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Full Bridge Transformer - should core be gapped?

Peter Baxter
Intellectual 390 points
Other Parts Discussed in Thread: UCC28950

Does the transformer for the magnetics on a Full-Bridge controller (UCC28950) require a gap?

In Keith Billing's book (Switchmode Power Supply Handbook), he gives reasons why there shouldn't be a gap, but then recommends against adding a series capacitor to block the DC.

Thanks

Peter 

  • 0
    Intellectual 970 points

    the textbooks tell us that its only flybacks that needs gaps.

     

    But in reality , all forward converters including bridge converters will end up needing gapping.........otherwise you end up needing a ridiculously large transformer

  • 0
    Intellectual 410 points

    Hi Peter,

     Gap is related with leaking inductivity and leaking inductivity  is related with power loss (in forward hard switch topologies).  Resonant topologies (soft switch topologies, ZVS, ZCS, LLC, CCL) take in to account leaking inductivity and significantly reduce switching losses. Hard switch forward ....no gap. So... depend of topologies, gap is required or not. 

    UCC28950 is phase shift driver and you cam make ZVS  by adding resonant circuit ( L C) . I made 1200W power supply, phase shift full bridge,and made resonant tank  but I got ZVS effect in relative narrow duty  cycle range.  Out of that rang efficiency decrease. Finally...Full-Bridge, Phase-Shift (no gaped transformer) Hard Switch with synchro rectifiers made efficiency above 90% @ 1200W.

    Dragan

     

     

  • 0 in reply to dragan svalina
    TI__Guru 73905 points

    An air gapped core helps to store energy in the magnetic field. If a converter requires the stored energy to be part of design, then the air gap should be made. In phase shifted full-bridge dc-dc converters, certain amount of energy should be stored in the magnetic field to facilitate ZVS. Such magnetic field energy can be stored in the gapped transformer or use additional inductor (shim inductor) to achieve this function. Either technique can be used in the phase shifted full-bridge dc-dc converters.