The Ween Semiconductors BUJ103AX,127 is an NPN power bipolar junction transistor rated for 400 V collector-emitter breakdown and 4 A continuous collector current, housed in a TO-220-3 Full Pack with an isolated tab.
Can BUJ103AX handle 400 V in a switching circuit?
The 400 V Vceo rating is the collector-emitter breakdown voltage at zero base drive — it defines the maximum voltage the transistor can block when off. In a typical flyback or forward converter running off a 240 V AC rectified bus (≈340 V DC), this part provides adequate headroom for the reflected voltage and leakage inductance spikes, provided the switching loop inductance is controlled. The 1 V saturation at 3 A (Vce(sat) max at 600 mA base current) keeps conduction losses manageable for a 4 A device, though the 12 minimum DC current gain at 500 mA means the base drive needs to be sized for forced-beta operation in saturation.