UCC28700: Need help reviewing the design including transformer

George Evdokimov

Part Number: UCC28700

I made CC power supply using UCC28700 (for input range 90-285VAC) and it can't start. Desired characteristics: output current (CC) 0.7A, output voltage is around 90V. IC generates the series of 3 pulses on DRV pin and nothing alse. Transformer based on PQ26/20 core. I tried 2 configurations: (Pri:Aux:Sec) 1) 28:7:36 and 2)30:5:33(Actual).

DRV (1) and AUX (2):

FET source pin:

Schematic:

over 5 years ago

0 VA Viswanathan over 5 years ago

TI__Prodigy 750 points

Thanks for your interest in this IC. I will review your schematic and get back to you.

0 George Evdokimov over 5 years ago in reply to VA Viswanathan

Prodigy 30 points

So what do you think about this?

0 VA Viswanathan over 5 years ago in reply to George Evdokimov

TI__Prodigy 750 points

Based on the schematic and the waveforms, I think you are hitting output OVP. The UCC28700 gets its output sample from the aux winding. Any large leakage inductance based spikes will get reflected on the Aux winding and can cause to trip the OVP. I suggest that you do the following 2 things:

1. Improve the leakage inductance performance of the transformer
2. Add a snubber on the drain node or the diode. You can look at the following link for reference. www.ti.com/.../sluubp3.pdf

Hope this helps
VA

0 George Evdokimov over 5 years ago in reply to VA Viswanathan

Prodigy 30 points

do the timing diagram look better? I worked on the quality of the transformer.

FET Gate pin and AUX:

Power management

Power management forum

UCC28700: Need help reviewing the design including transformer