Re: KA7500 vs TL494

On Thu, 27 Mar 2025 05:31:29 -0000 (UTC), "Don" <g@crcomp.net> wrote:

legg wrote:

Don wrote:

legg wrote:

Chinese commodity power supplies have tended to use recognizable
configurations from times gone by. In doing so, it's easy to
miss some of the 'small stuff' that actually produced a reliable
product, in the day.
>
Even more so, when pricing reaches the 'replace vs repair' threshold
- why even bother with burn-in, in that case? If no burn-in or field
return failure analysis is ever consudered, the small errors persist,
particularly if vendors play wack-a-mole with the same hardware
offered under different brand names and paperwork.
>
Case in point is a 5V 40A unit advertised 'for use in LED sign',
commonly used in Onbon product. In the application where a repair
or replace decision was made, actual consumption was in the 35W
range, though a test sequence could draw much higher power.
replacement with an identically rated unit was Cdn$22.00.
>
The replacement was physically and schematically identical, but
relaid as a mirror image for component placement. Different
brand name.
>
Anyways - a basic self-oscillating bipolar transistor half bridge
with forced beta, synchronized/steered and pwm'd by opening and
shorting the resistor-limited, center-tapped 'drive' winding.
Open collector drive out of a KA7500.
>
What's a KA7500 ? Turns out to be pin compatible to TL494, but
mfrd by Samsung/Fairchild/ONS.
>
http://ve3ute.ca/query/TL494_vs_KA7500.pdf
>
Oodles of data and apps for the 494, not so much for the 7500.
If anyone's got app info published for the KA7900, in any
language, I'd be interested to see it.

>
As you say, KA7500 specific notes are nearly non-existent; other than
this circuit schematic of an application available at DiodeGoneWild [1]:
>
   <https://danyk.cz/s_atx01h.png>
>

<snip>
>
That's basically the controller drive and regulation section found in
commodity single output jobs, but they use output copper track and
links to develop a bulk current limit, within the input compliance
 range of the error amp inputs (one would hope).

>

I note that in the Sunny Tech schematic, the slow start and aux
overvoltage line is disconnected from either regulation path.
>
The 12V 40A unit used MBRF30100 output rectifiers (fully insulated
TO220). I'll believe the part msrkings/ratings when I see it.

>
My knowledge of SMPS is limited. Where are the slow start and aux
overvoltage lines?
>
It seems like the thermister, tau set from R34 and C16, and the output
control (pin 13) ought to contribute something to a slow start.
>
Does the circuit around pin 15 sense overvoltage?
>
Danke,

PW is reduced as negative-going voltages on pin 15 approach 0.
The node is primarily aimed at current limit/regultion.

R28/C14 could provide slow-start if JN to D10 had contact
with (presumably) pin3. (0.7mA pull-up internally)
As is they do nothing.

Q6 can sense overvoltage on negative rails. Regulation
on +5 and/or +12 looks to be either selected or compounded.
If a zener, R21 could sort of limit +12.

RL