Analytical analysis of Quasi-Saturation Effect in PT and NPT IGBTs
During the turn-on process insulated gate bipolar transistors (IGBTs) traverse a quasi-saturation region from the off.state to the on-state (saturation). The additional voltage drop due to the quasi.saturation of the junction hardly depends on the DC-link voltage. Even if this voltage drop is low comparing to the DC link voltage, the losse due to this effect are rather big as its time constant is large for high-voltage high-speed IGBTs. Standard ZCS or ZVS circuits can't help to avoid those losses. The voltage tail will be analysed in detail for Punch-Through and Non-Punch-Through IGBTs for hard switching on an inductive load. Analytical results will be compared to 2D-simulation and measurements. The dependency of the voltage tail to the doping profile near the junction will be discussed. In order to minimise the size and weight of inductors and transformers, switching frequency increase is expected, espeially at high voltages. But we will show that when the switching frequency exceeds some kilo-hertz the on-state voltage drop may never reach the static value and the on-state losses are much bigger than expected.