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UCC27288: Bootstrap Capacitor Calculations

Subrahmanyam V
Intellectual 480 points
Part Number: UCC27288

Tool/software:

Hello there, 

The part number (UCC27288) was provided just as a reference while posting the question and the question is majorly related to a design equation in an application note on gate drivers as mentioned below.

My question is as follows - 

In the appendix C (Application Note: https://www.ti.com/lit/ml/slup170/slup170.pdf?ts=1722076093447), I believe that the equations for CBST,2 and CBST,3 have a slight error included in them and wanted to check with you.

In evaluating the CBST,2, a transient condition of continuous turn off period of 400 us was considered. However, as this corresponds to MOSFET turn off period, the highlighted term in the image attached below, should not be included in the equation, as the output of the gate driver is low during this period.

Similarly, in evaluating the equation for CBST,3, all the load currents delivered by the CBST are considered except for the initial gate charge required to turn on the MOSFET, as shown in the image attached below.

Can you please clarify on this?

Warm regards,

Siri

  • 0
    TI__Genius 13285 points

    Hey Siri,

    Thank you for reaching out to TI with your questions regarding bootstrap capacitors and the UCC27288.

    CBST,2 - They are looking at when turning back on after being off for an extended period of time so that portion of the equation should be included. "Moreover, at the end of this idle period, CBST still has to provide the gate charge to turn-on the MOSFET. Accordingly, the required capacitor value is: CBST,2 equation." So, with this statement the MOSFET has been off for an extended period of time and then turned on.

    CBST,3 - For this case, the MOSFET is already turned on and so it is looking for the capacitance needed to keep the FET on. The Qg is not included because its effective capacitance has already been charged as the MOSFET is ON.

    I am not familiar with the application note that you linked but I will be looking at it and reading it further. Typically for bootstrap capacitor calculations, this is the Application Note that I reference:

    Bootstrap Circuitry Selection for Half-Bridge Configurations

    Let me know if there are any further questions.

    Thank you,

    William Moore