400 V VCEO — the flyback and offline driver fit
The STD815CP40 is a complementary NPN/PNP Darlington transistor pair in a single 8-DIP package, designed for medium-voltage, medium-current switching. Its 400 V collector-emitter breakdown (VCEO) and 1.5 A continuous collector current cover the primary-side switch in low-power flyback converters, relay and solenoid drivers in 240 VAC industrial controls, and CFL or LED ballast circuits. The 2.6 W power dissipation ceiling in the 8-DIP through-hole package means you need to consider heatsinking or derating above ~500 mA continuous — the SOA is real-world limited by the package, not the silicon.
Vce(sat) and hFE — the drive margin check
Vce(sat) is specified at 1 V max with 50 mA base drive into 350 mA collector current. That 16 minimum hFE at 350 mA / 5 V is low — typical Darlington territory — so the base drive needs to be sized for worst-case gain. A microcontroller GPIO will not drive it directly; you need a pre-driver transistor or a resistor from a higher-voltage rail. The 1 mA max collector cutoff at rated VCEO matters in lightly loaded offline circuits where leakage can pull a gate or relay false.
Obsolete — sourcing via independent distribution
Official production has ended; there is no last-time-buy window remaining. Availability is through independent distribution and surplus channels only. Every unit we supply is sourced from vetted inventory with traceable date-code and marking consistency — the laser etch is a fingerprint, and we decap-sample lots where the marking looks off. If you need a drop-in through-hole replacement, the complementary pair in an 8-DIP is uncommon; a surface-mount migration to discrete NPN/PNP Darlingtons in SOT-223 or DPAK is the practical path for new designs.