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From: John R Walliker <jrwalliker@gmail.com>
Newsgroups: sci.electronics.design
Subject: Re: KA7500 vs TL494
Date: Sun, 6 Apr 2025 11:29:48 +0100
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On 05/04/2025 23:57, legg wrote:
> On Thu, 3 Apr 2025 19:11:03 +0100, John R Walliker
> <jrwalliker@gmail.com> wrote:
> 
>> 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
> 
> If they use 3 terminal barrel connectors, they will be
> negotiating output voltage and power settings, similar
> to USB-C 'PD' terminal traffic.
> 
> If negotiation isn't reached, they may revert to 5V low
> power. A USB-C mock load can usually convince a Dell
> 3-term 18-20V supply to regulate at 5,9 or 18-20V; 3 of
> the 5 voltages the USB regs cover.
> 
> Only the highest voltages produce rated power from
> the supply - as a characteristic of that supply (see label).
> 
> Dell were pretty good at squeezing maximum quality and
> lowest price out of their suppliers (Dell labeled or
> otherwise). Getting 10 years out of them at normal room
> temperature is quite common.
> 
> Don't be shy about getting an after-market battery
> replacement (or two).
> 
> RL

I'm certainly not shy about using an after-market power supply.
That is the one that works properly.

All these power supplies are the three-contact barrel type.  They
all deliver a fixed 19.5V, even into a resistive load.  They can
also all deliver well over their rated output currents.  The issue
seems to be that their failure mode is an inability to retry when
the current limit has been triggered.
I don't think they have any mechanism for negotiating like a USB-C
PD supply.  Instead they just have a memory accessed through a 1-wire
interface which tells the computer what their rating is.

John