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TIDA-010015: SYN rectification MOSFET selection

balasubramani r
Expert 1170 points
Part Number: TIDA-010015
Other Parts Discussed in Thread: CSD19505KCS, UCC24612

Hi ,

I just want to confirm the selection of secondary side sync rectification  MOSFET in TIDA-010015 model. In datasheet it is mentioned that the mosfet should have atleast 7mOhms with respect to  3A rms 14A peak .

For this TIDA-010015 model considering continuous current like 21A the minimum RDson is 4.7mOhms.So the selection of mosfet RDson should have more than 5mohms . But in this design the selected mosfet (CSD19505KCS) RDson is only 2.6mOhms. Can anyone explain the concept here since im using this design for our application want some clarification in the component selection. 

Seems like efficiency trade off for conduction loss. Correct me if im wrong .

What if i use the same mosfet for our design will it be ok ? shall i try parallel mosfet to minimize the heat dissipation ? If im using parallel mosfet do i need reconsider the RDSon parameters and i found related post suggesting not use syn rectification if RDson less than 3mohms. 

Link for the post: https://e2e.ti.com/support/power-management/f/196/t/691605

Please advise

Regards,

Bala

  • 0
    TI__Intellectual 925 points
    Hi balasubramani,
    Thanks for your interesting in TIDA-010015.
    I will invite the designer to answer your question as soon as possible.

    BR
    Cheng
  • 0
    TI__Intellectual 2815 points

    Hi Balasubramani,

    The recomendation for 7mOhm is based on maximizing SR Mosfet conduction time in DCM Flybacks. For LLC there are other techniques to maximize conduction time using lower Rdson Mosfet.

    The approach we use in this design is to operate above resonant frequency even under full load. This will maximize conduction time with lower Rdson Mosfet.

    Best Regards,

    Ram

  • 0 in reply to Ramkumar S
    Expert 1170 points

    Hi Ram,

    Thanks for the input. It means we can minimize the RDson as much as possible to operate the system above resonant frequency right? Shall i proceed with the same techniques?

    Shall i use two parallel mosfet with the same spec with UCC24612? Whether this UCC24612 is capable of driving two parallel mosfet at a same time ?Please advise

  • 0 in reply to balasubramani r
    TI__Intellectual 2815 points
    Hi Balasubramani,

    If you are going to use through hole mosfets, please ensure that the combined Rdson of the mosfets (in case of parallel mosfets) do not go below 3 to 4 mOhm. In case of SMD mosfets, you can reduce the Rdson slightly lower. Driving parallel mosfets is not a problem for UCC24612.

    Best Regards,
    Ram