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UCC28070: The AC current will be unstable during very short time.

Mamoru_Uozu
Prodigy 310 points
Part Number: UCC28070

In the evaluation of this device, when the AC input voltage is changing, the AC current will be unstable during very short time.

This behavior is due to that the AC voltage is reaching to threshold of the linear multiplier as showed on datasheet 7.3.10.

However, even though the AC voltage is held on this point, the AC current will be stable soon.

Can you respond to the following questions?

1. Why the AC current will be stable soon in this case? Is this due to feedback?

2. Is there a possibility to keep the AC current will unstable?

3. When the AC current will be unstable, is there any problem for the PFC working? (for example, the power factor will be bad)

  • 0
    TI__Genius 13660 points

    Hi Mamoru, an expert will get back to you tomorrow. Thanks. ——Teng

  • 0 in reply to Teng Feng
    TI__Genius 9140 points

    Hi Mamoru：

    1:if the error between the ac input voltage and the sampling voltage which we use to choose KVFF is under a reasonable range, the feedback loop will pulling the system back to stable.

    2:Be sure that ac voltage sampling should be correct.

    3: Yes, with Bad PF and ITHD.But it is easy to slow down current loop to keep system stable.

    hope this helps you.

  • 0 in reply to David Huang33
    TI__Prodigy 310 points

    >if the error between the ac input voltage and the sampling voltage which we use to choose KVFF is under a reasonable range, the feedback loop will pulling the system back to stable.

    For KVFF, what range is under a reasonable range? Does it show on the datasheet or app note?

    2:Be sure that ac voltage sampling should be correct.

    Let me correct may question.

    When the AC input voltage is changing, the AC current will be unstable during very short time at point.

    If the AC input voltage is held at this unstable point, Does it keep the AC output current unstable? 

    >3: Yes, with Bad PF and ITHD.But it is easy to slow down current loop to keep system stable.

    You mean the current loop should slow down to keep system stable.

    But if the current loop should slow down so much, we might find the other problem.

    Can you tell me what range can it slow down? If the current loop should slow down very much, what is happen?

  • 0 in reply to Mamoru_Uozu
    TI__Genius 9140 points

    Hi Mamoru:

    for Kvff, i suggest you can follow UCC28070 evm parameter, and the precise of the sampling resistor should be able to work well. Unfortunately, it do not show on the DS or app note.

    for second question, yes, if the ac input voltage is always unstable, the ac current will also unstable. UCC28070 is a PFC controller, the controller always wants to make sure ac current follows ac input votlage.

    For PFC current loop bandwith, my experimence is between 1kHz-5kHz is ok. slowing down the loop too much will lead to bad PF and higher Iithd on ac current.