Power Conversion Device

Abstract

To provide a power conversion device in which an erroneous determination of a short-circuit in a semiconductor switching element is prevented and a short-circuit detection time is shortened.SOLUTION: A power conversion device including a semiconductor switching element having a gate for performing on / off control of a current flowing between a first main electrode and a second main electrode, includes a first diode having a cathode connected to the first main electrode, a second diode having an anode connected to an anode of the first diode and a cathode connected to the gate, and a current source that supplies currents to the connection point between the anode of the first diode and the anode of the second diode, in directions from the anode of the first diode and the anode of the second diode to the respective cathodes. When the voltage at the connection point between the first diode and the second diode is higher than a threshold voltage, it is determined that the semiconductor switching element is short-circuited and an off voltage is set at the gate of the semiconductor switching element.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present application relates to a power conversion device. [Background technology]

[0002] Various methods have been proposed for detecting short circuits in semiconductor switching elements, which are key components of power conversion devices used to drive motors in automobiles and other vehicles. For example, as disclosed in Patent Document 1, a method for detecting short circuits in semiconductor switching elements is known, in which the voltage on the high-potential side of the semiconductor switching element is detected to be abnormally high, a voltage that would not occur in a normal on-state, and the method determines whether a short circuit has occurred. This method is called the DESAT method, and in this method, the drain voltage is high from the time the gate voltage of the semiconductor switching element starts to rise until it reaches the mirror voltage, so it is necessary to prevent erroneous short circuit detection during this period.

[0003] In response to this, for example, Patent Document 1 discloses a method for preventing erroneous determination by not performing a short circuit determination for a predetermined time after the gate voltage starts to rise. However, with this method, if the turn-on speed of a semiconductor switching element is fast due to variations in the semiconductor switching element, there will be a time period during which a short circuit determination is not performed even though a short circuit current has started to flow due to a fault, and as a result, the short circuit loss occurring in the semiconductor switching element will increase.

[0004] In response to this, for example, Patent Document 2 discloses a method that includes a circuit for determining the gate voltage and transitions to short-circuit detection after the gate voltage reaches the on-voltage. With this method, short-circuit detection begins according to the turn-on speed, and is therefore expected to detect short circuits in a relatively shorter time than the method of Patent Document 1, which sets a predetermined time. However, comparators typically used to determine voltage have a response delay of approximately 100 to 200 ns, depending on the components. This delay directly increases short-circuit loss, so there is still room for improvement in short-circuit detection time from the perspective of protecting semiconductor switching elements from short circuits. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 5861787 [Patent Document 2] Patent Publication No. 2021-57976 Summary of the Invention [Problem to be solved by the invention]

[0006] Semiconductor switching elements that must be protected by quickly detecting short circuits include wide bandgap semiconductors (SiC, GaN, etc.), which have become increasingly mainstream in recent years and have the characteristic of allowing more current to flow in the event of a short circuit.In the case of wide bandgap semiconductors, there is a need to further improve the delay in short circuit detection mentioned above.

[0007] The present application is intended to solve the above-mentioned problems, and has an object to provide a power conversion device that can prevent erroneous determination of a short circuit during the switching operation of a semiconductor switching element and detect a short circuit in a short time. [Means for solving the problem]

[0008] The power conversion device disclosed in the present application performs power conversion between DC power and AC power by controlling the on / off of a current flowing between a first main electrode and a second main electrode of a semiconductor switching element having a first main electrode, a second main electrode, and a gate, by controlling a voltage of the gate with respect to the second main electrode, the voltage being determined by the second main electrode being a reference. a first diode having a cathode connected to the first main electrode; and a second diode having an anode connected to the anode of the first diode and a cathode connected to the gate; a current source that supplies current to a connection point between the anode of the first diode and the anode of the second diode in a direction from the anode of the first diode to the cathode of the anode of the second diode; Controlling the voltage of the gate of the semiconductor switching element and controls the on / off. a drive unit; and a short circuit determination unit that determines whether the semiconductor switching element is short-circuited; a gate resistor connected between the connection point of the cathode of the second diode and the gate and the output of the driver; Equipped with The short circuit determination unit detects a voltage at a connection point between the first diode and the second diode. When the semiconductor switching element is turned on, the monitored voltage is set to a voltage higher than the sum of the mirror voltage of the gate when the semiconductor switching element starts to turn on and the voltage drop when the second diode is conductive. Higher than the threshold voltage It became When the semiconductor switching element is short-circuited, The drive section is configured to set the gate voltage to a voltage at which the semiconductor switching element is turned off when the short-circuit determination section determines that a short circuit has occurred. [Effects of the Invention]

[0009] According to the power conversion device disclosed in the present application, it is possible to provide a power conversion device that can prevent erroneous determination and detect a short circuit in a short time. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a circuit diagram showing a schematic configuration of a power conversion device according to a first embodiment. [Figure 2] 1 is a circuit diagram showing an example of a main part of a power conversion device according to a first embodiment. [Figure 3] 4 is a diagram illustrating an operation of the power conversion device according to the first embodiment when semiconductor switching elements are normal and on. FIG. [Figure 4] 10 is a diagram illustrating the operation of the power conversion device of the comparative example when the semiconductor switching elements are normal and on. FIG. [Figure 5A] 10 is a diagram illustrating the operation of the power conversion device of the comparative example when the semiconductor switching element is short-circuited and destroyed during ON state. FIG. [Figure 5B] 4 is a diagram illustrating the operation of the power conversion device according to the first embodiment when a semiconductor switching element is in a short-circuit state. FIG. [Figure 6]4 is a diagram illustrating the operation of the power conversion device according to the first embodiment when the semiconductor switching elements are normal and the turn-on speed is faster than that of FIG. 3. FIG. [Figure 7] 4 is a diagram illustrating the operation of the power conversion device according to the first embodiment when the semiconductor switching elements are normal and the turn-on speed is slower than that of FIG. 3. FIG. [Figure 8] 5B is a diagram illustrating the operation of the power conversion device according to the first embodiment when a semiconductor switching element is in a short-circuit state and when the turn-on speed is faster than that of FIG. 5B. FIG. [Figure 9] 5B is a diagram illustrating the operation of the power conversion device according to the first embodiment when a semiconductor switching element is in a short-circuit state and when the turn-on speed is slower than that of FIG. 5B. FIG. [Figure 10] 4 is a diagram illustrating the operation of the power conversion device according to the first embodiment when the semiconductor switching elements are normal, and explaining the operation due to differences in the parasitic capacitance of the first diode 22. FIG. [Figure 11] 4 is a circuit diagram showing another example of the main part of the power conversion device according to the first embodiment. FIG. [Figure 12] FIG. 10 is a circuit diagram showing an example of a main part of a power conversion device according to a second embodiment. [Figure 13] FIG. 10 is a circuit diagram showing another example of the main part of the power conversion device according to the second embodiment. [Figure 14] 10 is a diagram illustrating an operation of the power conversion device according to the second embodiment when the semiconductor switching elements are normal and on. FIG. [Figure 15] 10 is a diagram illustrating the operation of the power conversion device according to the second embodiment when a semiconductor switching element is in a short-circuit state. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0011] Embodiment 1 Fig. 1 is a circuit diagram showing a schematic configuration of a power conversion device 20 according to a first embodiment, and Fig. 2 is a circuit diagram showing a main part. As shown in Fig. 1, the power conversion device 20 has a rotating electric machine M as a control object, and includes a power conversion unit 10 composed of semiconductor switching elements of upper and lower arms for three phases (U phase, V phase, and W phase), and a drive control unit 21 that controls the on / off of each semiconductor switching element of the power conversion unit 10. The power conversion device 20 further includes a smoothing capacitor C that is electrically connected in parallel to the high potential side (drain) of the semiconductor switching element of the upper arm and the low potential side (source) of the semiconductor switching element of the lower arm, and smoothes the DC power supply voltage applied to the power conversion device 20. The power conversion device 20 converts DC power supplied from a DC power supply 30 into AC power and supplies it to the rotating electric machine M as a control object.

[0012] In the following, a MOSFET will be described as an example of a semiconductor switching element, in which main electrodes are a drain and a source and a current flowing between the drain and the source is controlled by controlling the gate voltage. However, the present invention can also be applied to an IGBT, in which main electrodes are a collector and an emitter and a current flowing between the collector and the emitter is controlled by controlling the gate voltage. In the present application, the drain or collector may be referred to as a first main electrode 12 (see FIG. 2), and the source or emitter may be referred to as a second main electrode 13 (see FIG. 2). That is, the semiconductor switching element 11 of the present application includes a first main electrode 12, a second main electrode 13, and a gate 14. The power conversion device of the present application performs power conversion between DC power and AC power by controlling the voltage of the gate 14 relative to the second main electrode 13 of the semiconductor switching element 11 to turn on and off the current flowing between the first main electrode 12 and the second main electrode 13.

[0013] Next, the detailed configuration of the power conversion device according to the first embodiment will be described with reference to Fig. 2. In the first embodiment, the control of the three phases and the upper and lower arms is basically the same configuration, so here, the lower arm of the U phase will be described. The U-phase lower arm of the power conversion unit 10 is provided with a semiconductor switching element 11, and the drive control unit 21 is provided with: a drive unit 28 that controls the voltage of the gate 14 of the semiconductor switching element 11 to control the on / off of the semiconductor switching element 11; a gate resistor 29; a first diode 22 having a cathode connected to a first main electrode (drain) 12 of the semiconductor switching element 11; a second diode 23 having a cathode connected to the gate 14 of the semiconductor switching element 11; wiring 24 (the connection point of the anode of the first diode 22 and the anode of the second diode 23) that connects the anode of the first diode 22 and the anode of the second diode 23; a short-circuit determination unit 25 that is connected to the wiring 24 and determines whether the semiconductor switching element 11 is short-circuited, and outputs a gate off command to the drive unit 28 if a short circuit is determined to exist; and a current source 26 that is connected to the wiring 24 and supplies current from the anode of the first diode 22 and the anode of the second diode 23 in the direction (forward direction) to their respective cathodes.

[0014] As shown in FIG. 2, a filter 27 for suppressing the influence of disturbance noise may be provided on the wiring 24 (between the connection point of the current source 26 and the first diode 22 and the second diode 23), but whether or not to implement it and at what position on the wiring 24 to implement it are optional.

[0015] The semiconductor switching element 11 is a transistor made of a wide bandgap semiconductor such as silicon (Si), silicon carbide (SiC), or gallium nitride (GaN). Regarding the gate voltage of the semiconductor switching element 11, in order to turn on the semiconductor switching element 11, a voltage equal to or higher than a predetermined voltage relative to the source of the semiconductor switching element 11 must be applied to the gate, and this predetermined voltage is called a mirror voltage. The mirror voltage depends on the type of semiconductor switching element 11 and the amount of current flowing through the rotating electric machine M, but here it is set to 10 V as an example.

[0016] When the gate is less than the mirror voltage, the drain voltage of the semiconductor switching element 11 is applied with a voltage approximately equal to the DC power supply voltage. This DC power supply voltage is, for example, 400 V for a DC power supply voltage used in an electric vehicle. On the other hand, in a region where the gate voltage is equal to or greater than the mirror voltage, the semiconductor switching element 11 turns on and the voltage drops to a voltage determined by the product of the current flowing through the rotating electric machine M and the on-resistance of the drain. In the first embodiment, the maximum current flowing through the rotating electric machine M is set to 10 V, for example.

[0017] Due to the connection configuration of the first diode 22 and the second diode 23, the voltage of the wiring 24 is determined by the lower of the gate voltage and the drain voltage. For example, when the gate is lower than the mirror voltage, the drain voltage is 400 V, so the second diode 23 turns on. If the on-voltage of the second diode 23 (voltage drop when conducting) is 0.5 V, the wiring 24 is at least 10.5 V or less. On the other hand, when the gate is higher than the mirror voltage, the semiconductor switching element 11 turns on, the drain voltage drops to 10 V or less, the first diode 22 turns on, and if the on-voltage of the first diode 22 is 0.5 V, the wiring 24 is at least 10.5 V or less.

[0018] Therefore, short-circuit determination unit 25 monitors the voltage of wiring 24, and when it detects that the voltage of wiring 24 has reached a voltage that would never be reached under normal operation, it can determine that semiconductor switching element 11 is in a short-circuit state. That is, if the voltage for determining that a short-circuit state exists is a threshold voltage, in the above example, the threshold voltage may be set to, for example, 10.8 V, which is slightly higher than the sum of 10.5 V of the mirror voltage of the gate and the on-voltage, which is the voltage drop when second diode 23 is conductive. When short-circuit determination unit 25 detects that the voltage of wiring 24 is higher than the threshold voltage, it determines that a short circuit has occurred, and drive unit 28 sets the gate voltage to an off voltage, thereby preventing short-circuit current from flowing through semiconductor switching element 11.

[0019] The amount of current supplied by current source 26 is arbitrary as long as it does not affect the normal operation of semiconductor switching element 11. A typical current amount used is about 1 mA. Note that current source 26 does not need to be a constant current source, and may instead supply a current amount that does not affect the normal operation of semiconductor switching element 11 via a resistor element from an arbitrary power source that is higher than the gate voltage.

[0020] First, FIG. 3 shows the gate voltage, drain voltage, drain current, and voltage of wiring 24 when semiconductor switching element 11 switches from off to on in a normal state (not short-circuited). The gate voltage, drain voltage, and drain current are each indicated by a solid line, and the voltage of wiring 24 is indicated by a dashed line along with the gate voltage indicated by a solid line. This also applies to the following FIGS. 4 to 10. In the following, the threshold voltage for determining a short circuit is set to 10.8 V, slightly higher than 10.5 V determined by the mirror voltage, drain voltage, and diode on-voltage, to account for variations in the diode on-voltage. Here, we will explain the case where the mirror voltage of semiconductor switching element 11 is the same as or higher than the drain voltage when on. However, if the mirror voltage of semiconductor switching element 11 is lower than the drain voltage when on, it is desirable to set the threshold voltage to a voltage higher than the sum of the drain voltage when on and the on-voltage of the first diode.

[0021] <Time t0~Time t1> The driver 28 starts to increase the gate voltage of the semiconductor switching element 11, and at time t1 the gate voltage reaches the mirror voltage of 10 V. The line 24 simultaneously increases with respect to the gate voltage with an offset equivalent to the on-voltage of the second diode 23, so that the line 24 reaches 10.5 V at time t1. During this time, the drain voltage remains at 400 V.

[0022] <Time t1~Time t2> As the gate voltage reaches the mirror voltage, the semiconductor switching element 11 turns on, and the drain voltage begins to drop sharply. During this time, the gate voltage is fixed at the mirror voltage of 10 V, and this interval is called the mirror interval. It takes a little longer than time t2 for the drain voltage to drop to the on voltage of 10 V, and it reaches this at time t22. This is because the on resistance of the semiconductor switching element 11 is still relatively high immediately after passing the mirror voltage at t2, and the drain voltage does not drop sufficiently.

[0023] What should be noted here is that the voltage on the wiring 24 starts to drop simultaneously with the sudden drop in the drain voltage at time t1, even though the drain voltage has not yet dropped to the on-voltage of 10 V. This is related to the parasitic capacitance (Cst) that occurs between the anode and cathode of the first diode 22, which is not shown in Fig. 2. A discharge current equivalent to the product of Cst and a steep voltage fluctuation (dV / dt) that occurs at the cathode of the first diode 22 due to the sudden drop in the drain voltage is generated from the wiring 24 toward the first main electrode 12 of the semiconductor switching element 11. If this discharge current is greater than the amount of current from the current source 26, the voltage on the wiring 24 starts to drop at time t1.

[0024] For example, if the parasitic capacitance Cst is 10 pF and the voltage fluctuation dV / dt is 1 kV / μs, the discharge current is 10 mA, so if the current amount supplied by the current source 26 is 1 mA, a discharge effect of 9 mA, which corresponds to the difference, is obtained. This discharge causes the voltage of the wiring 24 to drop at the same time as the drain voltage starts to drop.

[0025] 4 shows the voltage of wiring 24 when there is no parasitic capacitance in first diode 22. As shown in FIG. 4, there is no drop in the voltage of wiring 24 between time t1 and time t2, and therefore it can be seen that the time when first diode 22 turns on and the voltage of wiring 24 starts to drop is later than time t2 when the mirror voltage is passed, and the voltage when it starts to drop is 11.3 V, which is higher than 10.5 V in the mirror section. This indicates that if there is no discharge effect due to parasitic capacitance Cst, a threshold voltage of 10.8 V at which short-circuit determination unit 25 determines there is a short circuit will result in an erroneous determination; in this case, the threshold voltage needs to be set to 11.3 V or higher.

[0026] As will be described later, in order to quickly detect a short circuit and protect the semiconductor switching element 11, it is desirable that the threshold voltage for short circuit determination is low, and it can be said that discharge due to the parasitic capacitance Cst has the effect of shortening the detection time.

[0027] 5A and 5B show operation when the semiconductor switching element 11 is short-circuited. FIG. 5A shows operation when the short circuit continues without short-circuit detection as a comparative example of the operation of the power conversion device of the first embodiment. For example, assume that the first main electrode 12 of the semiconductor switching element 11 is electrically short-circuited to a 400 V DC voltage source for some reason. At this time, the gate voltage rises and the short-circuit current also rises along with the gate voltage. The short-circuit current becomes a large current far exceeding the maximum current flowing through the rotating electric machine M, and eventually exceeds the limit value of the semiconductor switching element 11, leading to a short-circuit breakdown state from which recovery is difficult. If the limit value is reached at time t13, short-circuit protection requires that the short circuit be detected and the gate voltage be turned off and the short-circuit current be interrupted within a time period shorter than time t13.

[0028] FIG. 5B illustrates the operation of the power conversion device according to the first embodiment. This figure shows the operation when the semiconductor switching element 11 is short-circuited, the short-circuit determination unit 25 determines that the short circuit has occurred, and the driver 28 turns off the gate voltage. As the gate voltage rises, the voltage on the wiring 24 also rises. When the semiconductor switching element 11 is short-circuited, unlike the case shown in FIG. 3 when the semiconductor switching element 11 is normal, the voltage on the wiring 24 rises to 10.5 V or higher, calculated by adding the mirror voltage (10.5 V) to the on-voltage of the second diode 23. Here, the threshold voltage for determining a short circuit is set to 10.8 V. Therefore, a short circuit is determined at time t11 when the voltage on the wiring 24 reaches 10.8 V. At the same time, the driver 28 reduces the gate voltage to the off voltage. This reduces the drain current. In the first embodiment, we obtained a time t1 of 575 ns, and a time t11 when the voltage on the wiring 24 reaches the threshold voltage of 10.8 V for determining a short circuit, i.e., a short-circuit detection time, of 596 ns. In other words, the detection delay time is 21 ns, allowing for high-speed detection.

[0029] Next, we will explain the behavior due to differences in turn-on speed. Figures 6 and 7 show the behavior during normal operation, while Figures 8 and 9 show the behavior during a short circuit. First, as behavior during normal operation, Figure 6 shows a state where the turn-on speed is 30% faster than that of Figure 3, and Figure 7 shows the behavior when the turn-on speed is 30% slower than that of Figure 3. As shown in the figures, it can be seen that the voltage on wiring 24 follows the turn-on speed, that is, the rate at which the gate voltage rises, and that at time t1 the voltage on wiring 24 begins to decrease and does not reach the threshold voltage, preventing erroneous determination.

[0030] Next, as for the behavior of short-circuit operation, Figure 8 shows the state where the turn-on speed is 30% faster than Figure 5B, and Figure 9 shows the behavior where the turn-on speed is 30% slower than Figure 5B. In both cases, it can be seen that the voltage on wiring 24 rises until time t11, just after the gate reaches 10.5V at time t1, and a short circuit can be detected when it reaches 10.8V.

[0031] Table 1 below summarizes times t1 and t11 in Figures 7, 8, and 9, which show the behavior when detecting a short circuit. The detection delay corresponding to times t1 to t11 varies slightly depending on the turn-on speed, so the effectiveness will increase or decrease depending on the target turn-on speed for drive control. However, this is sufficiently short compared to the detection delay of 100 to 200 ns that occurs with conventional technology, and when drive control is performed with turn-on on the order of 1 μs or less, it will have a significant effect over conventional technology.

[0032] [Table 1]

[0033] Table 2 summarizes the short circuit detection time when the threshold voltage for short circuit determination is changed in the present embodiment 1. The lower the threshold voltage, the easier it is for the voltage of the wiring 24 to reach the threshold voltage, resulting in a shorter short circuit detection time.

[0034] [Table 2]

[0035] 10 illustrates the transition of the voltage on the wiring 24 due to differences in the amount of discharge caused by differences in the parasitic capacitance Cst. As described above, the sudden voltage fluctuation dV / dt occurring in the first diode 22 and the discharge of the wiring 24 due to the parasitic capacitance Cst make it easier to lower the threshold voltage for short-circuit determination, contributing to shortening the time required to detect a short circuit.

[0036] As shown in FIG. 10 , in our device according to the first embodiment, we concluded that a parasitic capacitance of approximately 10 pF or more is required to maintain the voltage on wiring 24 below the threshold voltage without misjudgment after time t1 when semiconductor switching element 11 is in a normal state. However, the required parasitic capacitance value is likely to vary depending on the specifications of semiconductor switching element 11. Therefore, once the specifications of semiconductor switching element 11 are determined, the characteristics shown in FIG. 10 can be obtained when semiconductor switching element 11 is in a normal state, and the parasitic capacitance value required to maintain the voltage on wiring 24 below the threshold voltage can be determined in advance. This result indicates that the type of diode selected is an important factor. For example, first diode 22 could be a Zener type, which is considered to have a relatively large parasitic capacitance.

[0037] Alternatively, if the parasitic capacitance of the first diode 22 is smaller than the capacitance required to maintain the voltage of the wiring 24 at a voltage lower than the threshold voltage, an additional capacitor 40 may be provided in parallel with the first diode 22, as shown in FIG. 11, so that the capacitance value combined with the parasitic capacitance of the first diode is equal to or greater than the capacitance required to maintain the voltage of the wiring 24 at a voltage lower than the threshold voltage.

[0038] Embodiment 2 12 is a circuit diagram showing a main part of a power conversion device 20 according to a second embodiment. A U-phase lower arm of the power conversion device 10 includes a semiconductor switching element 11, and a drive control unit 21 includes a drive unit 28 that controls the voltage of a gate 14 of the semiconductor switching element 11 to turn the semiconductor switching element 11 on and off, a gate resistor 29, a first diode 22 having a cathode connected to a first main electrode (drain) 12 of the semiconductor switching element 11, a resistor 23b having one end connected to the gate 14 of the semiconductor switching element 11, a wiring 24 (a connection point between the anode of the first diode 22 and the resistor 23b) to which an anode of the first diode 22 and one end of the resistor 23b other than the gate 14 are connected, and a short-circuit determination unit 25 that is connected to the wiring 24 and determines whether the semiconductor switching element 11 is short-circuited and outputs a gate-off command to the drive unit 28 if a short-circuit is determined. Hereinafter, detailed descriptions of the basic configuration and control method of the power conversion device that are the same as those of the first embodiment will be omitted.

[0039] As in the first embodiment, the filter 27 provided on the line of the wiring 24 shown in FIG. 12 may or may not be mounted, and the position on the wiring 24 at which it is mounted is optional.

[0040] The difference from the first embodiment is that the second diode 23 is replaced with a resistor 23b and the current source 26 is not provided, but even with this configuration, a short circuit detection time equivalent to that of the first embodiment can be expected.

[0041] Specifically, we will explain how the voltage of the wiring 24 is determined based on the connection configuration of the first diode 22 and resistor 23b. Here, we will explain the case where the gate mirror voltage is 10 V and the drain voltage in the on state is 10 V, as in the first embodiment.

[0042] For example, if the gate voltage is lower than the mirror voltage, resistor 23b can raise the voltage of wire 24 in line with the rise in gate voltage, even without current source 26. During this time, the drain voltage is 400 V, so wire 24 and the gate remain at approximately the same potential. In other words, since the mirror voltage is 10 V, the voltage of wire 24 is also at least 10 V or less.

[0043] On the other hand, when the gate is higher than the mirror voltage, the semiconductor switching element 11 turns on, the drain voltage drops to 10V or less, the first diode 22 turns on, and if the on voltage of the first diode 22 is 0.5V, the wiring 24 is at least 10.5V or less.

[0044] If resistor 23b were not present and the gate and wiring 24 were directly connected, after the gate reached the mirror voltage and the first diode 22 turned on, the gate voltage would continue to rise, causing conduction between the gate and drain via the first diode 22, making it difficult to drive the semiconductor switching element 11 as desired. Therefore, resistor 23b serves to limit the conduction current between the gate and drain when the semiconductor switching element 11 is on, thereby ensuring continuous gate drive of the semiconductor switching element 11. On the other hand, if the resistance value is too large, the parasitic capacitance of wiring 24 (not shown in FIG. 12 ) and resistor 23b create a filtering effect, making it difficult for wiring 24 to rise in line with the gate voltage when the gate is turned on. This undesirably delays the time it takes for wiring 24 to reach the threshold voltage at which a short circuit is detected. Therefore, it is desirable to set the resistance value of resistor 23b as small as possible within the range that limits the conduction current and ensures continuous gate drive of the switching element 11.

[0045] Although an example in which the gate and wiring 24 are connected by one resistor 23b has been shown here, a configuration in which a third diode 23c is connected in series with resistor 23b so that the side closer to the gate serves as the anode, as shown in Fig. 13, may also be used. This configuration can prevent current from flowing from wiring 24 to the gate, and is therefore effective, for example, when turning off the gate, in cases where there is concern about a backflow of current from another circuit (not shown) connected to wiring 24 that would prevent the gate from being turned off.

[0046] For this reason, similarly to the first embodiment, short-circuit determination unit 25 monitors the voltage of wiring 24, and when it detects that the voltage of wiring 24 has reached a voltage that would never be reached under normal operation, it can determine that semiconductor switching element 11 is in a short-circuit state. That is, if the voltage at which a short-circuit state is determined to exist is a threshold voltage, then in the above example, the threshold voltage may be set to, for example, 10.8 V, which is slightly higher than 10.5 V. When short-circuit determination unit 25 detects that the voltage of wiring 24 is higher than the threshold voltage, it determines that a short circuit has occurred, and drive unit 28 sets the gate voltage to an OFF voltage, thereby preventing a short-circuit current from flowing through semiconductor switching element 11.

[0047] In the above example, the threshold voltage was set to 10.8 V because the voltage of wiring 24 determined from the drain voltage in the on state is higher than the voltage of wiring 24 determined from the mirror voltage. However, depending on the type of semiconductor switching element 11 or the maximum amount of current flowing through the rotating electric machine M, it is also possible that the voltage determined from the mirror voltage will be larger.

[0048] For example, if the drain voltage in the on state is assumed to be 8 V, the voltage of the wiring 24 when the semiconductor switching element 11 is in the on state is at least 8.5 V or less. Therefore, the voltage that the wiring 24 can reach under normal operation is 10 V, which is determined by the mirror voltage, and the threshold voltage should be set to 10 V or higher. In any case, it can be said that, in general, including the first embodiment, the threshold voltage should be set at least higher than the maximum voltage of the wiring 24, which is determined by the mirror voltage. Note that, in the configuration shown in FIG. 13 in which the third diode 23 c is connected in series with the resistor 23 b, the threshold voltage should be set to a voltage higher than the mirror voltage minus the voltage drop when the third diode 23 c is conductive.

[0049] 14 shows the gate voltage, drain voltage, drain current, and voltage of wiring 24 when semiconductor switching element 11 switches from off to on in a normal state (not short-circuited) in the second embodiment. The gate voltage, drain voltage, and drain current are each indicated by a solid line, and the voltage of wiring 24 is indicated by a dashed line together with the gate voltage indicated by a solid line. This is also true for the subsequent FIG. 15. In the following description, the threshold voltage for determining a short circuit is set to 10.8 V, taking into account variations in the diode on-voltage, compared to 10.5 V determined by the mirror voltage, drain voltage, and diode on-voltage described above.

[0050] Between time t0 and time t1, the gate voltage of semiconductor switching element 11 is increased by drive unit 28, and at time t1 the gate voltage reaches the mirror voltage of 10 V. Wiring 24 rises almost in line with the gate voltage, so it reaches 10 V at time t1. During this time, the drain voltage remains at 400 V.

[0051] After time t1, the gate voltage reaches the mirror voltage, turning on semiconductor switching element 11, and the drain voltage begins to drop sharply. At this time, the voltage of wiring 24 also begins to drop simultaneously with the sudden drop in the drain voltage at time t1. This is because, as in the first embodiment, the voltage of wiring 24 drops due to a discharge current corresponding to the product of a parasitic capacitance (Cst) generated between the anode and cathode of first diode 22 (not shown in FIG. 12) and a sudden voltage fluctuation (dV / dt) generated at the cathode of first diode 22 due to the sudden drop in the drain voltage. If this discharge current exceeds the amount of current supplied from the gate to wiring 24 via resistor 23b, the drop in wiring 24 begins at time t1, as shown in FIG. 14. In this way, erroneous determination of a short circuit is prevented in the second embodiment as well.

[0052] FIG. 15 shows the operation when the short-circuit determination unit 25 determines that the semiconductor switching element 11 is short-circuited and the driver 28 turns off the gate voltage. As the gate voltage rises, the voltage of the wiring 24 also rises. When the semiconductor switching element 11 is short-circuited, the voltage of the wiring 24 rises to 10 V or higher, unlike in FIG. 14 when the semiconductor switching element 11 is normal. Here, the threshold voltage for determining a short circuit is set to 10.8 V. Therefore, a short circuit is determined at time t11 when the voltage of the wiring 24 reaches 10.8 V. At the same time, the driver 28 reduces the gate voltage to the OFF voltage. This reduces the drain current. In the second embodiment, we obtained a time t1 of 575 ns, and a time t11 when the voltage of the wiring 24 reaches the short-circuit determination threshold voltage of 10.8 V, i.e., a short-circuit detection time of 625 ns. This means that the detection delay time is 50 ns.

[0053] The effect obtained is reduced compared to the detection delay time of 21 ns in the first embodiment, but this is because, whereas in the first embodiment the voltage on wiring 24 rises with a positive offset of the on voltage of second diode 23 relative to the gate voltage, in the second embodiment the voltage on wiring 24 rises at approximately the same potential as the gate voltage, and so it takes time to reach the same threshold voltage of 10.8 V. However, this is because, as described above, the mirror voltage and the drain voltage in the on state are both set to 10 V, and the threshold voltage determined from the drain voltage is set to 10.8 V; therefore, if the drain voltage in the on state is lower than 10 V, the threshold voltage can be lowered, and therefore it is possible to achieve a short-circuit detection time similar to that of the first embodiment, depending on the conditions.

[0054] In summary, a first diode is connected to the first main electrode (drain) of the semiconductor switching element so that the semiconductor switching element side is the cathode, and a resistor or a second diode is connected between the anode of the first diode and the gate of the semiconductor switching element so that the gate side is the cathode, and a function is provided to determine a short circuit when the voltage of the anode of the first diode is higher than the threshold voltage.This prevents erroneous determination of a short circuit during normal switching operation, and enables short circuit detection in a short time according to variations in turn-on speed without using a circuit with a delay element such as a gate voltage detection circuit.In addition, when a second diode is connected between the anode of the first diode and the gate of the semiconductor switching element so that the gate side is the cathode, a current source is required to supply current from the anode of the first diode and the anode of the second diode to their respective cathodes.

[0055] To further shorten the short circuit detection time, a diode is selected so that the voltage of the wiring drops immediately after the gate reaches the mirror voltage due to the discharge of the charge that had been stored in the parasitic capacitance of the diode, and the threshold voltage for determining a short circuit is set to the mirror voltage plus the conduction voltage of the diode.

[0056] These technologies can be applied to IGBTs (Insulated-Gate Bipolar Transistors), which are semiconductor switching elements primarily made of conventional silicon (Si), but they are more effective when applied to MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors), which are semiconductor switching elements made primarily of silicon carbide (SiC) or gallium nitride (GaN), which are expected to become mainstream in the future and have a wider bandgap than silicon.The reason for this is that wide-bandgap semiconductors have a low on-resistance, so short-circuit current flows rapidly in the event of a short-circuit fault, and in order to protect them, it is necessary to complete short-circuit detection as quickly as possible.

[0057] As described above, the power conversion device described in this application can prevent erroneous determination of a short circuit during normal switching operation of a semiconductor switching element, and can provide a power conversion device equipped with a short circuit determination circuit that improves the delay time required to determine a short circuit when a semiconductor switching element is short-circuited and has a short detection time.

[0058] Although various exemplary embodiments and examples are described in this application, the various features, aspects, and functions described in one or more embodiments are not limited to the application of a particular embodiment, but may be applied to the embodiments alone or in various combinations. Therefore, countless modifications not illustrated are contemplated within the scope of the technology disclosed in this specification. For example, this includes cases where at least one component is modified, added, or omitted, and even cases where at least one component is extracted and combined with components of other embodiments. [Explanation of symbols]

[0059] 11 semiconductor switching element, 12 first main electrode, 13 second main electrode, 14 gate, 22 first diode, 23 second diode, 23b resistor, 23c third diode, 24 wiring, 25 short circuit determination unit, 26 current source, 28 driver, 40 additional capacitor

Claims

1. A power conversion device performs power conversion between DC power and AC power by controlling the on / off of a current flowing between a first main electrode and a second main electrode of a semiconductor switching element having a first main electrode, a second main electrode, and a gate, by controlling a voltage of the gate with respect to the second main electrode, a first diode having a cathode connected to the first main electrode; and a second diode having an anode connected to the anode of the first diode and a cathode connected to the gate; a current source that supplies current to a connection point between the anode of the first diode and the anode of the second diode in a direction from the anode of the first diode to the cathode of the anode of the second diode; a drive unit that controls the gate voltage of the semiconductor switching element to perform the on / off control; a short-circuit determination unit that determines whether the semiconductor switching element is short-circuited; and a gate resistor connected between a connection point between the cathode of the second diode and the gate and an output of the drive unit, the short-circuit determination unit monitors a voltage at a connection point between the first diode and the second diode, and determines that the semiconductor switching element is short-circuited when the monitored voltage becomes higher than a threshold voltage that is set to a voltage higher than the sum of a mirror voltage of the gate when the semiconductor switching element starts to turn on and a voltage drop of the second diode when it is conductive, The power conversion device is configured such that, when the short-circuit determination unit determines that a short circuit has occurred, the drive unit sets the gate voltage to a voltage at which the semiconductor switching element is turned off.

2. A power conversion device performs power conversion between DC power and AC power by controlling the on / off of a current flowing between a first main electrode and a second main electrode of a semiconductor switching element having a first main electrode, a second main electrode, and a gate, by controlling a voltage of the gate with respect to the second main electrode, a first diode having a cathode connected to the first main electrode; and a series circuit of a resistor element and a third diode having an anode on the gate side connected between the anode of the first diode and the gate; a drive unit that controls the gate voltage of the semiconductor switching element to perform the on / off control; and a short-circuit determination unit that determines whether the semiconductor switching element is short-circuited, the short-circuit determination unit monitors the voltage of the anode of the first diode, and determines that the semiconductor switching element is short-circuited when the monitored voltage becomes higher than a threshold voltage set higher than a voltage obtained by subtracting a voltage drop of the third diode when the semiconductor switching element starts to turn on from a mirror voltage of the gate when the semiconductor switching element starts to turn on, The power conversion device is configured such that, when the short-circuit determination unit determines that a short circuit has occurred, the drive unit sets the gate voltage to a voltage at which the semiconductor switching element is turned off.

3. A power conversion device as described in claim 1 or 2, wherein the threshold voltage is a voltage lower than the voltage output by the driving unit when the semiconductor switching element turns on the gate voltage.

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