, the waveform of the voltage signal VE1 (see FIG. 1) supplied to the switching element 28 is disturbed. Thus, there is a possibility that the switching element 28 is turned off even though the diode DD1 is in the reverse recovery state. When such a state occurs, a large reverse recovery current flows through the diode DD1 due to the main DC voltage VE, and the reverse recovery loss also increases.

On the other hand, according to the present embodiment, the capacitor 32 is charged by the switching element 28, and the terminal voltage VC1 of the capacitor 32 can be applied to the diode DD1 via the diode 34. Thus, even when the switching element 28 is turned off due to noise or the like at the time of reverse recovery of the diode DD1, the terminal voltage VC1, which is a stable reverse voltage, can be continuously applied to the diode DD1. Further, in the present embodiment, since the switching element 28 is turned off at or before the time t2, it is possible to suppress interference of the reverse voltage application circuit 51 with the operation of the switching element QD1.

<Effect of Second Embodiment>
As described above, similarly to the first embodiment, the AC / DC converter 170 of the present embodiment includes a plurality of freewheel diodes (QD1 to QD4) connected in anti-parallel to each of the plurality of main circuit switching elements (QD1 to QD4). DD1 to DD4) and at least a part of the freewheeling diodes (DD1 to DD4). When the corresponding freewheeling diodes (DD1 to DD4) are cut off, the DC voltage (VE) output to the load device (164) is reduced. A reverse voltage application circuit (51-54) for applying a low reverse voltage to the corresponding freewheeling diodes (DD1-DD4); and a control unit (180) for controlling the reverse voltage application circuit (51-54). .
Thereby, when converting the AC voltage to the DC voltage, the reverse recovery current and the reverse recovery loss can be reduced, and the power loss can be suppressed.

The reverse voltage application circuit (51 to 54) in this embodiment is charged after the reverse voltage application switching element (28) is turned on, and is charged after the reverse voltage application switching element (28) is turned off. The control unit (180) further includes a discharged capacitor (32), and the control unit (180) performs a reverse operation during a dead time period (t0 to t2) in which both the first and second main circuit switching elements (QD1 and QD2) are turned off. The voltage application switching element (28) is set to the ON state, and the reverse voltage application switching element (28) is set to the OFF state before the dead time period (t0 to t2) ends.
Thereby, even if the operation of the reverse voltage application switching element (28) is disturbed at the timing when the freewheeling diodes (DD1 to DD4) reversely recover, the reverse voltage lower than the DC voltage (VE) is applied to the corresponding freewheeling diode (DD1). To DD4).

[Third embodiment]
FIG. 6 is a refrigeration cycle system diagram of an air conditioner 900 according to a third embodiment of the present invention.
As shown in FIG. 6, the air conditioner 900 of the present embodiment includes an outdoor unit 960 and an indoor unit 970, and further includes a gas pipe 982 connecting the two, and a liquid pipe 984.

外 The outdoor unit 960 includes a compressor 961, a four-way valve 962, an outdoor heat exchanger 963, and an outdoor expansion valve 964. These are sequentially connected by piping (no reference numeral). The outdoor unit 960 includes an outdoor fan 965 and an outdoor fan motor 966. The outdoor fan 965 is driven to rotate by an outdoor fan motor 966, and cools the outdoor heat exchanger 963.

室内 The indoor unit 970 includes an indoor heat exchanger 973 and an indoor expansion valve 974. Both are connected to each other by a pipe (unsigned). The indoor unit 970 includes an indoor fan 975 and an indoor fan motor 976. The indoor fan 975 is driven to rotate by the indoor fan motor 976, and cools the indoor heat exchanger 973. The four-way valve 962 provided in the outdoor unit 960 is a valve that switches the flow of the refrigerant, and switches between the cooling operation and the heating operation. The outdoor expansion valve 964 and the indoor expansion valve 974 reduce the pressure of the refrigerant to a low temperature and low pressure.

6, solid arrows shown along pipes such as the gas pipe 982 and the liquid pipe 984 indicate the flow of the refrigerant in the cooling operation of the air conditioner 900.
In the cooling operation, the four-way valve 962 communicates the discharge side of the compressor 961 with the outdoor heat exchanger 963, and communicates the suction side of the compressor 961 with the gas pipe 982, as indicated by the solid line. The refrigerant discharged from the compressor 961 is in a high-temperature and high-pressure gas state, passes through the four-way valve 962, and flows to the outdoor heat exchanger 963. The gaseous refrigerant that has flowed into the outdoor heat exchanger 963 exchanges heat with outdoor air supplied by the outdoor fan 965 and is condensed to be a liquid refrigerant. This liquid refrigerant flows into the indoor unit 970 through the outdoor expansion valve 964 and the liquid pipe 984 in the fully opened state.

(4) The liquid refrigerant flowing into the indoor unit 970 is decompressed by the indoor expansion valve 974 and becomes a low-temperature low-pressure gas-liquid mixed refrigerant. The low-temperature low-pressure gas-liquid mixed refrigerant flows into the indoor heat exchanger 973, exchanges heat with indoor air supplied by the indoor fan 975, evaporates, and turns into a gaseous refrigerant. At this time, the air in the room is cooled by the latent heat of vaporization of the gas-liquid mixed refrigerant, and cool air is sent into the room. Thereafter, the gaseous refrigerant flowing out of the indoor unit 120 passes through the gas pipe 982 and is returned to the outdoor unit 960. The gaseous refrigerant returned to the outdoor unit 960 passes through the four-way valve 962, is drawn into the compressor 961, and is compressed again here, thereby forming a series of refrigeration cycles.

The compressor 961 includes a compression mechanism 146 for compressing the refrigerant, and a motor 144 for driving the compression mechanism 146 to rotate. In addition, AC / DC converter 150 converts AC power supplied from AC power supply 162 into DC power and supplies the DC power to inverter 120. Inverter 120 converts the supplied DC power into three-phase AC power having an arbitrary frequency, and drives motor 144. Note that the configuration of the AC / DC converter 150 can be applied to any of the AC / DC converters 100 and 170 (see FIGS. 1 and 3) of the first and second embodiments.
Thus, according to the present embodiment, similarly to the first and second embodiments, the power loss of the AC / DC converter 150 can be suppressed, and a highly efficient air conditioner 900 can be realized.

[Modification]
The present invention is not limited to the embodiments described above, and various modifications are possible. The above-described embodiments are exemplarily illustrated for easy understanding of the present invention, and are not necessarily limited to those having all the configurations described above. Further, a part of the configuration of one embodiment can be replaced with the configuration of another embodiment, and the configuration of one embodiment can be added to the configuration of another embodiment. Further, a part of the configuration of each embodiment can be deleted, or another configuration can be added or replaced. Further, the control lines and information lines shown in the figure indicate those which are considered necessary for the description, and do not necessarily indicate all the control lines and information lines necessary for the product. In fact, it can be considered that almost all components are connected to each other. Possible modifications to the above embodiment are, for example, as follows.

(1) In each of the above embodiments, an example was described in which MOSFETs were applied as the switching elements 28 and QD1 to QD4. However, the switching elements are not limited to MOSFETs, but may be IGBTs, bipolar transistors, or the like.

(2) In each of the above embodiments (see FIGS. 1 and 3), the reverse voltage applying circuits 51 to 54 and 71 to 74 are connected in parallel to all the freewheel diodes DD1 to DD4. A reverse voltage application circuit may be connected only to the return diode. For example, only the reverse

voltage applying circuits

51, 52, 71, 72 corresponding to the switching elements QD1, QD2 whose on / off states are complementarily switched in the switching mode are provided, and the other reverse

voltage applying circuits

53, 54, 73 are provided. , 74 may be omitted. This is because the diodes DD1 and DD2 reversely connected to the switching elements QD1 and QD2 frequently perform reverse recovery as compared with the other diodes DD3 and DD4. Thus, by providing only some of the reverse voltage application circuits 51 to 54 and 71 to 74, the cost of the AC /

DC converters

100 and 170 can be reduced.

(3) In each of the above embodiments, the reverse voltage control signal SE1 has fallen at or before the time t2 (see FIG. 2). However, the reverse voltage control signal SE1 may fall after time t2.

(4) Although the AC /

DC converters

100 and 170 in each of the above embodiments have the operation modes of the synchronous rectification mode and the switching mode, in addition to these or instead of the synchronous rectification mode, the diode rectification mode May be selectable. Here, the diode rectification mode is an operation mode in which full-wave rectification is performed using four return diodes DD1 to DD4. In the diode rectification mode, converter control circuit 180 continuously sets main circuit control signals SD1 to SD4 to a low level, and thereby continuously turns off switching elements QD1 to QD4.

(5) The converter control circuit 180 in each of the above embodiments may perform control to suppress the reverse recovery loss not only in the switching mode but also in either the synchronous rectification mode or the diode rectification mode of the above-described modification. That is, the converter control circuit 180 applies a reverse voltage lower than the main DC voltage VE to the freewheeling diodes DD1 to DD4 when the freewheeling diodes DD1 to DD4 perform reverse recovery in the synchronous rectification mode or the diode rectification mode, Thereby, the reverse recovery loss may be suppressed.

(6) In the second embodiment, when the reverse recovery current generated in the diode 34 is not so large, the off timing of the reverse voltage control signal SE1 may be a timing later than the time t2.

(7) In the above embodiments, the return diodes DD1 to DD4 are separate from the switching elements QD1 to QD4. However, when the switching element has a parasitic diode, the parasitic diode may be applied as the return diodes DD1 to DD4.

28 Switching element (reverse voltage application switching element)
32 Capacitor 34 Diode (reverse voltage application diode)
51 to 54, 71 to 74 Reverse

voltage application circuit

100, 170 AC / DC converter (power conversion circuit)
162 AC power supply (AC system)
164 Load device 174 Reactor 180 Converter control circuit (control unit)
DD1 to DD4 freewheeling diode QD1 switching element (first main circuit switching element)
QD2 switching element (second main circuit switching element)
QD3 switching element (third main circuit switching element)
QD4 switching element (fourth main circuit switching element)
VE Main DC voltage (DC voltage)

Claims

A plurality of main circuit switching elements for converting AC power supplied from the AC system into DC power and supplying the DC power to a load device;
A reactor connected between the main circuit switching element and the AC system,
A plurality of freewheeling diodes connected in anti-parallel to each of the plurality of main circuit switching elements;
A reverse voltage application circuit that is connected to at least a part of the freewheeling diode and that applies a reverse voltage lower than a DC voltage output to the load device to the corresponding freewheeling diode when the corresponding freewheeling diode is cut off; ,
And a control unit for controlling the reverse voltage application circuit.
The plurality of main circuit switching elements are first and second main circuit switching elements that intermittently directly connect the reactor to the AC system by switching on / off at a frequency higher than the frequency of the AC system. And a third and fourth main circuit switching element whose on / off state is switched at the frequency of the AC system.
The reverse voltage application circuits each include a reverse voltage application switching element that applies the reverse voltage to the freewheeling diode, and are connected in parallel to the first and second main circuit switching elements. The power conversion circuit according to claim 1.
The plurality of main circuit switching elements are connected to the reactor and are connected in series with each other, and the third and fourth main circuits are connected in series with each other. And a switching element.
The control unit is configured to select any of a synchronous rectification mode and a switching mode as an operation mode,
The synchronous rectification mode is an operation mode for switching on / off states of the first to fourth main circuit switching elements at a frequency of the AC system,
The switching mode switches on / off states of the first and second main circuit switching elements at a frequency higher than the frequency of the AC system, and switches the third and fourth main circuits at a frequency of the AC system. An operation mode for switching the on / off state of the switching element,
The reverse voltage application circuits each include a reverse voltage application switching element that applies the reverse voltage to the freewheeling diode, and are connected in parallel to the first and second main circuit switching elements. The power conversion circuit according to claim 1.
The power conversion circuit according to claim 3, wherein the reverse voltage application circuit is not connected to the third and fourth main circuit switching elements.
The control unit sets the reverse voltage application switching element to an on state during a dead time period in which the first and second main circuit switching elements are both in an off state, and sets the reverse voltage after the dead time period ends. The power conversion circuit according to claim 3, wherein the applied switching element is set to an off state.
The reverse voltage application circuit,
A reverse voltage application diode for suppressing inflow of current from the corresponding return diode;
A capacitor that is charged after the reverse voltage applying switching element is turned on and is discharged via the reverse voltage applying diode after the reverse voltage applying switching element is turned off;
Further comprising
The control unit sets the reverse voltage application switching element to an on state during a dead time period in which both the first and second main circuit switching elements are in an off state, and before the dead time period ends. The power conversion circuit according to claim 3, wherein the reverse voltage application switching element is set to an off state.
The reverse voltage application circuit includes a reverse voltage application diode that suppresses inflow of current from the corresponding return diode,
The power conversion circuit according to claim 3, wherein the reverse voltage application diode is formed of a wide band gap semiconductor.
An air conditioner comprising the power conversion circuit according to any one of claims 1 to 7.