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I have a HiFi system that had some sort of failure which caused the class-D driver to blow as well as the DSP etc.

Already on board replacing those, I've found some pretty suspicious behaviour from the SMPS which might have been the cause of the fault in the first place.

It's intended to output +30 VDC but there seems to be a condition where it outputs a higher voltage (from +35 to +60 V). That happens if I turn off the mains power and then turn it back on at a specific time during the discharge of the capacitors.

From the datasheet the device is supposed to enter an UnderVoltage condition when power is removed. Though I don't understand why applying DC back in (330 V) at that specific time makes the regulator go crazy and ignore the Vc optocoupler feedback current.

It's a Philips system and it's been in use for a while which makes it probable it's not a design issue. I thought that would happen which an absence of a load at its output. The problem shows up with a light load at 0.3 W, and at a 30 W load the problem doesn't appear, which cannot be an option as I'm assuming an idle class-D amp probably draws very little.

Any ideas on what could be wrong?

Schematic

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  • \$\begingroup\$ Have you checked the electrolytic capacitors? You’ll need an ESR meter to do this. Measuring the capacitance is useless. I’d suspect C515 first. It may just be easier to replace it since it is just a $0.50 part. \$\endgroup\$
    – Kartman
    Commented Oct 31, 2022 at 22:56
  • \$\begingroup\$ My bet is on C520. It's a critical part setting the gain of the feedback loop; if it's dry and has high ESR, the optocoupler feedback gets messed up and the converter might lose regulation. \$\endgroup\$ Commented Oct 31, 2022 at 23:15
  • \$\begingroup\$ @Kartman and Jonathan S. I took them out and even though they measured ok as for capacitance, C520 had 40ohms ESR and C515 had 200Kohm ESR which sounds quite a lot and makes me want to swap them both. Hopefully that will solve it! \$\endgroup\$
    – Alex P
    Commented Nov 1, 2022 at 1:30
  • \$\begingroup\$ Regulation is ensured by injecting current into the shunt regulator. This current must be provided by the aux winding via the PC817 optocoupler. When the over-voltage occurs, check the voltage across \$C_{506}\$ reaches the shunt regulator 5.8-V level. If it does not meet that level, no injection can occur. It can be the rectified voltage, the optocoupler CTR that is too low, the LED not providing enough photons etc. Check if your behavior occurs with a short across the TL431 (3, 2) as the supply should go in hiccup mode with a fully-biased LED. Test with an isolated HV dc supply for safety. \$\endgroup\$ Commented Nov 1, 2022 at 11:09
  • \$\begingroup\$ @VerbalKint the behaviour of the TOP249 is pretty erratic when in that fault condition. The C pin (supply/feedback) goes higher to almost 6V (which in theory should be shunted to 5.8V by the controller), and it gave me the idea the optocoupler was working fine. Anyway I did try to parallel a capacitor to C520 and it seemed to help quite a bit, only getting a 35V overvoltage instead of 60V. Once I'll get the new caps I'll put them in and test! \$\endgroup\$
    – Alex P
    Commented Nov 2, 2022 at 12:18

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