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From: kmaryan on 9 Jul 2010 11:07
On Jul 9, 9:21 am, Jeroen Belleman <jer...(a)nospam.please> wrote:
> kmar...(a)gmail.com wrote:
> > On Jul 6, 4:18 pm, "Joel Koltner" <zapwireDASHgro...(a)yahoo.com> wrote:
> >> <kmar...(a)gmail.com> wrote in message
>
> >>news:246d906d-c984-41c5-b220-e5ea1203afc8(a)a30g2000yqn.googlegroups.com....
>
> >>> Most canned buck
> >>> converter designs quote high 80s to mid 90s efficiency when going from
> >>> 12V to 3.3V, but change the output to 1.0V and the efficiency drops by
> >>> 10% or more. It's generally worse at lower output voltages.
> >>> What are the properties of the regulator that cause this?
> >> 1) If a catch diode is used, even if it's only, say, 0.3-0.4V, that's a huge
> >> chunk relative to 1V whereas not necessarily that bad out of 3.3V (...and of
> >> course nothing at, say, 12V).
> >> 2) If instead a FET is used (a "synchronous rectifier"), it acts as a resistor
> >> and the loss is (mostly) I^2*R.  If you drop from 3.3V to 1V but want the same
> >> output *power*, the *current* will increase by a factor of 3.3 and hence the
> >> loss will increase by a factor of 3.3^2=10.9.  Ouch!
>
> > In the FET case (synchronous), why does the efficiency still drop so
> > substantially even at the same current? i.e. all else being equal.
> > [...]
>
> Isn't obvious yet? For a given output current, losses are
> more or less constant, while the output power scales with
> output voltage. So Pout/Ploss goes down with output voltage.
>
> Jeroen Belleman- Hide quoted text -
>
> - Show quoted text -

Got it, I was a bit dense there for a moment. I ran through the
numbers of a couple designs and everything agrees.

Thanks,

Chris
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