Hi,
I have implemented UCC28950 in peak current mode control, and under very low duty cycle operation, the controller gives me asymmetric QC QD duty cycle (much like the problem which existed with the UCC3895 controllers which I understand there is an app note for a fix). Can you tell me whether this problem should still exist with UCC28950, i.e. whether the problem with UCC3895 has been fixed for UCC28950? or am I more likely to have a different problem?
Another question is, if this is the same assymetric problem as UCC3895, this is caused by a very shallow current sense ramp. If I want to get rid of this problem by implementing Average Current Mode control, is this possible with this chip? I have read in a few other forum subject, and TI employees said with Average Current Mode control, you will not get cycle-by-cycle current limit which might saturate the transformer. But I believe this is not true, because on page20 of the datasheet, it says even if you operate under Voltage Mode control, the CS signal is still fed into the cycle-by-cycle Ilimit comparator.Can I have your opinion on this please.
Third question is, do you know if T.I. or anybody have successfully implemented UCC28950 under Average Current Mode control? Because the datasheet does said it can be implemented under Average Current Mode control, I want to know if this has been tested.
Thank you very much, I look forward to your answers.