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 TO-220AB metal tab is the primary thermal path — plan for a thermal pad or mica insulator and a properly sized heatsink. 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. The DC current gain (hFE) is a minimum of 23 at 800 mA collector current and 3 V Vce. 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.