400 V Vce, 5 A Ic — the switching transistor for mains-referenced circuits
The BUJ303B,127 from WeEn Semiconductor is an NPN power bipolar transistor in a TO-220AB through-hole package. Its 400 V collector-emitter breakdown voltage and 5 A continuous collector current make it a fit for line-voltage switching applications — flyback clamps, electronic ballasts, switch-mode power supply primary-side drivers, and motor control snubbers where a 400 V blocking rating is the minimum for 240 VAC rectified rails.
100 W power dissipation — thermal budget for the design
Rated for 100 W maximum power dissipation, this part needs a heatsink for any continuous load above a few watts. The junction temperature limit is 150°C, so derate the power based on the thermal resistance of your mounting arrangement.
The Vce(sat) is 1.5 V maximum at 1 A base current and 3 A collector current. That 1.5 V drop at 3 A means 4.5 W conduction loss in the transistor alone — a figure that directly factors into the heatsink sizing and the overall efficiency budget. This moderate gain means the base drive circuit must supply roughly 35 mA to saturate the transistor at 800 mA load; at the full 5 A rating the required base current will be higher, so the driver stage needs to be sized accordingly.