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UCC28710: The controller works with its minimum switching frequency during 30 ms

Thang Pham Ngoc
Prodigy 110 points
Part Number: UCC28710
Other Parts Discussed in Thread: UCC28700

Hello,

I desinged a Flyback with a multi-output windings transformer. There are fours outputs: two 24v, one 12v and one auxiliary 17v to supply the controller.

In the experiment, my problem occurs when the output 12v is charged up to 2A during 40ms. After this time, the controller enters in an operating mode with its minimum switching frequency of 700Hz. This operating

mode lasts about 30ms and the controller gets out of this mode and goes back to its previous operating mode. It is quite same as "burst mode".

Do you know about this operating mode or have same phenomena like mine?

Looking forward to getting many feedback :).

  • 0
    TI__Guru* 86286 points

    Hello,

    The UCC28700 does not have burst mode.  However, if you have overshoot during power the controller will lower the frequency to control the output voltage.

    That looks like what you are seeing. 

    Regards,

  • 0 in reply to Mike O'
    Prodigy 110 points

    Hello Mike O,

    My first approach was the voltage overshot on the auxiliary winding voltage (V17F) which has a resistor divider to VS pin. But in the other case, voltage on

    V17F decreased before the controller enters in this mode. This mode is shifted when one of the others output windings is charging. 

    The current on output P12 is 2A about 24W, this phenomena occurs. With 1A on P24 output ,same output power, it doest not occur. 

  • 0 in reply to Thang Pham Ngoc
    TI__Guru* 86286 points

    Hello,

    You have 4 outputs that you are trying to regulate on one aux winding.  When the 12V output is stepped from 0 to 2A the frequency should increase and when the 2A load is removed the frequency should decrease.  During this load step the 17V output increases.  These voltages should be clamped through the transformer turns ratio.  Maybe the 17 V is over shooting do to leakage energy.  These outputs should have a pre-load of 1 mA on each to ensure the output voltage do not over shoot when the converter is operating at the minimum switching frequency.  If the out puts do not have the pre-load please add them.

    The other issue is that your transformer turns ratio Np/NV17 and Np/V12x may not be setup correctly.  You might want to double check that to see if that is the case.  If they are not correct one output will pull down on the aux winding to control the switching frequency.  This will cause the other output to be out of regulation

    The following link will get you to a audio training presentation that gives techniques on how to improve cross regulation in multi output fly back converters that you may find helpful as well.

    .

    Regards,

  • 0 in reply to Mike O'
    Prodigy 110 points

    Hello Mike,

    There is already a 10k resistor on each output. 

    For turns ratio, I calculated these ratios based on the voltage desired with Flyback voltage set. For example: NP17/Nprimary= (17V +Vforward_diode)/ 80V , NP24/Nprimary= (24V +Vforward_diode)/ 80V.

    I tried to reduce maximum the leakage inductance between P12 with P17F but this did not resolve the phenomena.