Power conversion device and diagnosis method for power conversion device

Abstract

Provided is a power conversion device having a semiconductor switching element and capable of detecting a leakage current of the semiconductor switching element with a relatively simple configuration. The power conversion device is characterized by comprising: an upper arm including a first semiconductor switching element; a lower arm including a second semiconductor switching element connected in series to the first semiconductor switching element; a first control unit that controls the first semiconductor switching element and the second semiconductor switching element; a first current detector that is connected in parallel with the first semiconductor switching element and detects a leakage current generated in the lower arm; a second current detector that is connected in parallel with the second semiconductor switching element and detects a leakage current generated in the upper arm, wherein the first current detector has a first current detection element and a third semiconductor switching element connected in series to the first current detection element, and the second current detector has a second current detection element and a fourth semiconductor switching element connected in series to the second current detection element; and a second control unit that controls the third semiconductor switching element and the fourth semiconductor switching element.

Description

Power conversion device and method for diagnosing power conversion device

[0001] The present invention relates to a configuration of a power conversion device and a method for diagnosing the same, and more particularly to a technique that is effective when applied to diagnosing semiconductor switching elements that constitute a power conversion device.

[0002] Power conversion devices used to control electric motors for railways and large industrial equipment, and power conversion devices used for large-capacity frequency conversion in power systems, etc., use large-capacity power semiconductor elements to perform high-voltage, large-current power control.

[0003] If a failure occurs in such equipment during operation, it can cause damage to the system or an unplanned system shutdown, resulting in significant economic losses. To prevent such situations, it is necessary to detect signs of deterioration or abnormalities in the power conversion equipment, prevent damage due to malfunction, notify relevant parties of the need to update the equipment, and control the lifespan of the power conversion equipment.

[0004] One of the causes of failure in a power converter is known to be breakdown of the semiconductor switching elements that make up the power converter, which is known to be breakdown due to a breakdown in voltage caused by an increase in leakage current during voltage blocking.

[0005] In order to prevent breakdown of a semiconductor switching element due to a breakdown voltage defect, a technique for detecting a breakdown voltage defect of a semiconductor switching element is known. For example, Patent Document 1 discloses a technique for detecting breakdown voltage deterioration from information on a voltage applied to a semiconductor switching element.

[0006] International Publication No. 2022 / 153520

[0007] Incidentally, in order to detect deterioration or abnormality of semiconductor switching elements in advance, it is effective to detect the main withstand voltage leakage current of the power conversion device, that is, the leakage current of the switching elements.

[0008] In the method described in Patent Document 1, the operation of the switching elements provided on the high side and the low side is stopped, and when the temperature of both switching elements is higher than the ambient temperature, the control circuit determines the deterioration of the withstand voltage of both switching elements from the voltage value detected by the voltage detector.

[0009] However, Patent Document 1 does not mention at all a method for quantitatively measuring the leakage current of a switching element, and there is room for improvement in terms of detecting signs of deterioration or abnormalities in a power conversion device.

[0010] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a power converter having a semiconductor switching element, which is capable of detecting leakage current of the semiconductor switching element with a relatively simple configuration, and a method for diagnosing the same.

[0011] In order to solve the above problem, the present invention provides a power supply comprising: an upper arm including a first semiconductor switching element; a lower arm including a second semiconductor switching element connected in series to the first semiconductor switching element; a first control unit that controls the first semiconductor switching element and the second semiconductor switching element; a first current detector connected in parallel to the first semiconductor switching element and detecting a leakage current generated in the lower arm; and a second current detector connected in parallel to the second semiconductor switching element and detecting a leakage current generated in the upper arm, wherein the first current detector has a first current detection element and a third semiconductor switching element connected in series to the first current detection element, and the second current detector has a second current detection element and a fourth semiconductor switching element connected in series to the second current detection element, and a second control unit that controls the third semiconductor switching element and the fourth semiconductor switching element.

[0012] The present invention also provides a diagnostic method for a power conversion device having upper and lower arms, comprising: (a) a step in which a first control unit turns off a first semiconductor switching element of the upper arm and a second semiconductor switching element of the lower arm; (b) a step in which a second control unit turns off a third semiconductor switching element of a first current detector connected to the upper arm and turns on a fourth semiconductor switching element of a second current detector connected to the lower arm; and (c) a step in which a calculation unit calculates a leakage current generated in the upper arm based on a value obtained by subtracting the current value detected by the first current detector from the current value detected by the second current detector.

[0013] According to the present invention, it is possible to realize a power converter having a semiconductor switching element, capable of detecting leakage current of the semiconductor switching element with a relatively simple configuration, and a diagnostic method thereof.

[0014] This makes it possible to detect signs of deterioration or abnormalities in the power conversion device, thereby improving the reliability of the power conversion device.

[0015] Problems, configurations, and effects other than those described above will become apparent from the following description of the embodiments.

[0016] FIG. 4B is a diagram showing a schematic configuration of a power conversion device according to a first embodiment of the present invention. FIG. 5 is a timing chart showing an example of operation of the power conversion device of FIG. 1 and its current detector. FIG. 6 is a diagram showing a schematic configuration of a power conversion device according to a second embodiment of the present invention. FIG. 7 is a diagram showing an example of the configuration of a semiconductor switching element of a power conversion device according to a third embodiment of the present invention. FIG. 8 is a diagram showing a modification of FIG. 4B. FIG. 9 is a diagram showing a schematic relationship between the junction temperature and leakage current of a semiconductor switching element of a power conversion device according to a fourth embodiment of the present invention. FIG. 10 is a flowchart showing a method for diagnosing a power conversion device according to a first embodiment of the present invention. FIG. 11 is a diagram showing a schematic configuration of a power conversion device according to a fifth embodiment of the present invention.

[0017] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In the drawings, the same components are designated by the same reference numerals, and detailed description of overlapping parts will be omitted.

[0018] A power conversion device and a diagnostic method thereof according to a first embodiment of the present invention will be described with reference to FIGS. 1, 2, and 6. FIG.

[0019] Fig. 1 is a diagram showing a schematic configuration of a power conversion device according to this embodiment. Fig. 2 is a timing chart showing an example of the operation of the power conversion device shown in Fig. 1 and its current detectors 25 and 26. Fig. 6 is a flowchart showing a method for diagnosing the power conversion device according to this embodiment.

[0020] As shown in FIG. 1, in the power conversion device of this embodiment, commands output by a control unit 13 are input via gate drive circuits 7 to 12 to the gates of semiconductor switching elements 1 to 6, each of which has an IGBT (Insulated Gate Bipolar Transistor) and a diode connected in anti-parallel, and the semiconductor switching elements 1 to 6 are switched to convert DC power from a DC power supply 14 into AC power, output, and control a load 15 such as a motor.

[0021] Each of the gate drive circuits 7 to 12 is connected to the gate terminal and source terminal of each of the semiconductor switching elements 1 to 6, and applies a voltage to the gate terminal of the semiconductor switching elements 1 to 6 with the source terminal of the semiconductor switching elements 1 to 6 as a reference.

[0022] A current detector 25 is connected in parallel to the semiconductor switching element 1, and a current detector 26 is connected in parallel to the semiconductor switching element 2.

[0023] In this embodiment, current detector 25 and current detector 26 are connected to arms of the same phase, but they may also be connected to different phases, for example, current detector 25 being connected in parallel to semiconductor switching element 3 and current detector 26 being connected in parallel to semiconductor switching element 6.

[0024] The current detectors 25 and 26 each include semiconductor switching elements 16 and 17 each consisting of a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor), current detection elements (resistors) 18 and 19, and signal output units 23 and 24, and a command output from a control unit 22 is input to the gates of the semiconductor switching elements 16 and 17 via gate drive circuits 20 and 21.

[0025] The gate drive circuit 20 is connected to the gate terminal of the semiconductor switching element 16 and the source terminal of the semiconductor switching element 1, and applies a voltage to the gate terminal of the semiconductor switching element 16 with the source terminal of the semiconductor switching element 1 as a reference.

[0026] Furthermore, the gate drive circuit 21 is connected to the gate terminal of the semiconductor switching element 17 and the source terminal of the semiconductor switching element 2, and applies a voltage to the gate terminal of the semiconductor switching element 17 with the source terminal of the semiconductor switching element 2 as a reference.

[0027] In this embodiment, the current detection elements 18 and 19 are connected to the source sides of the semiconductor switching elements 16 and 17, respectively, but there is no problem if they are connected to the drain sides.

[0028] In this embodiment, resistors are connected as current detection elements 18 and 19 to the source side of semiconductor switching elements 16 and 17. Therefore, even if a short circuit occurs, for example, when semiconductor switching elements 16 and 17 are turned on simultaneously due to a malfunction at an unintended timing, when the current reaches a predetermined value, the electromotive force generated in current detection elements 18 and 19 reduces the gate-source voltage of semiconductor switching elements 16 and 17 to near the threshold value, and semiconductor switching elements 16 and 17 autonomously enter a state close to off, thereby providing the advantage of being able to limit the current.

[0029] Next, the operation of measuring the leakage current will be described with reference to FIG.

[0030] The leakage current is measured with all of the semiconductor switching elements 1 to 6 in the OFF state.

[0031] First, to measure the leakage current in the lower arm, only the semiconductor switching element 16 is turned on. Then, the voltage element capability of the current detector 25 is lost, and almost all of the voltage from the DC power supply 14 is applied to the semiconductor switching elements 2, 4, 6, and 17 on the lower arm side. At this time, leakage currents flowing through the semiconductor switching elements 2, 4, 6, and 17 and parasitic leakage currents flowing through the mounting portion of the power conversion device are generated.

[0032] On the other hand, at this time, since the resistance of the current detector 25 on the upper arm side is significantly lower than the resistance of the semiconductor switching elements 1, 3, and 5, most of the leakage current generated on the lower arm side flows through the current detector 25.

[0033] Therefore, by extracting the signal output from the current detection element 18 from the signal output section 23 via an isolated AD converter (not shown), it is possible to calculate the leakage current flowing through the semiconductor switching elements 2, 4, 6, 17 and the mounting section of the power conversion device.

[0034] At this time, the signal output from the current detection element 19 reflects the leakage current flowing through the semiconductor switching element 17, and by extracting the signal from the signal output unit 24 via an isolated AD converter (not shown), the leakage current flowing through the semiconductor switching element 17 can be calculated.

[0035] Then, by subtracting the leakage current flowing through semiconductor switching element 17 from the leakage current flowing through semiconductor switching elements 2, 4, 6, 17 and the mounting portion of the power conversion device, the total value of the leakage current flowing through semiconductor switching elements 2, 4, 6 and the mounting portion of the power conversion device can be calculated.

[0036] The calculation of the leakage current flowing through the semiconductor switching elements 2, 4, 6, 17 and the mounting portion of the power conversion device, the calculation of the leakage current flowing through the semiconductor switching element 17, and the subtraction of the leakage current flowing through the semiconductor switching element 17 from the leakage current flowing through the semiconductor switching elements 2, 4, 6, 17 and the mounting portion of the power conversion device may be performed by incorporating an arithmetic unit in the control units 13, 22, or by providing an arithmetic unit independent of the power conversion device.

[0037] The method for measuring the leakage current of the lower arm described above is shown in the flowchart of FIG.

[0038] First, in step S1, the control unit 13 outputs a command to control all of the semiconductor switching elements 1 to 6 to be turned off.

[0039] Next, in step S2, based on the command output from the control unit 22, only the semiconductor switching element 16 is controlled to be turned on.

[0040] Next, in step S3, the calculation unit (not shown) extracts the signal output from the current detection element 18 from the signal output unit 23 via an isolated AD converter, and calculates the leakage current flowing through the semiconductor switching elements 2, 4, 6, 17 and the mounting portion of the power conversion device.

[0041] Next, in step S4, a calculation unit (not shown) extracts the signal output from the current detection element 19 from the signal output unit 24 via an insulating AD converter, and calculates the leakage current flowing through the semiconductor switching element 17.

[0042] Finally, in step S5, the calculation unit (not shown) subtracts the leakage current flowing through semiconductor switching element 17 from the leakage current flowing through semiconductor switching elements 2, 4, 6, and 17 and the mounting portion of the power conversion device, calculates the total value of the leakage current flowing through semiconductor switching elements 2, 4, and 6 and the mounting portion of the power conversion device (i.e., the leakage current value of the lower arm), and ends the processing.

[0043] When measuring the leakage current of the upper arm, semiconductor switching element 16 is set to the OFF state and semiconductor switching element 17 is set to the ON state. As described above, the leakage current flowing through semiconductor switching elements 1, 3, 5, and 16 and the mounting portion of the power conversion device is calculated from signal output unit 24, and the leakage current flowing through semiconductor switching element 16 is calculated from signal output unit 23. Therefore, the leakage current flowing through semiconductor switching elements 1, 3, and 5 and the mounting portion of the power conversion device can be calculated by subtraction processing.

[0044] As described above, the configuration of the power conversion device and the diagnostic method of this embodiment make it possible to measure the leakage current of the upper and lower arms of the power conversion device with high accuracy, and to prevent element destruction due to deterioration of the withstand voltage of the semiconductor switching elements.

[0045] In this embodiment, as an example, n-type MOSFETs are used for the semiconductor switching elements 16 and 17, but p-type MOSFETs may also be used. Also, instead of MOSFETs, switching elements such as HEMTs (High Electron Mobility Transistors) and IGBTs may also be used.

[0046] Second Embodiment A power conversion device and a diagnostic method thereof according to a second embodiment of the present invention will be described with reference to Fig. 3. In this embodiment, an example in which leakage current is measured by variably controlling measurement accuracy will be described.

[0047] FIG. 3 is a diagram showing a schematic configuration of the power conversion device of this embodiment.

[0048] The difference from Example 1 (Figure 1) is that current detection elements 27 and 28 are provided in parallel with current detection elements 18 and 19, respectively, and that semiconductor switching elements 29 to 32 are provided in series with each of current detection elements 18, 19, 27, and 28, and gate drive circuits 33 to 36 that drive semiconductor switching elements 29 to 32 based on commands output by control unit 22.

[0049] By combining the on / off controls of the semiconductor switching elements 29 and 30, it is possible to control whether the leakage current on the lower arm side flows to the current detection element 18 or the current detection element 27. By changing the resistance values ​​of the current detection elements 18 and 27, it is possible to control the measurement range and measurement accuracy of the leakage current on the lower arm side.

[0050] Furthermore, it is possible to control whether the leakage current on the upper arm side flows to the current detection element 19 or the current detection element 28 by combining the on / off controls of the semiconductor switching elements 31 and 32. By changing the resistance values ​​of the current detection elements 19 and 28, it is possible to control the measurement range and measurement accuracy of the leakage current on the upper arm side.

[0051] Therefore, the configuration of the power converter and the diagnostic method thereof according to this embodiment make it possible to measure the leakage currents in the upper and lower arms of the power converter with both a wide measurement range and high resolution.

[0052] In this embodiment, as an example, two parallel connections of the current detection elements 18 and 27 and two parallel connections of the current detection elements 19 and 28 are shown, but each may be configured with three or more parallel connections.

[0053] 4A to 4C, a power conversion device and a diagnostic method thereof according to a third embodiment of the present invention will be described. In this embodiment, an example in which the present invention is applied to a high-voltage power conversion device will be described.

[0054] 4A to 4C are diagrams showing examples of the configuration of semiconductor switching elements of the power conversion device of this embodiment. Note that the configuration other than the semiconductor switching elements is the same as that of Example 1 (FIG. 1).

[0055] In a high-voltage power conversion device, the semiconductor switching elements 1 to 6 must have a high withstand voltage. At the same time, the semiconductor switching elements 16 and 17 included in the current detectors 25 and 26 connected in parallel to the semiconductor switching elements 1 and 2 must also have a similarly high withstand voltage.

[0056] Of course, it is possible to use high-voltage MOSFETs or the like as the semiconductor switching elements 16 and 17, but costs can be reduced by combining low-voltage semiconductor switching elements and treating them as high-voltage semiconductor switching elements.

[0057] 4A to 4C show an example of the connection relationship of multiple series-connected semiconductor switching elements that replace semiconductor switching element 16 and semiconductor switching element 17. In this embodiment, the number of series-connected low-voltage semiconductor switching elements is three, but the number of series-connected elements can be changed as desired. Also, as an example, an example of replacing an N-type MOSFET is shown, but a P-type MOSFET can also be used as a replacement.

[0058] FIG. 4A shows an example of a multi-stage cascode connection using N-type MOSFETs 37 to 39, where at least N-type MOSFETs 38 and 39 are normally-on type semiconductor switching elements.

[0059] The gate of N-type MOSFET 38 is connected to the source of N-type MOSFET 37, and the gate of N-type MOSFET 39 is connected to the source of N-type MOSFET 38, and the N-type MOSFETs 37 to 39 are controlled to be on and off by the voltage applied to the gate of N-type MOSFET 37.

[0060] When N-type MOSFET 37 is in the on state, the gate-source voltage of N-type MOSFET 38 is approximately 0 V, so N-type MOSFET 38 is in the on state. Similarly, N-type MOSFET 39 is also in the on state. When N-type MOSFET 37 transitions to the off state, the gate-source voltage of N-type MOSFET 38 becomes negative, and N-type MOSFET 38 is in the off state, and similarly, N-type MOSFET 39 is also in the off state. Therefore, the operation of N-type MOSFETs 37 to 39 can be controlled by the voltage applied to the gate of N-type MOSFET 37.

[0061] FIG. 4B shows an example of a multi-stage cascade connection, in which at least N-type MOSFETs 43 and 46 are normally-off type semiconductor switching elements.

[0062] P-type MOSFETs 41 and 42 are connected between the gate of N-type MOSFET 40 and the gate of N-type MOSFET 43, and P-type MOSFETs 44 and 45 are connected between the gate of N-type MOSFET 43 and the gate of N-type MOSFET 46.

[0063] When a positive voltage is applied to the gate of N-type MOSFET 40, a positive voltage is also applied to the gates of N-type MOSFETs 43 and 46, turning N-type MOSFETs 40, 43, and 46 on. When N-type MOSFET 40 transitions to the off state, the gate-source voltage of P-type MOSFET 42 rises, turning P-type MOSFET 42 off. When P-type MOSFET 42 turns off, the gate-source voltage of P-type MOSFET 41 drops, turning P-type MOSFET 41 on, and the gate-source of N-type MOSFET 43 is short-circuited, turning N-type MOSFET 43 off. Similarly, N-type MOSFET 46 also turns off. Therefore, the operation of N-type MOSFETs 40, 43, and 46 can be controlled by the voltage applied to the gate of N-type MOSFET 40.

[0064] Fig. 4C is a modification of Fig. 4B. As shown in Fig. 4C, by connecting a P-type MOSFET 47 between the gate of N-type MOSFET 40 and P-type MOSFET 42, and connecting a P-type MOSFET 48 between the gate of N-type MOSFET 43 and P-type MOSFET 45, it is possible to further improve the breakdown voltage.

[0065] Therefore, the power converter and its diagnostic method of the present embodiment can measure the leakage currents in the upper and lower arms of a high-voltage power converter with a low-cost configuration.

[0066] A power conversion device and a diagnostic method thereof according to a fourth embodiment of the present invention will be described with reference to Fig. 5. In this embodiment, an example of measuring the junction temperature of a semiconductor switching element and an example of measuring the leakage current of a specific semiconductor switching element will be described.

[0067] 5 is a diagram showing a relationship between the junction temperature and the leakage current of the semiconductor switching element of the power converter of this embodiment. The configuration of the power converter is the same as that of the first embodiment (FIG. 1).

[0068] As shown in FIG. 5, when the junction temperature of the semiconductor switching elements rises, the leakage current also rises, so the junction temperatures of the semiconductor switching elements 1 to 6 can be estimated from the leakage current calculated by the method of the first embodiment.

[0069] In the method described in Example 1, the leakage current on the lower arm side can be calculated as the sum of the leakage currents flowing through semiconductor switching elements 2, 4, and 6 and the mounting portion of the power conversion device, and the leakage current on the upper arm side can be calculated as the sum of the leakage currents flowing through semiconductor switching elements 1, 3, and 5 and the mounting portion of the power conversion device, but it is not possible to separate the leakage currents of the individual semiconductor switching elements.

[0070] Therefore, in this embodiment, for example, by passing current only through the semiconductor switching element 1 of the upper arm to increase the junction temperature and measuring the leakage current on the upper arm side, only the leakage increase due to the increase in junction temperature of the semiconductor switching element 1 is calculated as the difference. By passing current to increase the junction temperature under a plurality of conditions and measuring the leakage current, it is possible to calculate the temperature characteristics of the leakage current of the semiconductor switching element 1.

[0071] Therefore, the power conversion device and the diagnostic method of this embodiment can estimate the junction temperature of the semiconductor switching elements, and can measure the leakage current of each switching element separately.

[0072] A power conversion device and a diagnostic method thereof according to a fifth embodiment of the present invention will be described with reference to Fig. 7. In this embodiment, a current detector 25 is provided only in the upper arm of the power conversion device, without providing a current detector in the lower arm of the power conversion device.

[0073] FIG. 7 is a diagram showing a schematic configuration of the power conversion device of this embodiment.

[0074] The difference from the first embodiment (FIG. 1) is that the current detector 26 is not provided in the lower arm of the power converter, and the current detector 25 is provided only in the upper arm of the power converter. The other configurations are the same as those of the first embodiment (FIG. 1).

[0075] The leakage current of the lower arm is measured by the current detector 25 using the same method as in the first embodiment (FIG. 1).

[0076] On the other hand, with regard to the leakage current of the upper arm, if the circuit configuration of the upper and lower arms of the power conversion device is the same, the leakage current of the upper arm will be approximately the same as the leakage current of the lower arm, and therefore can be estimated from the leakage current of the lower arm measured by the current detector 25.

[0077] Therefore, the power converter and its diagnostic method of the present embodiment make it possible to measure the leakage currents in the upper and lower arms of the power converter with a lower cost configuration.

[0078] The present invention is not limited to the above-described embodiments, but includes various modifications. For example, the above-described embodiments have been described in detail to clearly explain the present invention, and the present invention is not necessarily limited to those including all of the described configurations. Furthermore, it is possible to replace part of the configuration of one embodiment with the configuration of another embodiment, or to add the configuration of another embodiment to the configuration of one embodiment. Furthermore, it is possible to add, delete, or replace part of the configuration of each embodiment with other configurations.

[0079] 1 to 6...semiconductor switching elements (IGBTs and diodes connected in anti-parallel), 7 to 12, 20, 21, 33 to 36...gate drive circuits, 13, 22...control unit, 14...DC power supply, 15...load, 16, 17, 29 to 32...semiconductor switching elements (MOSFETs), 18, 19, 27, 28...current detection elements (resistors), 23, 24...signal output unit, 25, 26...current detector, 37 to 40, 43, 46...N-type MOSFETs, 41, 42, 44, 45, 47, 48...P-type MOSFETs.

Claims

1. A power conversion device comprising: an upper arm including a first semiconductor switching element; a lower arm including a second semiconductor switching element connected in series to the first semiconductor switching element; a first control unit that controls the first semiconductor switching element and the second semiconductor switching element; a first current detector connected in parallel to the first semiconductor switching element and detecting a leakage current generated in the lower arm; a second current detector connected in parallel to the second semiconductor switching element and detecting a leakage current generated in the upper arm, wherein the first current detector has a first current detection element and a third semiconductor switching element connected in series to the first current detection element, the second current detector has a second current detection element and a fourth semiconductor switching element connected in series to the second current detection element, and a second control unit that controls the third semiconductor switching element and the fourth semiconductor switching element.

2. A power conversion device as claimed in claim 1, characterized in that, when the first control unit turns off the first semiconductor switching element and the second semiconductor switching element, and the second control unit turns off the third semiconductor switching element and turns on the fourth semiconductor switching element, a leakage current generated in the upper arm is measured based on a value obtained by subtracting the current value output by the first current detection element from the current value output by the second current detection element.

3. A power conversion device as claimed in claim 1, characterized in that, when the first control unit turns off the first semiconductor switching element and the second semiconductor switching element, and the second control unit turns on the third semiconductor switching element and turns off the fourth semiconductor switching element, a leakage current generated in the lower arm is measured based on a value obtained by subtracting the current value output by the second current detection element from the current value output by the first current detection element.

4. A power conversion device as described in claim 1, characterized in that the first semiconductor switching element, the second semiconductor switching element, the third semiconductor switching element, and the fourth semiconductor switching element are either a MOSFET, an IGBT, or a HEMT, or a combination thereof.

5. A power conversion device as claimed in claim 1, characterized in that at least one of the first semiconductor switching element, the second semiconductor switching element, the third semiconductor switching element and the fourth semiconductor switching element is a circuit formed by connecting a plurality of semiconductor switching elements in series.

6. A power conversion device as claimed in claim 1, characterized in that the first current detection element is connected between the source terminal of the third semiconductor switching element and the first semiconductor switching element, and the second current detection element is connected between the source terminal of the fourth semiconductor switching element and the second semiconductor switching element.

7. A power conversion device according to claim 1, wherein the first current detection element and the second current detection element are constituted by resistors.

8. A power conversion device as described in claim 1, wherein the first current detector has a fifth semiconductor switching element connected in series to the first current detection element, and a third current detection element and a sixth semiconductor switching element connected in parallel to the first current detection element and the fifth semiconductor switching element, and the second current detector has a seventh semiconductor switching element connected in series to the second current detection element, and a fourth current detection element and an eighth semiconductor switching element connected in parallel to the second current detection element and the seventh semiconductor switching element.

9. A power conversion device according to claim 2, characterized in that the junction temperature of said first semiconductor switching element is calculated from the value of said measured leakage current.

10. A power conversion device according to claim 3, characterized in that the junction temperature of said second semiconductor switching element is calculated from the value of said measured leakage current.

11. A power conversion device according to claim 2, characterized in that the temperature characteristic of the leakage current of said first semiconductor switching element is calculated from values ​​of the leakage current measured under a plurality of temperature conditions.

12. A power conversion device according to claim 3, characterized in that the temperature characteristics of the leakage current of said second semiconductor switching element are calculated from values ​​of the leakage current measured under a plurality of temperature conditions.

13. A method for diagnosing a power conversion device having upper and lower arms, comprising: (a) a step by a first control unit turning off a first semiconductor switching element of the upper arm and a second semiconductor switching element of the lower arm; (b) a step by a second control unit turning off a third semiconductor switching element of a first current detector connected to the upper arm and turning on a fourth semiconductor switching element of a second current detector connected to the lower arm; and (c) a step by an arithmetic unit calculating a leakage current generated in the upper arm based on a value obtained by subtracting the current value detected by the first current detector from the current value detected by the second current detector.

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