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UCC28950: Layout of current tranformer and DC blocking capacitors

user5927962
Prodigy 140 points
Part Number: UCC28950

Hi, 

I am designing a 3kW PSFB with UCC28950. Switching frequency 200kHz, and very fast MOSFETs. So I am really interested about turn off voltage overshots and ringing on power transistors. In https://www.ti.com/lit/an/slua560c/slua560c.pdf (app note about 600W PSFB with UCC28950) there is a scheme, where is the current transformer (CT) in series with DC bus. I think, that CT brings some extra stray inductance to fast switching current loop. In my other SMPS project with very fast MOSFETs there was very critical to minimaze stray inductance of switching current loop because of voltage overshoots. Have you any experience with this issue in this configuration (I mean the CT in series with DC bus) ?

Thank you for your reply 

Vladislav Šmarda

  • 0
    TI__Mastermind 40320 points

    Hello Vladislav,

    Thank you for your interest in the UCC28950 PSFB controller.

    You are correct to be concerned about excess stray inductance in any switching loop.  Good pcb layout design practice will minimize this.

    In this particular case, the added inductance of the CT primary is almost insignificant compared to the shim inductance, and the bulk-rail location (DC-Bus) for the CT is a good place free of significant voltage swings.  This configuration (with reasonable pcb layout) is not a problem for the PSFB.

    Regards,
    Ulrich