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UCC28950: UCC28950 mistaken leading leg ?

Catalin Muntean
Intellectual 280 points
Part Number: UCC28950

Hello,

I believe there is a mistake in the datasheet of the UCC28950 PSFB controller.

Looking at figure 46 (at page 36 of the latest revision of the datasheet of UCC28950), one can clearly see that "D" and "C" driving signals are the ones commanding the leading leg of the PSFB.(leading leg is the one that provides the active to passive transition). In other words, the leg formed by transistors "C" and "D" is the leading leg.

Then, I conclude that an improvement to the PSFB schematic (figure 48) can be made: swap places between Ls and T1.

Looking forward to your comments !

  • 0
    TI__Mastermind 25570 points

    Hello Catalin

    You make an interesting observation. To a first approximation the Ls can sit either side of T1. The main difference is the currents in the clamp diodes Dc and Db if they are used.  Redl suggests that Ls be placed in the Active/Passive side of the transformer (QD and QC) because the diode currents are lower there. If the Ls is placed in the Passive/Active side the diode currents are larger but the currents in the MOSFETs QA and QB are correspondingly lower. TI has normally placed the diodes in the Passive/Active side in our EVMs. The TI application note at  http://www.ti.com/lit/an/slua107a/slua107a.pdf         shows Ls in the AP side of the transformer   

     BTW, I try to avoid the terms leading and lagging legs because I don't think that they are applied consistently - the terms Active/Passive and Passive/Active seem to be used consistently.

    Switch transitions in the soft-switching full-bridge PWM phase-shift DC/DC converter: analysis and improvements Redl, Balogh, Edwards http://ieeexplore.ieee.org/document/388499/?reload=true

  • 0 in reply to Colin Gillmor
    Intellectual 280 points
    In my opinion, since the power FETs already need heatsink, a slight reduction in the RMS current of the QA,QB FETs is not worth because the heatsink will still be necessary, but connecting Ls to the QC,QD leg, can avoid heatsink in the clamp diodes.

    Again, this is just my opinion and everyone decides for their own app.

    Thank you for your time Colin.

    Best regards,
    Catalin