35 ns trr — the switching-speed anchor for PFC and flyback
The STTH3R04S is a 400 V, 3 A fast recovery diode from STMicroelectronics, built for the boost stage of power-factor correction and the secondary side of flyback converters where reverse recovery time sets the switching loss floor. Its 35 ns trr at rated current keeps the recovery charge low enough to run continuous-conduction-mode PFC above 100 kHz without excessive dissipation in the MOSFET or the diode itself.
Forward drop and leakage at the operating point
At 3 A forward current the maximum forward voltage is 1.5 V — this is the conduction-loss number for the thermal budget at full load. Reverse leakage is specified at 5 µA at the 400 V reverse standoff, which is the figure to use for off-state power dissipation in high-line PFC stages where the diode blocks the full rectified DC bus.
Package, marking, and reflow considerations
The SMC body is larger than the smaller SMA/SMB siblings, so the stencil aperture and paste volume should match the pad geometry in the ST application note for the STTH3 series — the tab's solder joint is the primary thermal path. The part is marked with the base number STTH3 and a date-code laser etch; the font consistency on the SMC epoxy body is a quick authenticity check against re-marked stock.
No official second-source cross-reference is published by ST, so for dual-sourcing the buyer should qualify a parametric equivalent from another fast-recovery diode family — the 35 ns trr and 400 V rating are the binding specs, not the package code.