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Original question:

UCC27200A: Bootstrap capacitor value

UCC27200A: current rating of the bootstrap diode

Martin Seeger
Expert 5391 points
Part Number: UCC27200A

Dear Team,

our customer would like to use the UCC2720xA with a large bootstrap cap because their schematics foresees MOSFETs being driven in parallel with large Qgate.

What is the average and peak current rating of the bootstrap diode in the UCC2720xA?

Thanks and best regards

Martin

  • 0
    TI__Genius 12990 points
    Hi Martin,

    Thanks for reaching out about the internal bootstrap diode current rating for UCC27200A. I work with this device however I will loop in Richard Herring who also works with this one for some further input.

    The average current depends on power dissipation limitation and can be found in the datasheet as 100mA when Vf = 1.1V.

    The peak current depends on many things including power dissipation. Bootstrap diode peak current can be broken up by forward peak current and reverse peak current ratings.

    Forward Peak current is greater, the larger the Cboot, the smaller the parasitics, the larger the VDD voltage, the smaller the Rdson of the lowside FET and dynamic impedance of the diode. The main factor being layout parasitics in the bootstrap charge and discharge paths as well as the bootstrap cap in the equation i_peak = (Cboot) (dv/dt). The peak current is of course dependent on the pulse width as well. For smaller pulse widths, the peak current wont reach as high.

    Reverse current depends on similar things as well as forward current during reverse bias and bus voltage.

    Thanks,
  • 0 in reply to Jeffrey Mueller
    Prodigy 50 points

    Hello,

    This is not the answer to the question. The designer needs to know the maximum allowed current and voltage drop in order to validate if the diode is ok. If you can specify the voltage drop at 5A and the thermal impedance instead this would also be ok. 

    regards

    Johan

     

  • 0 in reply to Johan Vissers
    TI__Mastermind 21475 points
    Hello Johan,
    I am an applications engineer at TI that works with Jeff, and will work to address your concerns.
    To give us a better understanding of your application, can you provide the bootstrap capacitor value that is of concern?
    In all half bridge applications with a bootstrap high side bias, there is the initial charging current through the boot diode that is much higher than the following cycle to cycle boot capacitor refresh charging pulses. The boot diode is capable of significant peak charging current but this is not a production tested parameter.
    I will have to research the request for the Vf at 5A as you requested, it will take estimated 1-2 days to confirm the Vf values.

    Regards,
    Richard Herring
  • 0 in reply to Richard Herring
    Prodigy 50 points

    Hello Richard,

    The total bootstrap capacity is 5.8uF. The charging voltage is 16VDC.

    Regards

    Johan

  • 0 in reply to Johan Vissers
    TI__Mastermind 21475 points

    Hello Johan,

    I confirmed the typical Vf of the bootstrap diode with 5A forward current, and reviewed the expected maximum Vf at 5A current.

    The typical Vf with 5A peak current is 1.65V. The estimated maximum Vf is 2.5V at 5A peak current.

    The thermal impedance can be found in the datasheet Section 6.4, junction to ambient thermal resistance.

    http://www.ti.com/lit/ds/symlink/ucc27201a.pdf

    For the boot capacitor value of 5.8uF we suggest you refer to the guidance in the following datasheet section 9.2.2 for sizing the bootstrap capacitor. Too large bootstrap capacitance values can create issues with the initial charging cycle of the bootstrap capacitor.

    http://www.ti.com/lit/ds/symlink/lm5101a.pdf

    Although the boot diode can have high peak currents during the initial charging of the bootstrap capacitor, the average current is determined by the current required to charge the MOSFET total gate charge and the converter switching frequency. The average IHB=Qg x Fsw. The average current should be the consideration for the operating condition of the bootstrap diode.

    Please confirm if this addresses your questions with the confirm button in the thread.

    Regards,

    Richard Herring