50 ns reverse recovery — switching-loss anchor for PFC and flyback
The STTH8R04DI is a 400 V, 8 A fast-recovery epitaxial diode from STMicroelectronics, housed in a TO-220AC insulated package. Its 50 ns reverse recovery time (trr) at 8 A forward current places it squarely in the boost-diode tier for power-factor-correction stages and single-switch flyback converters where switching losses dominate the thermal budget.
8 A average current and 400 V blocking — derating and heatsink sizing
At 8 A average rectified current and 1.5 V forward drop, the conduction loss is 12 W — that is the heat the TO-220AC tab must sink. The 400 V reverse voltage covers 240 VAC input rectified to ~340 VDC bus with margin for line transients. Reverse leakage is 10 µA at 400 V, negligible for the thermal calculation but worth noting for high-temperature standby power. The 50 ns trr keeps the switching energy per cycle low — in a 100 kHz PFC stage the turn-off loss is roughly proportional to trr times the bus voltage and current. That 50 ns figure is the datasheet typical; the actual loss depends on the di/dt the gate drive imposes, but it is the right ballpark for comparing against slower 200-500 ns recovery diodes.