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UCC256403: Doubt in the value of Bootstrap Capacitor used

Arun Thawait
Prodigy 155 points
Part Number: UCC256403
Other Parts Discussed in Thread: PMP40766, TL431

Dear sir ,

In the document  PMP47766  the value of bootstrap capacitor  (C21) used is 2.2uF. The Ciss  of MOSFET (IPA60R170CFD7XKSA1) is 1.2nF (from data sheet ) and  one generally has to choose the value of bootstrap capacitor such that it is at least 10 times greater than Ciss that is it has to be greater than 12nF . What is the reason behind using much  larger value of bootstrap capacitor ?

  • 0
    TI__Genius 9790 points

    Hi, Arun

    there are two condition to define bootstrap capacitor:

    1. as your comments, bootstrap capacitor should be at least 10 times greater than Ciss, this only make the MOSFET can be fully turned on. 

    2. burst mode is another condition to define bootstrap capacitor. During burst off period, bootstrap capacitor is discharged by the leakage current of IC. if the burst off period is too long, there is risk to trigger UVLO protection of bootstrap capacitor. 

    In general, burst mode is the major reason to use larger bootstrap capacitance. 

    Yunsheng 

  • 0 in reply to Yunsheng Qu
    Prodigy 155 points

    Thank you sir 

  • 0 in reply to Arun Thawait
    Prodigy 155 points

    Dear Sir ,

     How to design the value of R39,R40,and R41 in the document PMP40766 ?

    Regards

  • 0 in reply to Arun Thawait
    TI__Genius 9790 points

    It is not easy to explain how to design the value of R39/R40/R41 in short word. 

    R39 is the current limit resistor for TL431, it related to the supply current requirement of TL431.

    R40/R41 define the DC gain of feedback loop, it should be optimized based on gain/phase requirement of feedback loop. 

    I recommended you to follow the design of PMP40766.

    Yunsheng