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From: John R Walliker <jrwalliker@gmail.com>
Newsgroups: sci.electronics.design
Subject: Re: KA7500 vs TL494
Date: Thu, 3 Apr 2025 19:11:03 +0100
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On 03/04/2025 16:11, legg wrote:
>>> this circuit schematic of an application available at DiodeGoneWild [1]:
>>>
>>>     <https://danyk.cz/s_atx01h.png>
> 
> The circuitry around Q5 and Q6, far from being some kind
> of OVP protection, is actually an undervoltage latch.
> When either transistor turns off, the slow-start capacitor
> is discharged fairly quickly.
> 
> If the missing connection to pin 4 is present, the PW is
> inhibited and conversion is latched off, until the
> collapsing housekeeping supply turns the chip off.
> 
> Combined with a current limit, this can produce a hiccoughing
> response to overload or short circuit on the lower-powered
> outputs.Sort of a Hail Mary approach.
> 
> Q5 disables output undervoltage effects while the housekeeping
> supply is rising (at start-up), or has gross ripple.
> 
> Some of the commodity supplies of the type don't have this
> added circuitry, hence their sensitivity to output
> shorts and overload.
> 
> Single output units may monitor the one output or simply
> count on current limiting to reduce PW sufficiently to
> collapse the housekeeping supply.
> 
> RL

Slightly relevant to the above discussion - I recently
wanted to get an old Dell notebook PC running and found
that many of the (supposedly) genuine Dell power supplies
I had to hand did not work.  One was completely broken,
but several of the others allowed the notebook to run for
a few seconds before shutting down.  All were adequately rated.
I checked them all with a variable load resistor and
found that they would deliver significantly more current
than their rated output.  However, once they current limited
they latched off and could only be restarted by power cycling
the mains input.
The two supplies I found that would run the notebook were
very different.  One, labeled as a genuine Dell unit kept
delivering more and more current until I stopped as the
output dropped from 19.5V to 12V at about 8.5A.
The other one fold-back current limited at a sensible degree of
overload and restarted when the overload was removed.  This
was made by Lite-am.  This is the one I am now using.
The notebook PC may of course be drawing far too much current
at startup.  I will check this later.  The battery is dead.
However, I was quite surprised by these results.
John