Working switch mode power supply with overheating power resistor

[xavier60]

It's actually the primary self inductance that's ringing. The output diodes are unclamping very early.
It's common to see small flyback SMPS operate in extreme discontinuous mode.

[geekyoyd]

Hello: Just saw this post now.

Snubber power is a very old and well known question. It should be perhaps a few % of the max output power, so 5W is a lot for a small supply.

The ringing is a natural result of energy stored in the leakage inductance of the magnetics after switch turn off.

Minimum load is a worst case, as network damping is reduced.

The PRF does not affect the ring freq, only determined by the Llkg, and parasitic inductances of the magnetics, combined with the snubber R-C.

Many flyback and forward converters use active snubbers, diodes, TAZ or extra windings to reduce the lost power of the required snubber.

Old Vicor patents are an excellent example.

To learn more the OP could search sites about converter  forward and flyback design, and snubbers.

Enjoy,

Jon

Thanks for your insight.  I'm not that knowledgable on SMPS design and terminology, I usually avoid trying to repair them but this one is quite simple and through-hole parts are used.  I assume LLKG is the leakage inductance in T2 but what is PRF?  Reflected power?  I'm guessing TAZ = TVS = Transient Voltage Suppression diode?

[jonpaul]

Hello geekyoyd

Llkg is indeed leakage inductance, measure the primary ind with all secondary shorted. It represents the flux lines that do not link all windings, so appears as a series inductance.

TAZ= transient Absorbing Zener.

PRF = pulse repetition rate = switch frequency.

I would run the supply fully loaded and with no or minimum load. Then check the resistor dissipation (just put IR temp probe on the surface after warmup)

At least the snubber power at max and min load should give a clew.

Bon Chance,

Jon