35 ns reverse recovery — what it buys in the switching loop
The STTH3R04U is a 400 V, 3 A fast recovery rectifier from STMicroelectronics, built for continuous-conduction-mode PFC boost stages, flyback snubbers, and secondary-side output rectification where reverse recovery losses dominate the thermal budget. Its 35 ns trr at rated current keeps the turn-off energy low enough that the diode runs cooler than a standard recovery part in the same 3 A loop — a direct benefit for the heatsink area and the MOSFET's switching stress.
400 V / 3 A — sizing the primary-side rectifier
Rated 400 V reverse and 3 A average forward current, this diode fits the boost rectifier in a universal-input PFC (90–265 VAC) or the output clamp in a 48 V telecom brick. The 1.5 V forward drop at 3 A is typical for a fast-recovery epitaxial structure — expect about 4.5 W conduction loss at full load, which the SMB package can dissipate with adequate copper area on the cathode tab.
DO-214AA (SMB) — footprint and thermal reality
The SMB case is a JEDEC-standard surface-mount package with a cathode tab that carries the heat into the PCB copper. For 3 A continuous at 175 °C junction, the board layout needs adequate copper area on the cathode side.