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UCC28951: Phase-Shifted Full-Bridge primary peak current so high

Yuxi Liu(Tony)
Expert 3220 points
Part Number: UCC28951
Other Parts Discussed in Thread: UCC27714

Hi TIer

Customer use UCC28951 to build Phase-shifted full-Bridge power supply from the 2kV/2000W.

when the voltage at the input of the DC/DC stage is only 30V. the primary current of the transformer is already 50-A peak to peak.

(the output of the power supply is to charge a high-voltage capacitor of 33 UF / 2200 V).

Could you give us some sugguestions? Let us know what causes this high current peak.

Thank you very much.

  • 0
    TI__Guru* 86286 points

    Hello,

    It sounds like your cycle by cycle peak current limit is not setup correctly.

    The following link will bring you to an application note that goes through the step by step design process of PSFB using the UCC28951.

    There is a link that will bring you to an excel design file.  You and the customer can use it to review the design and setup the current sensing and cycle by cycle peak current limit correctly.

    https://www.ti.com/lit/pdf/slua560

    Regards,

  • 0 in reply to Mike O'
    TI__Expert 3220 points

    Hi Mike

    In ucc28951 datasheet, Do we have requirements for the voltage range of EA+? Will its voltage value affect the duty cycle of operation? 

  • 0 in reply to Yuxi Liu(Tony)
    TI__Genius 12795 points

    Yuxi,

    The range of the error amplifier output is 0.25 V to 4.25 V. The lower of the two noninverting inputs of the error amplifier is the dominant input and sets the duty cycle where the output signal of the error amplifier is compared with the internal ramp at the inputs of the PWM comparator.

    Best,

    Ning

  • 0 in reply to Ning Tan
    TI__Expert 3220 points

    Please also determine how the voltage range of the ucc28951 chip EA+ is set. According to your reference design case, this voltage value is 2.5V. Customer do not know if there is any special requirement. Whether the setting voltage at EA+ has an effect on the circuit duty cycle? How to effect?

  • 0 in reply to Yuxi Liu(Tony)
    TI__Genius 12795 points

    The whole error amplifier block can be used to close a feedback loop to control the Vout. Please refer to section block diagram in section 7.2 and application diagram in the first page of the datasheet for details. 

    Vsense pin feeds to EA- (with a resistor divider) and a user can set proper R1,2,3,4 values to determine the reference voltage on EA+. The voltage range on EA pin shall be 0.25 to 4.25V. It is recommend to set it to 2.5V but not mandatory. 

    During normal operation, the Vsense through the error amplifier network enable/disable the internal PWM signal, hence the duty cycle of the IC outputs.

  • 0 in reply to Ning Tan
    TI__Expert 3220 points

    Hi TIer

    There are a few things to help confirm about this PSFB full-bridge power supply:


    How the operating duty cycle of UCC28951 needs to be modified by the chip peripheral parameters.


    The full-bridge driving scheme of the UCC28951+UCC27714 we are using now is at risk, and there have been several HB-HS breakdown failures of the UCC27714. Please recommend a proven drive scheme.

  • 0 in reply to Yuxi Liu(Tony)
    TI__Genius 12795 points

    Yuxi,

    I am not sure what your question about UCC28951. If you need assistance on UCC27714 or other high side driver, please create another thread for UCC27714 and the right team will address it.

    Thanks,

    Ning

  • 0 in reply to Ning Tan
    TI__Expert 3220 points

    Hi Ning

    The customer would like to check:

    How to modify the operating duty cycle of the UCC28951 through the chip peripheral circuitry.

    How to modify the UCC28951 duty cycle.

  • 0 in reply to Yuxi Liu(Tony)
    TI__Genius 12795 points

    Yuxi,

    The duty cycle is determined by the load condition. Please refer to chapter "8.2.2.10.1 Voltage Loop Compensation Recommendation" in the datasheet for feedback loop design: https://www.ti.com/lit/ds/symlink/ucc28951.pdf

    Best regards,

    Ning