400 V NPN — the flyback / offline switcher transistor
The WeEn Semiconductors BUJ302A,127 is a high-voltage NPN power bipolar transistor built for offline power conversion — flyback converters, switch-mode power supply front-ends, and similar circuits where the DC bus sits at 300 V to 400 V after rectification. The 400 V collector-emitter breakdown rating gives it the voltage headroom for universal-input (85–265 VAC) flyback designs with a reasonable derating margin. The 4 A continuous collector current and 80 W maximum power dissipation define the practical output power envelope: in a typical flyback at 70 kHz, this part can deliver roughly 40–60 W depending on heatsinking and ambient temperature.
1.5 V Vce(sat) at 3.5 A — what the on-state drop means for the BOM
The saturation voltage is specified at 1.5 V maximum with a base current of 1 A and a collector current of 3.5 A. That 1.5 V drop at 3.5 A produces 5.25 W of conduction loss — the bulk of the dissipation in a hard-switched flyback. The 80 W power max is the package limit with an infinite heatsink at 25°C case temperature; real-world dissipation will be lower. The 25 minimum DC current gain at 800 mA collector current and 3 V Vce means the base drive circuit must supply at least 32 mA to saturate the transistor at 800 mA, and proportionally more at higher collector currents.
TO-220-3 — the standard through-hole power package
The BUJ302A comes in the TO-220-3 package, also designated TO-220AB. The metal tab is electrically connected to the collector, so the heatsink must be electrically isolated or the whole assembly floats. Through-hole mounting means it is a hand-solder or wave-solder part — no reflow profile needed, but the leads need to be bent and inserted before soldering.
WeEn Semiconductors still manufactures this part, so there is no last-time-buy deadline or forced redesign.