JP7088041B2 - Power converter - Google Patents

Description

The techniques disclosed herein relate to power converters.

Power conversion devices that perform power conversion between a power source and a load, such as a DC-DC converter and an inverter, are known. This type of power converter connects a power supply and a load via a plurality of switching circuits, and controls each switching circuit by, for example, a pulse width modulation (PWM) method. Power conversion is performed with the load.

For example, Patent Document 1 discloses a power conversion device. In this power conversion device, each switching circuit has two switching elements connected in parallel. One switching element is an IGBT (Insulated Gate Bipolar Transistor), and the other switching element is a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor). The IGBT and MOSFET are selectively used according to the current flowing through the switching circuit and the like. The MOSFET is configured using a wide bandgap semiconductor such as silicon carbide (SiC). A MOSFET composed of a wide bandgap semiconductor has the advantages of excellent withstand voltage resistance and high allowable current density, so that it is possible to reduce the size while maintaining high performance.

Japanese Unexamined Patent Publication No. 2014-027816

Usually, each switching circuit is provided with a freewheeling diode. The freewheeling diode may be prepared separately from the two switching elements, or may be provided integrally with the IGBT, for example. As a result, the reverse conduction current at the time of reverse conduction bypasses the switching element and flows through the freewheeling diode. However, a body diode is inherent in the MOSFET. Therefore, a reverse conduction current may flow not only in the freewheeling diode but also in the body diode of the MOSFET. In particular, if a large IGBT is used for one switching element and a small MOSFET is used for the other switching element, an excessive current will be generated in the MOSFET during reverse conduction while driving the IGBT. It may flow to the body diode. In view of this, the present specification provides a technique capable of preventing or suppressing an excessive reverse conduction current from flowing through the body diode of the MOSFET while driving the IGBT or other first switching element. do.

The power conversion device disclosed herein includes at least two switching circuits and a control device. These switching circuits are provided on the power supply path from the power supply to the load, and each switching circuit has a first switching element and a second switching element connected in parallel with each other. The control device selectively executes the first switching control for driving the first switching element and the second switching control for driving the second switching element based on the current flowing through the switching circuit and the like. The first switching element is configured by using the first semiconductor material. The second switching element is a MOSFET. The MOSFET is configured by using a second semiconductor material having a bandgap wider than that of the first switching element. The size of the MOSFET is configured to be smaller than the size of the first switching element. The control device keeps the gate voltage to the source of the MOSFET at a predetermined voltage during the execution of the first switching control. As a result, when a reverse conduction current exceeding a predetermined allowable value flows through the body diode of the MOSFET, the gate voltage with respect to the drain of the MOSFET is configured to be larger than the threshold voltage of the MOSFET.

In the power conversion device described above, the gate voltage with respect to the source of the MOSFET is held at a predetermined voltage during the execution of the first switching control. As a result, when a reverse conduction current exceeding a predetermined allowable value flows through the body diode of the MOSFET, the gate voltage with respect to the drain of the MOSFET becomes larger than the threshold voltage of the MOSFET. As a result, the MOSFET becomes conductive and the current flows between the source and the drain, which limits the current flowing through the body diode. As a result, it is possible to prevent an excessive current from flowing through the body diode of the MOSFET, or to suppress the occurrence of such a situation.

It is a block diagram which shows the structure of the power conversion apparatus 10. It is a graph which shows the gate drive voltage of each switching element which is determined according to the current which flows in a switching circuit. The flow of the reverse conduction current (IF1, IF2) at the time of reverse conduction is schematically shown. At point A in FIG. 2, a state in which the MOSFET 36 is turned on is schematically shown. It is a figure which shows the flow of the reverse conduction current (IF1, Ids, IF2-Ids) when the gate voltage Vges2 of the MOSFET 36 is held at 0V at the time of reverse conduction in a large current region.

The power conversion device 10 of the embodiment will be described with reference to the drawings. The power conversion device 10 of this embodiment can form a part of a power converter such as a DC-DC converter or an inverter that performs power conversion between the power supply 4 and the load 6. Although the power conversion device 10 of this embodiment is an example, it is mounted on an automobile such as a hybrid vehicle, a fuel cell vehicle, or an electric vehicle. However, the technique disclosed in this embodiment can be adopted not only in the power conversion device 10 mounted on the automobile but also in the power conversion device for various purposes.

FIG. 1 shows a block diagram of the power conversion device 10 of this embodiment. The power conversion device 10 includes an upper switching circuit 20, a lower switching circuit 30, a control device 12, and a plurality of drive circuits 42, 44, 46, 48. The upper switching circuit 20 and the lower switching circuit 30 are connected in series on the power supply path between the power supply 4 and the load 6. The upper switching circuit 20 is provided in a power supply path connecting the positive electrode of the power supply 4 and the load 6. The lower switching circuit 30 is provided on a power supply path connecting the negative electrode of the power supply 4 and the load 6. That is, the upper switching circuit 20 is located on the so-called upper arm, and the lower switching circuit 30 is located on the so-called lower arm. The load 6 is connected to the upper switching circuit 20 at the midpoint of the lower switching circuit 30. The control device 12 is connected to the switching circuits 20 and 30 via the drive circuits 42, 44, 46 and 48, respectively.

The upper switching circuit 20 includes an IGBT 22 and a MOSFET 26. The IGBT 22 and the MOSFET 26 are connected in parallel with each other. The IGBT 22 is a semiconductor device made of silicon (Si), and the MOSFET 26 is a semiconductor device made of silicon carbide (SiC). The IGBT 22 in this embodiment is an RC (Reverse-Conducting) -IGBT, and a freewheeling diode 24 is integrally provided. Further, the body diode 28 is inherent in the MOSFET 26. The freewheeling diode 24 may be provided as an independent semiconductor element separately from the IGBT 22.

The lower switching circuit 30 also includes an IGBT 32 and a MOSFET 36. The IGBT 32 and the MOSFET 36 are connected in parallel with each other. The IGBT 32 is a semiconductor device made of silicon, and the MOSFET 36 is a semiconductor device made of silicon carbide. Also in the lower switching circuit 30, the IGBT 32 is an RC (Reverse-Conducting) -IGBT, and a freewheeling diode 34 is integrally provided. Further, the body diode 38 is inherent in the MOSFET 36. The freewheeling diode 34 may be provided as an independent semiconductor element separately from the IGBT 32.

The silicon carbide constituting the MOSFETs 26 and 36 has a bandgap wider than that of silicon, and is one of those referred to as a wide bandgap semiconductor. Silicon carbide is an example of a second semiconductor material in this technique. The second semiconductor material constituting the MOSFETs 26 and 36 is not limited to silicon carbide, and may be other wide bandgap semiconductors such as gallium nitride (GaN), gallium oxide (Ga ₂ O ₃ ), or diamond. .. Further, the semiconductor material constituting the IGBTs 22 and 32 is not limited to silicon. The semiconductor material constituting the MOSFETs 26 and 36 may have a wider bandgap than the semiconductor material constituting the IGBTs 22 and 32. Further, the IGBTs 22 and 32 are not necessarily limited to the IGBT, and may be switching elements having other structures.

The control device 12 performs power conversion between the power supply 4 and the load 6 by controlling the upper switching circuit 20 and the lower switching circuit 30 by, for example, a PWM method. The IGBT 22 of the upper switching circuit 20 is connected to the control device 12 via the first drive circuit 42. Similarly, the MOSFET 26 of the upper switching circuit 20 is connected to the control device 12 via the second drive circuit 44, and the IGBT 32 of the lower switching circuit 30 is connected to the control device 12 via the third drive circuit 46. The MOSFET 36 of the lower switching circuit 30 is connected to the control device 12 via the fourth drive circuit 48. The control device 12 controls the operation of the upper switching circuit 20 and the lower switching circuit 30 via the plurality of drive circuits 42, 44, 46, 48.

In particular, the control device 12 in this embodiment selectively executes the first switching control and the second switching control according to the currents flowing through the switching circuits 20 and 30. In the first switching control, only the IGBTs 22 and 32 are driven, and the MOSFETs 26 and 36 are not used. On the other hand, in the second switching control, only the MOSFETs 26 and 36 are driven, and the IGBTs 22 and 32 are not used. That is, when executing the first switching control, the control device 12 controls the gate voltages Vge1 and Vge2 of the IGBTs 22 and 32 via the drive circuits 42 and 46. On the other hand, when executing the second switching control, the control device 12 controls the gate voltages Vgs1 and Vgs2 of the MOSFETs 26 and 36 via the drive circuits 44 and 48.

As described above, the MOSFETs 26 and 36 are configured using silicon carbide. Silicon carbide is a kind of wide bandgap semiconductor, and the MOSFETs 26 and 36 configured by using the silicon carbide have the advantages of excellent withstand voltage resistance and high allowable current density. Therefore, the MOSFETs 26 and 36 can be miniaturized while maintaining high performance. In this regard, in general, the manufacturing cost of switching elements 22, 26, 32, 36 such as IGBTs and MOSFETs increases in proportion to their sizes, and the tendency is remarkable in MOSFETs 26, 36 using wide bandgap semiconductors. It becomes. Therefore, in this embodiment, the sizes of the MOSFETs 26 and 36 are smaller than the sizes of the IGBTs 22 and 32, thereby reducing the manufacturing cost while maintaining high performance. The sizes of the switching elements 22, 26, 32, and 36 in the present specification are intended to be the size when viewed in a plan view, and are also referred to as, for example, the chip size.

The control device 12 performs the first switching control for driving the large-sized IGBT 22 during a period in which a large current exceeding a predetermined current flows through the upper switching circuit 20. In the present specification, a period in which a large current larger than a predetermined current flows is referred to as a "large current region". The control device 12 performs a second switching control for driving the small-sized MOSFET 26 during a period in which a small current smaller than a predetermined current flows through the upper switching circuit 20. In the present specification, a period in which a small current smaller than a predetermined current flows is referred to as a "small current region". Similarly, during the period in which a current in a large current region flows through the lower switching circuit 30, the control device 12 performs the first switching control for driving the large-sized IGBT 32. During the period when the current in the small current region flows through the lower switching circuit 30, the second switching control for driving the small size MOSFET 36 is performed.

FIG. 2 shows an example of control of the gate voltages Vge1, Vge2, Vgs1, and Vgs2 by the control device 12. As shown in FIG. 2, the first switching control is executed in the large current region, and the gate voltages Vge1 and Vge2 of the IGBTs 22 and 32 are controlled over time in order to drive the IGBTs 22 and 32. As an example, in the first switching control, the gate voltages Vge1 and Vge2 of the IGBTs 22 and 32 are controlled between 0 volt and 15 volt. On the other hand, the gate voltages Vgs1 and Vgs2 of the MOSFETs 26 and 36 are maintained at 0 volt. In the small current region, the second switching control is executed, and the gate voltages Vgs1 and Vgs2 of the MOSFETs 26 and 36 are controlled over time in order to drive the MOSFETs 26 and 36. As an example, in the second switching control, the gate voltages Vgs1 and Vgs2 of the MOSFETs 26 and 36 are controlled between −5 volt and 20 volt. On the other hand, the gate voltages Vge1 and Vge2 of the IGBTs 22 and 32 are maintained at zero volt.

Here, as can be understood from the graphs of the gate voltages Vgs1 and Vgs2 shown in FIG. 2, between the first switching control and the second switching control, the gate voltages Vgs1 and Vgs2 when the MOSFETs 26 and 36 are turned off are mutually exclusive. It's different. That is, in the first switching control, the gate voltages Vgs1 and Vgs2 when turning off the MOSFETs 26 and 36 are zero volts, while in the second switching control, the gate voltages Vgs1 and Vgs2 when turning off the MOSFETs 26 and 36 are -5. It is a bolt.

In the control device 12, all the switching elements 22, 26, 32, and 36 are powered by the power supply at the timing of switching from the state in which the upper switching circuit 20 is conducted to the state in which the lower switching circuit 30 is conducted (A in FIG. 2). A time is provided for the non-conducting state in the direction from the positive electrode side to the negative electrode side of 4. This time is called dead time. Immediately after the upper switching circuit 20 is in the non-conducting state, the current is not completely cut off. Therefore, at that timing, the lower switching circuit 30 is in the conductive state in the direction from the positive electrode side to the negative electrode side of the power supply 4. If it is set to, there is a risk of causing a short circuit. A dead time is set to prevent this from happening. In this dead time, a reverse conduction current may occur in the lower switching circuit 30. In this embodiment, the reverse conduction current at the timing of switching from the upper switching circuit 20 to the lower switching circuit 30 will be mainly described, but the same applies to the timing of switching from the lower switching circuit 30 to the upper switching circuit 20.

FIG. 3 shows the flow of the reverse conduction currents (IF1, IF2) generated in the lower switching circuit 30 at the timing of switching from the state in which the upper switching circuit 20 is made conductive to the state in which the lower switching circuit 30 is made conductive. Normally, it is assumed that the reverse conduction currents (IF1, IF2) flow through the freewheeling diode 34 provided integrally with the IGBT 32. However, the body diode 38 is also inherent in the MOSFET 36 connected in parallel with them. Therefore, as shown in FIG. 3, the reverse conduction current IF2 may flow not only in the freewheeling diode 34 but also in the body diode 38 of the MOSFET 36.

Here, since the IGBT 32 is responsible for switching in a large current range, a relatively large size is adopted. On the other hand, since the MOSFET 36 is responsible for switching in a small current region, a MOSFET 36 having a relatively small size is adopted. Therefore, the allowable current of the MOSFET 36 is relatively small with respect to the allowable current of the IGBT 32. Therefore, the reverse conduction current in the large current region is excessive for the body diode 38 of the MOSFET 36, and may damage the MOSFET 36, for example.

Regarding the above problem, in the power conversion device 10 of the present embodiment, the gate voltage Vgs2 with respect to the source of the MOSFET 36 is held at zero volt during the execution of the first switching control. As a result, as shown in FIG. 4, a reverse conduction current exceeding a predetermined allowable value flows through the body diode 38 of the MOSFET 36, and when the forward voltage Vf reaches a predetermined level, the gate voltage with respect to the drain of the MOSFET 36 ( Vgs2-Vd, where Vd = −Vf) is larger than the threshold voltage of the MOSFET 36. As a result, as shown in FIG. 5, the MOSFET 36 becomes conductive and the current Ids flows between the source and the drain, so that the current IF2 flowing through the body diode 38 is limited (that is, IF2-Ids).

In the power conversion device 10 of this embodiment, the gate voltage Vgs1 of the MOSFET 36 is maintained at zero volt in the first switching control. As a result, when a reverse conduction current exceeding a predetermined allowable value flows through the body diode 38 of the MOSFET 36, the voltage Vd of the gate with respect to the drain of the MOSFET 36 becomes larger than the threshold voltage of the MOSFET 36. In other words, in the first switching control, the specific value of the gate voltage Vgs1 given to the MOSFET 36 can be determined according to the allowable value for the current of the MOSFET 36 and the threshold voltage of the MOSFET 36. Therefore, for example, since the threshold voltage of the MOSFET 36 changes according to the temperature of the MOSFET 36, the gate voltage Vgs1 given to the MOSFET 36 in the first switching control may also be adjusted according to the temperature.

In any case, when the gate voltage with respect to the source of the MOSFET 36 is Va, the gate-drain voltage Vgd when the reverse conduction current IF2 flows is Vgd = Va + VF. If the gate-drain voltage Vgd exceeds the threshold voltage Vth of the MOSFET 36, that is, if Va + VF> Vth is satisfied, the MOSFET 36 is turned on and becomes a conduction state between the source and the drain. Therefore, the gate voltage Vgs1 applied to the MOSFET 36 in the first switching control is not limited to zero volt, and may be any value that satisfies Va + VF> Vth.

Further, in this embodiment, the reverse conduction current generated in the lower switching circuit 30 at the timing of switching from the upper switching circuit 20 to the lower switching circuit 30 has been described. On the other hand, at the timing of switching from the lower switching circuit 30 to the upper switching circuit 20, a reverse conduction current is generated in the upper switching circuit 20 from the load 6 toward the positive electrode side of the power supply 4. Even in this case, the same effect can be obtained by reversing the roles of the upper switching circuit 20 and the lower switching circuit 30. That is, by holding the voltage output by the second drive circuit 44 at a predetermined voltage and conducting the MOSFET 26 between the source and the drain, it is possible to prevent an excessive reverse conduction current from flowing through the body diode 28. Or it can be suppressed.

Although specific examples of the techniques disclosed in the present specification have been described in detail above, these are merely examples and do not limit the scope of claims. The techniques described in the claims include various modifications and modifications of the specific examples exemplified above. The technical elements described in the present specification or the drawings exhibit technical usefulness alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. Further, the techniques exemplified in the present specification or the drawings can achieve a plurality of purposes at the same time, and achieving one of the purposes itself has technical usefulness.

4: Power supply 6: Load 10: Power conversion device 12: Control device 20, 30: Switching circuit 22, 32: IGBT
24, 34: Reflux diode 26, 36: MOSFET
28, 38: Body diode 42, 44, 46, 48: Drive circuit

Claims (1)

A power conversion device that converts power between a power source and a load.
At least two switching circuits provided on the power supply path from the power supply to the load and each having a first switching element and a second switching element connected in parallel to each other.
A control device that selectively executes a first switching control for driving the first switching element and a second switching control for driving the second switching element.
Equipped with
The first switching element is configured by using the first semiconductor material.
The second switching element is a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor), which is configured by using a second semiconductor material having a bandgap wider than that of the first semiconductor material, and its size is described above. Smaller than the size of the first switching element,
The control device is configured to hold the gate voltage with respect to the source of the MOSFET at a predetermined voltage during the execution of the first switching control, whereby a predetermined allowable value is set in the body diode of the MOSFET. When a reverse conduction current exceeding the above is applied, the gate voltage with respect to the drain of the MOSFET becomes larger than the threshold voltage of the MOSFET.
Power converter.

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