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UCC28711: Flyback Controller & Control Behaviour

Part Number: UCC28711
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


 

  • Hi Martin,

    Since all of regulation information were detected from VS pin for PSR controller.  a degraded VS signal will lead to un-expected behavior . when do the PCB layout , minimize stray capacitance on the VC node , and place the voltage sense divider resistors close to the VS pin . for your currently PCB design . you can try to add a 2~3pF capacitor on VS and GND . but this is not recommended if the controller working good . since extra cap on VS pin will delay the signal design.

    Other proposals are increase output capacitance and dummy load if standby power still has some margin.

    Thanks.

  • Hi Jaden,

    Thanks for the reply.

    The voltage sense resistors are now as close to the VS and GND pin as possible (almost directly on top), this did not solve the problem.

    I have tried adding small amount of capacitance (range few pF up to 100pF), although the control behavior changed a bit, it did not solve the problem.

    As output capacitance we have 150uF and already tried to add 470uF up to 3mF. Again the control behavior changed a bit, but not solving the problem.

    There is a bit of dummy load (1kOhm at 26V output).

    Made a shield for the auxiliary windings from the primary windings in the transformer, sadly no change was observed.

    If I measure on the VS pin the voltage seems quite stable cycle by cycle, however the control behavior can change rapidly.

    I agree this has to be parasitic interference, causing common-mode current to mess up with the control, but am a bit out of options to try.

  • Hi Martin,

    There is a another concept for solving  VS detection issue.  you can try to parallel a 1-2pF cap on VS pin upper divider resistor . the purpose of this cap is to attenuate the existing parasitic cap on VS to GND.

    And you can try UCC28704 which has more stronger noise immunity .

    Thanks.