JP2020120435A - Power conversion apparatus - Google Patents

Abstract

To suppress an excessive reverse conducting current flowing through a body diode of a MOSFET.SOLUTION: A power conversion apparatus includes switching circuits that respectively have first and second switching elements, and a control device that selectively performs a first switching control and a second switching control. The second switching element is a MOSFET, and is made from a second semiconductor material that has a wider band gap than a first semiconductor material of the first switching element. The control device is configured to maintain a gate voltage to a source of the MOSFET at a prescribed voltage while performing the first switching control. Accordingly, when a reverse conducting current exceeding a prescribed allowable value flows through a body diode of the MOSFET, a gate voltage to a drain of the MOSFET becomes higher than a threshold voltage of the MOSFET.SELECTED DRAWING: Figure 4

Description

The technique disclosed in the present specification relates to a power conversion device.

There is known a power conversion device such as a DC-DC converter or an inverter that converts power between a power source and a load. In this type of power conversion device, a power supply and a load are connected via a plurality of switching circuits, and each switching circuit is controlled by, for example, a pulse width modulation (PWM) method, so that Power conversion is performed with the load.

For example, Patent Document 1 discloses a power conversion device. In this power converter, 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. The MOSFET is configured using a wide band gap semiconductor such as silicon carbide (SiC). Since the MOSFET composed of the wide band gap semiconductor has the advantages of excellent withstand voltage and high allowable current density, it is possible to achieve miniaturization while maintaining high performance.

JP, 2014-027816, A

Usually, each switching circuit is provided with a freewheeling diode. The free wheeling diode may be prepared separately from the two switching elements, or may be provided integrally with the IGBT, for example. Thereby, the reverse conduction current at the time of reverse conduction bypasses the switching element and flows through the return diode. However, the body diode is inherent in the MOSFET. Therefore, a reverse conduction current may flow not only in the free wheeling diode but also in the body diode of the MOSFET. In particular, with a configuration in which a large-sized IGBT is used for one switching element and a small-sized MOSFET is used for the other switching element, an excessive current of the MOSFET is generated during reverse conduction when 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 in the body diode of the MOSFET while the IGBT or other first switching element is being driven. To do.

The power conversion device disclosed in the present specification includes at least two switching circuits and a control device. The switching circuits are provided on a 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 a first switching control for driving the first switching element and a second switching control for driving the second switching element based on a current flowing through the switching circuit or the like. The first switching element is composed of the first semiconductor material. The second switching element is a MOSFET. The MOSFET is composed of a second semiconductor material having a bandgap wider than that of the first switching element. The size of the MOSFET is smaller than the size of the first switching element. The control device holds the gate voltage for the source of the MOSFET at a predetermined voltage during execution of the first switching control. Thereby, when a reverse conduction current exceeding a predetermined allowable value flows in the body diode of the MOSFET, the gate voltage to the drain of the MOSFET becomes higher than the threshold voltage of the MOSFET.

In the above-described power converter, the gate voltage with respect to the source of the MOSFET is held at a predetermined voltage during 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 higher than the threshold voltage of the MOSFET. As a result, the MOSFET becomes conductive, and a current flows between the source and the drain, thereby limiting the current flowing through the body diode. As a result, an excessive current is prevented from flowing in the body diode of the MOSFET, or the occurrence of such a situation is suppressed.

3 is a block diagram showing the configuration of the power conversion device 10. FIG. 6 is a graph showing the gate drive voltage of each switching element, which is determined according to the current flowing in the switching circuit. The flow of reverse conduction currents (IF1, IF2) at the time of reverse conduction is schematically shown. A state in which the MOSFET 36 is turned on is schematically shown at a point A in FIG. It is a figure which shows the flow of the reverse conduction current (IF1, Ids, IF2-Ids) when the gate voltage Vges2 of MOSFET36 is hold|maintained at 0V at the time of reverse conduction in a large current area.

A power converter 10 according to an embodiment will be described with reference to the drawings. The power conversion device 10 of the present embodiment can configure 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. The power conversion device 10 of the present embodiment is mounted on a vehicle such as a hybrid vehicle, a fuel cell vehicle, or an electric vehicle, for example. However, the technology disclosed in the present embodiment can be applied not only to the power conversion device 10 mounted on an automobile but also to power conversion devices for various purposes.

FIG. 1 shows a block diagram of a 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 source 4 and the load 6. The upper switching circuit 20 is provided in a power supply path that connects the positive electrode of the power supply 4 and the load 6. The lower switching circuit 30 is provided on a power supply path that connects the negative electrode of the power supply 4 and the load 6. That is, the upper switching circuit 20 is located in the so-called upper arm, and the lower switching circuit 30 is located in the so-called lower arm. The load 6 is connected to the upper switching circuit 20 and the middle point of the lower switching circuit 30. The control device 12 is connected to the respective switching circuits 20 and 30 via drive circuits 42, 44, 46 and 48.

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 element made of silicon (Si), and the MOSFET 26 is a semiconductor element made of silicon carbide (SiC). The IGBT 22 in this embodiment is an RC (Reverse-Conducting)-IGBT, and a free wheeling diode 24 is integrally provided. A body diode 28 is included in the MOSFET 26. The free wheeling diode 24 may be provided as an independent semiconductor element, separately from the IGBT 22.

Similarly, 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 element made of silicon, and the MOSFET 36 is a semiconductor element made of silicon carbide. Also in the lower switching circuit 30, the IGBT 32 is an RC (Reverse-Conducting)-IGBT, and the free wheeling diode 34 is integrally provided. A body diode 38 is included in the MOSFET 36. The free wheeling diode 34 may be provided as an independent semiconductor element separately from the IGBT 32.

Silicon carbide forming the MOSFETs 26 and 36 has a wider bandgap than silicon, and is one of the so-called wide bandgap semiconductors. Silicon carbide is an example of the second semiconductor material in the present technology. The second semiconductor material forming the MOSFETs 26 and 36 is not limited to silicon carbide and may be, for example, gallium nitride (GaN), gallium oxide (Ga ₂ O ₃ ), or another wide band gap semiconductor such as diamond. .. The semiconductor material forming the IGBTs 22 and 32 is not limited to silicon. It suffices that the semiconductor material forming the MOSFETs 26 and 36 has a wider band gap than the semiconductor material forming the IGBTs 22 and 32. Moreover, the IGBTs 22 and 32 are not necessarily limited to the IGBTs, and may be switching elements having other structures.

The control device 12 performs power conversion between the power source 4 and the load 6 by controlling the upper switching circuit 20 and the lower switching circuit 30 by, for example, the 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 controller 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 the present embodiment selectively executes the first switching control and the second switching control according to the current 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, the control device 12 controls the gate voltages Vge1 and Vge2 of the IGBTs 22 and 32 via the drive circuits 42 and 46 when executing the first switching control. 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 made of silicon carbide. Silicon carbide is a kind of wide band gap semiconductor, and MOSFETs 26 and 36 configured by using it have the advantages of excellent withstand voltage and high allowable current density. Therefore, the MOSFETs 26 and 36 can be downsized while maintaining high performance. In this regard, in general, the manufacturing costs of the switching elements 22, 26, 32, 36 such as IGBTs and MOSFETs increase in proportion to their sizes, and this tendency is remarkable in the MOSFETs 26, 36 employing wide band gap semiconductors. Becomes Therefore, in the present embodiment, the sizes of the MOSFETs 26 and 36 are made smaller than the sizes of the IGBTs 22 and 32, thereby reducing manufacturing costs while maintaining high performance. Note that the sizes of the switching elements 22, 26, 32, and 36 in the present specification are intended to be sizes when viewed in plan, and are also referred to as chip sizes, for example.

The controller 12 performs the first switching control for driving the large-sized IGBT 22 during the period when a large current exceeding the predetermined current flows in the upper switching circuit 20. In this 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 the second switching control for driving the MOSFET 26 having a small size during a period in which a small current smaller than a predetermined current flows through the upper switching circuit 20. In this 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 when the current in the large current region flows through the lower switching circuit 30, the control device 12 performs the first switching control for driving the IGBT 32 having a large size. 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-sized MOSFET 36 is performed.

FIG. 2 shows an example of control of the gate voltages Vge1, Vge2, Vgs1, Vgs2 by the controller 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. The second switching control is executed in the small current region, and the gate voltages Vgs1 and Vgs2 of the MOSFETs 26 and 36 are controlled with time in order to drive the MOSFETs 26 and 36. As one example, in the second switching control, the gate voltages Vgs1 and Vgs2 of the MOSFETs 26 and 36 are controlled between −5 and 20 volts. On the other hand, the gate voltages Vge1 and Vge2 of the IGBTs 22 and 32 are maintained at zero volts.

Here, as understood from the graph of the gate voltages Vgs1 and Vgs2 shown in FIG. 2, the gate voltages Vgs1 and Vgs2 when turning off the MOSFETs 26 and 36 are mutually different between the first switching control and the second switching control. Be 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.

The control device 12 causes all of the switching elements 22, 26, 32, and 36 to turn on at the timing (A in FIG. 2) at which the lower switching circuit 30 is turned on from the state where the upper switching circuit 20 is turned on. In the direction from the positive electrode side to the negative electrode side of No. 4, there is provided a time in which the non-conduction state is established. This time is called dead time. Immediately after the upper switching circuit 20 becomes non-conductive, the current is not completely cut off, and therefore the lower switching circuit 30 is made conductive in the direction from the positive side to the negative side of the power supply 4 at that timing. If you do so, you may cause a short circuit. In order to prevent this, dead time is provided. During this dead time, 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 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 conducting to the state in which the lower switching circuit 30 is conducting. Normally, it is assumed that the reverse conduction currents (IF1, IF2) flow in the free wheeling diode 34 provided integrally with the IGBT 32. However, the body diode 38 is also present 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 free wheeling diode 34 but also in the body diode 38 of the MOSFET 36.

Here, the IGBT 32 has a relatively large size because it is responsible for switching in a large current region. On the other hand, since the MOSFET 36 is responsible for switching in the small current region, a relatively small size MOSFET is used. Therefore, the allowable current of the MOSFET 36 is relatively smaller than the allowable current of the IGBT 32. Therefore, the reverse conduction current in the large current region is too large 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, when a reverse conduction current exceeding a predetermined allowable value flows in the body diode 38 of the MOSFET 36 and the forward voltage Vf thereof reaches a predetermined level, the voltage of the gate to the drain of the MOSFET 36 ( Vgs2−Vd, and Vd=−Vf) becomes 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, becomes IF2-Ids).

In the power conversion device 10 of the present 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 that exceeds a predetermined allowable value flows through the body diode 38 of the MOSFET 36, the gate voltage Vd with respect to the drain of the MOSFET 36 becomes higher 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 of the current of the MOSFET 36 and the threshold voltage of the MOSFET 36. Therefore, for example, the threshold voltage of the MOSFET 36 changes according to the temperature of the MOSFET 36. Therefore, the gate voltage Vgs1 applied 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 the source-drain becomes conductive. Therefore, the gate voltage Vgs1 applied to the MOSFET 36 in the first switching control is not limited to zero volts 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 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 from the second drive circuit 44 at a predetermined voltage and making the MOSFET 26 conductive between the source and the drain, it is possible to prevent an excessive reverse conduction current from flowing in the body diode 28. Alternatively, it can be suppressed.

Specific examples of the technology disclosed in the present specification have been described above in detail, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above. The technical elements described in the present specification or the drawings exert technical utility alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. Further, the technique illustrated in the present specification or the drawings can simultaneously achieve a plurality of purposes, and achieving the one purpose among them has technical utility.

4: Power supply 6: Load 10: Power conversion device 12: Control devices 20, 30: Switching circuits 22, 32: IGBT
24, 34: Free wheeling diodes 26, 36: MOSFET
28, 38: body diodes 42, 44, 46, 48: drive circuits

Claims (1)

A power conversion device for converting power between a power source and a load,
At least two switching circuits, which are provided on a power supply path from the power source to the load and each have a first switching element and a second switching element connected in parallel with each other,
A control device that selectively executes first switching control that drives the first switching element and second switching control that drives the second switching element;
Equipped with
The first switching element is composed of a first semiconductor material,
The second switching element is a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor), is composed of a second semiconductor material having a wider bandgap than the first semiconductor material, and has a size of 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 execution of the first switching control, whereby the body diode of the MOSFET has a predetermined allowable value. A reverse conduction current of more than, the gate voltage to the drain of the MOSFET becomes larger than the threshold voltage of the MOSFET,
Power converter.

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