Re: gEDA-user: Switching regulator question

[Peter Clifton <pcjc2@xxxxxxxxx>]

On Sat, 2007-06-16 at 22:23 -0400, DJ Delorie wrote:
> > Doesn't seem that you gain or loose anything here - assuming the
> > switching time is not affected.
> 
> The FET helps because there's an ohmic loss across the diode on the
> order of Io*Vf when the switcher is in the off state.  The FET has a
> lower Von (Vf) than the diode, which reduces this particular loss.

Sorry - I wasn't being clear, I was thinking of the main switching
element, not the synchronous rectifier FET. I know I commented on that
right before.

> You still need the diode, though, in case there's current during the
> transitions.

Yep, but depending on the exact application - you "Might"(TM) be able to
use the synchronous rectifier FET's body diode. Most FETs don't have
great characteristics for their body diodes though, as they are
essentially a parasitic.

[snip]

> And choosing caps with higher voltage ratings helps them last longer,
> too.

I didn't know that for electrolytics, but I can well believe it. I've
heard of sizable de-rating need for reliability in rolled-film
capacitors used at high voltage. (UPS etc..)

Peter C




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