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UCC24624: Dual synchronous rectifier for Push-pull half bridge converter.

Han asdf
Prodigy 30 points

Part Number: UCC24624

Hello,

We design a push-pull half bridge converter 50V, 500W and would like to use active rectification for output.

Can the UCC24624 be used for this matter, although it is dedicated for LLC resonant converters?

Thank you,

Best regards.

  • 0
    TI__Mastermind 40580 points

    Hello Han,

    Thank you for your interest in the UCC24624 dual-SR controller.

    It is possible to use this part for your push-pull application, but it will not provide optimal performance.  Because it is optimized for LLC applications (with low current levels at turn-on and turn-off), the UCC24624 has a turn-on delay and may have sluggish turn-off with the high forward current. This high current at the turn-off edge "fools" the controller into providing high gate-drive voltage when it should be reducing Vgs (as it does with LLC operation).  That takes longer to turn off the SR MOSFET and can result in some brief cross-conduction. 

    With this controller, it would be preferable if your push-pull application runs at less than 100kHz.  Higher frequency than this may lead to excess cross-conduction from the turn-off delays.  

    It would be better if you can self-drive the SR Fets using the opposite winding of the push-pull output for each FET.  Even better would be to use the primary-side PWM signals (through an isolator) to provide the SR drivers with advanced "warning" that the current is about to commute to the other side.

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Prodigy 30 points

    Thank you Ulrich for the detailed answer.

    Yes, we want to use a µC for the PWM signal generation for pushpull transformer.

    Now the reason for using a synchronous rectifier IC is that it will "take care"of all eventualities. You suggest self-driving the SR FET or using the PWM signal for warning. Since we use a controller, in your eyes, is driving the SR FET from the same controller possible, without additional converter output sensing?

    Thank you!

    Best regards,

    Hannes

  • 0 in reply to Han asdf
    TI__Mastermind 40580 points

    Hello Hannes,

    Yes, the SR control can be derived from the same uC that generates the PWM. In fact it would be advantages to do so, since that controller "knows" exactly when it is going turn on one of the primary Fets.  It can pre-emptively turn off the appropriate SR Fet with enough advanced warning time to ensure that the SR is completely off before turning on the primary Fet.

    This would eliminate any chance of cross conduction due to slow turn off.  If an SR controller was left "In charge", it can only detect a change in phase when its drain current suddenly reverses.  By then it is too late to discharge the gate voltage and turn off the FET to avoid significant shoot-through. 

    The uC control algorithm can build in the necessary delay between turning off the SR and turning on the opposite primary Fet to eliminate any overlap of conduction.  If the uC is on the primary side, it will need isolators to convey the SR control to the secondary FETs and buffer drivers to convert the signal into gate drive.  If the uC is on the secondary side, you can have direct connection from uC ot the SR buffer/drivers. (Of course, then you'll need isolation between the uC and the primary Fet drivers.) 

    Regards,

    Ulrich