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Inrush current limit in 5kw PFC PMP4311

tuan doan
Prodigy 155 points
Other Parts Discussed in Thread: TL431

Hi all,

I have got problem to understand the operation of relay for inrush current limit in PMP4311 design.

Here is my understanding: During power up, the coil of relay will be supplied with 13V, thus the NO contact will close to bypass the thermistors RT4 & 5. When the output voltage reaches 400V, the coil of relay is not supplied any more, and the NO contact is released. But that is undesired operation, could anybody point out my misunderstanding of that   circuit?

Thank in advance

  • 0
    TI__Mastermind 34465 points

    The schematic is shown with the relay contacts open and this is the standard drawing method for unpowered relays.

    This is the sequence of events at power-up:

    1. The bulk capacitors on the PFC (VBULK in schematic are uncharged) and the 13V Bias Supply is zero.
      The relay contacts are open

    2. AC power is applied to the input:
      Charging current through thermistors RT4,RT5 result in an increasing value for VBULK
      When VBULK reaches about 77V the p channel mosfet (Q5) turns on.
      Also by this time the 13V Bias Supply will already be up and running and so Q5 can supply power to the relay coil.

    3. The relay contacts close
       The bulk capacitor voltage can increase at a fast rate and the whole system will start up.

    The relay limits the input current surge at startup. It needs to do this because the large value of bulk capacitance can blow fuses, trip circuit breakers or even cause instabilities in the boost pfc.

    Startup of the pfc should be reliable, robust and repeatable.

    John Griffin

  • 0 in reply to John Griffin1
    Prodigy 155 points
    There are still some points that I am not clear. Plz help me to understand it.
    1. I think the p mosfet will turn on whenever the VKA of TL431 is smaller enough than 13V, meanwhile that VKA is proportional to the Vbulk. Increasing Vbulk will increase VKA. Thus when Vbulk reaches 400V, VKA reaches nearly 13V. At that point the p mosfet will turn off, the relay coil is not supplied, the NO contact will open in steady state operation?
    2. As your explanation, the output capacitor is charged directly from 77V. Is that too early when I plug that PFC to high line 265VAC?
    Is any mistakes in my thinking? thank for your help
  • 0 in reply to tuan doan
    TI__Mastermind 34465 points
    The TL431 begins to draw current through its anode pin when there is 77V on the bus capacitors.
    That event turns on Q5
    Then the relay is energized.
  • 0 in reply to John Griffin1
    Prodigy 155 points
    Thank for your reply,
    I understand that when Vbulk = 77V, the VREF of TL431 = 2.48V (the lowest operating value), the relay starts to be energized.
    After that Vbulk will increase to 400V, thus VREF will increase to 13V, Q5 will turn off, relay coil will be de-energied, and the contact will open. Do I understand that correctly?