450 V NPN for line-powered switching
The BUX87 is an NPN silicon power transistor in a through-hole TO-126 / SOT-32-3 package, rated for 450 V collector-emitter breakdown and 500 mA continuous collector current. With 40 W power dissipation and a 20 MHz transition frequency, it is sized for flyback converters, offline auxiliary supplies, and low-power switching regulators where the primary-side switch sees rectified mains voltage.
Ratings that define the switching budget
The 450 V Vceo is the headline number — it gives enough headroom for a universal-input flyback (85–265 VAC rectified to ~375 VDC) with a reasonable derating margin. The 500 mA Ic and 40 W Pd together bound the output power: at 300 V bulk, a flyback can deliver roughly 15–20 W continuous before hitting the current or thermal ceiling, depending on heatsinking. Saturation voltage is specified at 1 V max at 200 mA, which keeps conduction loss manageable at moderate duty cycles. Transition frequency of 20 MHz means the part is comfortable in the 20–100 kHz switching range typical of cost-optimised offline converters. Above 100 kHz the switching losses start to dominate the 40 W budget. DC current gain is specified at a minimum of 12 at 40 mA and 5 V — a forced-beta drive of 10–15 is typical for saturating the transistor in a self-oscillating or driven-base topology.
Active, ROHS3, through-hole
It is ROHS3 compliant.