Smoothing caps across rectifiers?

Eric at Svetlana Electron Devices svetengr at
Thu Jan 8 07:13:22 CET 1998

>     I remember there being a discussion a while ago on how it was a good
> idea to put a 100n cap across the bridge rectifer in a power supply, to
> prevent reverse recovery spikes. My question is this: which way should
> this cap go; across the AC inputs to the bridge, or across the rectified
> outputs? Or doesn't it matter?

The location varies depending on the engineer you ask. However:
The usual places are:
1) across the electrolytic filter cap (OK)
2) across the transformer secondary (better)
3) one cap in parallel with EACH rectifier diode (best!)

The latter is how it's done on RF linear amplifiers. Gotta keep the RF
out of the power line, and the switching transients out of the RF
The caps must be rated for at least TWICE the transformer secondary
voltage, NOT the AC voltage.

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