JP4755915B2 - Inverter device - Google Patents

Description

  The present invention relates to an inverter device for driving a motor, for example.

  An inverter device for driving a load including an inductive component, for example, a brushless DC motor, includes a plurality of series circuits of two switching elements on the upstream side and the downstream side in the voltage application direction. The interconnection point of the downstream switching element is connected to each phase winding of the brushless DC motor. Each switching element has a freewheeling diode.

  Recently, many IGBTs and MOSFETs have been adopted as switching elements. When a MOSFET is used, there is a merit that high-frequency switching is possible because the on / off speed of the MOSFET is fast, and it is frequently used when driving a motor with a small output such as a fan motor because the loss at low voltage output is small. Is done.

  However, the MOSFET has a problem in that reverse recovery characteristics of a freewheeling diode (also referred to as a parasitic diode) inevitably formed on the same element in the process of manufacturing the element are poor. In recent years, super junction MOSFETs have been developed that have low on-state resistance and excellent switching characteristics. However, in this super junction MOSFET, the reverse recovery characteristics of the freewheeling diode formed on the device become worse. ing.

  If the reverse recovery characteristic of the freewheeling diode is poor, the following problems occur. That is, when the MOSFET is turned off, the forward current (also called the reflux current) flows to the MOSFET's freewheeling diode due to the energy stored in the inductive load. In this state, the other switching element of the same series circuit is turned on. When a DC voltage is applied to the MOSFET, a large reverse recovery current due to the charge stored in the freewheeling diode flows through the freewheeling diode. This reverse recovery current results in a large power loss. In addition, most of the power loss is generated when the other switching element paired with the MOSFET is turned on, and when this is increased, the other switching element is destroyed by the heat.

  For this reason, it is very difficult to employ a MOSFET for an inverter through which a large current flows, for example, to drive a compressor motor of an air conditioner.

Therefore, conventionally, a reverse voltage application circuit for applying a reverse voltage to the MOSFET is turned on prior to turning on the other switching element after the MOSFET is turned off, and before the other switching element is turned on by the application of the reverse voltage. In some cases, a reverse recovery current is supplied to the freewheeling diode in advance, thereby reducing the charge stored in the freewheeling diode in advance, so that a large reverse recovery current does not flow when the other switching element is turned on (for example, Patent Documents). 1).
Japanese Patent Laid-Open No. 10-327585

  The application timing of the reverse voltage from the reverse voltage application circuit to the MOSFET is set to a so-called dead time from when the MOSFET is turned off until the other switching element is turned on. This dead time is for preventing a short circuit of the series circuit composed of the MOSFET and the other switching element, and is desirably set as short as possible from the viewpoint of efficiency.

  However, if the dead time is short, it is difficult to sufficiently reduce the charge stored in the freewheeling diode until the other switching element is turned on. As a result, there arises a disadvantage that the reverse recovery current that flows when the other switching element is turned on cannot be sufficiently suppressed.

  In consideration of the above circumstances, the present invention can sufficiently reduce the charge stored in the freewheeling diode, and can sufficiently suppress the reverse recovery current that flows when the other switching element is turned on. An object is to provide an inverter device.

  The inverter device of the invention according to claim 1 includes a plurality of series circuits of two switching elements using at least one of MOSFETs having a freewheeling diode, and an interconnection point of each switching element of these series circuits is connected to a load. A switching circuit; first control means for controlling on / off operation of each switching element; and a reverse voltage application circuit for applying a reverse voltage lower than the driving voltage of each MOSFET to the free wheel diode of each MOSFET; Second control means for starting the operation of the reverse voltage application circuit while the MOSFET is on and ending the operation after the other switching element is turned on.

  According to the inverter device of the present invention, the reverse voltage application circuit starts operating while the MOSFET is still on. Thereby, the electric charge stored in the freewheeling diode can be sufficiently reduced, and the reverse recovery current that flows when the other switching element is turned on can be sufficiently suppressed.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
In FIG. 1, M is a brushless DC motor (load) used as a compressor motor of an air conditioner. It is configured. The rotor rotates due to the interaction between the magnetic field generated by the current flowing through the phase windings Lu, Lv, and Lw and the magnetic field generated by the permanent magnet.

  The brushless DC motor M is accommodated in the compressor 20 as a power source. During cooling, the refrigerant discharged from the compressor 20 flows to the outdoor heat exchanger 22 via the four-way valve 21 as indicated by the solid line arrow, and the refrigerant passing through the outdoor heat exchanger 22 passes through the expansion valve 23 to the room. It flows to the heat exchanger 24. Then, the refrigerant that has passed through the indoor heat exchanger 24 is sucked into the compressor 20 through the four-way valve 21. Thereby, the outdoor heat exchanger 22 functions as a condenser, and the indoor heat exchanger 24 functions as an evaporator. At the time of heating, the four-way valve 21 is switched, whereby the refrigerant flows in the direction indicated by the broken-line arrow, and the indoor heat exchanger 24 functions as a condenser and the outdoor heat exchanger 22 functions as an evaporator.

  The brushless DC motor M is connected to the inverter device of the present invention. The inverter device includes a switching circuit 1 that receives an output (for example, DC 280 V) of a DC power source 30 rectified from a commercial AC power source and performs energization to the phase windings Lu, Lv, and Lw and switching of the energization. It is comprised by the control part 10 which controls drive.

  The switching circuit 1 has a series circuit of U, V, and W for three phases of switching elements, for example, low-loss power MOSFETs (superjunction MOSFETs, etc.) on the upstream side and the downstream side along the DC voltage application direction. MOSFET 2u is provided upstream of the U phase, MOSFET 3u is provided downstream of the U phase, MOSFET 2v is provided upstream of the V phase, MOSFET 3v is provided downstream of the V phase, MOSFET 2w is provided upstream of the W phase, and MOSFET 3w is provided downstream of the W phase. Yes. The free-wheeling diodes Du +, Dv +, and Dw + are connected in reverse parallel to the MOSFETs 2u, 2v, and 2w, and the free-wheeling diodes Du−, Dv−, and Dw− are connected in reverse parallel to the MOSFETs 3u, 3v, and 3w, respectively. These free-wheeling diodes are built in corresponding MOSFETs (element bodies) as parasitic diodes.

  The unconnected ends of the phase windings Lu, Lv, and Lw are respectively connected to an interconnection point between the MOSFETs 2u and 3u, an interconnection point between the MOSFETs 2v and 3v, and an interconnection point between the MOSFETs 2w and 3w. And the drive voltage of MOSFET2u-2w and 3u-3w is set to about 10V-15V.

  In addition, the switching circuit 1 is configured such that when forward current (return current) flows through the freewheeling diodes Du + to Dw + due to the energy stored in the phase windings Lu, Lv, and Lw that are inductive loads, In order to suppress reverse recovery current flowing through the freewheeling diodes Du + to Dw + when 3w is turned on, a reverse voltage application circuit 4u that applies a reverse voltage to the freewheeling diodes Du + to Dw + of the MOSFETs 2u to 2w prior to turning on the MOSFETs 3u to 3w, respectively. , 4v, 4w.

  Similarly, reverse-side voltage application circuits 5u, 5v, and 5w are provided in the downstream side MOSFETs 3u, 3v, and 3w, respectively.

  The reverse voltage application circuit 4 u applies a voltage (for example, 2.5 V) of the DC power supply 40 to the reverse voltage application capacitor 42 via the resistor 41, and uses the voltage of the reverse voltage application capacitor 42 of the reverse voltage application MOSFET 43. The reverse voltage is applied to the MOSFET 2u between the drain and source and via the diode 44. Further, the reverse voltage application circuit 4 u has a pulse generation circuit 45 and a gate drive circuit 46 for driving the reverse voltage application MOSFET 43. The pulse generation circuit 45 generates a pulse signal for applying a reverse voltage in response to a command from the control unit 10. The gate drive circuit 46 outputs a one-shot pulse signal for turning on the reverse voltage application MOSFET 43 based on the pulse signal generated by the pulse generation circuit 45. This output is supplied to the gate of the reverse voltage application MOSFET 43.

  In particular, the output voltage of the reverse voltage application circuit 4u, that is, the reverse voltage applied to the freewheeling diode Du + of the MOSFET 2u is a direct current so that the voltage is lower than the voltage drop due to the conduction channel resistance at the rated current of the MOSFET 2u (for example, 1 V or less). The voltage of the power supply 40 is preset.

  The other reverse voltage application circuits 4v and 4w and the reverse voltage application circuit 5u have the same configuration as the reverse voltage application circuit 4u. Therefore, the description is omitted. The reverse voltage application circuits 5v and 5w are the same as the reverse voltage application circuit 4u, except that the reverse voltage application circuits 5v and 5w are different from the other in that a direct current voltage is taken from the direct current power source 40 of the reverse voltage application circuit 5u.

  On the other hand, resistors 11u, 11v, and 11w for current detection are inserted and connected to the downstream sides of the MOSFETs 3u, 3v, and 3w in each series circuit. A voltage of a level corresponding to the current flowing through the MOSFETs 3u to 3w, that is, the current flowing to the motor windings Lu to Lw is generated in these resistors 11u to 11w, and this voltage is supplied to the control unit 10.

  The control unit 10 detects the current flowing through each phase winding of the brushless DC motor M from the voltages generated in the resistors 11u to 11w, grasps the rotation of the rotor of the brushless DC motor M from the detected current, and adjusts to the rotation. Drives each MOSFET and each reverse voltage application circuit of the switching circuit 1 and controls at least one of the MOSFETs in the switching circuit 1 by controlling on / off of each MOSFET as a main function. First control means for sequentially switching energization of each phase winding by the MOSFET on the upstream side and the downstream MOSFET of the other at least one series circuit, and the other of the same series circuit as the MOSFET after the MOSFET is turned off Prior to turning on the switching element, reverse voltage is applied while the MOSFET is still on. To initiate the operation of the road, the operations other switching element and a second control means to terminate after one. The principal part of this control part 10 is shown in FIG.

  In FIG. 2, reference numeral 50 denotes a current detection unit that detects a current flowing through each phase winding of the brushless DC motor M from the voltage generated in the resistors 11 u to 11 w. The detection result is supplied to the rotation speed calculation unit 51 and is also supplied to the reference signal generation unit 54. The rotation speed calculation unit 51 calculates the rotation speed of the brushless DC motor M from the detection result of the current detection unit 50. This calculation result is supplied to the speed difference calculation unit 53. The speed difference calculation unit 53 obtains a difference between the target rotation number commanded from the target rotation number command unit 52 and the actual rotation number calculated by the rotation number calculation unit 51.

  The reference signal generation unit 54 has a frequency corresponding to the rotation speed of the rotor of the brushless DC motor M obtained from the detection result of the current detection unit 50, and a voltage level according to the calculation result of the speed difference calculation unit 53. And three-phase sine wave signals Eu, Ev, Ew whose phases are different from each other by 120 degrees are generated and output as reference signals Eu, Ev, Ew. The reference signal Eu is supplied to the PWM basic signal Vu generator 60, the reference signal Ev is supplied to the PWM basic signal Vv generator 70, and the reference signal Ew is supplied to the PWM basic signal Vw generator 80, respectively.

  Reference numeral 55 is a reference clock signal generator for generating a reference clock signal necessary for the operation of each part. A triangular wave signal generator 56 generates a triangular wave signal (also called a carrier signal) Eo having a predetermined frequency and voltage level based on the reference clock signal. The triangular wave signal Eo is supplied to the PWM basic signal Vu generator 60, the PWM basic signal Vv generator 70, and the PWM basic signal Vw generator 80. The PWM basic signal Vu generator 60, the PWM basic signal Vv generator 70, and the PWM basic signal Vw generator 80 are output from the reference signals Eu, Ev, Ew and the triangular wave signal generator 55 output from the reference signal generator 54. The PWM basic signals Vu, Vv, Vw for switching for each MOSFET are generated by voltage comparison with the triangular wave signal Eo.

  The generated PWM basic signal Vu is supplied to the upper element driving unit 61, the lower element driving unit 62, and the application timing determining units 63 and 64. The PWM basic signal Vv is supplied to the upper element driving unit 71, the lower element driving unit 72, and the application timing determining units 73 and 74. The PWM basic signal Vw is supplied to the upper element driving unit 81, the lower element driving unit 82, and the application timing determining units 83 and 84. In addition, the detection result of the current detection unit 50 is supplied to the application timing determination units 63, 64, 73, 74, 83, and 84.

  FIG. 3 shows the waveforms of the PWM basic signal Vu and the output signals of the upper element driving unit 61, the lower element driving unit 62, and the application timing determining units 63 and 64.

  Based on the PWM basic signal Vu and the reference clock signal, the upper element driving unit 61 is delayed by a predetermined time (dead time) td from the falling edge of the PWM basic signal Vu (after a predetermined time T0 from the lowest level point of the triangular wave signal Eo). A signal having a waveform that falls and rises in synchronization with the rise of the PWM basic signal Vu (after a predetermined time T1 from the fall of the PWM basic signal Vu) is output as a drive signal A for the MOSFET 2u. The lower element driving unit 62 is synchronized with the falling edge of the PWM basic signal Vu (after a predetermined time T0 from the lowest level point of the triangular wave signal Eo) based on the PWM basic signal Vu and a predetermined timing corresponding to the reference clock signal. A signal having a waveform that rises and falls after a predetermined time (dead time) td from the rise of the PWM basic signal Vu (after a predetermined time T1 from the fall of the PWM basic signal Vu) is output as a drive signal B for the MOSFET 3u.

  Based on the PWM basic signal Vu, the reference clock signal, and the output of the detection result of the current detection unit 50, the application timing determination unit 63 performs a predetermined time “T0 + T1 from the lowest level point of the triangular wave signal Eo before the PWM basic signal Vu rises. A signal having a waveform that falls after “ta” and rises after the rise of the PWM basic signal Vu (after a predetermined time “T0 + T1 + td + tb” from the lowest level point of the triangular wave signal Eo) is an upper operation signal for the reverse voltage application circuit 4u. Output as C.

  Based on the output of the PWM basic signal Vu, the reference clock signal, and the detection result of the current detection unit 50, the application timing determination unit 64 (before the falling of the PWM basic signal Vu (a predetermined time from the lowest level point of the triangular wave signal Eo) A signal having a waveform that falls after (T0-ta ") and rises after the fall of the PWM basic signal Vu (after the lowest level point of the triangular wave signal Eo for a predetermined time" T0 + td + tb ") is applied to the reverse voltage application circuit 5u. Output as side operation signal D.

  FIG. 3 is a schematic diagram for understanding the operation. In practice, all signal generation timings are determined by digital arithmetic processing. That is, the PWM basic signal generation unit 60 calculates the comparison between the generation signal of the triangular wave signal generation unit 56 and the reference signal of the reference signal generation unit 54 at the start timing of time T0, determines time T0 and time T1, The data is output to the element driving unit 61, the lower element driving unit 62, and the application timing determining units 63 and 64. The upper element driving unit 61, the lower element driving unit 62, and the application timing determining units 63 and 64 that have received this time data calculate the above-described various timings based on the time data, and the reference clock signal generating unit 55 receives the reference clock. The signal is counted, and the signal is changed according to the calculated timing. For example, the lower element drive unit 62 outputs a signal having a waveform that rises after T0 time from the start timing of T0, that is, the lowest level point of the triangular wave signal Eo, and falls after a predetermined time “T1 + td” as the drive signal B for the MOSFET 3u.

  FIG. 3 shows a signal waveform of low level drive (Low Active). The MOSFET is turned on when the upper element drive signal A and the lower element drive signal B are low levels, and the MOSFET is turned on when the level is high. Turn off. Similarly, the reverse voltage application MOSFET is turned on when the upper operation signal C and the lower operation signal D are each at a low level, and the reverse voltage application MOSFET is turned off when it is at a high level.

  Further, the predetermined time ta for determining the falling timing of the upper operation signal C and the lower operation signal D becomes longer as the current increases in accordance with the magnitude of the current detected by the current detector 50 ( The application timing determining units 63 and 64 set the operation so that the operation start timing of the reverse voltage application circuit is advanced.

  Upper element driving unit 71, lower element driving unit 72, application timing determining units 73 and 74 to which PWM basic signal Vv is supplied, upper element driving unit 81, lower element driving unit 82 to which PWM basic signal Vw is supplied, application The timing determination units 83 and 84 also have the same function.

Next, the operation will be described.
The upper element drive signal A for turning on and off the upstream side MOSFETs 2u, 2v, and 2w and the lower element drive signal B for turning on and off the downstream side MOSFETs 3u, 3v, and 3w are created at the timing described above. The

  With the upper element drive signal A and the lower element drive signal B, among the series circuits in the switching circuit 1, the upstream MOSFET of at least one series circuit is turned on and the downstream MOSFET of another at least one series circuit is turned on. The multi-phase energization is sequentially switched. By switching the multiphase energization, three interphase voltages are generated, and the interphase voltages are applied to the phase windings Lu, Lv, Lw of the brushless DC motor M. Thereby, a sinusoidal current flows through Lu, Lv, and Lw, and the brushless DC motor M operates.

  By the way, it is assumed that a current based on energy stored in one of the phase windings flows in the forward direction through the free-wheeling diode Du− of the MOSFET 3u on the downstream side. When this forward current so-called reflux current flows, the reverse voltage application MOSFET 43 of the reverse voltage application circuit 5u on the downstream side is turned on for a predetermined time “Ta + Tb” before the upstream MOSFET 2u is turned on, and the reverse voltage application The voltage stored in the capacitor 42 is applied as a reverse voltage to the freewheeling diode Du− of the downstream MOSFET 3u. The application of the reverse voltage starts while the downstream side MOSFET 3u is still on, and includes the timing at which the MOSFET 3u is turned off and the constant time td of the dead time, and further, the constant time after the upstream side MOSFET 2u is turned on. End after tb. That is, the reverse voltage application circuit 5u is turned on and the MOSFET 3u is turned off for the Ta time.

  When a reverse voltage is applied from the reverse voltage application circuit 5u to the freewheeling diode Du− of the MOSFET 3u in a state where the freewheeling current is flowing, a reverse recovery current flows to the freewheeling diode Du− of the MOSFET 3u. In this case, the reverse voltage applied to the free-wheeling diode Du− of the MOSFET 3u is a voltage based on the DC power supply 40 in the reverse voltage application circuit 5u, and is a voltage lower than the voltage drop due to the conduction channel resistance at the rated current of the MOSFET 3u. Thus, even if a reverse recovery current flows through the freewheeling diode Du− of the MOSFET 3u, the value of the reverse recovery current is extremely small.

  As shown in FIG. 4, the reverse voltage applied to the free-wheeling diode Du− of the MOSFET 3u is Vd, the current flowing through the element body of the MOSFET 3u is Id, the current flowing through the free-wheeling diode Du− of the MOSFET 3u is Id1, and the reverse voltage in the reverse voltage applying circuit 5u. How Vd, Id, Id1, and Im change when the current flowing through the application MOSFET 43 and the diode 44 is Im and the predetermined time ta, which is a determinant of the application start timing of the reverse voltage Vd, is 8 μsec, for example. This is simulated in FIG. At this time, it is assumed that a current Iu flows as a reflux current.

  In this simulation, prior to turning on the upstream MOSFET 2u after the downstream MOSFET 3u is turned off, the operation of the reverse voltage application circuit 5u starts while the MOSFET 3u is still turned on, and the operation ends after the MOSFET 2u is turned on. is doing.

  While the MOSFET 3u is still on, the reverse voltage application circuit 5u operates to apply the reverse voltage Vd to the free-wheeling diode Du-, thereby starting a reverse recovery current. During the reverse recovery operation, Vd is maintained at substantially the same potential as before the reverse recovery operation (before the reverse voltage application circuit 5u is operated) due to the voltage drop due to the reverse recovery current, and when the reverse recovery is completed, the freewheeling diode Du− is completely turned off. Thus, the current Im supplied from the reverse voltage application circuit 5u is only the return current Iu or the return current Iu + the current Id (forward direction) of the MOSFET 3u, and Vd is the reverse voltage with respect to the return diode Du− at this time, that is, the forward direction of the MOSFET 3u. Voltage. Therefore, the charge stored in the free-wheeling diode Du− of the MOSFET 3u can be sufficiently reduced before the upstream-side MOSFET 2u is turned on. As a result, it is possible to eliminate the problem that a large reverse recovery current flows when the MOSFET 2u is turned on.

  According to this embodiment, the voltage of the DC power supply 40 is set so that the output voltage of the reverse voltage application circuit 5u, 0.2V in FIG. 5, is lower than the voltage drop due to the conduction channel resistance at the rated current of the MOSFET 3u. Is set to a low value (about 2.5 V) in advance, the period during which the reverse recovery current flows becomes long. However, the integrated power value in the free-wheeling diode Du− of the MOSFET 3u is the product of power consumption and time, and since the applied reverse voltage Vd is low and the reverse recovery current is extremely small, the integrated power is greatly reduced. Even if reverse voltage application circuit is turned on early and reverse recovery ends when MOSFET 3u is on as shown in FIG. 5, from reverse voltage application circuit 5u as in this embodiment. If the reverse voltage Vd is set low, the current Id when this voltage is short-circuited by the MOSFET 3u is very small, and the power loss in this portion is extremely small. Therefore, even when a MOSFET is used, power loss can be greatly reduced, and efficiency can be improved.

  Moreover, the predetermined time ta, which is a determinant of the application start timing of the reverse voltage Vd while the MOSFET 3u is turned on, is advanced as the return current detected by the current detection unit 50 increases, that is, the return diode of the MOSFET 3u. Since the amount of charge stored in Du− increases as the amount of charge increases, the charge stored in the free-wheeling diode Du− can be surely eliminated by the end of the dead time.

  The broken line in FIG. 5 sets the power supply voltage of the reverse voltage application circuit 5u to the same as the MOSFET drive voltage (about 15V) and starts the operation of the reverse voltage application circuit 5u during the dead time after the MOSFET 3u is turned off. The simulation results are shown for reference, and it can be seen that the reverse recovery current that flows during the dead time is large.

  For reference, FIG. 6 shows a simulation result in the case where the return current is small and the power supply voltage of the reverse voltage application circuit 5u is increased to about 15 V and the operation of the reverse voltage application circuit 5u is started while the MOSFET 3u is on. It can be seen that the current flowing during the dead time increases as the power supply voltage increases.

  In the above embodiment, the case where all of the switching elements of each series circuit in the switching circuit 1 are MOSFETs has been described as an example. However, if at least one of the series circuits is a MOSFET, the other switching element is, for example, an IGBT. It is possible to implement similarly in the case of.

  In addition, the present invention is not limited to the above embodiment, and various modifications can be made without departing from the scope of the invention.

The figure which shows the structure of one Embodiment of this invention, and the structure of a refrigerating cycle. The block diagram which shows the structure of the principal part of the control part in the embodiment. The figure which shows the signal waveform of each part in the embodiment. The figure which shows a part of MOSFET and reverse voltage application circuit in the same embodiment, the voltage applied there, and the flowing electric current. The figure which shows the simulation result of how Vd, Id, Id1, and Im in FIG. 4 change. The figure which shows the simulation result in case the reverse voltage Vd in FIG. 4 is not low for reference.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Switching circuit, 2u, 2v, 2w ... Upstream side MOSFET (switching element), 3u, 3v, 3w ... Downstream side MOSFET (switching element), 4u, 4v, 4w ... Reverse voltage application circuit, 5u, 5v, 5w ... reverse voltage application circuit, Du +, Dv +, Dw +, Du-, Dv-, Dw -... freewheeling diode, 10 ... control unit, 30 ... DC power supply, 42 ... reverse voltage application capacitor, 43 ... reverse voltage application MOSFET , 50 ... current detection unit, 51 ... rotation speed calculation unit, 52 ... rotation speed command unit, 53 ... speed difference calculation unit, 54 ... reference signal generation unit, 55 ... reference clock signal generation unit, 56 ... triangular wave signal generation unit, 60 ... PWM basic signal Vu generator, 70 ... PWM basic signal Vv generator, 80 ... PWM basic signal Vw generator, M ... Brushless DC motor, Lu, Lv, Lw ... Phase winding

Claims (3)

A switching circuit in which a plurality of series circuits of two switching elements each using a MOSFET having a free-wheeling diode are used, and an interconnection point of each switching element of these series circuits is connected to a load;
First control means for controlling on / off operations of each of the switching elements;
A reverse voltage application circuit for applying a reverse voltage lower than the drive voltage of each MOSFET to the freewheeling diode of each MOSFET;
Second control means for starting an operation of the reverse voltage application circuit while the MOSFET is on, and ending the operation after the other switching element is turned on;
An inverter device comprising: 2. The inverter device according to claim 1, wherein an output voltage of the reverse voltage application circuit is a voltage lower than a voltage drop due to a conduction channel resistance at a rated current of the MOSFET. A current detection unit configured to detect a current flowing through the MOSFET, wherein the control unit is configured to start the operation of the reverse voltage application circuit as the current increases according to the magnitude of the current detected by the current detection unit; The inverter device according to claim 1, wherein the inverter device is advanced.

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