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UCC256303: Circuit Design and Precautions

KIM Andy
Expert 2530 points
Part Number: UCC256303

Our customer is designing a 400W SMPS.


UCC256303 will be applied. Please review the circuit diagram below.

Additional questions

1) I don't want to use 8 pin BW terminal. Is there a way to disable?

I have a separate OVP circuit on the secondary side, so I don't want to remove the auxiliary winding from the primary transformer.


2) In the demo board, 2 resonance caps are used. Ref. I'm curious about the role of CM4.(What is the side effect if you delete CM4?)

3) In the guide data, Lr and Lm were used separately when designing the resonant transformer.

Should I design separately? Or can it be used as a general section type 1ea?

4) Is it possible to design 1KW?

8662.SCH-MMIS-190808(TI_24V).pdf

  • 0
    TI__Mastermind 27135 points

    Hi Andy,

    Nice hearing from you.

    1. It is ok to ground the BW pin to disable the controller's OVP function

    2. CM4 along with CM12 form a split resonant capacitor structure where half of the resonant capacitance is connected from the transformer pin to ground and the other half of the resonant capacitance is connected from the transformer pin to Vin. The idea behind using a split resonant capacitor structure is that the transformer will be driven more symmetrically sooner during startup. The disadvantage is having to route the resonant capacitance to primary ground as well as to Vin.

    3. It is acceptable to use an external resonant inductor or use the leakage of the transformer as the resonant inductor. Some designs will favor using the integrated leakage but this does make the transformer design more complex. Other designs prefer only having to source 1 magnetic component.

    4. I would say the power limit will depend heavily on the power stage and also the design requirement. Generally speaking, I think phase shifted full bridge would be better equipped to handle the power stage losses at 1kW output vs a half bridge LLC. I'm sure it is possible to do a 1kW half bridge LLC but it may not be the optimal topology choice.

    Best Regards,

    Ben Lough