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UCC24624: high current full bridge SR

Ben Guo1
Prodigy 20 points
Part Number: UCC24624
Other Parts Discussed in Thread: CSD19536KCS, UCC24612

Hi, I am trying to design a full-bridge synchronous rectifier on the secondary side of LLC with high output current >100A

A few questions on UCC24624:

  1. Will UCC24624 fit the full-bridge application? If so, any suggestions on the gate drives of high-side MOSFETs?
  2. For MOSFETs with TO-220 package (2-3 in parallel, e.g. TI csd19536kcs), the parasitic inductance is expected to be large. Will UCC24624 be able to drive paralleled MOSFETs and handle the impact of parasitic inductance?

Thank you

Best regards

Ben

  • 0
    TI__Mastermind 26790 points

    Hello Ben,

    Thank you for your interest in the UCC24624 Dual SR driver.

    I don't know the answer to your question off-hand, and need to check with an LLC expert.
    I will try to have an answer for you in a day or two.

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    TI__Mastermind 26790 points

    Hello Ben,

    I'm sorry it took so long to get back to you.  Checking with an expert on this matter I received this reply:

    "This is a tough question. Basically, we haven’t done any test with UCC24624 together with full bridge rectifier. Therefore, these ideas could have some issues.

      Here are some thoughts.

    1. UCC24624 can’t be used directly drive the full bridge. However, it can potentially drive the full bridge with the help from two high-side drivers. If you let the UCC24624 drive the two low-side SR and send the signal to the cross SR FET using high-side driver, it might work. The issues I see here mainly coming from the turn-off delay of the high-side driver. If it has significant delay, we might see issues when the turn-off speed is important. I suspect too much delay might shoot through the two high-side SR FET and short the transformer. This needs to be watched out...

    2.  You can always use UCC24612 to drive individual MOSFET. The concern in this case is how low the Rdson is. It appears they are using MOSFETs in parallel and the Rdson must be small. The UCC24624 is not very good to deal with low-Rdson designs. But when Rdson is relatively higher, it works just fine. I think we set the threshold at about 4mohm.

    The UCC24624 (and the UCC24612) is not able to compensate for significant parasitic inductance.  Please connect the VD and VS sense pins as close to the MOSFET terminals as possible to minimize the effects of stray inductance. 

    I hope this information will help you.

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Prodigy 20 points

    Thanks Ulrich. 

    1. For UCC24624 to drive the high-side gate drive, will its proportional gate drive be a problem, since it will try to regulate Vds to 50 mV? For my application, Vds = 2mohm*100A =  0.2 V > 50 mV. 

    2. For my application, Vds = 2mohm*100A =  0.2 V > 50 mV. Will UCC24612 work in this case? 

    Best regards

    Ben

  • 0 in reply to Ben Guo1
    TI__Mastermind 26790 points

    Hello Ben,

    The UCC24624 should have no problem to drive the high-side isolator, since the Vgs will be high enough to stay above the isolator's input threshold voltage. 
    Note that 2mV will reach 50mV when the current falls to about 25A (or a bit lower if Rds(on) rises due to higher Tj).  To turn off at a lower current, the net resistance should be higher than 2mR or the SR Fets may turn off too soon and incur too much body-diode conduction.

    The UCC24612 is essentially the same as the UCC24624.  The -624 has some additional circuits to avoid cross-conduction with the other half of the bridge.

    Regards,

    Ulrich