UCC28742: Issue in start-up

Akash Harsh

Part Number: UCC28742

Hello Sir/Madam,

I have designed 25W,18V flyback converter using UCC28742. We are facing issue during startup. IC is turning off after around 900ms.

Below are the attached waveform of CS, VDD, AUX(VS gain is 10/57) and VDS.

I believe all the protection are within the limit.

Could you please help us in rectifying the issue. I have already referred the troubleshoot document (https://www.ti.com/lit/an/sluaac5/sluaac5.pdf?ts=1681906025110&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FUCC28742)

Rlc=4.7k, Ccs=1nF.

Please let me know if there is any further data is needed.

Thanks and Regards

Akash

over 2 years ago

0 Claire Chang over 2 years ago

TI__Intellectual 2750 points

Hi Akash,

Thanks for reaching out! From the first picture it seems like your VDD is slowly dropping below UVLO threshold (7.8V) and therefore it shuts down. This VDD drop seems to result from not getting power from aux winding circuit.

What does your aux winding circuit look like?

Thanks.

0 Akash Harsh over 2 years ago in reply to Claire Chang

Prodigy 130 points

Hi Claire,

Thanks for the kind support. Below is the circuit requested,

Some Resistors and capacitors value mentioned in the schematic are different. Below are the exact values

R163=47k, R170=33k, R176=4k, R177=30k, R173=4.7k, R228=Not populated, R174=0.68ohm, R167=1ohm, R164=22ohm, R160=2.2ohm, C137=1nF

Thanks and Regards

Akash

0 Claire Chang over 2 years ago in reply to Akash Harsh

TI__Intellectual 2750 points

Hi Akash,

I am looking into it and will get back to you shortly. Thank you for your patience.

Best,

Claire

0 Mike O' over 2 years ago in reply to Claire Chang

TI__Guru* 86286 points

Hello Akash,

Your secondary to aux to secondary turns ratio (Ns/Na) may be off. You need to double check this.

The following equation should help you.

(Vout+Vd)*Na/Ns > or = VDDoff+Vd

Regards,

Mike

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UCC28742: Issue in start-up