JP6666174B2 - Power converter - Google Patents

Description

  The present invention relates to a power converter.

  Conventionally, an inverter drive system that converts electric power of a motor by two inverters is known. For example, in Patent Document 1, the phase of a fundamental wave component of a pulse width modulation signal (hereinafter, pulse width modulation is referred to as “PWM”) of a first inverter system and a second inverter system is shifted by 180 ° to be 2 degrees. Two power supplies are electrically connected in series and drive the motor by the sum of the two power supply voltages. In one inverter, any three phases of the upper and lower arms are simultaneously turned on, and the other inverter performs pulse width modulation driving.

JP 2006-238686 A

By the way, if the state where power is supplied to a specific element continues, for example, when the rotating speed of the rotary electric machine is low, the output is limited according to the heat resistance performance of the element, and there is a possibility that a desired torque cannot be output.
SUMMARY An advantage of some aspects of the invention is to provide a power conversion device capable of suppressing heat generation in a specific element.

The power converter of the present invention controls a rotating electric machine (10) having coils of a plurality of phases (11, 12, 13), and includes a first inverter (20), a second inverter (30), A control unit (65).
The first inverter is provided between one end (111, 121, 131) of the coil and the first voltage source (41).
The second inverter is provided between the other end (112, 122, 132) of the coil and the second voltage source (42).

  The control unit controls all phases of the upper arm element (21 to 23, 31 to 33) or the lower arm element (24 to 26, 34 to 36) provided on the high potential side in one of the first inverter and the second inverter. Turn on to neutral point, and perform one-side drive control to control the other according to the drive request of the rotating electric machine.

The control unit, in the neutral point side inverter which is an inverter to be neutralized, the upper arm ON fixed state in which all phases of the upper arm element are turned ON, and the lower arm ON fixed state in which all phases of the lower arm element are turned ON. , With the up / down switching frequency.
A control unit configured to control the first inverter as a driving inverter that controls the driving inverter in response to a driving request, to set the second inverter to a neutral point inverter, to set the first inverter to a neutral point inverter, and to set the first inverter to a neutral point inverter; And the second state in which is the drive-side inverter is switched at the inverter switching frequency.
In this specification, the number of times control is switched per unit time is referred to as a “switching frequency”.
When switching from the drive-side inverter to the neutral-point-side inverter, the control unit first fixes the arm on the upper and lower side opposite to the arm including the maximum heat generation location to ON.
Thereby, heat generation in a specific element can be suppressed.

1 is a schematic configuration diagram illustrating a power conversion device according to an embodiment of the present invention. FIG. 3 is an explanatory diagram illustrating a drive region of the motor generator according to the embodiment of the present invention. FIG. 3 is an explanatory diagram illustrating a phase current according to an embodiment of the present invention. It is an explanatory view explaining the maximum heat generation place by one embodiment of the present invention. It is an explanatory view explaining the maximum heat generation place by one embodiment of the present invention. FIG. 7 is an explanatory diagram illustrating an arm that is first turned on and fixed in the neutral point side inverter in one embodiment of the present invention. 6 is a flowchart illustrating a mode switching process according to an embodiment of the present invention. 6 is a time chart illustrating a mode switching process according to an embodiment of the present invention. 6 is a time chart illustrating a mode switching process according to an embodiment of the present invention.

Hereinafter, a power converter according to the present invention will be described with reference to the drawings.
(One embodiment)
1 to 9 show a power converter according to an embodiment of the present invention.
As shown in FIG. 1, the rotating electric machine drive system 1 includes a motor generator 10 as a rotating electric machine, and a power converter 15.

  The motor generator 10 is a so-called “main motor” that is applied to, for example, an electric vehicle such as an electric vehicle or a hybrid vehicle, and generates torque for driving drive wheels (not shown). The motor generator 10 has a function as an electric motor for driving the driving wheels and a function as a generator for generating electric power by being driven by kinetic energy transmitted from an engine and driving wheels (not shown). In the present embodiment, a case will be mainly described in which the motor generator 10 functions as an electric motor.

Motor generator 10 is a three-phase AC rotating machine, and has U-phase coil 11, V-phase coil 12, and W-phase coil 13. Hereinafter, the U-phase coil 11, the V-phase coil 12, and the W-phase coil 13 are appropriately referred to as "coils 11 to 13".
In the present embodiment, the current flowing through the U-phase coil 11 is defined as a U-phase current Iu, the current flowing through the V-phase coil 12 is defined as a V-phase current Iv, and the current flowing through the W-phase coil 13 is defined as a W-phase current Iw. Hereinafter, regarding the current flowing through the coils 11 to 13, the current flowing from the first inverter 20 side to the second inverter 30 side is positive, and the current flowing from the second inverter 30 side to the first inverter 20 side is negative. Further, the phase current having the largest absolute value is defined as “maximum phase current”. When the maximum phase current flows from the first inverter 20 to the second inverter 30, the first inverter 20 is on the upstream side and the second inverter 30 is on the downstream side. On the other hand, when the maximum phase current flows from the second inverter 30 to the first inverter 20, the second inverter 30 is on the upstream side and the first inverter 20 is on the downstream side.

The power conversion device 15 converts the power of the motor generator 10 and includes a first inverter 20, a second inverter 30, a control unit 65, and the like.
The first inverter 20 is a three-phase inverter that switches energization of the coils 11 to 13 and has switching elements 21 to 26. The second inverter 30 is a three-phase inverter that switches energization of the coils 11 to 13 and has switching elements 31 to 36.
The switching element 21 has a transistor 211 and a diode 221. Similarly, the switching elements 22 to 26 and 31 to 36 have transistors 212 to 216 and 311 to 316 and diodes 222 to 226 and 321 to 326, respectively. In the present embodiment, the transistors 211 to 216 and 311 to 316 correspond to an “element unit”, and the diodes 221 to 226 and 321 to 326 correspond to a “return unit”. Further, the transistor and the diode are appropriately referred to as a “device”.

  The transistors 211 to 216 and 311 to 316 are IGBTs (insulated gate bipolar transistors), and the on / off operation is controlled by the control unit 65. When the transistors 211 to 216 and 311 to 316 are turned on, energization from the high potential side to the low potential side is allowed, and when turned off, the energization is cut off. The transistors 211 to 216 and 311 to 316 are not limited to IGBTs, but may be MOSFETs or the like.

  The diodes 221 to 226 and 321 to 326 are free-wheeling diodes connected in parallel with the transistors 211 to 216 and 311 to 316, respectively, and which allow current to flow from the low potential side to the high potential side. For example, the diodes 221 to 226 and 321 to 326 may be built in the transistors 211 to 216 and 311 to 316 like a parasitic diode of a MOSFET, for example, or may be externally mounted. .

  In the first inverter 20, the switching elements 21 to 23 are connected to the high potential side, and the switching elements 24 to 26 are connected to the low potential side. A first high-potential-side wiring 27 connecting the high-potential sides of the switching elements 21 to 23 is connected to the positive electrode of the first battery 41, and a first low-potential-side wiring connecting the low-potential sides of the switching elements 24 to 26. 28 is connected to the negative electrode of the first battery 41.

  One end 111 of the U-phase coil 11 is connected to the connection point of the U-phase switching elements 21 and 24, one end 121 of the V-phase coil 12 is connected to the connection point of the V-phase switching elements 22 and 25, One end 131 of the W-phase coil 13 is connected to a connection point between the switching elements 23 and 26. That is, the first inverter 20 is connected between the one ends 111, 121, 131 of the coils 11, 12, 13 and the first battery 41.

  In the second inverter 30, the switching elements 31 to 33 are connected to the high potential side, and the switching elements 34 to 36 are connected to the low potential side. Also, a second high-potential-side wiring 37 connecting the high-potential sides of the switching elements 31 to 33 is connected to the positive electrode of the second battery 42, and a second low-potential-side wiring connecting the low-potential sides of the switching elements 34 to 36. 38 is connected to the negative electrode of the second battery 42.

The other end 112 of the U-phase coil 11 is connected to the connection point of the U-phase switching elements 31 and 34, the other end 122 of the V-phase coil 12 is connected to the connection point of the V-phase switching elements 32 and 35, The other end 132 of the W-phase coil 13 is connected to a connection point between the W-phase switching elements 33 and 36. That is, the second inverter 30 is connected between the other ends 112, 122, 132 of the coils 11, 12, 13 and the second battery 42.
Hereinafter, the switching elements 21 to 23 and 31 to 33 connected to the high potential side are appropriately referred to as “upper arm elements”, and the switching elements 24 to 26 and 34 to 36 connected to the low potential side are appropriately referred to as “lower arm elements”.

  The switching elements 21 to 26 and 31 to 36 are provided with temperature detecting elements 231 to 236 and 331 to 336 for detecting the temperatures of the elements 21 to 26 and 31 to 36, respectively. The detected values of the temperature detecting elements 231 to 236 and 331 to 336 are output to the control unit 65 and used for calculating the element temperature in the control unit 65.

A first battery 41 as a first voltage source, which is a chargeable / dischargeable DC power supply such as a lithium ion battery, is connected to the first inverter 20 so that power can be exchanged with the motor generator 10 via the first inverter 20. Provided.
A second battery 42 as a second voltage source, which is a chargeable / dischargeable DC power supply such as a lithium ion battery, is connected to the second inverter 30 and can exchange power with the motor generator 10 via the second inverter 30. Provided.
The voltage of the first battery 41 and the voltage of the second battery 42 may be equal or different.

The first capacitor 43 is connected to the first high potential side wiring 27 and the first low potential side wiring 28. The first capacitor 43 is a smoothing capacitor that smoothes a current from the first battery 41 to the first inverter 20 or a current from the first inverter 20 to the first battery 41.
The second capacitor 44 is connected to the second high potential side wiring 37 and the second low potential side wiring 38. The second capacitor 44 is a smoothing capacitor that smoothes a current from the second battery 42 to the second inverter 30 or a current from the second inverter 30 to the second battery 42.
The current detection unit 50 detects the phase currents Iu, Iv, Iw. In FIG. 1, a current detection element such as a Hall element is provided for each phase. However, a current detection element may be provided for two phases, and the remaining one phase may be calculated from the sum of three phases = 0.

The control signal generator 60 includes a first driver circuit 61, a second driver circuit 62, and a controller 65.
The control unit 65 is mainly configured by a microcomputer, and performs various arithmetic processes. Each process in the control unit 65 may be a software process by executing a program stored in advance by the CPU or a hardware process by a dedicated electronic circuit.
The control unit 65 controls the first inverter 20 and the second inverter 30. Specifically, the transistors 211 to 216 of the switching elements 21 to 26, 31 to 36 based on the command values related to the driving of the motor generator 10, such as the torque command value trq ^* and the current command values Iu ^* , Iv ^* , Iw ^*. , 311 to 316, and outputs the control signals to the driver circuits 61 and 62.

  The first driver circuit 61 generates and outputs a gate signal for controlling on / off operations of the transistors 211 to 216 according to a control signal from the control unit 65. The second driver circuit 62 generates and outputs a gate signal for controlling the on / off operation of the transistors 311 to 316 according to a control signal from the control unit 65. When the transistors 211 to 216 and 311 to 316 are turned on and off according to the control signal, the DC power of the batteries 41 and 42 is converted to AC power and supplied to the motor generator 10. Thereby, the driving of motor generator 10 is controlled by control unit 65 via first inverter 20 and second inverter 30. Hereinafter, appropriately controlling the on / off operation of the transistors 211 to 216, 311 to 316 of the switching elements 21 to 26, 31 to 36 is simply referred to as controlling the on / off operation of the switching elements 21 to 26, 31 to 36.

  The drive control of the motor generator 10 will be described. The drive control in the rotating electrical machine drive system 1 of the present embodiment includes “one-side drive control” in which the drive is performed using the power of the first battery 41 or the second battery 42, and the power of the first battery 41 and the second battery 42. And “double-sided drive control”. As shown in FIG. 2, in the present embodiment, the driving region is divided into a one-side driving region Ra and a both-side driving region Rb according to the rotation speed N and the torque trq of the motor generator 10, and the rotation speed N and the torque trq are changed to the one-side driving region. If it is Ra, it is one-side drive control, and if it is both-side drive region Rb, it is both-side drive control. The boundary B between the one-side drive region Ra and the both-side drive region Rb is set according to the upper limit value that can be output by the one-side drive control. In the present embodiment, the rotation speed N and the torque trq of the motor generator 10 are referred to as “drive request”.

  In the both-side drive control, the phase of the first fundamental wave related to the drive control of the first inverter 20 and the phase of the second fundamental wave related to the drive control of the second inverter 30 are inverted. In other words, the first fundamental wave and the second fundamental wave are out of phase by approximately 180 [°]. By inverting the phases of the first fundamental wave and the second fundamental wave, controlling the first inverter 20 based on the first fundamental wave, and controlling the second inverter 30 based on the second fundamental wave, the first battery It can be considered that the first battery 41 and the second battery 42 are electrically connected in series. Thereby, a voltage (that is, V1 + V2) corresponding to the sum of first power supply voltage V1 that is the voltage of first battery 41 and second power supply voltage V2 that is the voltage of second battery 42 can be applied to motor generator 10. is there. Although the phase difference between the first fundamental wave and the second fundamental wave is 180 [°], a voltage corresponding to the difference between the first power supply voltage V1 and the second power supply voltage V2 can be applied to the motor generator 10. Any deviation is tolerated.

  The amplitude of the first fundamental wave and the amplitude of the second fundamental wave may be equal or different. The first fundamental wave and the second fundamental wave may have similar waveforms so that they are both sine waves. Also, as in the case where one of the first inverter 20 and the second inverter 30 is controlled by the sine wave PWM and the other is controlled by the overmodulation PWM, the waveforms of the first fundamental wave and the second fundamental wave F2 are different. Is also good. Alternatively, rectangular wave control may be used in which the amplitude is regarded as infinite and each element is switched on and off every half cycle of the fundamental wave. In the rectangular wave control, the energization phase may be other than 180 °, for example, 120 ° energization.

In the one-side drive control using the electric power of the first battery 41, one of all the phases of the upper arm elements 31 to 33 or the lower arm elements 34 to 36 of the second inverter 30 is turned on, and the other is turned off. (2) The inverter 30 is neutralized. Further, the switching elements 21 to 26 of the first inverter 20 are switched according to a drive request.
In the one-side drive control using the power of the second battery 42, one of all the phases of the upper arm elements 21 to 23 or the lower arm elements 24 to 26 of the first inverter 20 is turned on, and the other is turned off. One inverter 20 is neutralized. Further, the switching elements 31 to 36 of the second inverter 30 are switched according to the drive request.
Hereinafter, turning on all phases of the upper arm element in the inverter that is neutralized is referred to as “upper arm ON fixed”, and turning on all phases of the lower arm element as “lower arm ON fixed”. In addition, the inverter that changes to a neutral point is referred to as a “neutral-side inverter”, and the inverter that switches according to a drive request is referred to as a “drive-side inverter”.

  When the voltage of the first battery 41 and the voltage of the second battery 42 are different, the range that can be output by the low-voltage battery is such that the high-voltage inverter is neutralized and the low-voltage battery is It is preferable to give priority to one-side drive control using electric power. As a result, switching loss, iron loss of motor generator 10, and the like are reduced, and the loss of the entire system can be reduced.

  When the motor generator 10 is the main motor of the vehicle, for example, when trying to climb a slope, the motor generator 10 may be in the “torque locked state”. The torque locked state is a state in which a relatively large torque is output when the rotation speed N of the motor generator 10 is smaller than the rotation speed threshold value Nth. The rotation speed threshold value Nth is set to a value at which the motor generator 10 can be regarded as substantially stopped. The rotation speed threshold value Nth is smaller than the maximum rotation speed Na capable of outputting the maximum torque in the one-side drive control. That is, in the present embodiment, when the motor generator 10 is in the torque locked state, the motor generator 10 is controlled by the one-side drive control.

When the motor generator 10 is rotating, the phase at which the maximum phase current is obtained changes with the rotation, so that it is difficult for the heat generation to be biased among the devices. On the other hand, when a relatively large torque is to be output in a state where the rotation speed N of the motor generator 10 is low, the energization of a specific device is continued, and the heat generation amount of the device may exceed the allowable heat generation amount. . The amount of heat generation is proportional to the product of current and time. When the temperature of the device exceeds the allowable temperature Tth, the amount of current is limited to prevent the device from being damaged, and the output is reduced.
In the present embodiment, the modes M1, M2, and M3 are appropriately switched to prevent uneven heating.

  In the present embodiment, an example will be described in which the torque lock state is established at the electrical angle indicated by the broken line La in FIG. When the torque lock state is established at the location indicated by the broken line La, as shown in FIG. 3B, the state where the positive U-phase current Iu is the maximum phase current is continued. When the V-phase current Iv or the W-phase current Iw is the maximum phase current, the phase that becomes the maximum heat generation location changes, but the control is the same as when the U-phase current Iu is the maximum phase current.

  The modes M1, M2, and M3 will be described with reference to FIGS. 4 and 5, in the neutral-point-side inverter, a transistor that is fixed at ON is indicated by a solid line, and a transistor that is fixed at OFF is indicated by a broken line. A device through which the maximum phase current (here, the U-phase current Iu) flows is surrounded by a two-dot chain line. One of the devices through which the maximum phase current flows is the maximum heat generation location according to the direction of the maximum phase current flow, the heat generation characteristics of the device, and the like. In FIG. 4 and FIG. 5, for simplification, the control signal generation unit 60 and some reference numerals are omitted as appropriate.

First, the mode M1 will be described. Mode M1 is a neutral point fixed mode in which the inverter to be neutralized is fixed and the arm that is turned on in the inverter to be neutralized is fixed. FIG. 4A shows an example in which the upper arm of the second inverter 30 is fixed on.
As shown in FIG. 4A, in the mode M1, the U-phase current Iu, which is the maximum phase current, includes the U-phase upper transistor 211 and the U-phase lower diode 224 of the first inverter 20, which is the driving inverter. Then, the current flows to the diode 321 on the U-phase side of the second inverter 30 which is the neutral point side inverter. On the first inverter 20 side, the transistors 211 to 216 are switched at the carrier frequency. Therefore, in the U phase, when the upper transistor 211 is on and the lower transistor 214 is off, a current flows through the transistor 211, When the upper transistor 211 is off and the lower transistor 214 is on, current flows through the diode 224. That is, in the U phase of the first inverter 20, the device through which the current flows is switched at the carrier frequency.

On the other hand, on the neutralized second inverter 30 side, when the upper arm element is fixed to ON, current continues to flow through the diode 321 on the upper side of the U phase. Therefore, when this state is continued, the U-phase upper diode 321 of the second inverter 30 generates the most heat.
When the lower arm of the second inverter 30 is fixed to the ON state in the mode M1, the transistor 314 becomes a maximum heat generating portion. In addition, when the upper arm of the first inverter 20 is fixed on in the mode M1, the transistor 211 becomes a maximum heat generating part, and when the lower arm is fixed on, the diode 224 becomes a maximum heat generating part (see FIG. 5).

Here, when the temperature of the diode 321 that is the maximum heat generation point exceeds the allowable temperature Tth, it is necessary to limit the current. If the current is limited, the maximum torque that can be output as a system cannot be output.
Therefore, in the present embodiment, when the temperature of the diode 321 reaches the allowable temperature Tth, the temperature of the diode 321 is lowered by switching from the mode M1 to the mode M2 instead of the current limitation. In the mode M2, the state where the second inverter 30 is the neutral point side inverter is continued, and in the second inverter 30, the state where the upper arm element is fixed on and the state where the lower arm element is fixed on are switched. This is an up / down switching mode. That is, in the mode M2, the state of FIG. 4A and the state of FIG. 4B are switched.

  As shown in FIG. 4B, when the lower arm element of the second inverter 30 is fixed to the ON state, a current flows through the lower transistor 314 in the U phase. In other words, when the lower arm element is fixed in the ON state, no current flows through the diode 321 on the upper side of the U-phase, which was the maximum heat generation point in the mode M1, and the temperature of the diode 321 decreases. That is, in the neutral point side inverter, by switching the arm that is fixed ON, the device in which the U-phase current Iu that is the maximum phase current flows can be switched, and heat generation can be dispersed.

  Here, in the neutral point side inverter, the ratio of the period PH during which the upper arm is fixed to ON and the period PL during which the lower arm is fixed ON is determined by the temperature of the diode 321 through which the U-phase current Iu flows when the upper arm is fixed ON. And the temperature of the transistor 314 through which the U-phase current Iu flows when the lower arm is fixed to ON, is set according to the temperature characteristics of the transistor and the diode so as to be substantially uniform.

  In addition, the number of times of switching of the arm to be fixed on in the neutral point side inverter up and down is smaller than the number of times of on / off switching of each switching element in the drive side inverter. Assuming that the number of up / down switchings per unit time in the neutral point side inverter is an up / down switching frequency Fn and the number of on / off switchings of each switching element in the driving side inverter per unit time is a switching frequency Fd, the up / down switching frequency Fn in the neutral side inverter is Is smaller than the switching frequency Fd of the driving inverter. When the drive-side inverter is subjected to PWM control based on a comparison between a carrier wave and a fundamental wave, the switching frequency Fd is the carrier frequency.

  If Fd> Fn, the switching loss of the drive side inverter becomes larger than the switching loss of the neutral point side inverter. Here, assuming that the switching elements 21 to 26 and 31 to 36 have substantially the same conduction loss, the element that generates the most heat in the mode M2 is the transistor 211 or the diode 224. Considering that the transistor 211 and the diode 224 are not the maximum heat generating portions in the mode M1, by setting the mode M2 instead of the mode M1, the maximum heat generating portion is switched and the heat generation is dispersed.

  When the transistor 211 or the diode 224 reaches the allowable temperature Tth in the mode M2, the mode is switched from the mode M2 to the mode M3. Mode M3 is an inverter switching mode for switching between the drive side inverter and the neutral point side inverter. In the mode M3, in the neutral-point-side inverter, similarly to the mode M2, the arm to be fixed to ON is switched at the up-down switching frequency Fn.

  The number of times of switching between the neutral point side inverter and the driving side inverter is smaller than the number of times of switching of the arm fixed to ON in the neutral point side inverter up and down. Here, assuming that the number of times of switching between the neutral point side inverter and the driving side inverter per unit time is an inverter switching frequency Fi, the inverter switching frequency Fi is lower than the up / down switching frequency Fn of the neutral point side inverter. That is, Fi <Fn <Fd.

In FIG. 5, the first inverter 20 is a neutral point side inverter, and the second inverter 30 is a drive side inverter. That is, in the mode M3, a first state in which FIGS. 4A and 4B are switched at the up / down switching frequency Fn and a second state in which FIGS. 5A and 5B are switched at the up / down switching frequency Fn are switched.
As described with reference to FIG. 4, in the mode M2, the transistor 211 or the diode 224 is a maximum heat generating portion. Which of the transistor 211 and the diode 224 becomes the maximum heat generation point is determined by the duty, the loss characteristics of the device, the thermal resistance, and the like.

  In the present embodiment, when the neutral point side inverter is switched from the second inverter 30 to the first inverter 20 in the mode M3, the current does not flow through the transistor 211 or the diode 224, whichever generates more heat. First, determine the arm to be fixed ON. In other words, when switching from the drive-side inverter to the neutral-point-side inverter, if the upper-arm element includes a device that becomes the largest heat-generating part when the drive-side inverter is used, the lower-arm element is first fixed to ON, Is included in the lower arm element, the upper arm element is first fixed on.

  The heat generation amount of the transistor per unit time is Ht, and the heat generation amount of the diode is Hd. If Ht <Hd, the maximum heat generation point in the mode M2 is the diode 224. Therefore, immediately after switching the neutral point side inverter to the first inverter 20, the upper arm is set so that no current flows through the diode 224. Fixed on. On the other hand, if Ht> Hd, the maximum heat generation point in the mode M2 is the transistor 211. Therefore, immediately after the neutral-side inverter is switched to the first inverter 20, the lower part is set so that no current flows through the transistor 211. Fix the arm on.

When the second inverter 30 is the driving inverter, if Ht <Hd, the maximum heat generation location is the diode 321. Therefore, immediately after the neutral point inverter is switched to the second inverter 30, the diode 321 is connected to the diode 321. The lower arm is fixed ON so that no current flows. On the other hand, if Ht> Hd, the highest heat generating portion is the transistor 314. Therefore, immediately after switching the neutral-point-side inverter to the second inverter 30, the upper arm is fixed ON so that no current flows through the transistor 314. I do.
As a result, the temperature at the point of maximum heat generation can be reduced, so that the time during which the maximum torque can be output can be extended.

Here, when switching from the drive side inverter to the neutral point side inverter, the arm to be turned on first is determined according to the heat generation amounts Ht and Hd, which are the temperature characteristics of the transistor and the diode, and the direction in which the maximum phase current flows.
The arm which is turned on first by the neutral point side inverter when switching from the drive side inverter to the neutral point side inverter will be described with reference to FIG. FIG. 6A shows the phase current for one cycle of the electrical angle when the motor generator 10 is driven at a constant speed. As shown in FIG. 6A, in the section Za, the direction of the maximum phase current is positive, and in the section Zb, the direction of the maximum phase current is negative.

First, the case where the transistor generates more heat than the diode (that is, Ht> Hd) will be described.
As shown in FIG. 6B, when the neutral point side inverter is switched from the first inverter 20 to the second inverter 30, in the second inverter 30, in the section Za where the maximum phase current is positive, first the upper arm Is fixed to ON, and in the section Zb where the maximum phase current is negative, the lower arm is first fixed to ON.
When the neutral point side inverter is switched from the second inverter 30 to the first inverter 20, in the first inverter 20, in the section Za in which the maximum phase current is positive, the lower arm is first fixed at ON and the maximum phase current is negative. In the section Zb, the upper arm is first fixed on.
In other words, when Ht> Hd, and the inverter that switches from the drive inverter to the neutral point inverter is on the upstream side of the maximum phase current, the lower arm is fixed ON first, and when it is on the downstream side, Fix the upper arm to ON.

Next, a case where the diode generates heat more than the transistor (that is, Ht <Hd) will be described.
When switching the neutral point side inverter from the first inverter 20 to the second inverter 30, in the second inverter 30, in the section Za where the maximum phase current is positive, the lower arm is first fixed at ON, and the maximum phase current is negative. In the section Zb, the upper arm is first fixed on.
When the neutral point side inverter is switched from the second inverter 30 to the first inverter 20, in the first inverter 20, in the section Za where the maximum phase current is positive, the upper arm is first fixed to ON first, and the maximum phase current is negative. In the section Zb, the lower arm is first fixed on.
In other words, when Ht <Hd, and the inverter that switches from the drive-side inverter to the neutral-point-side inverter is on the upstream side of the maximum phase current, the upper arm is first fixed at ON, and when it is on the downstream side, Fix the lower arm on.

The mode switching process will be described based on the flowchart shown in FIG. This process is executed by the control unit 65 at a predetermined cycle. For example, in the case where the one-side drive control is performed immediately after the start switch is turned on, the mode is started from the mode M1. In the description of the flowchart, “step” of “step S101” is omitted, and is simply written as “S”. The same applies to other steps.
In S101, the control unit 65 calculates an element temperature that is the temperature of the switching elements 21 to 26, 31 to 36 based on the detection values of the temperature detection elements 231 to 236, 331 to 336. Among the calculated element temperatures, the temperature of the element having the highest temperature is defined as the maximum temperature Tmax.

  In S102, the control unit 65 determines whether or not the maximum temperature Tmax is higher than the allowable temperature Tth. When it is determined that the maximum temperature Tmax is higher than the allowable temperature Tth (S102: YES), the process proceeds to S105. When it is determined that the maximum temperature Tmax is equal to or lower than the allowable temperature Tth (S102: NO), the process proceeds to S103.

  In S103, the control unit 65 determines whether the rotation speed N and the torque trq are in the one-side drive range Ra. When it is determined that the rotation speed N and the torque trq are not in the one-side drive range Ra (S103: NO), the process proceeds to S110. When it is determined that the rotation speed N and the torque trq are in the one-side drive range Ra (S103: YES), the process proceeds to S104.

  In S104, the control unit 65 continues the driving mode under execution. In addition, for example, when the maximum temperature Tmax becomes lower than the mode return determination temperature Tb set to a temperature lower than the allowable temperature Tth, if the drive mode being performed is the mode M3, the mode M2 or the mode M1 or the current drive mode is performed. If the drive mode is mode M2, the mode may be returned to mode M1.

  When it is determined that the maximum temperature Tmax is higher than the allowable temperature Tth (S102: YES), in S105, similarly to S103, the control unit 65 determines whether the rotation speed N and the torque trq are in the one-side drive range Ra. I do. When it is determined that the rotation speed N and the torque trq are not in the one-side drive region Ra (S105: NO), that is, when the rotation speed N and the torque trq are in the both-side drive region Rb, the process proceeds to S111. When it is determined that the rotation speed N and the torque trq are in the one-side drive range Ra (S105: YES), the process proceeds to S106.

  In S106, the control unit 65 determines whether or not the driving mode under execution is the mode M1. When it is determined that the drive mode is the mode M1 (S106: YES), the process proceeds to S107. When it is determined that the drive mode is not the mode M1 (S106: NO), the process proceeds to S108.

  In S107, the control unit 65 changes the drive mode to the mode M2. That is, in the neutral-point-side inverter, the arm that is fixed ON is switched up and down. In consideration of the time required for the maximum temperature Tmax to start decreasing after the drive mode is switched, the drive mode may be continued for at least a predetermined period immediately after the mode switch. The same is true when changing from the mode M2 to the mode M3.

In S108, when it is determined that the drive mode is not the mode M1 (S107: NO), the control unit 65 determines whether the drive mode being executed is the mode M2. When it is determined that the drive mode is the mode M2 (S108: YES), the process proceeds to S109. When it is determined that the drive mode is not the mode M2 (S108: NO), that is, when the drive mode is the mode M3, the process proceeds to S111.
In S109, the control unit 65 changes the drive mode to the mode M3. That is, switching between the neutral point side inverter and the drive side inverter is performed.

If a negative determination is made in S103, or in S110, which proceeds to S104, S107 or S109, the control unit 65 sets the current command value I ^* to a value smaller than the current upper limit value Ia. The current command value I ^* is, for example, the sum of squares of the current command values Iu ^* , Iv ^* , and Iw ^* of each phase. The current upper limit value Ia is an upper limit value at which current can be supplied to the rotating electric machine drive system 1. In other words, in S110, the current limitation at the output limitation value Ib described below is not performed.

When the maximum temperature Tmax is higher than the allowable temperature Tth (S102: YES), and when the rotation speed N and the torque trq are in the both-side drive region Rb (S105: NO), or when the drive mode of the one-side drive control is the mode M3 ( In S111, which proceeds to (S108: NO), the control unit 65 limits the current command value I ^* to the output limit value Ib. Output limit value Ib is set to a value smaller than current upper limit value Ia.

  That is, even if the maximum temperature Tmax is higher than the allowable temperature Tth, if the drive mode in the one-side drive control is the mode M1 or the mode M2, the drive mode is changed, and no current flows to the maximum heat generation point in the immediately preceding mode. By doing so, the maximum temperature Tmax can be reduced without changing the current flowing through the coils 11 to 13. On the other hand, when the drive mode in the one-sided drive control is the mode M3 or the both-sided drive control, the heat generating portions cannot be dispersed, so that the current is limited to prevent the switching elements 21 to 26 and 31 to 36 from overheating. I do.

Switching of the drive mode in the one-side drive control will be described based on the time charts of FIGS. 8 and 9. 8 and 9, (a) shows the switching pattern of each element, and (b) shows the temperature of each element. Specifically, (a-1) shows the switching pattern of the switching element 21, (a-2) shows the switching element 24, (a-3) shows the switching element 31, and (a-4) shows the switching pattern of the switching element 34. (B-1) is the transistor 211 of the switching element 21, (b-2) is the diode 224 of the switching element 24, (b-3) is the diode 321 of the switching element 31, and (b-4) is the switching element 34. Of the transistor 314 of FIG. In the figure, the switching element 21 is described as “U1 above”, the switching element 24 is described as “U1 below”, the switching element 31 is described as “U2 above”, and the switching element 34 is described as “U2 below”. In (b), the temperature of the transistor is described as T_de, and the temperature of the diode is described as T_di. 8 and 9, it is assumed that a diode generates heat more easily than a transistor, that is, Ht <Hd.
The phase currents Iu, Iv, and Iw are assumed to be in the state shown in FIG. That is, it is assumed that the maximum phase current is the U-phase current Iu and the U-phase current Iu is positive.

FIG. 8 is an example of a case where the first inverter 20 is a driving inverter and the second inverter 30 is a neutral point inverter, and the mode M2 is continued. As shown in FIGS. 8A to 8D, the up / down switching frequency Fn of the second inverter 30 that is the neutral point inverter is lower than the switching frequency Fd of the first inverter 20 that is the driving inverter.
In the U phase of the second inverter 30, when the lower arm is fixed to ON, a current flows through the transistor 314, so that the temperature of the transistor 314 increases and the temperature of the diode 321 decreases. When the upper arm is fixed ON, a current flows through the diode 321, so that the temperature of the diode 321 increases and the temperature of the transistor 314 decreases. Here, when Ht <Hd, the period PH during which the upper arm is fixed ON is the period PL during which the lower arm is fixed ON so that the heat generation amount of the diode 321 and the heat generation amount of the transistor 314 are substantially equal. The periods PH and PL are set so as to be shorter. Thereby, the temperature rise of a specific device in the second inverter 30 is suppressed.

  On the other hand, in the first inverter 20, which is the drive-side inverter, the on / off states of the switching elements 21 and 24 are switched complementarily. The switching control of the drive-side inverter may be any control such as PWM control or control using a pulse pattern. In this example, since the U-phase current Iu is positive, in the first inverter 20, when the switching element 21 is on, the U-phase current Iu flows through the transistor 211, and when the switching element 21 is off, U-phase current Iu flows through diode 224. Here, in the case of Ht <Hd, the diode 224 becomes the maximum heat generating part because the diode generates heat more easily than the transistor. In FIG. 8, at time x10, the temperature of the diode 224 becomes the allowable temperature Tth.

  Therefore, in the present embodiment, when the maximum temperature Tmax, which is the temperature of the maximum heat generation point in the mode M2, reaches the allowable temperature Tth, the mode shifts to the mode M3. As shown in FIG. 9, when the mode shifts from the mode M2 to the mode M3 at the time x10, the drive side inverter and the neutral point side inverter are exchanged. In the present embodiment, before time x10, the driving inverter is the first inverter 20 and the neutral point inverter is the second inverter 30 (see FIG. 8). The inverter 30 and the neutral point side inverter are changed to the first inverter 20. At this time, by first fixing the upper arm to the ON position, current does not flow through the diode 224, which is the maximum heat generation location before switching. By preventing current from flowing through the diode 224, the temperature of the diode 224 decreases.

At time x11, the driving inverter is changed to the first inverter 20, the neutral inverter is changed to the second inverter 30, and the driving inverter and the neutral inverter are exchanged. When the second inverter 30 is the drive-side inverter, the maximum heat generation location is the diode 321. Therefore, when switching the second inverter 30 to the neutral-point-side inverter, the lower arm is first fixed to ON, and the current flows through the diode 321. Avoid flowing. By preventing current from flowing through the diode 321, the temperature of the diode 321 decreases.
At time x12, the drive side inverter and the neutral point side inverter are replaced again.

  Accordingly, at least during the period shown in FIG. 9, the element temperature changes at a temperature lower than the allowable temperature Tth, so that the current is not limited by the output limit value Ib, and the state in which the maximum torque can be output can be continued. For example, when the state shown in FIG. 3B continues for a long time and the element temperature exceeds the allowable temperature Tth even in the mode M3, the current is limited by the output limit value Ib.

  In the present embodiment, a first state period Pi1 in which the first inverter 20 is the neutral-point-side inverter and the second inverter 30 is the driving-side inverter, and the first inverter 20 is the driving-side inverter and the second inverter 30 is the neutral-side inverter It is equal to the period Pi2 of the second state in which the point-side inverter is used. The periods Pi1 and Pi2 may have different lengths according to the characteristics of the switching elements 21 to 26 and 31 to 36 and the like.

  In addition, the number of times of switching between the drive side inverter and the neutral point side inverter is smaller than the number of times of switching of the arm of the neutral point side inverter that is fixed to ON in the vertical direction. In addition, the number of times of switching of the arm to be fixed to ON in the neutral point side inverter is smaller than the number of times of switching in the drive side inverter.

  In the mode M3, the drive-side inverter and the neutral-point-side inverter are switched for each of the periods Pi1 and Pi2, and the arm that is fixed ON in the neutral-point-side inverter is switched for each of the periods PH and PL. Thereby, at the time of one-side drive control, without changing the energized state of the coils 11 to 13, it is possible to disperse the heat generation locations and prevent a temperature rise due to a continuous current flowing to a specific device. Therefore, in the one-side drive control, the period during which the maximum torque can be output can be made longer than in the case where, for example, the mode M1 or the mode M2 is continued.

As described above, the power converter 15 of the present embodiment controls the motor generator 10 having the coils 11, 12, and 13 of a plurality of phases, and includes the first inverter 20, the second inverter 30, A control unit 65.
The first inverter 20 is provided between one ends 111, 121, 131 of the coils 11, 12, 13 and the first battery 41.
The second inverter 30 is provided between the other ends 112, 122, 132 of the coils 11, 12, 13 and the second battery 42.

In one of the first inverter 20 and the second inverter 30, the control unit 65 turns on all phases of the upper arm element or all phases of the lower arm element provided on the high potential side to neutralize the other, and sets the other to the neutral point. One-side drive control is performed in accordance with the drive request of motor generator 10.
The control unit 65 is configured such that, in the neutral point side inverter which is an inverter to be neutralized, an upper arm on fixed state in which all phases of the upper arm element are turned on, and a lower arm on fixed state in which all phases of the lower arm element are turned on. At the upper and lower switching frequency Fn.

  The control unit 65 sets the first inverter 20 to a drive-side inverter that controls according to a drive request, sets the second inverter 30 to a neutral-point-side inverter, and sets the first inverter 20 to a neutral-point-side inverter. And the second state in which the second inverter 30 is set as the drive-side inverter, and switching is performed at the inverter switching frequency Fi.

  Further, when switching from the drive-side inverter to the neutral-point-side inverter, the control unit 65 first fixes the arm on the upper and lower sides opposite to the arm including the maximum heat generating portion to ON. Specifically, if the device included in the upper arm element is the maximum heat generating part when controlled as the drive side inverter, the lower arm is first fixed to ON when switching to the neutral point side inverter. Further, if the device included in the lower arm element is the maximum heat generating part when controlled as the drive side inverter, when switching to the neutral point side inverter, the upper arm is first fixed ON.

In this embodiment, the neutral-point-side inverter, the driving-side inverter, and the arm that is turned on and fixed in the neutral-point-side inverter are switched.
For example, a state where a relatively large current is supplied to a specific phase, such as a torque lock state, may be continued. Even in such a case, the bias of the heat generation is suppressed by switching the neutralized point in the one-side drive control and switching the maximum heat generating point. As a result, heat generation in the specific device is suppressed, and the temperature rise of the specific device can be prevented. Further, by preventing the bias of the heat generation, it is possible to delay the temperature from reaching the temperature at which the current limitation is required, so that the period during which the maximum torque can be output at the time of torque lock or the like can be extended.

  In the inverter that switches from the drive-side inverter to the neutral-point-side inverter, the arm on the side opposite to the arm including the maximum heat generation point is first fixed on. In other words, when switching to the neutral-point-side inverter, the arm that is first turned on is selected so that current does not flow to the device that becomes the maximum heat generating point when the drive-side inverter is used. As a result, the temperature of the maximum heat generating portion can be reduced.

  When the maximum temperature Tmax in the first inverter 20 or the second inverter 30 is equal to or lower than the allowable temperature Tth, the control unit 65 sets the mode to the neutral point fixed mode M1. In the neutral point fixing mode, the neutral point side inverter is fixed to the first inverter 20 or the second inverter 30. In the neutral point side inverter, the upper arm is fixed or the lower arm is fixed.

  When the maximum temperature Tmax becomes higher than the allowable temperature Tth in the neutral point fixing mode, the mode shifts from the mode M1 that is the neutral point fixing mode to the mode M2 that is the up / down switching mode. In the mode M2, the control unit 65 switches between the upper arm ON fixed state and the lower arm ON fixed state at the up-down switching frequency Fn while the neutral point side inverter is continued. “Continue the neutral point side inverter” means that if the second inverter 30 is the neutral point side inverter in the mode M1, the state where the second inverter 30 is the neutral point side inverter also in the mode M2. Means to do.

Further, when the maximum temperature Tmax becomes higher than the allowable temperature Tth in the up / down switching mode, the control unit 65 switches the driving-side inverter and the neutral-point-side inverter at the inverter switching frequency Fi, and sets the neutral-point-side inverter to Then, the mode shifts to a mode M3 which is an inverter switching mode for switching the upper arm ON fixed state and the lower arm ON fixed state at the upper and lower switching frequency Fn.
Thereby, the drive mode can be appropriately switched based on the maximum temperature Tmax. By switching the drive mode so that the maximum temperature Tmax decreases, it is possible to prevent the temperature of a specific device from rising and to extend the period during which the maximum torque can be output.

  The upper and lower switching frequency Fn is lower than the switching frequency Fd of the driving inverter. That is, the number of times of up / down switching per unit time is smaller than the number of times of switching in the drive-side inverter. The inverter switching frequency Fi is lower than the upper and lower switching frequency Fn. That is, the number of times of inverter switching per unit time is smaller than the number of times of inverter switching. This makes it possible to appropriately disperse the heat-generating portions.

  The switching elements 21 to 26 and 31 to 36 include transistors 211 to 216 and 311 to 316 and diodes 221 to 226 and 321 to 326, respectively. When the transistors 211 to 216 and 311 to 316 are turned on / off, energization from the high potential side to the low potential side is allowed or cut off. The diodes 221 to 226 and 321 to 326 are connected in parallel to the transistors 211 to 216 and 311 to 316, respectively, and are allowed to conduct from the low potential side to the high potential side.

  In the inverter that switches from the driving-side inverter to the neutral-point-side inverter, the arm that is fixed ON first has the temperature characteristics and absolute values of the transistors 211 to 216 and 311 to 316 and the diodes 221 to 226 and 321 to 326. It is determined based on the direction of conduction of the largest phase current, which is the largest phase current. Here, the “temperature characteristics of the transistor and the transistor” is, for example, the amount of heat generated per unit time when a certain current is applied. Based on the temperature characteristics of the transistor and the diode, and the direction in which the maximum phase current flows, the device that becomes the maximum heat generating part when the drive inverter is used can be specified, so that the arm that is first fixed ON can be appropriately determined. .

  In the neutral point side inverter, the ratio of the period PH in which the upper arm is fixed to the fixed state and the period PL in which the lower arm is fixed is determined by the transistors 211 to 216 and 311 to 316 and the diodes 221 to 226 and 321 to 326. It is determined according to the temperature characteristics. By setting the periods PH and PL so that the temperatures of the upper arm element and the lower arm element become substantially equal, it is possible to reduce the bias of the heat generation in the neutral point side inverter.

  The control unit 65 determines the temperature of the switching elements 21 to 26 and 31 to 36 based on the detection values of the temperature detection elements 231 to 236 and 331 to 336 that detect the temperatures of the switching elements 21 to 26 and 31 to 36. Is calculated. By switching the control according to the element temperature, it is possible to prevent breakage or the like due to an increase in the element temperature.

(Other embodiments)
(A) Mode Switching In the above embodiment, the switching of the driving mode and the start of the current limitation are determined by the same threshold value. In another embodiment, the temperature at which the drive mode is switched may be set to a temperature lower than the temperature at which the current limit is started. Further, a temperature threshold value for switching from the neutral point fixing mode to the up / down switching mode may be different from a temperature threshold value for switching from the up / down switching mode to the inverter switching mode.
In the above embodiment, when the maximum temperature exceeds the allowable temperature, the drive mode is switched. In another embodiment, even when the maximum temperature is lower than the allowable temperature, the inverter switching mode may be set.

  In the above embodiment, in the up / down switching mode, the ratio of the period in which the upper arm is fixed to the ON state to the period in which the lower arm is fixed to the ON state is determined according to the temperature characteristics of the element section and the reflux section. In another embodiment, the ratio between the period in which the upper arm is fixed on and the period in which the lower arm is fixed on may be a predetermined ratio (for example, 1: 1) irrespective of the temperature characteristics of the element section and the reflux section. . Even with such a configuration, heat generation can be dispersed as compared with the case where the upper arm or the lower arm is kept in the ON fixed state.

  Further, in the above embodiment, the up / down switching frequency when the first inverter is set to the neutral point and the up / down switching frequency when the second inverter is set to the neutral point are the same. In another embodiment, for example, when the first inverter and the second inverter use different switching elements, the upper and lower switching frequency when the first inverter is neutralized, and the second inverter is neutralized. The upper and lower switching frequency in this case may be different. Also, the period in which the upper arm is fixed on and the period in which the lower arm is fixed on may be different between the first inverter and the second inverter.

(A) Element temperature In the above embodiment, a temperature detecting element for detecting the element temperature is provided for each switching element. In another embodiment, the temperature detecting element may not necessarily be provided for each switching element, and some of the temperature detecting elements may be omitted. For example, each inverter may be provided with one temperature detecting element. In another embodiment, the temperature detection element may be omitted, the element temperature may be estimated based on the phase current or the like, and control may be performed based on the estimated element temperature. The phase current may be a detected value or a command value.

  Further, in the above embodiment, the maximum heat generation point is specified based on the temperature characteristics of the element section and the reflux section, and the arm to be turned on first in the inverter switched to the neutral point side inverter is determined. In another embodiment, the maximum heat generation point is specified based on the actual element temperature, and in the inverter that switches to the neutral point side inverter, an arm to be fixed to ON is selected so that the current does not flow first to the maximum heat generation point. You may do so.

(C) First voltage source and second voltage source In the above embodiment, a lithium ion battery or the like is illustrated as the first voltage source and the second voltage source. In another embodiment, the first voltage source and the second voltage source may be a lead storage battery other than a lithium ion battery, a fuel cell, or the like. Further, the first voltage source and the second voltage source may have the same type and characteristics, or may have different types and characteristics. Further, one of the first voltage source and the second voltage source may be a capacitor such as an electric double layer capacitor or a lithium ion capacitor. Further, one of the first voltage source and the second voltage source may be a generator or the like driven by a drive source such as an engine to generate power.

(D) Rotating electric machine In the above embodiment, the rotating electric machine is a motor generator. In another embodiment, the rotating electric machine may be a motor having no generator function, or may be a generator having no motor function. Further, the rotating electric machine of the above embodiment has three phases. In another embodiment, the rotating electric machine may have four or more phases.
Further, in the above embodiment, the rotating electric machine is the main motor of the electric vehicle. In another embodiment, the rotating electric machine is not limited to the main motor, and may be a so-called ISG (Integrated Starter Generator) having both a starter function and an alternator function, or an auxiliary motor. Further, the power conversion device may be applied to a device other than the vehicle.
As described above, the present invention is not limited to the above embodiment, and can be implemented in various forms without departing from the spirit of the invention.

1 ... rotating electric machine drive system 10 ... motor generator (rotating electric machine)
11-13 ... Coil (winding)
15 Power converter 20 First inverter 30 Second inverter 41 First battery (first voltage source)
42... Second battery (second voltage source)
65 Control unit

Claims (8)

A power converter for controlling a rotating electric machine (10) having coils of multiple phases (11, 12, 13),
A first inverter (20) provided between one end (111, 121, 131) of the coil and a first voltage source (41);
A second inverter (30) provided between the other end (112, 122, 132) of the coil and a second voltage source (42);
In one of the first inverter and the second inverter, all phases of the upper arm elements (21 to 23, 31 to 33) or the lower arm elements (24 to 26, 34 to 36) provided on the high potential side are turned on. A control unit (65) that performs one-sided drive control that sets the neutral point and controls the other side in accordance with the drive request of the rotating electric machine;
With
The control unit includes:
In the neutral point side inverter which is an inverter to be neutralized, the upper arm ON fixed state in which all phases of the upper arm element are turned ON and the lower arm ON fixed state in which all phases of the lower arm element are turned ON are up and down. Switching at the switching frequency,
A first state in which the first inverter is a drive-side inverter that is controlled according to the drive request, a second state in which the second inverter is the neutral point-side inverter, and the first inverter is the neutral point-side inverter, A second state in which a second inverter is used as the driving-side inverter is switched at an inverter switching frequency;
A power converter for first turning on an arm on the side opposite to the arm including the maximum heat generation point when switching from the driving inverter to the neutral point inverter. The control unit includes:
When the maximum temperature of the first inverter and the second inverter is equal to or lower than an allowable temperature, the neutral point side inverter is fixed to the first inverter or the second inverter, and the upper arm is fixed or the lower arm is fixed. And the neutral point fixed mode
When the maximum temperature becomes higher than the allowable temperature in the neutral point fixed mode, the upper arm ON fixed state, the lower arm ON fixed state, and the upper and lower switching frequency in the state where the neutral point side inverter is continued. Switch to the up / down switching mode
When the maximum temperature is higher than the allowable temperature in the up / down switching mode, the drive inverter and the neutral point side inverter are switched at the inverter switching frequency, and the neutral point side inverter The power converter according to claim 1, wherein the power converter shifts to an inverter switching mode for switching between an arm-on fixed state and the lower arm-on fixed state at the up / down switching frequency.   The power converter according to claim 1, wherein the up / down switching frequency is lower than a switching frequency of the drive-side inverter.   The power converter according to any one of claims 1 to 3, wherein the inverter switching frequency is lower than the up / down switching frequency.   The upper arm element and the lower arm element are respectively arranged in an element section (211-216, 311-316) in which energization from a high potential side to a low potential side is allowed or cut off by turning on and off, and in parallel with the element section. The power converter according to any one of claims 1 to 4, further comprising a reflux section (221 to 226, 321 to 326) connected to the power supply and allowing current to flow from the low potential side to the high potential side.   In the inverter that switches from the driving-side inverter to the neutral-point-side inverter, the arm that is first fixed ON is based on the temperature characteristics of the element section and the reflux section, and the direction of flow of the phase current having the largest absolute value. The power converter according to claim 5, which is determined.   6. The neutral point-side inverter, wherein a ratio between a period in which the upper arm is fixed and a period in which the lower arm is fixed is determined according to temperature characteristics of the element portion and the reflux portion. Or the power converter according to 6.   The control unit calculates an element temperature based on a detection value of a temperature detection element (231 to 236, 331 to 336) that detects a temperature of at least a part of the upper arm element and the lower arm element, or a phase current. The power converter according to any one of claims 1 to 7.

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