Switching PS self oscillation frequency

[Bud]

I am repairing a Philips 40'' smart TV power supply and have replaced a bunch of semis which were gone after a mains line surge or something, mainly on the primary side. The power supply board is currently on the bench with the secondary outputs loaded with resistors. The problem is it is noticeably squealing (not loud but still annoying) and I only give it like less than 5W load. Not sure if the transformer was damaged by the failure   or some other frequency defining component. The transformer looks ok and has no smell. The secondary rectifiers are good, secondary caps are also good. So question is in this topology what components define the frequency of oscillation ?

[T3sl4co1l]

Is that actually a forward converter?  (What does the secondary side look like?)

C608 and C609 are the primary timing components, I think.  This is a variable peak current control, but the off time is probably variable as well, so it may be normal for it to whine at light load.  Try a heavier load.  I'm guessing there's a TL431 on the secondary side, just behind the opto, which will have compensation, which is the other thing to look at.

Tim

[Bud]

That is right, there is a TL431 on the secondary side, attached is a fuller schematic. But the heavier the load the louder the whine. I havn't looked at C609 yet. C608 is a tiny SMD ceramic cap. R611 and D608 were blown, I replaced them.

Edit: for completeness, the following parts failed: Q601, Q602, D608, R611, and D655 on the secondary side (shorted)

[Bud]

I gave it more load , about 20W , and the whine went away. Apparently this is what is supposed to be.
However, the PS secondary voltage goes too high at light loads - the 21V secondary goes as high as 40 V and the 12V line goes 20V or something. I think it is a poor design just waiting for a bad thing to happen.

Is a way to improve voltage regulation at light loads in this topology ?  There does not seem to be much of overvoltage protection on the secondary beside a simple zener crowbar.

[Armadillo]

Hi;

your power supply is similar to this power supply with scope traces and measurements, you can refer to it.

https://www.eevblog.com/forum/repair/please-help-me-repair-a-smps/msg1345909/#msg1345909

Edit: Read the above particular on the opto-coupler. If you apply more load the opto should be off, the anode and cathode ~3V same voltage, if you apply less load the opto should be more "ON", both the anode and the cathode will be more different voltage.

hope it helps.

[Bud]

Edit: Read the above particular on the opto-coupler. If you apply more load the opto should be off, the anode and cathode ~3V same voltage, if you apply less load the opto should be more "ON", both the anode and the cathode will be more different voltage.

That is right, but at light loads the opto is fully open, struggling to reduce the voltage on the secondary, but it can't anymore. The loop goes out of wack at light loads.

[Armadillo]

Edit: Read the above particular on the opto-coupler. If you apply more load the opto should be off, the anode and cathode ~3V same voltage, if you apply less load the opto should be more "ON", both the anode and the cathode will be more different voltage.

That is right, but at light loads the opto is fully open, struggling to reduce the voltage on the secondary, but it can't anymore. The loop goes out of wack at light loads.

So you should do voltage measurement at the TL431 and the opto, so that we can further advise you as to whether the TL431 is defective or the opto or the other connecting devices. Try doing with 2 different loads for comparison. With written voltage measurements, it would be easier for you.
Edit: And also measure the voltage across R611 in the two cases above. The voltage represents what the mosfet is actually seeing the actual loads regardless of what loads you put at the secondary.

[james_s]

Does the power supply ever experience light loads in normal operation? It's not uncommon for power supplies integrated into equipment to require a minimum load to operate correctly. Since it's never intended to be operated without being connected to the load, it doesn't have to be designed to tolerate that condition.

[Bud]

the PS secondary voltage goes too high at light loads - the 21V secondary goes as high as 40 V and the 12V line goes 20V or something.

Found the fault - in this case it was my fault. When replacing the original burned transistors Q601 and Q602 (per the above schematic), I could not find a 2SC5344 for Q602, Digi did not have it either, so I slapped a mmbt3904 which I had a bunch of, it also had a higher CE breakdown voltage. It eventually turned out to be the culprit of the observed regulation problems at light loads. After I suspected it may be what was causing the problem, perhaps because of the specific Beta, I got from Digi a couple transistors with a closer Beta to try. Both worked wonders and the power supply began to behave. The replacement transistors I tried were BC846B and mmbt5551. I left the 5551 because it has a higher CE voltage (160V).
I now have a working 40' Philips Smart TV    with an excellent LCD panel