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TMS320F280049: LLC doubts when output short

Jordan Zhou
Genius 10470 points

Part Number: TMS320F280049
Other Parts Discussed in Thread: TIDM-1001

Hi,

Customer is asking some questions about LLC, especially when LLC output short:

1) When short happens, or over current, what usually to do? Decrease period time, increase the switching frequency?

2) when short happens, customer wants to keep the output currrent at a fixed value (for example 5A, less than over current), how can they do to accomplish?

Thanks a lot.

Br, Jordan

  • 0
    TI__Intellectual 855 points

    Hi Jordan,

    In general the switching frequency will have to increase, to reduce the gain and power transfer. it may even require limiting the duty cycle of the PWMs. There are a couple problems, though.

    First the output current will be uncontrolled as the secondary capacitance quickly discharges through the short circuit no much you can do about this without adding some additional current limiting circuitry on the output stage.

    The second problem will be with the primary tank current, which I believe will need to be limited similar to operation during a soft start.

    Let me see if I can find some literature related to this and I'll get back to you.

    Thanks,

    Clayton

  • 0
    TI__Intellectual 855 points

    Hi Jordan,

    The TIDM-1001 design includes some good references at the end. One of those is this app note that serves as a good introduction to LLC converters. The short circuit / over load condition is explored in some detail there.

    As I mentioned, increasing the operating frequency may not be sufficient to limit the secondary current because there are some practical limitations to the maximum frequency.

    So it seems that either a current limiting circuit will be required, or possibly some duty modulation and or a burst mode to limit the output current when frequency modulation alone is not sufficient.

    Please let me know if this helps.

    Best,

    Clayton