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I would to identify the cause of a problem affecting a faulty board based on an existing design based on the TOP270EG (see attached schematic).
Characteristics of the supply: mains ~240V, output ~27.5V 3A

Problem description:
- applying the full load causes an overheating of the 200V surge suppressor D7.
- holding the full load results in braking the following components: D7, R10,R108 and U1
- the switching frequency on the faulty board seems to be lower than on an healthy board (~58KHz vs ~122KHz)
- the drain of the TOPSWITCH is driven for longer on the faulty board (~2.5us vs 1.5us)

Could you help me to identify why the TOPSWITCH is not operating at the expected frequency?

Thanks for your support

첨부 파일Size
27.5V 3A TOP270 PSU40.58 KB

Hi ,

Please re-check transformer magnetizing inductance and leakage inductance. For quick verification replace the transformer from good unit to fault unit. Excessive leakage energy causes higher losses in RCD snubber compoents.

Thanks for you answer, I have already tried to replace the transformer and checked the primary and leakage inductance were in range.

I did a scope capture of the drain voltage and current on a faulty unit and healthy unit and it appeared the current limit on the faulty unit is higher than on the healthy unit (2.6A vs 3.9A).
I think the TOPSWITCH is operating in the variable frequency area and only reaches the full frequency at full load (3A).
I don't understand why the current limit "kPS x ILIMIT(set)" is not the same the same on both units (I have checked the external resistor which limit the current to ~90% of the maximum current).

Is it possible I could have a problem with the feedback on the control pin? (outputs voltages are in range)


Top270, typical current limit is 4.2 A. Based on x-pin resistance it is limited to 92% of 4.2A at full load approx. Based on the waveform , it shows that unit is reaching the peak current limit. Check frequency pin properly shorted to the source pin or not . Replace the TOP switch IC from the good board and check. Measure the control pin current. Connect small value of resistance (100 ohm) in between opto and control pin, measure voltage across it and compute the current. If control current is high, then check feedback circuit. Please send full schematic.

Thanks for your answer.

1) The frequency pin is shorted to source.

2) It appears that the TOP270 used on faulty and healthy units are not from the same batch.
The TOPSWITCH of a faulty unit moved on an healthy unit brings it faulty.
The TOPSWITCH of an healthy unit moved on a faulty unit bring it healthy.

3) Control pin current (scope capture in FeedBackCurrent.pdf).
The tests have been performed on the same unit just changing the TOPSWITCH.
The 100R resistor has been placed on the emitter of the optocoupler as the PSU failed to start-up when connected to the control pin of the TOPSWITCH.
With the TOPSWICH from faulty unit, the feedback current reaches ~8mA when it reaches ~4mA with the TOPSWITCH from an healthy unit.

4) I have added a schematic including the feedback (Schematic.JPG)

Schematic.jpg161.33 KB
FeedBackCurrent.pdf106.89 KB


Thank you for sharing the information. Please send the date code of both samples.

1) The "faulty" samples have the date code 1710, those working fine 1945 (see attached picture). I also tried samples with date code 1205 and 1403 without getting problem.

2) Am I right in thinking that with a switching frequency of ~50kHz when the nominal frequency is 132kHz, the TOPSWITCH is operating in the variable frequency area where the peak current is constant and limited to 55% of Ilimit(set)?

3) Is the drain current in the variable frequency area only controlled by the drain voltage? Is there a contribution from the control pin?

4) Could you give me a link to download PIXls usable for TOP270?

Thanks for your support

TOP270EG_Not_working.jpg346.29 KB
TOP270EG_Works.jpg442.15 KB

Please contact local FAE for RMA request. Howmany fault ICs you have and whats the PPM.
You are right, at full load TOP switch operating at nominal frequency and with reducing load frequency goes down.
The drain current is controlled by control pin voltage.
Pi expert online link:
Open above link and click on topswitch Jx family.

Hi PI-csbabu,
Thanks for your support, which was very helpful. I have now forwarded the information to my manager who is going to deal directly with the problem.

What's about post #9 and #10 are they spams or useful information? I was just a bit squared to follow the links.