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LMG2610: BST Capacitor

Jim G
Intellectual 695 points
Part Number: LMG2610

Tool/software:

I'm reviewing a design based around LMG2610 and see that the bootstrap capacitor used is 220nF. The maximum frequency of this flyback based design is roughly 250kHz.

Am I correctly to believe that this is too large of a value? The UCC28782EVM appears to use a value about 1/10 that size, 22nF. 

Would this just make initial turn-on slow, or could it cause other issues?

  • 0
    TI__Intellectual 845 points

    Hello Jim,

    Let me check with the team about the recommended value for the BST capacitors.

    Regards,

    Adithya

  • 0 in reply to Adithya Ankam
    Intellectual 695 points

    Thanks Adithya, I can see in table 6.3 of the datasheet that it suggests Cbst to be a minimum value of 10nF, and then Caux needs to be about 3x Cbst. Normally Cbst would be based on switching frequency and the high side FET characteristics, but those devices are integrated here so we should have a more accurate idea of what's necessary. I just want to make sure 220nF isn't so large that it could potentially cause issues.

    Appreciate your help!

  • 0 in reply to Jim G
    TI__Intellectual 845 points

    Hello Jim,

    For practical applications it is recommended to have a Bootstrap Capacitor to have a value of 10 times the minimum value of the Capacitor required to charge the High Side FET.

    Considering the minimum value mentioned in the datasheet to be 10nF, using a 100nF capacitor can be used for robustness.

    Just to make sure if 220nF is suitable we would need to have some understanding about the design.

    Can you briefly explain what are the operating conditions, supply voltages  and other details which can be helpful to analyze.

    Regards,

    Adithya 

  • 0 in reply to Adithya Ankam
    Intellectual 695 points

    Yes, it's an active clamp hybrid flyback to convert 120VAC input to 28V DC output, 140W maximum. 

    The LMG2610 is the main switching device for the primary side of the flyback.

    It is driven by an Infineon controller, XDPS2201 up to 250kHz.

    Let me know what additional information is helpful.

  • +1 in reply to Jim G
    TI__Intellectual 845 points

    Hello Jim,

    First, I apologize for the delay in getting back to you.

    Based on the test conditions and our calculations, a 100 nF capacitor is a suitable choice. However, using a 220 nF capacitor does not pose any issues and can be used without concern. Capacitor values exceeding 1 µF may lead to potential complications, but 220 nF remains well within safe limits for the intended application.

    Please don’t hesitate to reach out if you have any further questions or concerns.

    Best regards,
    Adithya

  • 0 in reply to Adithya Ankam
    Intellectual 695 points

    Thanks Adithya, much appreciated!