About the Overcurrent Protection in IGBT Power Supply

IGBT modules provide technical assurance for uninterruptible power supply technology and are becoming a mainstream technology in uninterruptible power supply technology.

In the IGBT module power supply technology, the protection of the circuit is very necessary. In this article, the overcurrent protection in the IGBT protection circuit will be explained comprehensively.

Overcurrent Protection

Manufacturers have strict restrictions on the safe working area provided by IGBTs, and the time for IGBTs to withstand overcurrent is only a few microseconds (SCRs, GTRs and other devices withstand overcurrent for tens of microseconds), so overcurrent protection is the primary concern when using IGBTs.

The causes of overcurrent are: transistor or diode damage, control and drive circuit failure or interference causing misoperation, output line misconnection or insulation damage, etc. to form a short circuit, output short circuit to ground and motor insulation damage, inverter bridge arm short circuit, etc.

Overcurrent detection protection for IGBT is divided into two cases.

1. There is no protection function in the driving circuit. In this case, an overcurrent detection device should be set in the main circuit.

For small-capacity inverters, the resistor R is usually connected directly in series in the main circuit, and the voltage across the resistor is used to reflect the magnitude of the current;

For large and medium-capacity inverter, because of the large current, need to use current transformer TA (such as Hall sensor, etc.). Current transformer is connected to the location: one is connected in series in the main circuit like a series resistor. The second is connected in series on each IGBT. 

The former uses only one current transformer to detect the total current flowing through the IGBT, economical and simple, but the detection accuracy is poor; the latter directly reflects the current of each IGBT, the measurement accuracy is high, but requires six current transformers. The current signal detected by the overcurrent is output to the control circuit by the optocoupler tube to block the signal, thus shutting down the triggering of the IGBT and realizing overcurrent protection.

2. There is a protection function in the driver circuit. 

For example, the HR065 of Intec, the EXB840-844 of Fuji Electric, and the M57962L of Mitsubishi are integrated circuits that combine drive and protection functions (known as hybrid drive modules), and their current detection is based on the characteristic that the voltage drop of the forward conductor Uce (ON) is proportional to the collector current Ie under a certain forward gate voltage Uge. ON) to determine the size of Ie, the product is highly reliable.

Different models of hybrid driver modules have different output capacity, switching speed and du/dt tolerance, which should be properly selected according to the actual situation when used.

As the hybrid drive module itself overcurrent protection critical voltage action value is fixed (generally 7 ~ 10V), there is a problem with IGBT cooperation.

The method usually used is to adjust the number of diodes V connected in series between the collector of the IGBT and the driver module, so that the sum of the pass-state voltage drop of these diodes is equal to or slightly greater than the difference between the overcurrent protection action voltage of the driver module and the pass-state saturation voltage drop of the IGBT Uce (ON).

The method of using the number of diodes to adjust the overcurrent protection action point, although simple and practical, but the accuracy is not high. This is because the through-state voltage drop of each diode is a fixed value, which makes the voltage between the driver module and the IGBT collector c not continuously adjustable. In practice, there are two improvement methods.

(1) change the type of diode combined with the number of diodes. For example, the IGBT pass saturation voltage drop of 2.65V, the drive module overcurrent protection critical action voltage value of 7.84V, then the sum of the entire diode on the pass voltage drop should be 7.84-2.65 = 5.19V, when the choice of seven silicon diodes and a germanium diode in series, the sum of its pass voltage drop of 0.7 × 7 + 0.3 × 1 = 5.20V (silicon tube as 0.7 V, germanium tube is regarded as 0.3V), then it can be better achieved with (2) diode combined with resistance.

Due to the difference of diode through-state voltage drop, the above improved method is difficult to accurately set the critical action voltage value of IGBT overcurrent protection If 1 to 2 diodes are replaced by resistors, an accurate fit can be achieved.

In addition, due to the same bridge arm on the two IGBT control signal overlap or the switch device itself delay too long and other reasons, so that the upper and lower two IGBT through, bridge arm short circuit, when the current rise rate and inrush current are very large, very easy to damage the IGBT For this reason, you can also set the bridge arm interlock protection, with two and the gate on the same bridge arm of the two IGBT drive signal interlock, so that each Only after one $IGBT is confirmed to be turned off, the other IGBT can be turned on, which strictly prevents overcurrent caused by short circuit of the bridge arm.

Overcurrent protection in the IGBT module is very necessary because IGBTs have poor overcurrent and overvoltage resistance, so any accident may cause damage to the overall circuit, resulting in a waste of time and cost, so it is necessary to protect the IGBTs effectively to avoid this situation.