400 V / 5 A NPN power transistor in TO-220
The BUL138 is an NPN bipolar power transistor rated for 400 V collector-emitter breakdown and 5 A continuous collector current, housed in a TO-220-3 through-hole package. With 80 W maximum power dissipation, it is suited for line-voltage switching applications such as flyback converters, offline power supplies, and motor-drive snubber circuits where the TO-220 package can be bolted to a heatsink.
Saturation voltage and base drive
Vce(sat) is specified at 700 mV maximum with 1 A base current driving 5 A collector current. That means the base drive circuit must supply roughly 625 mA to saturate the device at 5 A, which needs a driver stage capable of that current.