KA7500 vs TL494

As you say, KA7500 specific notes are nearly non-existent; other than this circuit schematic of an application available at DiodeGoneWild [1]:

The switch supply section at his website is worth a look:

The Badcaps SMPS forum is also worth a look:

Note.

Danke,

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.

RL

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

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

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

On 4/6/25 3:29 AM, John R Walliker wrote: <...>

The first time I cam across one of these I accidentally shorted the inner ring to the centre pin while measuring the output voltage.

Subjecting the One-wire device to that 19V destroyed it so the computer refused to recognize the power supply and would only work at a lower CPU speed and not charge the battery.

I disassembled the supply and I was surprized to find there is no protection circuitry to prevent this type of damage.

So be careful when measuring the voltage output of these supplies.

kw

Burn-in? Doesn't that happen at the customer? :-)

You mean a datasheet? It's here:

No, burn-in is a well-defined process control step used in the manufacturing of equipment to achieve and maintain low failure rates (ppm).

The only thing a customer can do is avoid suppliers whose products are not proven reliable. Even rumours of such a condition can put a name brand out of business - hence the plethora of short-lived off-shore vendor names in this market (and the bargain pricing).

No, I don't mean the data sheet.

RL

What fraction of the parts and equipment that you buy has been burned-in? And how do you know?

I'd expect 0%, and that you don't know.

Purchases of assembled hardware, here, are generally consumer grade, with no obvious indication that infant mortality has been addressed.

Some incoming aql levels are specified in the data sheets at the component level. Things like pumps, motors and power supply units are included in this category.

Designs or products that go out the door can only achieve ppm failure rates if a burn-in strategy is included after final assy. Nuts and bolts can fail just as often at this stage.

RL

With modern electronics, burnin isn't necessary or feasible.

Temperature cycling and vibration would improve reliability a bit, but that's not practical either.

The biggest failure cause is bad engineering.

Modern electronics, except for the obvious cheap junk, is remarkably reliable.

Apart from the price, there's nothing 'obvious' about modern electronic reliability. Modern's got nothing to do with it.

Off-shore hardware can suffer 200% tarrifs and still be competative at the retail level. There's plenty of room for quality control.

Those margins are, instead, being absorbed by shareholders; hence the stock market sensitivity at that end.

RL

ICs are more reliable than tubes.

Transistors don't have their wire bonds sheared by bad epoxy shrinking much these days, or breaking from Purple Plague. Fraction-of-a-cent surface mount resistors don't fail at all.

Great, let's have some.

The CCP, mostly.