Gate drive circuit

A gate drive circuit for driving an IGBT serving as a power semiconductor device includes a constant-current gate drive circuit that charges a gate capacity of the IGBT at a constant current, and a constant-voltage gate drive circuit that is connected in parallel to the constant-current gate drive circuit between input and output terminals thereof via a series circuit constituted by a MOSFET and a resistor, and charges the gate capacity of the IGBT at a constant voltage, wherein the gate drive circuit charges the gate capacity of the IGBT using both the constant-current gate drive circuit and the constant-voltage gate drive circuit at the time of driving the IGBT.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No. PCT/JP2011/063240 filed Jun. 9, 2011, the contents of all of which are incorporated herein by reference in their entirety.

FIELD

The present invention relates to a gate drive circuit for driving a power semiconductor device.

BACKGROUND

As a conventional gate drive circuit, the following technique has been disclosed. That is, to reduce noise caused by ringing or the like in a switching circuit including an IGBT (Insulated Gate Bipolar Transistor) that is configured to connect diodes formed of silicon carbide (SiC) having low recovery current as a material (hereinafter, “SiC diodes”) in parallel (in anti-parallel, to be more precise) and that serves as a power semiconductor device while reducing the device loss of the IGBT at the time of turning on the IGBT and the diode loss during a recovery time, the conventional gate drive circuit increases resistance of resistors connected in series to the gate of the IGBT right before the IGBT is turned on so as to control a current change rate at the time of turning on the IGBT to become gradual from the middle (for example, see Patent Literature 1).

CITATION LIST

Patent Literature

SUMMARY

Technical Problem

However, this conventional technique has a problem that because the control is executed to make a current change rate at the time of turning on the IGBT to be gradual from the middle, the starting time since a start signal (a command signal) is output until the power semiconductor device actually starts operating is elongated.

Furthermore, this conventional technique is a method of changing the resistance by on-off controlling the switching device connected to both ends of one of the two resistors connected in series. Accordingly, it is necessary to make the resistance value of two resistors different from each other to some extent so as to effectively execute this control. However, when the difference of resistance value between the two resistors is large, a gate voltage disadvantageously greatly changes before and after switching the resistance value. Because the gate voltage change is one of noise increasing factors, it is preferable to avoid this gate voltage change.

The present invention has been achieved to solve the above problems, and an object of the present invention is to provide a gate drive circuit capable of reducing the starting time while suppressing the change in a gate voltage during the transition to turning on a power semiconductor device.

Solution to Problem

In order to solve the aforementioned problems, a gate drive circuit for driving a power semiconductor device according to one aspect of the present invention is configured to include: a constant-current gate drive circuit that charges a gate capacity of the power semiconductor device at a constant current; and a constant-voltage gate drive circuit that is connected in parallel to the constant-current gate drive circuit between input and output terminals thereof via a series circuit constituted by a switching device and a resistor, and charges the gate capacity at a constant voltage, wherein at the time of driving the power semiconductor device, the gate drive circuit charges the gate capacity of the power semiconductor device using both the constant-current gate drive circuit and the constant-voltage gate drive circuit.

Advantageous Effects of Invention

According to the present invention, it is possible to shorten a starting time while suppressing the change in a gate voltage (hereinafter may be referred to just as “gate voltage change”) at the time of turning on a power semiconductor device.

DESCRIPTION OF EMBODIMENTS

Exemplary embodiments of a gate drive circuit according to the present invention will be explained below in detail with reference to the accompanying drawings. The present invention is not limited to the embodiments.

First Embodiment

FIG. 1depicts a configuration of the gate drive circuit according to the first embodiment of the present invention. As shown inFIG. 1, the gate drive circuit according to the first embodiment is configured to include a constant-current gate drive circuit1that is connected to the gate of an IGBT2serving as a power semiconductor device to limit the gate current of the IGBT2for driving the IGBT2, a power-semiconductor-device control circuit3that outputs an ON command signal (an ON command voltage) to the constant-current gate drive circuit1, a constant-voltage gate drive circuit5that is connected in parallel to the constant-current gate drive circuit1for driving the IGBT2at a constant voltage, and a reference voltage source7that applies a reference voltage16to the constant-voltage gate drive circuit5.

As shown inFIG. 1, the constant-current gate drive circuit1is configured to include resistors1-3and1-4, transistors (PNP bipolar transistors in an example ofFIG. 1)1-1and1-2, and a diode1-5connected in series to the collector of the transistor1-1. The cathode of the diode1-5serves as an output terminal of the constant-current gate drive circuit1and is connected to the gate of the IGBT2. The constant-current gate drive circuit1includes a function of limiting the gate current10at the time of turning on the IGBT2to a predetermined upper limit.

As shown inFIG. 1, the constant-voltage gate drive circuit5is configured to include a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor)4that is an example of the switching device, a resistor5-1connected in series to the MOSFET4, a MOSFET control circuit6that controls the MOSFET4as a switching device control circuit, and a gate-voltage detection comparator8-1that serves as a first comparator to which a gate voltage11of the IGBT2and the reference voltage16applied from the reference voltage source7are input as input signals and that controls the MOSFET control circuit6according to a magnitude relation between these signals. The constant-voltage gate drive circuit5is connected in parallel to the constant-current gate drive circuit1, and supplies a necessary gate current to the IGBT2together with the constant-current gate drive circuit1. In the example ofFIG. 1, the gate drive circuit is configured so that the source of the MOSFET4that constitutes one end of a series circuit constituted by the MOSFET4and the resistor5-1is connected to one end of the resistor1-3of the constant-current gate drive circuit1, and one end of the resistor5-1of the constant-voltage gate drive circuit5that constitutes the other end of the series circuit is connected to the cathode of the diode1-5. Alternatively, the gate drive circuit can be configured so that a connection relation of the series circuit constituted by the MOSFET4and the resistor5-1is reversed, whereby one end of the resistor5-1of the constant-voltage gate drive circuit5is connected to one end of the resistor1-3of the constant-current gate drive circuit1and the drain of the MOSFET4is connected to the cathode of the diode1-5.

Operations performed by the gate drive circuit according to the first embodiment are explained next. With reference toFIG. 2, the operation performed by the constant-current gate drive circuit1is explained first.FIG. 2depicts a circuit configuration in which a part corresponding to the constant-voltage gate drive circuit5is omitted fromFIG. 1.

When the IGBT2is to be turned on, an ON command signal9is input to the constant-current gate drive circuit1from the power-semiconductor-device control circuit3. When the ON command signal9is input to the constant-current gate drive circuit1, the transistor1-1enters into a conductive state, an emitter current flows via the resistor1-3, and a base current flows via the resistor1-4. Furthermore, a collector current flows to the transistor1-1via the diode1-5, and this collector current becomes the gate current10flowing to the IGBT2and charges the gate capacity of the IGBT2.

When the emitter current of the transistor1-1increases, a voltage drop increases in the resistor1-3. Because this voltage drop applies a forward bias voltage between the base and the emitter of the transistor1-2, the transistor1-2enters into a conductive state. When the transistor1-2is conductive, the current (the emitter current) flowing to the transistor1-1flows to the transistor1-2, and the voltage drop in the resistor1-3decreases. On the other hand, when the voltage drop decreases in the resistor1-3, the bias voltage between the base and the emitter of the transistor1-2decreases, and the transistor1-2changes from the conductive state into a cutoff state. In short, such an operation is performed instantaneously, and a constant current at a value obtained by dividing a forward voltage drop (0.6 volt, for example) between the base and the emitter (at a PN junction part) of the transistor1-2by a resistance value of the resistor1-3flows to the emitter of the transistor1-1. The gate current that charges the IGBT2also becomes a constant current because the collector current is substantially equal to the emitter current due to transistor characteristics. In this way, the constant-current gate drive circuit1drives the IGBT2serving as the power semiconductor device at a constant current.

Next, the operation performed by the gate drive circuit according to the first embodiment, that is, the operation in a case of using both the constant-current gate drive circuit1and the constant-voltage gate drive circuit5are explained with reference toFIGS. 1 and 3.FIG. 3is a time chart for explaining the operation performed by the gate drive circuit according to the first embodiment. In the explanation ofFIG. 3, the power semiconductor device is appropriately abbreviated as “PSD”.

First, at the beginning of gate driving (before the operation time A shown inFIG. 3), the MOSFET4has been turned on. That is, the constant-voltage gate drive circuit5has been set to an operable state. In this state, when the ON command signal9is input to the constant-current gate drive circuit1from the power-semiconductor-device control circuit3(at the operation time A), the current supplied from the constant-voltage gate drive circuit5is superimposed on the current supplied from the constant-current gate drive circuit1at the gate of the IGBT2, thereby charging the gate capacity of the IGBT2. Regarding the waveform shown in the second from the top (the PSD gate current10) inFIG. 3, a solid-line part19indicates a current waveform at the time of driving both the constant-current gate drive circuit1and the constant-voltage gate drive circuit5, and a dashed-line part18indicates a current waveform at the time of controlling the MOSFET4to be turned off (at which time, the constant-voltage gate drive circuit5is isolated) and of driving only the constant-current gate drive circuit1.

The gate voltage (PSD gate voltage)11of the IGBT2is input to the gate-voltage detection comparator8-1. The gate-voltage detection comparator8-1compares the PSD gate voltage11with the reference voltage16applied from the reference voltage source7. As shown inFIG. 3, this reference voltage16is set to be lower than a threshold voltage15(PSD turn-on threshold voltage) at the time of turning on the IGBT2. With this setting, when the PSD gate voltage11gets closer to the PSD turn-on threshold voltage15and exceeds the reference voltage16, the output from the gate-voltage detection comparator8-1is inverted, the MOSFET control circuit6operates to control the MOSFET4to be turned off (the voltage across MOSFET12) rises from “LOW” to “HIGH”), the constant-voltage gate drive circuit5is disconnected from the constant-current gate drive circuit1, and the gate drive circuit enters into a state where only the constant-current gate drive circuit1operates (at an operation time B).

Thereafter, when the PSD gate voltage11exceeds the PSD turn-on threshold voltage15, a PSD collector current13starts to flow (at an operation time C), the PSD collector current13abruptly rises, and after it reaches the peak, then settles down to a predetermined value. Furthermore, the voltage (PSD collector-emitter voltage)14between the collector and the emitter of the IGBT2falls toward a zero potential after the PSD collector current13reaches its peak.

Due to the control described above, the time (a turn-on time) T1, which is until the IGBT2is turned on (A to C), is made shorter than the time in a case in which the gate capacity of the IGBT2is charged using only the constant-current gate drive circuit1, and the time (an actual operation time) T2since a start command is input until the IGBT2actually starts operating is also made shorter. The reason for shortening the turn-on time T1and the actual operating time T2is as follows. By charging the gate capacity of the IGBT2using both the constant-current gate drive circuit1and the constant-voltage gate drive circuit5, more electric charge amounts corresponding to the hatched area shown inFIG. 3can be charged.

As attention is paid to the current waveform indicated by the solid part19inFIG. 3, it can be understood that the current value becomes smaller in accordance with the lapse of time. Such a current waveform is obtained because the PSD gate voltage11rises by charging the gate capacity of the IGBT2and as a result, the voltage across the resistor5-1falls. This result is due to the function of the resistor5-1electrically connected between input and output terminals of the constant-current gate drive circuit1when the MOSFET4is conductive, and can be obtained by allowing the constant-voltage gate drive circuit5to literally drive the IGBT2at a constant voltage. Furthermore, this function can suppress the gate voltage change during the transition to turning on the IGBT2.

As described above, the gate drive circuit according to the first embodiment charges the gate capacity of the IGBT2using both the constant-current gate drive circuit1and the constant-voltage gate drive circuit5at the time of driving the IGBT2serving as the power semiconductor device. Therefore, it is possible to shorten the starting time while suppressing the gate voltage change during the transition to turning on the IGBT2.

Second Embodiment

FIG. 4depicts a configuration of the gate drive circuit according to the second embodiment. The gate drive circuit shown inFIG. 4differs from that shown inFIG. 1according to the first embodiment in that the reference voltage source7has been replaced by a reference-voltage generation circuit7a. Other configurations of the second embodiment are identical or equivalent to those according to the first embodiment shown inFIG. 1and the common constituent elements are denoted by the same or similar reference signs, and common explanations thereof will be appropriately omitted.

A configuration and an operation of the reference-voltage generation circuit7aare explained next. Basic operations performed by the gate drive circuit according to the second embodiment are equivalent to those performed by the gate drive circuit according to the first embodiment and the operation waveforms of principal constituent elements of the gate drive circuit according to the second embodiment are equivalent to those shown inFIG. 3.

With reference toFIG. 4, the reference-voltage generation circuit7ais configured to include a current detection resistor25serving as a current detection unit that detects an emitter current of the IGBT2, a triangular-wave power supply24that is a first power supply circuit, an output circuit8-3constituted by a capacitor and a resistor, and a current detection comparator8-2that serves as a second comparator for comparing the output from the triangular-wave power supply24with that from the current detection resistor25.

In the second embodiment, the reference voltage16input to the gate-voltage detection comparator8-1is generated by the current detection comparator8-2. The current detection comparator8-2generates the reference voltage16by comparing a triangular wave generated by the triangular-wave power supply24with an output from the current detection resistor25, and inputs the generated reference voltage16to the gate-voltage detection comparator8-1via the output circuit8-3. Subsequent operations are identical or equivalent to those according to the first embodiment.

The reference voltage16generated by the reference-voltage generation circuit7ais set to be lower than the PSD turn-on threshold voltage15similarly to the first embodiment. This PSD turn-on threshold voltage15fluctuates depending on characteristics of the IGBT2and also depending on the current flowing to the collector of the IGBT2. On the other hand, the gate drive circuit according to the second embodiment detects the collector current of the IGBT2and generates the reference voltage16using the detected collector current. Accordingly, even when the turn-on threshold voltage of the IGBT2changes, the gate drive circuit can execute the control to follow this change. This control makes it possible to generate the reference voltage16according to the characteristics of the IGBT2and to effectively execute a control for shortening the starting time. Alternatively, the gate drive circuit can detect an emitter current instead of the collector current. Furthermore, any detection unit other than the current detection resistor can be used.

As described above, the gate drive circuit according to the second embodiment charges the gate capacity of the IGBT2using both the constant-current gate drive circuit1and the constant-voltage gate drive circuit5at the time of driving the IGBT2serving as the power semiconductor device. Further, the gate drive circuit detects the current flowing to the IGBT2and controls the operation of the constant-voltage gate drive circuit5using the reference voltage generated by using the detected current. Therefore, in addition to the effects of the first embodiment, it is possible to achieve an effect of controlling the starting time to be shortened according to the characteristics of the IGBT2.

Third Embodiment

FIG. 5depicts a configuration of the gate drive circuit according to a third embodiment. The gate drive circuit shown inFIG. 5is configured, differently from that of the first embodiment shown inFIG. 1, so that not the gate voltage of the IGBT2but a voltage signal generated in a self-contained manner is input to an inverted input terminal of the gate-voltage detection comparator8-1. Specifically, the gate drive circuit according to the third embodiment is configured so as to include a square-wave power supply20that is a second power supply circuit and an RC filter21that serves as a filter circuit for smoothing a square wave voltage22output from the square-wave power supply20, and configured such that a smoothed square wave voltage23output from the RC filter21is input to the inverted input terminal of the gate-voltage detection comparator8-1. Other configurations of the third embodiment are identical or equivalent to those according to the first embodiment shown inFIG. 1and common constituents are denoted by the same reference signs, and common explanations thereof will be appropriately omitted.

Operations performed by the gate drive circuit according to the third embodiment are explained next with reference toFIGS. 5 and 6.FIG. 6is a time chart for explaining the operations performed by the gate drive circuit according to the third embodiment. The time chart shown inFIG. 6is basically identical or equivalent to that shown inFIG. 3except that the square wave voltage22and the smoothed square wave voltage23are added in the middle of the time chart.

In the third embodiment, a time constant (=a product between R and C) of the RC filter21is set such that the smoothed square wave voltage23crosses the reference voltage16right before the IGBT2is turned on (see the smoothed square wave voltage23at a point B). That is, the RC filter21smoothes the square wave voltage22and the gate drive circuit operates so that the smoothed square wave voltage23reaches the reference voltage16before the IGBT2is turned on. Therefore, differently from the first embodiment, it is possible to drive the IGBT2without detecting the gate voltage (the PSD gate voltage11) of the IGBT2.

As described above, the gate drive circuit according to the third embodiment charges the gate capacity of the IGBT2using both the constant-current gate drive circuit1and the constant-voltage gate drive circuit5at the time of driving the IGBT2serving as the power semiconductor device. In addition, the constant-voltage gate drive circuit5is switched between an operating state and a non-operating state using a control signal (voltage) generated in a self-contained manner. Therefore, it is possible to achieve effects equivalent to those of the first embodiment without detecting the gate voltage.

The material of the power semiconductor device is explained as a matter common to the first through third embodiments. A semiconductor transistor device made of silicon (Si) (such as an IGBT or a MOSFET, hereinafter, “Si—SW”) is generally used as the power semiconductor device. The technique described above is suited for use in this general Si—SW.

However, the above technique is not limited to a switching device formed of Si as a material. Needless to mention, the above technique is also applicable to a power semiconductor device made of silicon carbide (SiC) (hereinafter, “SiC—SW”) to which attention is paid because of its capability of a high-speed switching operation and of which development is underway in place of Si.

The reason that the SiC—SW can perform a fast switching operation is as follows. Because the SiC—SW can be used at a high temperature and has a high heat resistance, it is possible to increase an allowable operating temperature of a device module that accommodates the SiC—SW to a high temperature side. Even if a carrier frequency is set to high to accelerate the switching rate, it is possible to prevent a cooler that cools the device module from becoming larger.

However, although being effective from the viewpoint of improving efficiency, the accelerated switching rate has a problem that since the change of a collector-emitter voltage (Vice) and a collector current (Ic) along with the time lapse (dv/dt and di/dt) is made abrupt, noise increases at the time of driving the SiC—SW.

On the other hand, the gate drive circuit according to the present embodiment uses both the constant-current gate drive circuit and the constant-voltage gate drive circuit at the time of driving the power semiconductor device, and suppresses the gate voltage change during the transition to turning on the power semiconductor device. Therefore, it is possible to suppress the noise caused by switching as compared with the conventional technique. That is, it is no exaggeration to state that the gate drive circuit according to the present embodiment of the present application functions effectively when using the SiC—SW as the power semiconductor device and can serve as one of techniques flexibly applicable to future trends.

The SiC is an example of semiconductors referred to as a “wide bandgap semiconductor” because of the characteristic that its bandgap is wider than that of Si. For example, semiconductors formed of a gallium nitride (GaN)-based material or diamond (C) also belong to the wide bandgap semiconductor in addition to this SiC. Characteristics of those semiconductors have many similarities to those of the SiC. Accordingly, the configuration using a wide bandgap semiconductor other than a SiC semiconductor is also in the scope of the present invention and can achieve effects identical to those of the case of SiC.

The configuration described in each of the first to third embodiments is only an example of the configuration of the present invention, and it is possible to combine the configuration with other publicly-known techniques, and it is needless to mention that the present invention can be configured while modifying it without departing from the scope of the invention, such as omitting a part of the configuration.

INDUSTRIAL APPLICABILITY

As described above, the present invention is useful as a gate drive circuit that can shorten the starting time while suppressing a gate voltage change at the time of turning on a power semiconductor device.

REFERENCE SIGNS LIST