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UCC24624: Switching Frequency above 625kHz - what's going to happen?

Andre Haspel
Prodigy 20 points
Part Number: UCC24624
Other Parts Discussed in Thread: PMP21251

Hello Everyone,

my LLC-Converter is working up to 1MHz. I'd like to use SR MOSFETs with the UCC24624 Controller to reduce the losses. 

Thats going to happen, when the switching frequency increases above the specified 625kHz? 

Will the Controller just stop operation or will there be any dangerous malfunction. 
Will the Controller turn back to normal Operation as soon as the frequency drops below 625kHz again?

Since the LLC will only reach this frequencies during light-load conditions, a simple passive rectification in this situations is okay for my application. 

Thanks for your help!

Best regards

André

  • 0
    TI__Mastermind 27095 points

    Hi Andre,

    My concern would be the minimum on time is too long for switching frequencies greater than 625kHz and you end up with shoot through because the wrong SR FET is still conducting even through the primary has already changed. That being said, it would be acceptable to use an external circuit to disable UCC24624 at lighter loads and allow the secondary currents to ride the body diodes of the SR MOSFETs. 

    Best Regards,

    Ben Lough

  • 0 in reply to Benjamin Lough
    Prodigy 20 points

    Hey Ben,

    thanks for your reply. I'm not sure how i could design such an external disable circuit. Do you have any suggestion?

    Otherwise I'll try to limit the frequency in the LLC-Software.

    Thanks again!

    Best regards

    André 

  • 0 in reply to Andre Haspel
    TI__Mastermind 27095 points

    Hi Andre,

    You could place a PNP transistor in between the VDD pin of UCC24624 and its voltage bias. You could then use a circuit such as what is shown in PMP21251 but instead of doing OCP, use it to turn on or off the transistor to enable/disable UCC24624.

    Best Regards,

    Ben Lough