There is an insulating silicon dioxide structure between the emitter and gate of the IGBT device, and DC power cannot pass through it, so the low-frequency static driving power is close to zero. However, a gate capacitance CGs is formed between the gate and the emitter, so a certain amount of dynamic driving power is required when alternating on and off at high frequencies. The CGs of low-power IGBTs are generally within 10~100pF. For high-power insulated gate power devices, because the gate capacitance CGs is large, ranging from 1~100pF or even larger, larger dynamic driving power is required.

IGBT gate voltage can be generated by different drive circuits. The design of the gate drive circuit is directly related to the long-term operational reliability of the system composed of IGBTs. The value of the forward gate voltage should be sufficient to completely saturate the IGBT and minimize on-state losses. It should also limit the short-circuit current and the power stress it brings.

The greater the IGBT positive gate voltage VGE, the lower the on-resistance and the smaller the loss. However, if VGE is too large, once the IGBT over-currents, it will cause the static pillar effect of the internal parasitic thyristor, causing the IGBT to fail. On the contrary, if VGE is too small, the operating point of the IGBT may fall into the linear amplification zone, eventually causing overheating damage to the device. In any case, the gate drive voltage at turn-on should be between 12 and 20V.

The gate of the IGBT is electrically isolated from the emitter through an oxide film. Because this oxide film is very thin, its breakdown voltage can generally only reach 20-30V. Therefore, gate breakdown is one of the common causes of IGBT failure. In applications, sometimes although it is ensured that the gate drive voltage does not exceed the maximum rated voltage of the gate, the parasitic inductance of the gate connection and the capacitive coupling between the gate collector will also produce an oscillation voltage that damages the oxide layer. to this end. Twisted wires are usually used to transmit drive signals to reduce parasitic inductance. Adding a small resistor in series to the gate connection can also suppress the oscillation voltage.

Several consequences of IGBT load short-circuit

Exceeding the thermal limit: The intrinsic temperature limit of the semiconductor is 250°C. When the junction temperature exceeds the intrinsic temperature, the device will lose its blocking capability. When the IGBT load is short-circuited, the junction temperature rises due to the short-circuit current. Once its thermal limit is exceeded, the gate-level protection will also fail accordingly.

Current locking effect: Under normal operating current, IGBT has no current locking phenomenon because the sheet resistance Rs is very small. However, in the short-circuit state, due to the large short-circuit current, when the voltage drop on Rs is higher than 0.7V, J1 is forward biased, causing current locking, and the gate level loses voltage control.

Shut-off over-voltage: In order to suppress the short-circuit current, when a fault occurs, the control circuit immediately removes the positive gate voltage, turns off the IGBT, and the short-circuit current decreases accordingly. Due to the large short-circuit current, the current drop rate during shutdown is very high, which will induce a very high voltage in the wiring inductance. Especially, this induced voltage on the package lead inductance in the device is difficult to suppress, which will cause the device’s over-current to turn off over-voltage and fail.

IGBT over-current protection method

Voltage reduction method: refers to reducing the gate voltage when a fault occurs. Since the short-circuit current is proportional to the external positive gate voltage Ug1, the positive gate voltage can be reduced in the event of a fault.

• Pulse cutting method: Since the Uce voltage rises during over-current, we use the method of detecting the collector voltage to determine whether there is over-current. If there is over-current, the trigger pulse is cut off. At the same time, try to use soft turn-off mode to alleviate the decline rate of short-circuit current and avoid damage to IGBT caused by over-voltage.

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