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Original question:

UCC28950: Synchronous Rectification as H-Brdige

UCC28950: Full Bridge Synchronous Rectifier Clarification

Noah Martin
Prodigy 90 points
Part Number: UCC28950

Tool/software:

Hi,

So I asked this question yesterday, but the thread is now locked. There is conflicting information on the E2E page, so I cannot draw a concrete conclusion.

(1) https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1462410/ucc28950-synchronous-rectification-as-h-brdige/5612446#5612446

(2) https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1418014/ucc28951-ucc28951-psfb-with-full-bridge-sr

(3) https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/927631/ucc28950-high-output-voltage

In these 3 posts states this is not possible in links (1) and (2). However in the link (3)  states this is possible.

Link (1) and (2) coincide with what other documentation says that the switching pattern must match the primary side as stated in other TI resources using external logic by AND'ing A and D and AND'ing B and C, but I need more clarification. Why can the secondary not short circuit when gate signals E and F overlap as they would in a Center Tapped configuration. The center tap configuration during the freewheeling period shorts both FETs causing a shorted secondary and (0.5*Inductor current) flowing through each MOSFET. During freewheeling period PSFB there will be no primary winding power delivery and the Power transformer will be short circuited on the Primary side. So why would having a short circuit secondary winding matter during the Freewheeling diode period?

Regards,

-Noah

  • 0
    TI__Guru* 85616 points

    Hello,

    The first 2 options recommended not doing option 3 presented by Colin.  The paper that was discussed also recommends not doing this.  This was to avoid transformer saturation and not shorting out the transformer.  This is something you are also worried about.

    I did look at option 3 and the more, I think about it, this could actually work as well.  During the freewheeling period the primary of the transformer is shorted with FET pair QA, QC or QB, QD depending on weather E or F is high.  What this means even if E and F are on at the same time during freewheeling this will not short the input across the transformer.  The output filter inductor will prevent the output from being shorted across the transformer during freewheeling.

    Thank you for interest in Texas Instruments (TI) products.  If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

    Regards,