ATX SMPS (TL494) output oscillating

[Srbel]

I have an ATX SMPS (DTK 400W) that is oscillating on the output. It uses TL494.

PSU works, but this 800 Hz oscillations are present.

It had a failed output filter caps, I have replaced them all with high-quality low-ESR Japanese caps (this was like 2 years ago).

This is how 12V rail looks like under 100W load.

1,2Vpp 800Hz oscillations:



And when I zoom-in, 40mVpp 50KHz ripple:



Something is wrong with the regulation or feedback? What can cause this?

[tautech]

Do I see a green pointer for Ch 2 triggering?

[Srbel]

Those are old screenshots, from 9 months ago. I don't have time to do that again. Here is what it looks like when measured inside the computer (also old screenshot):

[T3sl4co1l]

Well there's your problem...

Add probably 10 to 100 milliohms in series with one or more caps and you'll probably have it solved.  Ya, I know... the solution is worse than the problem

Can also try improving the pole-zero or lead-lag compensation on the error amplifier (probably going back to the TL494 error amps, look for a voltage divider from one or more outputs back to the TL494 input(s)), but the root problem is a voltage mode forward converter has a complex double pole in the loop response, and wrapping a control loop around that usually makes either a uselessly slow loop (an integrator with large response time, so the output over/undershoot is massive) or an oscillator (which is the case here).

Capacitor ESR is beneficial because it puts a zero in the control loop.  If it happens to be on top of the controller pole, or near the complex poles, that gives the loop a boost in phase margin, keeping it stable.

I suppose aging of the filter choke(s) is a possibility, too.  Let me guess, the core is yellow on three sides, white on the other?  Those things are almost better as resistors than inductors... but again, loss is kind of a helper on this topology.  Except that the internal heating leads to breakdown in the core, which makes it hotter and so on.  If it looks like it's cooking, better to replace it (at which point, you might as well replace the whole power supply, eh?).

Tim

[Srbel]

Thanks. PSU has been replaced, I was just interested to know what could cause this.

This PSU has been used for like 10 years. Here is how it looks like:

[diyaudio]

I have an ATX SMPS (DTK 400W) that is oscillating on the output. It uses TL494.

PSU works, but this 800 Hz oscillations are present.

It had a failed output filter caps, I have replaced them all with high-quality low-ESR Japanese caps (this was like 2 years ago).

This is how 12V rail looks like under 100W load.

1,2Vpp 800Hz oscillations:



And when I zoom-in, 40mVpp 50KHz ripple:



Something is wrong with the regulation or feedback? What can cause this?

TL 494 is easy to debug, probe the output section and oscillator sections directly, a sawtooth waveform should be realized and a complementary push pull with 1% or so dead-time should be present, if no output is present check the 2x ERROR amplifiers, feedback or dead-time pin, they may take control of the voltage loop required to start up the supply. current, voltage, protection or soft-start.

That ATX PSU looks like a generic ATX supply and there are lots of generic ATX schematics on the internet.

I had my fair share with these IC`s

   

[Srbel]

@ diyaudio

Unfortunately, I don't have time for probing. As I said, the PSU has been replaced more than half an year ago, but I did not found out what is causing these oscillations back when I was trying to fix it. These days I got curious again, so I made this thread to see if anyone could give me some clue. At the moment, I don't plan nor have time to further investigate it.

@ blueskull

These waveforms are with around 100W resistive load, as I have mentioned.

[tautech]

@ diyaudio

Unfortunately, I don't have time for probing. As I said, the PSU has been replaced more than half an year ago, but I did not found out what is causing these oscillations back when I was trying to fix it. These days I got curious again, so I made this thread to see if anyone could give me some clue. At the moment, I don't plan nor have time to further investigate it.

@ blueskull

These waveforms are with around 100W resistive load, as I have mentioned.

Well unless you post accurate waveforms that the scope IS triggering on, nobody will be of the real help you seek.
The only one that is anything like accurate, I think by luck only, is the screenshot displaying a pulse @ 52 KHz.
You NEED to do these again, this time be sure to set the trigger to the channel used.

[Srbel]

Well, this is it, I have already posted it.

It has been probed while inside a computer, so there is a lot of noise, but you can see 800Hz oscillations:

Those are old screenshots, from 9 months ago. I don't have time to do that again. Here is what it looks like when measured inside the computer (also old screenshot):

[Srbel]

How would this influence the other components in the computer (basically DC-DC converters), if I use this PSU in it?

Will they "see" it as ripple? Or just a fluctuation of input voltage? DC-DC converters will have to adjust their output as the input voltage changes (1,2Vpp), but will that decrease the life span of it?

It is a relatively new computer. I use some cheap LC-Power PSU (like 15$), and I will probably need to go beck to using this one soon.

[Srbel]

Would 12V rail stabilise if I load the 5V rail? I have only loaded 12V rail, when measuring this. And if yes, should it be 5Vsb (standby) or regular 5V rail?

[T3sl4co1l]

Did this start before, during or after the apparent recapping?

Forward converters, especially voltage-mode converters such as using TL494, are very sensitive to ESR.  You don't want low ESR because it provides a zero in the control loop that keeps it stable.

Tim

[Srbel]

I haven't looked it with the scope with old caps. I didn't have a scope back then.

[Srbel]

I have 22 milliOhm ESR @ 100KHz on 12V rail.

P.S. I am 99% sure that the problem is in too low ESR of new caps. :/