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'[EE] low side buck possibility'
2005\08\16@225436 by Martin Klingensmith

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Assume there is a transformer with an arbitrary secondary voltage, isolated.
Is it inappropriate to run a low-side buck SMPS off of this secondary?
It seems to me that it makes no difference because it is already
isolated. The fact that the low side is not at the traditional ground
level seems irrelevant. The control IC ground potential would be the
negative end of the secondary and the positive would be a low power
linear housekeeping supply.

I bought two rackmount Sorensen power supplies on eBay that were 'as-is'
and as such not working. They are linear supplies with a main
transformer output of about 30VAC followed by a bank of 2N3055's on a
massive heatsink. I thought the rectifiers were burned out so I replaced
one and tried it. The lights dimmed nicely until the fuse blew. I'm just
not sure it's worth it to try to reverse engineer what looks like a 20
year old linear power supply circuit. I don't have any high-side MOSFET
driver chips at the moment though I have everything that could make a
low-side SMPS which is why I am considering that.

Thanks for any help.

--
Martin Klingensmith
http://wwia.org/
http://nnytech.net/

2005\08\17@201115 by Dwayne Reid

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At 08:54 PM 8/16/2005, Martin Klingensmith wrote:
>Assume there is a transformer with an arbitrary secondary voltage, isolated.
>Is it inappropriate to run a low-side buck SMPS off of this secondary?
>It seems to me that it makes no difference because it is already
>isolated. The fact that the low side is not at the traditional ground
>level seems irrelevant. The control IC ground potential would be the
>negative end of the secondary and the positive would be a low power
>linear housekeeping supply.

Your assumptions are spot on.  The transformer provides whatever isolation
it was rated for, the output of the transformer is rectified and filtered,
the SMPS buck convertor changes the filtered DC with lots of ripple into
the desired output voltage.

Make sure that the reservoir cap is large enough to ensure the buck
convertor does not run out of headroom during the low points of the DC ripple.

dwayne

--
Dwayne Reid   <spam_OUTdwaynerTakeThisOuTspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

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2005\08\19@084532 by Martin Klingensmith

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Dwayne Reid wrote:

{Quote hidden}

Hi Dwayne,
I didn't think of how to sense voltage though. The control IC expects a
2.5v voltage error signal. It is unclear how I would do this with a
low-side buck. Any ideas?

--
Martin Klingensmith
http://wwia.org/
http://nnytech.net/

2005\08\19@144005 by Dwayne Reid

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At 06:45 AM 8/19/2005, Martin Klingensmith wrote:
{Quote hidden}

Actually, what I envisioned is the transformer/rectifier/filter block
simply feeding a DC-DC buck convertor.  In other words, design the DC-DC
convertor as normal.  Then feed it from the unregulated supply.

dwayne

--
Dwayne Reid   <.....dwaynerKILLspamspam@spam@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

Celebrating 21 years of Engineering Innovation (1984 - 2005)
 .-.   .-.   .-.   .-.   .-.   .-.   .-.   .-.   .-.   .-
    `-'   `-'   `-'   `-'   `-'   `-'   `-'   `-'   `-'
Do NOT send unsolicited commercial email to this email address.
This message neither grants consent to receive unsolicited
commercial email nor is intended to solicit commercial email.

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