Other Parts Discussed in Thread: UCC28704
Dear Team,
our customer reached out regarding a question about the control behaviour of the UCC flyback controller.
I will be on vacation, hence the follow-up via the forum as a direct link between customer/TI.
Thanks and best regards
Martin
Is there more information about control stability, as I am experiencing unstable control behavior?
In the attached pictures you can see what I mean. As the input voltage and output power are fixed the flyback controller should be stable in 1 single control position.
However it is not. It seems that the input of the control law V_CL is going very quickly from high to low and back.
As extra information, there are combinations of input voltage and output power where the control is stable, see Tek037.
I have checked all inputs of the controller, they seem all ok. Control behavior is mainly based on the voltage sense pin, if I read the datasheet correctly.
The flyback has no external capacitance with 68k/33kOhm divider on the auxiliary winding / VS pin.
But measuring the VS pin directly on the pin itself with a TIVH probe (10pF) the input voltage seems stable (Tek078).
Is the VS pin very sensitive to common-mode current due to capacitive coupling from the auxiliary winding to the primary winding?
And if so, what do you recommend to suppress this?
Tek037:
Tek066
Tek078