Power conversion device

Abstract

A power conversion device comprising: an inverter circuit configured by connecting switching elements of an upper arm and a lower arm in series; an output current detector outputting a detection value of an alternating current output from a connection point between the switching elements of the upper arm and the lower arm; a driver circuit driving each of the switching elements by means of a driver signal via a switched gate resistor; an overcurrent detection circuit comparing the detection value with any one of a plurality of predetermined overcurrent thresholds and outputting an overcurrent detection signal in a case where the detection value is greater than the overcurrent threshold;and a controller that transmits the driver signal, transmits a gate resistor switching signal to the driver circuit for switching the gate resistor, and transmits an overcurrent threshold switching signal for switching the overcurrent threshold, wherein the controller sets the overcurrent threshold to a small value by means of the overcurrent threshold switching signal in a case where the controller switches to the gate resistor having a lower resistance value by means of the gate resistor switching signal.

Description

Power Conversion Device

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

[0002] A power conversion device that converts DC power to AC power has multiple phases of arms each configured by connecting an upper arm switching element and a lower arm switching element in series. The switching elements are driven by a drive signal input via a gate resistor connected to the gate of the switching element.

[0003] Generally, switching elements generate switching losses when energized, but by switching the gate resistor according to the operating conditions of the switching element, such as the applied voltage and current, the switching speed of the switching element can be increased and the loss in the switching element reduced.On the other hand, when the switching element is turned off, a surge voltage is generated in proportion to the rate at which the collector current decreases.

[0004] Patent Document 1 describes a power conversion device that varies the gate resistance under conditions where surge voltages become high.

[0005] Japanese Patent Application Publication No. 2016-059089

[0006] In the technology described in Patent Document 1, if the resistance value of the gate resistor is set low and the motor current suddenly increases due to some abnormality, there is a risk that the switching element will break down due to a surge voltage, reducing the reliability of the power conversion device.

[0007] a power conversion device according to the present invention comprising: an inverter circuit having arms for multiple phases, each of which has an upper arm switching element and a lower arm switching element connected in series between a DC positive terminal and a DC negative terminal; an output current detector that detects an AC current output from a connection point between the upper arm switching element and the lower arm switching element and outputs a detected value; a driver circuit having a gate resistor switching circuit that switches a gate resistor connected to the gate of each of the switching elements and drives each of the switching elements by a drive signal input via the switched gate resistor; an overcurrent detection circuit that compares the detected value detected by the output current detector with any of a plurality of predetermined overcurrent thresholds and outputs an overcurrent detection signal if the detected value becomes larger than the overcurrent threshold; and a controller that transmits the drive signal to the driver circuit, as well as transmits a gate resistor switching signal to the driver circuit that switches the gate resistor, and transmits an overcurrent threshold switching signal to the overcurrent detection circuit to switch the overcurrent threshold; and when the gate resistor with a lower resistance value is switched in response to the gate resistor switching signal, the controller sets the overcurrent threshold to a smaller value by the overcurrent threshold switching signal.

[0008] According to the present invention, it is possible to improve the reliability of the power conversion device even when the resistance value of the gate resistor is reduced.

[0009] Fig. 1 is an overall configuration diagram of a power conversion device according to a first embodiment; Fig. 2 is a detailed configuration diagram of a main part of the power conversion device according to the first embodiment; (a) (B) are graphs showing the relationship between the collector-emitter voltage of a switching element and switching of gate resistance, and the relationship between an overcurrent threshold and AC current; Fig. 3 is a detailed configuration diagram of a main part of a power conversion device according to a second embodiment; Fig. 4 is a detailed configuration diagram of a main part of a power conversion device according to a third embodiment;

[0010] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The following description and drawings are examples for explaining the present invention, and some omissions and simplifications have been made as appropriate for clarity of explanation. The present invention can be implemented in various other forms. Unless otherwise specified, each component may be singular or plural.

[0011] 1 is an overall configuration diagram of a power conversion device 1000 according to a first embodiment. The power conversion device 1000 converts DC power supplied from a DC power supply 2000, such as a battery, into AC power to drive a motor 3000. The DC power supply 2000 supplies DC power between a positive terminal P and a negative terminal N of the power conversion device 1000 via a contactor 2001. The motor 3000 is, for example, a three-phase induction motor, and is used as a drive source for a vehicle.

[0012] The power conversion device 1000 includes a voltage detector 100 connected in parallel between a positive terminal P and a negative terminal N to detect a DC voltage between the positive terminal P and the negative terminal N, a capacitor module 200 to smooth the DC current, and an inverter circuit 300 to perform power conversion. The power conversion device 1000 further includes an output current detector 400, an overcurrent detection circuit 500, a controller 600, a shutoff circuit 700, and a driver circuit 800.

[0013] The inverter circuit 300 includes a power module 310 having an upper and lower arm series circuit including a switching element 311 and a diode 312 that operate as an upper arm, and a switching element 321 and a diode 322 that operate as a lower arm. Three phases (U phase, V phase, and W phase) of this power module 310 are provided corresponding to the respective phase windings of the motor 3000. That is, the inverter circuit 300 includes three phases of arms each formed by connecting an upper arm switching element 311 and a lower arm switching element 321 in series between a DC positive terminal P and a DC negative terminal N. Here, an example of a three-phase configuration will be described, but a configuration may also be provided in which multiple phases of power modules 310 are provided to match the number of phases of the motor 3000.

[0014] The output current detector 400 detects, for each phase, the AC current output from the connection point between the upper arm switching element and the lower arm switching element, and outputs each detected value Ei to the overcurrent detection circuit 500 and the controller 600. The detected value Ei represents the magnitude of the detected AC current as a voltage value.

[0015] An overcurrent detection circuit 500 is provided for each phase, and when the detected value of the AC current becomes larger than the overcurrent threshold, outputs an overcurrent detection signal Em to the controller 600 and the interrupter circuit 700. Details of the overcurrent detection circuit 500 will be described later.

[0016] The controller 600 is connected to the low-voltage power supply 4000 from outside the power conversion device 1000, and receives a low voltage from the low-voltage power supply 4000 when the vehicle ignition key is turned on. The controller 600 outputs a drive signal Pw to the driver circuit 800 via the interrupter circuit 700 in response to a torque command from a higher-level controller (not shown), a detected voltage Vdc from the voltage detector 100, a detected value Ei from the output current detector 400, and an overcurrent detection signal Em from the overcurrent detection circuit 500. The controller 600 also outputs an overcurrent threshold switching signal Ts to the overcurrent detection circuit 500. Furthermore, the controller 600 transmits a gate resistance switching signal Rs to the driver circuit 800, which switches the gate resistance in response to the detected value from the output current detector 400. Details of the controller 600 will be described later.

[0017] The shutoff circuit 700 is provided corresponding to the driver circuit 800, and in response to the overcurrent detection signal Em output from the overcurrent detection circuit 500, shuts off the drive signal Pw from the controller 600 to the switching elements 311, 321. Details of the shutoff circuit 700 will be described later. The driver circuit 800 is provided corresponding to each of the switching elements 311, 321, and has a gate resistance switching circuit that switches the gate resistance connected to the gate of each of the switching elements 311, 321, and drives the switching elements 311, 321 with the drive signal Pw input via the switched gate resistance. Details of the driver circuit 800 will be described later.

[0018] 2 is a detailed configuration diagram of the main parts of the power conversion device 1000 according to the first embodiment. This diagram shows an overcurrent detection circuit 500 corresponding to one power module 310 for one phase, and a shutdown circuit 700 and a driver circuit 800 corresponding to the lower arm switching element 321 of the power module 310 for one phase. That is, although not shown, the shutdown circuit 700 and the driver circuit 800 corresponding to the upper arm switching element 311 of the power module 310 for one phase have the same configuration as the shutdown circuit 700 and the driver circuit 800 corresponding to the lower arm switching element 321. Furthermore, the overcurrent detection circuits 500 corresponding to the power modules 310 of the other phases, and the shutdown circuits 700 and the driver circuits 800 corresponding to the switching elements 311, 321 of the power modules 310 of the other phases also have the same configuration as in FIG. 2.

[0019] The detected value Ei from the output current detector 400 is input to the overcurrent detection circuit 500 and the controller 600. The detected voltage Vdc from the voltage detector 100 is input to the controller 600.

[0020] The controller 600 transmits a drive signal Pw to the driver circuit 800 via the cutoff circuit 700. The controller 600 also transmits a gate resistance switching signal Rs for switching the gate resistance to the driver circuit 800. The drive signal Pw is a PWM signal. The cutoff circuit 700 immediately cuts off the drive signal Pw when an overcurrent detection signal Em, which will be described in more detail later from the overcurrent detection circuit 500, is input. This causes the driver circuit 800 to stop driving the switching element 321.

[0021] The driver circuit 800 includes a drive circuit 810 and a gate resistance switching circuit 820. The gate resistance switching circuit 820 switches between gate resistances Rf1 and Rf2 connected to the gate of the switching element 321 in response to a gate resistance switching signal Rs. The gate resistance Rn0 is a resistance that is connected when the switching element 321 is turned on. The gate resistances Rf1 and Rf2 are resistances that are connected when the switching element 321 is turned off, and the resistance value of the gate resistance Rf1 is lower than the resistance value of the gate resistance Rf2. In response to the input of the gate resistance switching signal Rs, the resistance when the switching element 321 is off is switched to the gate resistance Rf1.

[0022] Generally, switching loss occurs in the switching element 321, which reduces the efficiency of the inverter circuit 300 and, ultimately, the vehicle's cruising range. Therefore, depending on the operating conditions of the switching element 321, such as voltage and current, when the switching element 321 is turned off, the switching speed is increased by switching to a gate resistor Rf1 with a low resistance value, thereby reducing the switching loss of the switching element 321. Specifically, when the DC voltage, i.e., the detection voltage Vdc, detected by the voltage detector 100 is low and the output current detector 400 detects that the current flowing through the motor 3000 is low, the allowable surge voltage becomes high. In this case, the gate resistor Rf1 is switched to a resistor with a low resistance value to increase the switching speed and reduce the switching loss.

[0023] However, if the current flowing through the motor 3000 suddenly increases due to some abnormality while the resistance value of the gate resistor is low, a surge voltage may cause the switching element 321 to malfunction. Therefore, when the controller 600 switches to the gate resistor Rf1 with a low resistance value, the controller 600 transmits an overcurrent threshold switching signal Ts to the overcurrent detection circuit 500 (described below) to set the overcurrent threshold to a low value. That is, the controller 600 outputs the overcurrent threshold switching signal Ts at approximately the same time as switching to the gate resistor Rf1 by the gate resistor switching signal Rs, thereby setting the overcurrent threshold of the overcurrent detection circuit 500 to a low value. This prevents malfunction of the switching element 321 and improves the reliability of the power conversion device 1000 even when the resistance value of the gate resistor is low.

[0024] As shown in FIG. 2 , the overcurrent detection circuit 500 includes a threshold switching unit 510 and a comparison unit 520. The threshold switching unit 510 switches the threshold between overcurrent thresholds VH1 and VL1 or overcurrent thresholds VH2 and VL2 using a switch SW. Specifically, the switch SW operates in response to an overcurrent threshold switching signal Ts from the controller 600, and the overcurrent thresholds VH1 and VL1 are selected. When the overcurrent threshold switching signal Ts is removed, the overcurrent thresholds VH2 and VL2 are selected. The thresholds are expressed in voltage, with the relationship being overcurrent threshold VH1<overcurrent threshold VH2 and overcurrent threshold VL1<overcurrent threshold VL2. The overcurrent thresholds VH1, VL1, VH2, and VL2 are supplied from multiple power sources with different voltage values.

[0025] The comparator 520 includes a comparator CH that compares the detection value Ei from the output current detector 400 with either the overcurrent threshold VH1 or the overcurrent threshold VH2, and a comparator CL that compares the detection value Ei with either the overcurrent threshold VL1 or the overcurrent threshold VL2, and outputs an overcurrent detection signal Em according to the comparison result. When the overcurrent threshold switching signal Ts is not present and the overcurrent thresholds VH2 and VL2 are selected, the comparator 520 does not output the overcurrent detection signal Em if the detection value Ei is within the range of the overcurrent thresholds VH2 and VL2. When the detection value Ei exceeds the range of the overcurrent thresholds VH2 and VL2, the comparator 520 outputs the overcurrent detection signal Em. When the overcurrent thresholds VH1 and VL1 are selected by the overcurrent threshold switching signal Ts, the comparator 520 does not output the overcurrent detection signal Em if the detection value Ei is within the range of the overcurrent thresholds VH1 and VL1. When the detection value Ei exceeds the range of the overcurrent thresholds VH1 and VL1, the comparator 520 outputs the overcurrent detection signal Em. The overcurrent detection signal Em is input to the controller 600 and the interrupter circuit 700 .

[0026] The overcurrent thresholds VH2 and VL2 are set to values ​​indicating a current value at which, when the switching element 321 is driven at the resistance value of the gate resistor Rf2, the voltage applied to the switching element 321 does not exceed the tolerance of the switching element 321. Moreover, the overcurrent thresholds VH1 and VL1 are set to values ​​indicating a current value at which, when the switching element 321 is driven at the resistance value of the gate resistor Rf1, the voltage applied to the switching element 321 does not exceed the tolerance of the switching element 321.

[0027] When the overcurrent detection signal Em is input, the controller 600 performs control to prevent an overcurrent in the motor 3000, such as by stopping the output of the drive signal Pw. Because the overcurrent detection signal Em is also input to the cutoff circuit 700, the drive signal Pw can be cut off immediately without waiting for the controller 600 to stop outputting the drive signal Pw.

[0028] 3A is a graph showing the relationship between the voltage Vce applied between the collector and emitter of the switching element 321 and the current value I detected by the output current detector 400. The horizontal axis represents the current value I, and the vertical axis represents the voltage Vce. FIG. 3B is a graph showing the relationship between the overcurrent threshold and the detected AC current value Ei. The horizontal axis represents time, and the vertical axis represents voltage.

[0029] Normally, when the switching element 321 is turned off, the voltage Vce applied between the collector and emitter of the switching element 321 increases as the current flowing through the switching element 321 increases. The magnitude of this voltage increases more rapidly when the switching speed is fast. Furthermore, the switching speed at turn-off is inversely proportional to the resistance value of the gate resistor, and tends to be slower when the resistance value is high and faster when the resistance value is low. Therefore, the surge voltage decreases when the resistance value is high, and increases when the resistance value is low.

[0030] On the other hand, when the switching speed is fast, the switching loss of the switching element 321 can be reduced, so that the resistance value of the gate resistor can be lowered in the low current region, thereby reducing the loss while suppressing the surge voltage below the allowable value.

[0031] 3A, the current value for switching the gate resistor Rf1 with the low resistance is I0, the overcurrent detection level for the gate resistor Rf1 with the low resistance is I1, and the overcurrent detection level for the gate resistor Rf2 with the high resistance is I2. Vm is the allowable withstand voltage of the switching element 321.

[0032] The graph shown in FIG. 3( a) shows an example in which the controller 600 switches to the gate resistor Rf2 when the current value I is greater than the current value I0, and switches to the gate resistor Rf1 when the current value I is equal to or less than the current value I0, based on the detection value Ei from the output current detector 400.

[0033] Now consider the case where the gate resistance remains at Rf2 without switching the gate resistance regardless of the current value I. In this case, as shown by the dotted line Vf2 in Fig. 3(a), the voltage Vce applied to the switching element 321 gradually increases as the current increases from 0 (A) to I0 (A), I1 (A), and I2 (A).

[0034] However, if the motor 3000 or the like experiences some kind of abnormality, causing the current to increase suddenly and exceed the maximum current, the voltage Vce applied to the switching element 321 may exceed the allowable withstand voltage Vm, potentially causing the switching element 321 to fail. To prepare for this situation, overcurrent thresholds VH2 and VL2 are set. The overcurrent thresholds VH2 and VL2 correspond to the overcurrent detection level I2. When the current reaches the overcurrent detection level I2, the controller 600 prevents an increase in the current flowing through the motor 3000 by, for example, stopping the transmission of the PWM signal, thereby controlling the voltage Vce applied to the switching element 321 so that it does not exceed the allowable withstand voltage Vm.

[0035] On the other hand, when switching between gate resistors Rf1 and Rf2, the gate resistor Rf1 with a low resistance is used in the low-current region where the current is below I0, thereby increasing the switching speed of the switching element 321. If an overcurrent state occurs suddenly when the gate resistance Rf1 is low, the surge voltage increases more rapidly than when the resistance is high. Therefore, if the overcurrent detection level remains at I2, the voltage Vce will exceed the allowable breakdown voltage Vm when the current is greater than I1, as shown by the dotted line Vf1 in Figure 3(a).

[0036] In this embodiment, when the gate resistance is set to a low value, the overcurrent detection level is set to overcurrent detection level I1, which is lower than I2. The overcurrent detection level I1 corresponds to the overcurrent thresholds VH1 and VL1. This is a value that prevents the voltage applied to the switching element 321 from exceeding the tolerance of the switching element 321 when the switching element 321 is driven by the gate resistance Rf1. As shown by the thick solid line Vf in FIG. 3A, the overcurrent detection level is set to overcurrent detection level I1 when the gate resistance Rf1 is switched to a low value, and to overcurrent detection level I2 when the gate resistance Rf2 is switched to a high value.

[0037] 3(b), when the gate resistance Rf1 is switched to a low value and the detected value Ei from the output current detector 400 exceeds the range of the overcurrent thresholds VH1 and VL1, the overcurrent detection circuit 500 detects this and outputs an overcurrent detection signal Em. This makes it possible to prevent failure of the switching element 321 and improve the reliability of the power conversion device 1000, even when the switching speed of the switching element 321 is increased by switching the resistance value of the gate resistor.

[0038] In this example, the gate resistance is switched in two stages, but it may be switched in multiple stages, such as three or more stages. In this case, the gate resistance switching circuit 820 switches between multiple gate resistances, and the overcurrent detection circuit 500 switches between multiple overcurrent thresholds in accordance with the switched gate resistance.

[0039] [Second Embodiment] Fig. 4 is a detailed configuration diagram of the main parts of a power conversion device 1000 according to a second embodiment. As with Fig. 2, this diagram shows an overcurrent detection circuit 500 corresponding to one phase of power modules 310, and a shutdown circuit 700 and a driver circuit 800 corresponding to the lower arm switching elements 321 of one phase of power modules 310. The same components as those in Fig. 2 are assigned the same reference numerals and will be described briefly. The overall configuration diagram of the power conversion device 1000 is the same as Fig. 1. In the second embodiment, the configuration of the overcurrent detection circuit 500 is different from that of the first embodiment.

[0040] 4, the overcurrent detection circuit 500 includes a threshold value switching unit 510 and a comparison unit 520. Similar to FIG. 2, the comparison unit 520 includes a comparator CH that compares the detection value Ei from the output current detector 400 with the overcurrent threshold value VH1 or the overcurrent threshold value VH2, and a comparator CL that compares the detection value Ei with the overcurrent threshold value VL1 or the overcurrent threshold value VL2, and outputs an overcurrent detection signal Em in accordance with the comparison result.

[0041] The threshold switching unit 510 includes resistors R1, R2, and R3 connected in series to a reference power supply, a resistor R4 connected in parallel to resistor R1 via FET 511, and a resistor R5 connected in parallel to resistor R3 via FET 512. The multiple overcurrent thresholds are supplied by dividing the voltage from a single power supply. When the overcurrent threshold switching signal Ts from the controller 600 is at a low level (not output), the FET 511 connected via NOT gate 513 and the FET 512 connected without the NOT gate 513 are off. In this case, the overcurrent threshold VH2 divided by the resistors R1, R2, and R3 is input to the comparator CH, and the overcurrent threshold VL2 is input to the comparator CL. When the overcurrent threshold switching signal Ts from the controller 600 is at a high level (output), the FETs 511 and 512 are on, turning on resistor R4 and resistor R5, respectively, in parallel to resistor R1 and resistor R3, thereby reducing their resistance values. Therefore, the overcurrent threshold VH1 is input to the comparator CH, and the overcurrent threshold VL1 is input to the comparator CL.

[0042] When the gate resistor switching signal Rs switches to a gate resistor with a lower resistance, the controller 600 sets the overcurrent threshold to a lower value using the overcurrent threshold switching signal Ts. Then, when the overcurrent detection signal Em is input, the controller 600 performs control to prevent an overcurrent in the motor 3000, such as by stopping the output of the drive signal Pw. According to this embodiment, even when the switching speed of the switching element 321 is increased by switching the resistance value of the gate resistor, it is possible to prevent failure of the switching element 321 and improve the reliability of the power conversion device 1000, and also to simplify the configuration of the overcurrent detection circuit 500.

[0043] [Third Embodiment] Figure 5 is a detailed configuration diagram of the main parts of a power conversion device 1000 according to a third embodiment. This diagram shows a shutdown circuit 700 and a driver circuit 800 corresponding to the lower arm switching elements 321 of one phase of a power module 310. An overcurrent detection circuit 500 is provided within the driver circuit 800. The same components as those in Figure 2 are assigned the same reference numerals and will be described briefly. The overall configuration diagram of the power conversion device 1000 is the same as that in Figure 1. In the third embodiment, the configurations of the output current detection circuit 410 and the overcurrent detection circuit 500 are different from those of the first embodiment.

[0044] 5, the switching element 321 has a current sense terminal that divides the current flowing through the switching element 321. The output current detection circuit 410 converts the divided current into a voltage value using a sense resistor Rj and outputs this as a detection value Ej to the overcurrent detection circuit 500.

[0045] The overcurrent detection circuit 500 includes a threshold value switching unit 510 and a comparison unit 520. When there is no overcurrent threshold value switching signal Ts from the controller 600, the threshold value switching unit 510 selects the overcurrent threshold value V2 by a switch SW, and when there is an overcurrent threshold value switching signal Ts, the threshold value switching unit 510 selects the overcurrent threshold value V1. The relationship is overcurrent threshold value V1<overcurrent threshold value V2. The comparison unit 520 compares the detection value Ej from the output current detection circuit 410 with the overcurrent threshold value V1 or the overcurrent threshold value V2, and outputs an overcurrent detection signal Em according to the comparison result.

[0046] When the gate resistor switching signal Rs switches to a gate resistor with a lower resistance, the controller 600 sets the overcurrent threshold to a lower value using the overcurrent threshold switching signal Ts. Then, when the overcurrent detection signal Em is input, the controller 600 performs control to prevent an overcurrent in the motor 3000, such as by stopping the output of the drive signal Pw. According to this embodiment, even when the switching speed of the switching element 321 is increased by switching the resistance value of the gate resistor, it is possible to prevent failure of the switching element 321 and improve the reliability of the power conversion device 1000, and also to simplify the configurations of the output current detection circuit 410 and the overcurrent detection circuit 500.

[0047] According to the embodiment described above, the following advantageous effects can be obtained. (1) The power conversion device 1000 includes an inverter circuit 300 having arms for a plurality of phases, each of which is formed by connecting an upper arm switching element 311 and a lower arm switching element 321 in series between a positive terminal and a negative terminal of a DC line, an output current detector 400 that detects an AC current output from a connection point between the upper arm switching element 311 and the lower arm switching element 321 and outputs a detected value Ei, a gate resistance switching circuit 820 that switches gate resistances Rf1 and Rf2 connected to the gates of the switching elements 311 and 321 and drives the switching elements 311 and 321 with a drive signal Pw input via the switched gate resistances Rf1 and Rf2, and a driver circuit 800 that converts the detected value Ei detected by the output current detector 400 into a predetermined and a controller 600 that transmits a drive signal Pw to a driver circuit 800, transmits a gate resistance switching signal Rs to the driver circuit 800 for switching between gate resistors Rf1 and Rf2, and transmits an overcurrent threshold switching signal Ts to the overcurrent detection circuit 500 for switching between the overcurrent thresholds VH1, VL1, VH2, and VL2. When the gate resistance switching signal Rs switches to a gate resistor Rf1 with a lower resistance, the controller 600 sets the overcurrent thresholds VH1, VL1, VH2, and VL2 to lower values ​​using the overcurrent threshold switching signal Ts. This makes it possible to improve the reliability of the power conversion device even when the resistance values ​​of the gate resistors are reduced.

[0048] The present invention is not limited to the above-described embodiments, and other embodiments that are conceivable within the scope of the technical concept of the present invention are also included within the scope of the present invention as long as they do not impair the characteristics of the present invention. Furthermore, configurations that combine the above-described embodiments may also be used.

[0049] 100: Voltage detector, 200: Capacitor module, 300: Inverter circuit, 310: Power module, 311, 321: Switching elements, 312, 322: Diodes, 400: Output current detector, 410: Output current detection circuit, 500: Overcurrent detection circuit, 510: Threshold value switching unit, 520: Comparison unit, 600: Controller, 700: Shutdown circuit, 800: Driver circuit, 810: Drive circuit , 820...Gate resistance switching circuit, 1000...Power conversion device, 2000...DC power supply, 2001...Contactor, 3000...Motor, 4000...Low voltage power supply, Rn0, Rf1, Rf2...Gate resistance, Rs...Gate resistance switching signal, Pw...Drive signal, Vdc...Detection voltage, Ei, Ej...Detection value, VH1, VL1, VH2, VL2, V1, V2...Overcurrent threshold, Ts...Overcurrent threshold switching signal, Em...Overcurrent detection signal.

Claims

1. A power conversion device comprising: an inverter circuit having arms for multiple phases, each formed by connecting upper arm switching elements and lower arm switching elements in series between a positive terminal and a negative terminal of a DC; an output current detector that detects an AC current output from a connection point between the upper arm switching elements and the lower arm switching elements and outputs a detected value; a driver circuit having a gate resistor switching circuit that switches a gate resistor connected to the gate of each of the switching elements, and that drives each of the switching elements by a drive signal input via the switched gate resistor; an overcurrent detection circuit that compares the detected value detected by the output current detector with one of a plurality of predetermined overcurrent thresholds, and outputs an overcurrent detection signal if the detected value becomes larger than the overcurrent threshold; and a controller that sends the drive signal to the driver circuit, and also sends a gate resistor switching signal to the driver circuit that switches the gate resistor, and sends an overcurrent threshold switching signal to the overcurrent detection circuit to switch the overcurrent threshold, wherein the controller sets the overcurrent threshold to a lower value by the overcurrent threshold switching signal when the gate resistor with a lower resistance value is switched by the gate resistor switching signal.

2. A power conversion device according to claim 1, wherein the overcurrent threshold is set to a value indicating a current value at which the voltage applied to the switching element does not exceed the allowable value of the switching element when the switching element is driven by the switched gate resistor.

3. A power conversion device according to claim 1 or 2, wherein the overcurrent detection circuit comprises a threshold switching unit that switches among the plurality of overcurrent thresholds in response to the overcurrent threshold switching signal, and a comparison unit that compares the detection value with the overcurrent threshold.

4. A power conversion device according to claim 3, wherein the plurality of overcurrent thresholds are supplied from a plurality of power sources having different voltage values.

5. A power conversion device according to claim 3, wherein the plurality of overcurrent thresholds are supplied by dividing a voltage from a single power source.

6. A power conversion device according to claim 1 or claim 2, further comprising a cutoff circuit that cuts off the drive signal transmitted from the controller to the driver circuit, the cutoff circuit cutting off the drive signal in response to the overcurrent detection signal output from the overcurrent detection circuit.

7. A power conversion device comprising: an inverter circuit having arms for a plurality of phases, each of which has an upper arm switching element and a lower arm switching element connected in series between a positive terminal and a negative terminal of a DC line; an output current detection circuit that detects a current flowing in each of the switching elements and outputs a detection value; a driver circuit having a gate resistance switching circuit that switches a gate resistance connected to the gate of each of the switching elements and drives each of the switching elements with a drive signal using the switched gate resistance; an overcurrent detection circuit that compares the detection value detected by the output current detection circuit with one of a plurality of predetermined overcurrent thresholds and outputs an overcurrent detection signal when the detection value becomes larger than the overcurrent threshold; and a controller that sends the drive signal to the driver circuit, and also sends a gate resistance switching signal to the gate resistance switching circuit that switches the gate resistance, and sends an overcurrent threshold switching signal to the overcurrent detection circuit, wherein the controller sets the overcurrent threshold to a smaller value when the gate resistance switching signal has switched to a gate resistance with a lower resistance.

8. A power conversion device according to claim 7, wherein the output current detection circuit detects a current from a current sense terminal that branches off the current flowing through the switching element, and outputs the detected value.

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