Path: news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail
From: legg <legg@nospam.magma.ca>
Newsgroups: sci.electronics.design
Subject: KA7500 vs TL494
Date: Tue, 25 Mar 2025 09:25:15 -0400
Organization: A noiseless patient Spider
Lines: 76
Message-ID: <h9b5uj1g7bl5ie2l321qoduc2a7tu7d8it@4ax.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Injection-Date: Tue, 25 Mar 2025 14:21:53 +0100 (CET)
Injection-Info: dont-email.me; posting-host="d388c9e6a8c8481637748a657440fc98";
	logging-data="3590457"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1+h86eSaMyXgm3Df2IacySG"
Cancel-Lock: sha1:cPzWXudnGDFrY7JMCK6FFOcXPBg=
X-Newsreader: Forte Agent 4.2/32.1118

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.

The TL494 was an interesting choice for a chip to clone, 
considering the perceived importance of pulse-by-pulse 
current limiting in subsequent control chip designs. 
It modulates the turn-on time in the drive period.

It's possible to turn the switch off, after turn-on, before 
the end of a conduction period. The error amps are extremely 
fast, but you have to latch this decision somehow, in order 
not to produce multiple pulses on the phase, before the period 
ends.

The commodity app simply adds a slow control loop for average 
current limit from a crude output sensor. I was unable (and 
unwilling) to provoke a current limit below 70A of test load, 
prefering to adjust the circuit to get some kind of limiting 
response before component and fuse ratings were exceeded.

Two attempts to adjust a current limit in a 12V 40A version 
of this particular design resulted in primary switch smoke 
and one output rectifier short.

Basic production test has to include output voltage 
adjustment and current limit . . . the latter one I would 
assume to include simple output short cct. Never got that 
far with these fellows.
.. . . . .

The actual 'failure' in the pulled unit was an electrolytic 
capacitor in the bootstrap housekeeping supply. If ESR rises 
above a certain level in this part, the unit cannot start.
In 24/7 service (or static burn-in), you'd not notice till 
the last power cycle, power failure or cold snap.

I used to be quite sniffy when it came to specifying parts 
for this kind of position - ratings seldom reflecting the 
standard use; it was hard to ensure ESR below 10 ohms (the 
practical upper limit for guranteed start-up) in small 
electrolytics over their intended environmental range and 
lifetime.

RL