JP3337796B2 - Drive circuit for voltage-driven elements - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an IGBT
The present invention relates to a drive circuit for a voltage-driven element having an element protection function when an overcurrent flows to a voltage-driven element such as a MOSFET or a MOSFET.

[0002]

2. Description of the Related Art First, in the present invention, an IGBT (Insulated Gate Bipolar Transistor) characterized by high speed operation and large capacity, which is widely used in recent years as a voltage driven element.
Will be described as an example. In a power converter represented by an inverter circuit, an overcurrent flows through an element forming a circuit due to an accident such as output grounding or output short circuit. This will be described with reference to FIG.

FIG. 3 shows an inverter circuit to which a drive circuit of a voltage drive type element is applied. 1 is a DC power supply, 2 is a wiring inductance, 3 to 6 are IGBTs, 7 to 10 are drive circuits, and 11 to 14. Is an overvoltage suppression circuit (so-called snubber circuit)
It is. In this type of circuit, the drive circuit 7 normally charges and discharges the gate (G) and the emitter (E) of the IGBT 3 at a high speed (about 1 μs) to thereby achieve an IGB.
T is turned on and off to generate an alternating current at the output.

[0004] Due to an accident or the like, for example, IGB
When an output short circuit occurs while T3 and T6 are on, as shown by the dotted line, a short circuit occurs in the path of DC power supply 1, wiring inductance 2, IGBT3, output terminal T1, output terminal T2, IGBT6, and DC power supply 1. Electric current flows. If the short-circuit current continues to flow, the internal loss of the element increases, eventually destroying the element. Therefore, it is necessary to detect the short-circuit state by some means and cut off the current.

However, the wiring inductance 2 exists in the path of the short-circuit current, and an overvoltage due to the energy accumulated in the wiring inductance due to the interruption of the current is applied to the IGBT. This overvoltage is increasing in proportion to the magnitude of the short-circuit current and the speed of the current breaking operation. In the case of an IGBT, the short-circuit current is 10 times the rated current of the element, and since the IGBT is high-speed, if the short-circuit current is cut off in a normal operation, the overvoltage exceeds the withstand voltage of the IGBT element, and the element is destroyed.

In general, therefore, when a short-circuit current is cut off, a so-called soft cut-off operation is performed, in which a voltage between a gate and an emitter (GE) is gradually discharged to make a device current cut-off operation slow. This will be described with reference to FIGS.

FIGS. 4 and 5 show a driving circuit of a conventional voltage-driven element and its operation waveforms.
BT, 16 is a current detector. Here, PUs is a short-circuit state detection circuit, DRb is a base drive circuit, and TR1 and TR
2, TR3 are transistors, Von, Vof are DC power supplies, R
1, R2 is a resistor, CD is a capacitor, D1 is a diode, RG is a charging resistor, and transistors TR1, TR1
2 is complementary connected.

That is, in the normal operation, the base drive circuit DRb operates in response to the control signal CTR, and when the IGBT 15 is turned on, the transistor TR1 is turned on and the IGB
G of T15 is charged by the DC power supply Von rather than having a positive potential with respect to E (Ton period). When IGBT15 is off,
The transistor TR2 is turned on, and G is charged by the DC power supply Vof so as to have a negative potential with respect to E (Tof). In principle, it is sufficient to discharge the voltage between GEs when the IGBT is turned off. However, in order to prevent a malfunction at the time of turning off the IGBT, the G potential is made negative as described above.

When the short-circuit state detection circuit PUs detects the IGBT overcurrent state by detecting the collector (C) current of the IGBT 15 with the current detector 16 (time Ts), the output of the base drive circuit DRb is opened, Further, the transistor TR3 is turned on. By turning on the transistor TR3, the capacitor CD, which has been charged to the potential obtained by adding the DC power supply Von and the DC power supply Vof in advance, becomes the resistor R2.
Is discharged. This discharge waveform is as shown in the figure, and the potential fluctuation of the capacitor CD indicates that the output of the base drive circuit DRb is in an open state.
The base voltages of R1 and TR2 fluctuate, and finally the output of the base drive circuit DRb has a waveform (gate-emitter voltage) that gradually decreases.

As a result, the short-circuit current flowing through the IGBT 15 gradually attenuates (collector current), and suppresses an overvoltage generated between CEs. A so-called soft cutoff operation is performed. The degree of the soft cutoff (gate emitter voltage) makes it possible to change the cutoff state of the overcurrent. The capacity of the capacitor CD in the circuit is increased to make the soft cutoff more gradual and the cutoff to be gradual. This suppresses an overvoltage generated between the elements.

[0011]

However, in a short-circuit state in which the wiring inductance of the short-circuit is relatively large and the rise of the short-circuit current is relatively low, the following problems occur when the soft cut-off state is made slower. I will. This will be described with reference to FIG. That is, IGB
T has a current transfer characteristic (constant CE voltage) by a gate-emitter (GE) voltage as shown in FIG. 6. When the GE voltage exceeds the threshold voltage Vth, a current flows through the element.
When the voltage between GEs exceeds that, a collector (C) current determined by the voltage flows.

The same goes for the short-circuit operation, of course. In principle, the shut-off operation does not start until the short-circuit current reaches a current value determined by the voltage between GEs. I having such characteristics
In the application of GBT, if the soft breaking condition is slowed down due to the short circuit operation with relatively large wiring inductance,
In spite of the overcurrent protection operation of the drive circuit, the current flowing through the element generates a time during which it is not interrupted (called a dead time).

In the operation explanatory diagram of the conventional example shown in FIG. 7, a soft shut-off operation is started at time TF1, and the gate emitter (GE) voltage gradually starts to discharge as shown.
At this time, if the degree of soft interruption is gentle, the time T
The peak current IP1 of the short-circuit (collector) current and the GE voltage VR on the current transfer characteristic curve shown in FIG.
The point C determined by the relationship of 1 does not reach the point A, and the C current starts to be cut off after the time TF2. The time TEN from time TF1 to time TF2 is the dead time.

During this dead time, the short-circuit current continues to flow irrespective of the drive circuit and, as a result, the cutoff current increases, so that the energy stored in the wiring inductance increases and the overvoltage generated between the elements increases. . On the other hand, if soft shutoff is performed rapidly for the purpose of shortening the dead time and reducing the cutoff current, the current cutoff is also rapid, and the overvoltage applied to the element is also increased.

[0015]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and more specifically, the first capacitor is connected in parallel with the first capacitor via a diode . And a second capacitor charged to a voltage lower than that of the second capacitor.

[0016]

According to this solution, in the short-circuit protection operation of the voltage driving element, the increase in the short-circuit current is reduced by shortening the time during which the short-circuit current is not interrupted even though the drive circuit is in the soft shut-off operation. The soft shutoff operation can be performed slowly while suppressing the current, and the overvoltage applied to the element can be reduced due to the wiring inductance, and the overcurrent interruption can be performed safely without destruction of the element. Hereinafter, the present invention will be described in more detail with reference to the drawings.

[0017]

1 and 2 show an embodiment of the present invention shown in FIGS. 4 and 5, wherein R3 and R4 are resistors,
D2 is a diode, CD is the first capacitor, CA is the
Reference numeral 2 denotes a capacitor, and reference numeral 17 denotes a drive circuit according to the present invention similar to FIG. Here, the second capacitor CA is charged to a potential VS slightly lower than the combined potential of the DC power supplies Von and Vof by the voltage division of the resistors R3 and R4. Also,
As will be described later, the capacitance of the first capacitor CD is relatively small, and that of the second capacitor CA is the first capacitor.
It is bigger than Sa CD. Here, in this circuit configuration, the normal operation is the same as that described with reference to FIG. 4, and the circuit portion including the added resistors R3 and R4, the diode D2, and the second capacitor CA is blocked by the diode D2. Has no effect.

Next, the short-circuit protection operation will be described. I
When an overcurrent flows through the GBT 15, a short-circuit state detection circuit PU
4 is the same as that in FIG. 4 until s detects this and releases the output of the base drive circuit DRb to turn off the transistor TR3.

[0019] When the transistor TR3 is turned on at time TH1, circuit first capacitor CD is discharged by the resistor 2 is formed, the transistor TR1 potential variation of the first capacitor CD via a diode D1,
It becomes the base voltage of TR2, and controls its operation. When the first capacitor CD is discharged and its potential becomes the potential Vs of the second capacitor CA which has been charged in advance (time TH2), the diode D2 becomes conductive and the second capacitor CA and the first capacitor CA are connected to each other . , A resistor R2 is connected to the second capacitor CA and the first capacitor CD after time TH2 .
It will discharge the capacitor CD.

Accordingly, since the discharge time constant changes after time TH2, the CD voltage, that is, the base voltage of the transistors TR1 and TR2 is discharged in two stages as shown in FIG. Accordingly, the output of the drive circuit is also governed by this potential, and as shown in the figure, the gate-emitter voltage drops to the potential Vs with a relatively fast time constant after detecting a short circuit, and thereafter attenuates with a gentle slope. It becomes a waveform.

When the gate-emitter voltage has a waveform as shown in the figure, the short-circuit current flowing through the IGBT 15 is reduced by the GE in the region IP2 where the C current is lower than the point A shown in the problem in the current transfer characteristic curve shown in FIG. Reaching the point B of the restriction by the inter-voltage VR2, the shutoff operation is started (time TH3). As a result, the dead time TEN is significantly short-circuited, and the cutoff current peak is also reduced. Moreover, after the time TH3, the soft cutoff operation is performed with the time constant determined by the capacitors CD and CA and the resistor R2.
Since the short-circuit current has already begun to break, this soft breaking can be slowed down without any restrictions.
Obviously, the short circuit current interruption can be made more gradual.

As can be understood from the above operation, the capacitances of the capacitors CA and CD are selected for the second capacitor CA so as to realize the required soft cut-off state, and the capacitance of the first capacitor CD is set to the second capacitor CA. May be sufficiently small (for example, about 10%) with respect to the capacitor CA. Strictly speaking, the potential Vs must be selected according to the wiring inductance of the short-circuit current and the detection speed of the drive circuit. In other words, when the drive circuit detects the overcurrent from the time when the overcurrent starts to flow and shifts to the protection operation, it grasps how much the short-circuit current determined by the wiring inductance is flowing, and the voltage between GEs at which the short-circuit current can flow. It is necessary to set the potential Vs slightly higher than that.
However, the wiring inductance changes depending on what short-circuit path is assumed, and it is difficult to specify the wiring inductance. Therefore, simply, by setting the potential Vs to a potential several volts lower than the DC current V0n in FIG. 1 (based on the E potential), the function can be exhibited.

Further, in the example shown in FIG. 1, the voltage of the capacitor CA is based on the voltage dividing method of the resistors R3 and R4. Instead of the resistor R4, a Zener diode or a series circuit of a Zener diode is used. For example, a potential setting method of such a type may be used.

[0024]

As described above, according to the present invention, the dead time during the overcurrent protection operation can be suppressed to reduce the peak current flowing to the element, and the soft cutoff can be slowed down without restriction. It is possible to provide an apparatus having a simple configuration capable of making the interruption more gradual.

[Brief description of the drawings]

FIG. 1 is a connection diagram showing a configuration of a main part of an embodiment of the present invention.

FIG. 2 is a waveform diagram of each part shown for explanation of FIG. 1;

FIG. 3 is a circuit diagram showing an example of an inverter circuit using a voltage-driven element drive circuit.

FIG. 4 is a connection diagram showing a drive circuit of a conventional voltage-driven element.

FIG. 5 is a waveform diagram of each part shown for explanation of FIG. 4;

FIG. 6 is a diagram showing a current transfer characteristic of the IGBT.

FIG. 7 is a diagram for explaining the operation of a conventional voltage-driven element drive circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 DC power supply 2 Wiring inductance 3 IGBT 4 IGBT 5 IGBT 6 IGBT 7 Drive circuit 8 Drive circuit 9 Drive circuit 10 Drive circuit 15 IGBT 16 Current detector17 Drive circuit  DRb Base drive circuit PUs Short circuit detection circuit TR1 transistor TR2 transistor TR3 transistor Von DC power supply Vof DC power supply R1 resistor R2 resistor R3 resistor R4 resistor D1 diode D2 diode RG charging resistor CDFirstCapacitor CASecondCapacitor

Claims (1)

(57) [Claims] 1. A first capacitor and a resistor are connected by a switch, and a time constant determined by the first capacitor and the resistor is:
In a drive circuit for a voltage-driven element that performs an overcurrent protection operation by detecting a current flowing in the voltage-driven element and performing soft cutoff, the drive circuit is connected in parallel with the first capacitor.
A second capacitor is provided and a first capacitor is provided therebetween.
A diode for the capacitors side and cathode is provided, further a second
The voltage of the capacitor is lower than the voltage of the first capacitor.
A driving circuit for a voltage-driven element, wherein the driving circuit is configured to charge to a voltage.