UCC28712: can you keep the device out of Frequency modulation at low current loads

Hello Ajayt,

Thank you for your interest in the UCC2871x family of PSR flyback controllers.

The short answer would be "No" given that the fixed-frequency portion of the Control Law operates in Amplitude Modulation for a limited range, dropping the peak primary current from the maximum level to the minimum level (typically a 3:1 or 4:1 range, depending on the device).  Once the minimum peak level is reached there is no recourse but to drop switching frequency as load decreases further, to maintain voltage regulation. 

That said, if the object is to "trick" the controller into staying in the AM region, then an external means of reducing the peak current would be necessary to force the feedback to demand higher Ipp to satisfy a diminishing load.  This could be done by adding an offset signal to the CS input to effectively make Vcs reach the commanded threshold at a lower real Ipp (seen at Rs) for the reduced load.
The caveat is that this idea has a limited range, and could not extend the "AM" operation all the way down to 0A. I would expect that this offset could be varied continuously or applied in fixed steps (with hysteresis), but as the offset voltage approaches the normal Vcs threshold for that region, control over the actual peak becomes more jittery and may become unstable. Especially at high line, where the di/dt can be very fast and turn-off delay makes Ipk control rather non-linear.

So I suggest that the AM region may be artificially extended to some reduction of load (to, say, 1/4 the usual load (from 1/2 the Ippk) at the lower AM/FM corner, for instance), but it cannot be extended indefinitely to 0A.  Whether this extension is worth the extra cost and complexity is another consideration.

Regards,
Ulrich