Re: How do isolated DC/DC converters get feedback from output?
- From: Tim Wescott <tim@xxxxxxxxxxxxxxxx>
- Date: Fri, 28 Dec 2007 18:57:44 -0600
On Fri, 28 Dec 2007 12:05:59 -0800, Michael wrote:
On Dec 28, 9:57 am, Tim Wescott <t...@xxxxxxxxxxxxxxxx> wrote:
On Fri, 28 Dec 2007 09:09:01 -0800, Michael wrote:
Hi - this has been bugging me for a while: How exactly do isolated
output DC/DC converters get feedback about the output voltage? To me,
this would seem to be quite the challenge. I read somewhere that in
some systems they'll use a device that has two matched photodiodes
looking at an LED, and so on the output side you drive the LED with a
circuit that uses one of the photodiodes for feedback, and use the
other photodiode for feedback for the DC/DC converter chip. But then
I've also heard of some oddities that involve extra windings on the
transformer, or something like that, which confuses me as I have
trouble understanding how that would be of use.
So - can anybody shine some light on this subject? Thanks!
-Michael
Some designs put the controller on the output side, with transformers
to control the driver on the input side and a driver that will self-
oscillate before the driver comes on-line.
The 2004 ARRL Handbook has such a design for a 12V, 100W, off-line
converter.
--
Tim Wescott
Hmm I had thought about doing it that way but decided that the power on
condition would be annoyingly complex to deal with. You don't happen to
have a schematic handy, do you? It would be interesting to see how
others dealt with that.
Also - what exactly does "off-line" mean in this context? I occasionally
run into that term but am not familiar with it.
Thanks,
-Michael
I don't have a schematic in electronic form -- that's why I referenced
the Handbook.
"Off-line" in this context means that the supply is designed to rectify
the AC from the wall and generate isolated regulated DC straight from
that, with no intervening linear supplies. It's what's in your PC and
all the newer wall-warts.
--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com
Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
.
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