JPS6395722A - Overcurrent protection circuit for igbt element - Google Patents

Abstract

PURPOSE:To evade latchup of an IGBT (insulated gate bipolar mode transistor) by connecting a voltage limiting circuit between the gate and emitter so as to prevent a gate voltage from exceeding a prescribed value, thereby suppressing an abnormal current increase at the occurrence of short-circuit. CONSTITUTION:A Zener diode 4 as a voltage limit means is provided between the gate and emitter of the IGBT 1. In this case, the limit voltage of the Zener diode is selected to a value slightly exceeding the output voltage of the drive circuit. the gate of the IGBT might cause electrostatic destruction the same as a conventional power MOSFET and a Zener diode is inserted between the gate and emitter to prevent it and the function is covered in common by the Zener diode 4.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an I
GBT (In5lated Gate Bipol)
The present invention relates to an overcurrent protection circuit for an ar modeTranaiator) element.

[Conventional technology]

IGBT elements have the advantages of bipolar transistors, such as high breakdown voltage and easy increase in capacity, and the '7-M08
Monote, IGT, and GOMPE have recently been attracting attention as new devices that combine the advantages of FETs, such as high-speed switching and easy drive.
The equivalent circuit and circuit symbol are shown in Figure 3.
As shown in the same figure (a), the GBT is a MOS-F E
T (F), PNP) transistor TRl, NP
N) It consists of a transistor TR2 and a base-emitter shorting resistor R, and is characterized by having a built-in thyristor circuit made up of transistors TRI and TR2.

In addition, the circuit symbol gold of IGBT is shown in the same figure (b).

It is said that the biggest drawback of these IGBTs is that a phenomenon called launch-up (a phenomenon in which a parasitic thyristor turns on) occurs when the collector current exceeds a predetermined value, and the flow cannot be cut off. In other words, Figure 3 (a)
Since the thyristor circuit is completely built-in, the collector current can be maintained at a predetermined value t? If it exceeds this, the thyristor circuit will turn on and the IGBT will not be able to turn off. Since this launch-up phenomenon is directly linked to the destruction of the IGBG element, it is necessary to prevent this from occurring. In particular, IGBT overcurrent protection i! If you do it, will this latch amplifier handle the overcurrent? The launch amplifier phenomenon can be said to be an extremely important indicator since the generated current (also referred to as launch amplifier current) must be suppressed to below.

What is the output characteristic of IGBT in Figure 4? The characteristic diagram shown is shown below.

As is clear from the figure, the IGBT has a collector current I
When c exceeds a predetermined value, the collector-emitter voltage ■α suddenly increases. Therefore, the overcurrent value of the IGBT is limited by its output characteristics and becomes the original value. Therefore, if a short-circuit accident occurs in an inverter device that uses IGBT as a switching element, assuming all cases, a DC power supply voltage will be applied between the collector and emitter of the IGBT, and the current at this time will be the launch amplifier current. If not, overcurrent protection can be achieved by turning off the IGBT gold.

Now, in Figure 4, the gate voltage VGx? Examples of the characteristics of three parameters are shown, and as is clear from the graph, it is clear that the higher the gate voltage, the greater the current flow. On the other hand, in current IGBTs, this launch amplifier current is not at an extremely large level, but rather at a level where the collector current when high voltage is applied will exceed the launch amplifier current unless the gate voltage is kept below a certain level. be on the level. Therefore, when performing overcurrent protection, it is necessary to limit the total gate voltage to a predetermined value or less. However, lowering the gate voltage is the commonly used collector 11! ! On voltage at Ryubin? It will increase
Unfavorable for application. For this reason.

Currently, the gate voltage is set as high as possible within the range that allows overcurrent protection.

[Problem that the invention seeks to solve]

However, when an overcurrent occurs in an actual device, the gate voltage at this time exceeds the voltage applied to the gate drive circuit 1, and as a result.

It has been found that a current larger than the expected original value flows.

Figure 5 is a circuit diagram showing the equivalent circuit in the event of a short circuit accident to explain all of this, and Figure 6 is its operation. This is the tth waveform diagram of each part to be explained. In addition, in FIG. 5, l is IG
BT, 2 is an on-gate power supply, 3 is a resistor, 7a is a switch, and 8 is a main circuit power supply.

Now, the voltage between the collector and emitter of IGBTl immediately before the short circuit is approximately O, and when switch 7 & is turned on in this state and a short circuit is formed, the voltage between the collector and emitter of IGBTl as shown in Fig. 6 (a) is generated. A step-like voltage VCE is applied. IGBTI has a capacitor between each terminal as shown in the figure, so the capacitance t between the gate and collector is
Assuming that t-ccc, the capacitance between the gate and emitter 'xcaz, and the voltage applied between the collector and emitter kEa, the gate voltage due to the application of the main circuit power supply 8 is calculated by the following formula, as shown in Fig. 6 (b). It will rise.

Here, for example, CGC/CGE = 0.01, Ed
= 300", a voltage rise of Δvcgz3v will occur, and as a result, the goo14 voltage will exceed the output voltage of the drive circuit9, and a large collector current will flow. Even in such a case, be careful not to use a launch amplifier. In order to achieve this, the output voltage of the drive circuit should be lowered in consideration of this voltage increase public money, but if this is done, the on-voltage in the normal collector current range will increase as described above.

On the other hand, in the explanation up to f'1-, what if the impedance on the drive circuit side is high, that is, the resistor 3 in FIG. 5 is large? I am assuming that. If the resistor 3t-0 in Figure 5 is used, the gate voltage will not exceed the output voltage of the drive circuit, that is, the voltage of the on-gate power supply 2, but in IGBT, the launch amplifier is activated even at turn-off. Then, this phenomenon occurs, and in order to avoid this, 50 to 10
This is due to the fact that it must be driven through a relatively large resistance such as 0Ω. Therefore, if there is a resistance of this level, it is sufficient to ignore the drive circuit side and only consider the increase in gate voltage.

In addition, the reason why the collector current decreases as time passes after the short circuit as shown in Figure 6 (c) is that the accumulated charge in the capacitor mentioned above is due to resistance. is discharged through.

The idea is that the gate voltage decreases toward the output voltage of the drive circuit.

Therefore, this invention makes it possible to set the output voltage of the drive circuit to a high level by preventing the reached value of the overcurrent from exceeding the value determined by the output voltage value of the drive circuit, and thereby prevents overcurrent protection. The overall objective is to obtain a low on-state voltage with a common collector current, while still being possible.

[Means to solve all problems]

The entire voltage limiting circuit serves to limit the voltage between -t between the gate and emitter of the IGBT element so that it does not exceed a predetermined value.

[Effect]

Is the gate voltage increased by clamping the voltage between the gate and emitter using the above voltage limiting circuit? 〇 [Embodiment] Fig. 1 is a circuit diagram showing an embodiment of the present invention.

As is clear from the figure, this embodiment is characterized in that a Zener diode 4 is provided as a voltage limiting means between the gate and emitter of the IGBTl.

At this time, the limiting voltage of the Zener diode 4 is selected to be slightly higher than the output voltage of the drive circuit. Also, does this type of IGBT suffer from electrostatic discharge damage to the gate like normal power MO3FETs? This is likely to occur, and in order to completely prevent this, a Zener diode is sometimes inserted between the gate and the emitter, but the Zener diode 4 shown in FIG. 1 can also serve as the coa function.

FIG. 2 is a circuit diagram showing another embodiment of the invention. This uses diode 5 and capacitor 6 as voltage limiting means.
It is characterized by the provision of a clamp circuit consisting of: 31 in the figure represents a resistor.

In addition to the above, it is also possible to use a nonlinear resistance element or the like as the voltage limiting means.

〔Effect of the invention〕

According to this invention, the voltage limiting circuit is fully connected between the gate and emitter to prevent the gate voltage from exceeding a predetermined value, thereby suppressing abnormal current increase in the event of a short-circuit accident.
T's launch amplifier can be avoided. Nineteenth, since the output voltage of the gate drive circuit can be raised to the limit of the operating voltage of the voltage limiting circuit, the on-state voltage at the normal collector current can be reduced. As a result, IGBTt
-It is possible to apply it in a state where the on-state voltage is lower and overcurrent protection is easier.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of this invention, FIG. 2 is a circuit diagram showing another embodiment fMJt of this invention, and FIG. 3 is an I
An explanatory diagram to explain the equivalent circuit of a GBT element and its circuit symbol. Figure 4 is a characteristic diagram showing the output characteristics of the IGBT. Figure 5 is a circuit diagram showing the equivalent circuit at the time of a short circuit accident. It is a waveform diagram of each part for explaining the operation. Description of symbols 1... IGBT element, 2... On-gate power supply, 3°3a... Resistor, 4... Zener diode, 5...
・Diode, 6...Capacitor, 7.7m...Switch, 8...Main circuit power supply, TRI...PNP transistor. TR2...NPN transistor, F...MO3F
ET%R...Resistance for shorting between base and emitter. Agent Patent Attorney Akio Namiki Agent Patent Attorney Kiyoshi Matsuzaki Figure 1 Figure 4

Claims (1)

[Claims] 1) Turn off the element itself when an overcurrent flows;
In an overcurrent protection circuit for an IGBT element that prevents element destruction by cutting off overcurrent, a voltage limiting circuit is provided between the gate and emitter of the IGBT element to limit the voltage therebetween from exceeding a predetermined value, An overcurrent protection circuit for an IGBT element, characterized in that the circuit suppresses a peak value of overcurrent. 2) The overcurrent protection circuit for an IGBT element according to claim 1, wherein a Zener diode is used as the voltage limiting circuit.