UCC28702: output resets in some boards

Prabhu R56

Part Number: UCC28702

Hi,

We have been using UCC28702DBVT in a design for couple of years, but we were reported in some boards recently the the DC output voltage resets continuously when the input AC voltage increased beyond 232VAC. But this design is basically to support 85V to 305VAC and it has been tested as well. But only in few boards (so far 2 nos out of 100) has this issue.

I am suspecting the transformer core, I am yet to replace with new one but before that I wanted to check in the forum if anybody have any other suggestions.

I have attached the schematic for your reference.

Thanks,

Prabhu

over 1 year ago

0 Claire Chang over 1 year ago

TI__Intellectual 2420 points

I am looking into it and will get back to you soon.

0 Claire Chang over 1 year ago

TI__Intellectual 2420 points

Hi Prabhu,

THanks for reaching out, can you provide a clearer schematic picture? Some of the numbers here are hard to see.

What is the turns ratio of your transformer? (Np:Ns:Na)

You can probe Vdrain, Vcs, Vaux, and Vout (all respective to GND) on the failed board to check each signals if they trigger any protection fault.

Here is an attachment (here) that helps explain the basic troubleshooting for our flyback controllers.

Best,

Claire

0 Prabhu R56 over 1 year ago in reply to Claire Chang

Prodigy 180 points

Hi Claire Chang,

Thanks for quick response.

I have enclosed the schematic in pdf, hope this is legible.

Below you can find the transformer turns ratio.

TURNS RATIO	(2-1):(5-4)	3.83:1, ±1%
TURNS RATIO	(2-1):(9-6)	15.33:1, ±1%

I will probe those points and will get back to you.

Regards,

PrabhuUCC28702 schematic.pdf

0 Claire Chang over 1 year ago in reply to Prabhu R56

TI__Intellectual 2420 points

Hi Prabhu, I don't see any big issues from the schematic. Please probe the waveforms as suggested and let us know if you detect the issue.

0 Prabhu R56 over 1 year ago in reply to Prabhu R56

Prodigy 180 points

Hi Claire,

I was trying to probe the points we discussed but unfortunately I couldn't probe when the issue occurs since the power supply immediately goes normal soon after I touch VS pin either with DMM or DSO probes or any metal objects.

However, I resolved the issue by adding a snubber across output diode D4. Board works absolutely fine even up to 277VAC.

Thanks a lot for your support.

Regards,

Prabhu

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UCC28702: output resets in some boards