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LM3478: How to design isolated flyback

Mike Jung
Intellectual 700 points
Part Number: LM3478
Other Parts Discussed in Thread: LM3481

Hi all

My customer is trying to design an isolated flyback with LM3478.

but there is only LM3481 reference design.

Please review attached file for circuit & 20V & 8V outputs on a 12V input

Question

1)  where would it be advantageous to get a feedback from both output power?

2) what should I do with the feedback connections as the pin configuration on the LM3478 and  LM3481 are different?

Thanks.

Schematic review.pptx

  • 0
    TI__Mastermind 46905 points
    Hi Mike

    The expert supporting this device is out of office. He will be back to office this week.
  • 0
    TI__Mastermind 21720 points

    Hi Mike,

    This application note covers how to design the LM3478 as a flyback. 

    1)  where would it be advantageous to get a feedback from both output power?

    Splitting the feedback between both outputs helps to keep both output voltage closer to the target when there is unbalanced loading. For example if one rail is near no load and the other rail is heavily loaded the light loaded rail will stay closer to the target output voltage. 

    2) what should I do with the feedback connections as the pin configuration on the LM3478 and  LM3481 are different?

    I'm not sure that I follow here. There is an FB pin on both devices. The FB pin should just be tied to ground.

    -Garrett

  • 0
    TI__Mastermind 46905 points
    Hi Mike

    We closed the question as no further feedback. Let us your further question. The post open again if you reply.