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LM3478: Isolated Flyback circuit

tateo yamashiro
Guru 10890 points
Part Number: LM3478
Other Parts Discussed in Thread: LM3481

Hi,

We got a question from customer about LM3478.
Could you help us?

[Question]
Our customer is considering to use LM3478 for isolated Flyback convertor like LM3481. Is there any concerns about this usage?

 FB=GND
 COMP=connected the transistor of photocoupler

Best Regards,
tateo

  • 0
    TI__Guru* 78150 points
    You should not use the opto coupler the same way as with the LM3481: the error amplifier is different in the LM3478 and it is a gm amplifier, and the opto coupler cannot override the COMP pin for loop control. For isolated flyback, please use the LM3481.
  • 0 in reply to Youhao Xi
    Guru 10890 points

    Hi, Thank you for your reply. The datasheet recommends to use for flyback converter. And specifications of error AMP in EC table is similar to LM3481.
    So I think LM3478 can operate flyback converter. Is my understanding of this correct?
    If the IC can operate flyback converter, is there any concerns about this usage?

    Best Regards,
    tateo

  • 0 in reply to tateo yamashiro
    TI__Guru* 78150 points

    Yes you are correct, and I was confused with another part.  Please discard my previous comment.  It should work And there are a lot customers used it that way.  Sorry for the confusion.  

  • 0 in reply to Youhao Xi
    Guru 10890 points

    Thank you for your support. Also they are considering to use for forward converter. Can the IC operate forward converter?

      Topology: forward converter

      FB: connected to GND

      comp: connected to the transistor of photocoupler

    Best Regards,

    tateo

  • 0 in reply to tateo yamashiro
    TI__Guru* 78150 points
    I would not recommend it for the forward. Forward should use a controller having 50% duty cycle limit to fully reset the power transformer in each cycle. I would recommend the LM5020-2 , which is mainly developed for forward converter applications.