Other Parts Discussed in Thread: UCC28704
Hi support team,
My customer is using UCC28700 to design a wide VIN range aux power supply. We found during the mode transition the switching become unstable and caused audible noise (especially at low line). We already tried to add small filter cap in parallel with VS pin, which helped a little but still having some issue at certain load current, is there any other way to increase the stability or adjust the feedback loop to make switching more stable during mode transition?
Please see waveform and test condition below:
Design Spec:
VIN RANGE: 60VDC ~ 600VDC
VOUT: 7.2V @ 1.5A
- Unstable waveform:
From FM to AM mode:
Vin = 60V
Vout = 7.2V @ 0.3A
CH1 CS
CH2 VDS
CH1 CS
CH2 VDS
- From AM mode to FM mode
VIN = 60V
Vout = 7.2V @ 1.5A
CH1 CS
CH2 VDS
CH1 CS
CH2 VDS
Please note that at 0.3A load the controller entered AM mode but switching at 39KHz? May I know what condition does UCC28700 switch at 39kHz under AM mode? suppose AM mode the fsw should be fixed at 44KHz (Please see second waveform, all previous pulses are at 44kHz except for the last pulse)
Please provide the email to me so I can share the schematic to you, but would you mind letting me know first what can we try to increase/decrease the loop gain and phase margin for PSR flyback?
Thanks a lot.
Best Regards,
Wei-Hao
