The holding effect and safe working area of insulated gate bipolar transistor

The holding effect and safe working area of insulated gate bipolar transistor

It can be found from the structure of the IGBT that a parasitic thyristor composed of an NPN+ transistor and a P+NP transistor as the main switching device is parasitic inside the IGBT, as shown in Figure 1. Rbr is the body region short-circuit resistance between the base and emitter of the NPN transistor. When the collector current IC of the IGBT increases to a certain extent, the parasitic NPN transistor emitter junction is forward conduction, so that the NPN and PNP transistors are in a saturated conduction state at the same time, causing the parasitic thyristor turn-on effect, causing the IGBT gate to lose its control of the collector current, which is called the holding effect or self-locking effect. The cause of the seizure effect may be excessive collector current (static seizure effect) or excessive duCE/dt (dynamic seizure effect). Temperature rise will also aggravate the risk of seizure effect.

The holding effect and safe working area of insulated gate bipolar transistor
Figure 1 – IGBT actual equivalent circuit model

In order to avoid the holding effect of the IGBT, the maximum collector current must be specified. When the IGBT is turned off, apply a certain back pressure to the gate to reduce duCE/dt, or connect a small capacitor in parallel between the collector C and the emitter E to reduce the duCE/dt during turn-off to avoid the occurrence of dynamic seizing effects.

According to the maximum collector current, the maximum collector-emitter voltage and the maximum collector power consumption, the parameter limit range of the IGBT in the conduction state can be determined, that is, the forward biased safe operating area (FBS30A); according to the maximum collector current, the maximum collector-emitter voltage and the maximum allowable voltage rise rate duCE/dt, the parameter limit range of the IGBT in the blocking state can be determined, that is, Reverse Biased Safe Operating Area (RBSOA).

RBSOA is an important parameter of IGBT, which represents the device’s turn-off current capability. It means that under specified conditions, ICBT can withstand the collector current and collector-emitter voltage during the turn-off time without holding up.

For example, Infineon’s F600ME4 driver chip is an IGBT module with a rating of 600A and 1200V. According to its RBSOA curve, the 1200A current can be repeatedly turned off. That is, the Infineon IGBT module with a rated current (nominal current) of 600A can repeatedly turn off the 1200A current under the condition that heat dissipation permits, as long as the junction temperature does not exceed 150°C, the maximum current can be designed to be twice the nominal current.

The characteristics and parameters of IGBT can be summarized as follows (basic characteristics of IGBT and main parameters of IGBT):

(1) IGBT is the most commonly used fully controlled power electronic device. The characteristics of the device, such as saturation voltage drop, turn-off loss, etc., vary greatly among devices of different voltage levels, and there are also devices with different performances developed for different applications under the same voltage level. At present, 600V IGBT can work at 100kHz, the performance is equivalent to MOSFET, 6500V IGBT can only work at a few hundred Hz.

(2) IGBT can have strong short-circuit withstand capability. Under the same voltage and current rating, the safe working area of IGBT is larger than that of GTR, and it has the ability to withstand the impact of pulse current.

(3) The on-state voltage drop of the IGBT is lower than that of the VDMOSFET at high voltage, especially in the area with larger current. Compared with MOSFET, IGBT has weak avalanche energy resistance. In any case, it cannot exceed the collector-emitter voltage to take the voltage.

(4) The input impedance of IGBT is high, and its input characteristic is similar to that of power MOSFET.