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 losses 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. We will show that at relatively low switching frequency the on-state voltage drop may never reach the static value and the on-state losses are much bigger than expected.