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UCC28056EVM-296: 10Hz load transient response evaluation <plus UCC25630-1EVM-291 (IC is UCC256302)>

Part Number: UCC28056EVM-296


I think that it is highly likely that the PFC output overvoltage detection circuit operates and the PFC is stopped as a cause of the instantaneous stop of the LLC output when the load suddenly changes (0 to 100%) of the LLC output.
As a countermeasure, the capacitor C13 (10 pF) connected to the IC input terminal (ZCD / CS) of the PFC (UCC 28056 EVM) evaluation board was further upgraded to capacity of 15 p to 120 pF, and the effectiveness of the countermeasure was verified.

However, by increasing the capacitance of this capacitor, although it is possible to confirm the effect of avoiding the overvoltage protection OV (which seems to be a malfunction), while the startup test is being carried out by adjusting the capacitor capacity, the circuit does not start up After that, I reset the input many times and did the gradual raise operation but it stopped starting.

At first glance, this IC seems to be broken (damaged?) Often occurs, every time
We are replacing the device with a new one and continuing the test. At this time, it is confirmed that the power supply voltage (Vcc) of the IC and the voltage applied to the input terminal (ZCD / CS) are not beyond the rating.

So I ask you the following questions.

① Does increasing the capacity of the capacitor C13 of the evaluation board cause damage or stress to the IC?
In that case, for what reason?
② As the rating condition of the input terminal (ZCD / CS), not only is the input voltage rating 7 Vmax, but there are other input conditions so as not to damage the IC?
③ If there is a problem with increasing the capacity of the capacitor C13, what is the upper limit of the adjustable capacity possible up to pF?
④ If the capacity of C13 above is increased and changed, what is the evaluation that needs to be verified to confirm the degree of influence on normal circuit operation?

  • Hi Doi-san,

    1. I would not expect increasing C13 to damage the controller. R12 and C13 form a RC filter feeding the ZCD/CS pin. Increasing the capacitance would only dampen high frequency more.
    2. The ZCD/CS pin must stay below 7V to avoid damage to the IC. it is possible to see unexpected behavior if the ZCD/CS pin waveform is misbehaving but it would not cause damage to the IC.
    3. There is not an explicit range on the RC filter values. If the filter is too aggressive it may distort the ZCD/CS pin waveform and cause odd behavior however.

    Are you seeing the PFC controller stop switching? If you are truly hitting OVP2, you would see the ZCD/CS pin waveform go above 1.125V and stop switching for at least 1s.

    Best Regards,
    Ben Lough
  • Hi Doi-san,

    I believe this post is related to your other post

    If yes, I will go ahead and close this thread.

    Best Regards,
    Ben Lough