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Original question:

UCC25600: Inrush current

UCC25600: Inrush current with LLC

user5339789
Intellectual 590 points
Part Number: UCC25600

Dear 

I have sent before below question. Could you please reply to me ?

In fact LLC power supply with the UCC25600 generally, Inrush current will be occured basically, then designer has to consider RC network to avoid such as problem, is that correct?

  • 0
    TI__Guru* 85616 points

    Hello,

    All power supplies will have an inrush current issue not just LLCs.  This is generally handled by an NTC or relaying in a current limiting resistor.

    It is a good idea to keep the UCC25600 off during inrush until the bulk capacitors have fully charged.

    Regards,

  • 0 in reply to Mike O'
    Intellectual 590 points

    Hello,

    Customer indicated waveform when the LLC power supply activated.

    CH2 is current sence signal line ( around RC network ) to the OC pin, CH3 is soft start pin. 

    We we do not think  in soft start mode, inrush current come up rapidly during increasing soft start level.

    That is why OC is operate during 5-6mS then freesed all of operation.

    Could you please is it normal? 

  • 0 in reply to user5339789
    TI__Guru* 85616 points

    Hello,

    Could you double check the waveforms that you sent?

    The UCC25600 should shut down both gate driver if the OC (CH2) is greater than 1V. 

    The UCC25600 if the soft start pin is pulled below 1V switching should stop as well.

    Pleas note the UCC25600 will soft start the output voltage but is not controlling the current.  This is a voltage mode control device.

    Section 7.3.2 of the data sheet explains how to setup the OC protection.  This might be worth reviewing and double checking.

    Also there is an excel design tool for the UCC25600 that you can use to double check your OC circuitry.

    If the tools don't help resolve the issue.  Please send a schematic with corrected waveforms and we will help further?

    Regards,