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UCC28781-Q1: Use of SR controller

Valentinas Pancerovas
Intellectual 410 points
Part Number: UCC28781-Q1
Other Parts Discussed in Thread: UCC28782, UCC24612, UCC28781, UCC5304

Can a SR controller be used with this IC? I am thinking of UCC24612 since its suggested for UCC28782 in Webench design tool. Or is it a must to use an isolated gate driven in the same way as the EVM is ?

Second question: why does EVM use high side and not low side SR controlling? High side has advantage of lower CM noise.

Thanks

  • +1
    TI__Mastermind 29720 points

    Hello Valentinas, 

    The UCC28781 EVM does not have a high-side clamp and primary switch from which to derive the negative primary current for ZVS.  Instead, it uses the PWMH signal to drive the secondary-side SR MOSFET for synchronous rectification AND for additional on-time to allow some current from the output capacitor to flow backwards and generate the negative  current needed for ZVS. 

    To couple the secondary SR FET to the primary PWMH signal, we use an isolating driver.
    We need to meet the pri-to-sec safety isolation requirements.  We need a driver with sufficient current to drive the MOSFET.  And we need to keep propagation delay from PWML signal to MOSFET response to a reasonable minimum.  The UCC5304 meets all three criteria.

    An alternative could be a transformer coupled circuit, but that could increase complication.  

    The EVM uses low-side SR control because it is able to bias the driver directly form the output.  A high-side SR would require an additional secondary-side bias winding to power the driver. 

    Regards,
    Ulrich