UCC28740: Abnormal Waveform on DRV when load current increase

Fery,

Thanks for your note here.

It appears that the device is going into current limit. They may need to look at sizing of CS signal especially.

Can you provide Vout, CS, Aux winding waveform, and Vdd signals as well?

If you can share schematic that will be very helpful.

What input voltage and output voltage/current is this occurring?

Regards

John

Hi John,

Thanks for your quick response, please find attached detail test result.

the input voltage is 110VAC, and when the output current larger than 5V@2A, it will occur.

UCC28740 Testing Result_08_17.xlsx

Hello Fery,

The PWM DRV pattern seen in the waveform posted is indicative of an unstable loop response and this is supported by the FB waveform in the Test Result file. Looking at your schematic diagram, I see that the feedback divider R27 (at IC202-A input) is bypassed by a large feed-forward capacitance in series with a small resistance. I suspect that this provides too much feed forward signal and may be the source of the instability. Generally feed-forward of this type is more subtle, using smaller C and larger R across R27, although I'm not sure what the actual values should be. This R-C network adds additional positive phase shift normally used to compensate for an L-C pole, but this power stage does not have such a pole.

For debug purposes, I suggest that you remove C21 and conduct your testing to see if the loop remains stable for all operating conditions.
Once this is established, then check your gain and phase plots to determine where you may need additional phase boost, if any, and how much. If needed, you can calculate the values of C and R necessary to achieve this.

Regards,
Ulrich

Hello Ferry,

I reviewed your e2e inquiry and have attached my feedback and suggestions in the attached PDF.

Regards,

Mike

2742.Fery Feng E2E.pdf