What makes the STTH108 a fast recovery diode — and why it matters
That trr figure — well under the 500 ns threshold that defines the 'fast recovery' class — means this diode switches off quickly, reducing turn-off losses in circuits like power-factor-correction (PFC) boost stages, flyback snubbers, and continuous-conduction-mode (CCM) boost converters. The 1.65 V forward voltage drop at 1 A is a trade-off: you get fast switching at the cost of higher conduction loss compared to a standard recovery rectifier. The DO-41 axial package is a through-hole workhorse, easy to hand-solder and retrofit into existing boards.
For a buyer managing a BOM line, this means the STTH108 is a candidate for a last-time-buy (LTB) or a planned migration to a pin-compatible successor. The 800 V / 1 A / 75 ns trr combination is common in PFC and flyback circuits, so an alternative with the same DO-41 footprint and similar trr should be sourced before the NRND window closes.
Reverse recovery time and forward drop — the switching-loss trade-off
The 75 ns trr is the headline switching parameter. In a 100 kHz PFC boost stage, a 75 ns trr diode will have significantly lower turn-off losses than a standard 2 µs recovery rectifier, which means less heat to manage and a smaller heatsink. The 1.65 V forward drop at 1 A is on the higher side for a 1 A diode — a standard recovery part might drop 1.0 V at the same current — but that is the price of fast recovery. The 5 µA reverse leakage at 800 V is low enough to not affect efficiency in most offline applications. The junction temperature rating of 175°C maximum gives thermal headroom for cramped layouts or high ambient temperatures.