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UCC21520: Parallel MOSFET drive with using UCC21520 , what is the maximum Bootstrap capacitor to use

Muthukrishnan P
Prodigy 110 points
Part Number: UCC21520

Hi ,

We using UCC21520 for Motor control along with MOSFETs.

3phase motor we using Parallel MOSFET for each phase  6MOSFET using

High side Gate 3MOSFET/ /Low side Gate 3MOSFETs.

Current we using 20uF Bootstrap capacitor for 1kHz - 40kHz switching frequency. VCC=5V, VDD =12V, Bus supply 48V

We have question to select Bootstrap capacitor for our requirement. From data sheet and simulation compared to use 20uf Capacitor used now.

Please let us what we used 20uf right capacitor for Bootstraping.

Regards

Muthukrishnan P

  • 0
    TI__Genius 13445 points

    Hi Muthukrishnan,

    My name is Mamadou Diallo, I am an applications engineer in MID(Mosfet and IBGT Drivers) and I will be working to help you address your concerns..

    Determining this maximum value for the bootstrap capacitor is dependent on many factors such as minimum and maximum duty cycle requirements of the high side and low side as well as your choice of external bootstrap capacitor and other factors.  

    Using the 20uF capacitor may be fine as long as you choose the right bootstrap diode to handle the peak currents from this bootstrap capacitor. This peak current is related to the bootstrap capacitor by the relationship Ipeak = Cdv / dt. Keeping in mind that higher values of bootstrap capacitor leads to longer charging time at start up.


    This material will help in selecting the bootstrap capacitor and considerations to take into account.

    Regards, 

    -Mamadou

  • 0 in reply to Mamadou Diallo
    Prodigy 110 points
    Hi Mamadou diallo,
    Thank you for your reply.

    The datasheet have mentioned Sink and Source Peak current CVDD is 10uF so that we have small confusion.
    As of now we using 20uf there is no issue we want confirm the same Bootstrap Capacitor. Thank you once again.


    Regards
    Muthukrishnan P
  • 0 in reply to Muthukrishnan P
    TI__Genius 13445 points
    Hi Muthukrishnan,

    Thank you for your response!
    Is there any specific reason as to why you chose 20uF for your application?

    It seems very high due to the previously mentioned peak current that can damage bootstrap diode at start-up. You could add a series resistor to mitigate the effects of that current peak but on the other hand, it would slow your start-up time.

    Though you're not observing any issue right now, I would suggest first determining the minimum required bootstrap capacitor based on the FET you're using and then choosing the safe appropriate level of bootstrap cap accordingly. Additionally, you also want to maintain a 10:1 ratio for your bootstrap vs decoupling (Cvdd) capacitor.

    Regards,

    -Mamadou
  • 0 in reply to Mamadou Diallo
    Prodigy 110 points
    Hi,
    Thank you for your reply,
    We used 3MOSFETs Parallel drive per gate singnal with 121nC total gate charge each MOSFET. we operating 1kHz to 40kHz switching frequency.
    Thease are reasons we using 20uf Bootstrap capacitor. 3A BOOT Diode & 0.7W series resistor we using now.

    Regards
    Muthukrishnan P
  • 0 in reply to Muthukrishnan P
    TI__Genius 13445 points
    Hi Muthukrishnan,

    Thank you for the update...
    Based on what the gate charge, the ESTIMATED value for the absolute minimum bootstrap cap would be 0.106uF. A good rule of thumb is to make sure your actual value is AT LEAST 10X larger (1uF-2uF) than the estimated value, additionally, as previously mentioned, you want to choose the bootstrap capacitor and the decoupling capacitor such that CVDD is at least 10X larger than Cboot.
    I would highly suggest choosing smaller values of bootstrap capacitance first then adjusting accordingly based on your requirements and needs.

    Please click the green button if this addresses your concern or let us know if you have further questions!

    Regards,

    -Mamadou