Hello,
I'm working on a 2 outputs Flyback DC/DC converter based on UCC28700. The design is based on reference design PMP10415, but with lower input voltage (300-400 VDC). I have attached the schematic. The two outputs are targeted to deliver 30W each at 15V. The transformer has 60 turns on the primary, 6 turns on each secondary and 9 turns on the auxiliary output). We have tried two primary inductance values: 300uH and 500uH. The result in both cases is an unstable behaviour, with the duty and frequency moving, even with occurences of time slots with no swithcing. (The test was performed at 400V input).The only way to get a stable behaviour so far is replacing the sense resistors (2 x 0.39 ohm in parallel) with a single 1 ohm resistor). But in this way the output power is limited.
What can we try to get a stable working ? Is it possible to have more information on the control law (loop gain, poles and zeros) ?
Thanks.
Regards,
Ottavio DI Sauro.