500 W Zener TVS in SQ-MELF — transient protection for general-purpose rails
The 1N6109US: The 9.9 V reverse standoff voltage protects a 12 V rail, clamping transients at 19.11 V max while handling 26.13 A peak pulse current. The SQ-MELF (B, DO-213AB) package is a hermetic glass body suited for surface-mount assembly — the metalized end caps reflow directly to the PCB pad, no leads to bend.
Clamp voltage and peak current — the surge budget
At 26.13 A peak pulse current the diode clamps at 19.11 V max. For a 12 V nominal rail, that clamp margin (~7 V above standoff) means downstream components need a 20 V minimum breakdown rating to stay inside the safe operating area. The 500 W rating is the product of clamp voltage and peak current — derate above 25 °C per the datasheet curve.
Active production — no LTB pressure
Microchip lists the 1N6109US as Active. The SQ-MELF package is a standard Microchip TVS platform — same footprint as the 1N61xxUS family, so a BOM change to a different voltage variant (e.g., 1N6108US, 1N6110US) requires only a reel swap, not a board respin.
SQ-MELF footprint — layout checklist
The SQ-MELF (B, DO-213AB) is a two-terminal end-cap package. PCB land pattern: rectangular pads matching the metallized band width — typically 0.060″ × 0.100″ per end with a 0.100″ gap. Reflow profile follows standard SnAgCu solder; the hermetic glass body handles 175 °C junction temperature without moisture sensitivity concerns. No MSL classification — the glass seal is impervious to humidity.