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LM3478: Web bench refuses

Part Number: LM3478
Other Parts Discussed in Thread: LM5022, , POWERSTAGE-DESIGNER

I asked Webbench for a SEPIC converter using the LM5022 or LM3478 20V-60V input 48V @ 1A output, and it says it can't, but it doesn't say WHY it can't.

I would like to extend the input voltage to 132V, which I presumably can do if I limit the supply voltage to the IC.

  • Hello Ian,

    Thanks for reaching out to us via e2e.

    Using an external regulator to bring down the supply voltage for the controller IC will work when the ET is external as well.
    It will also help to move the power dissipation of the (linear) regulator to an external part and to better distribute the thermal power dissipation.

    Webbench Power designer does not know about this concept and is strict with regards to the supply voltage.
    Therefore, it cannot propose an LM3478 based design for a VIN above 40V.

    For the LM5022, Webbench will only create a SEPIC when the sum of input voltage and output voltage (plus 20% margin) will stay below 80V.
    Above that it will only propose a Flyback topology. (For some reason, Webbench prefers Flyback over SEPIC for most converters.)
    It is somewhat strange that it does not find any FET with a VDS rating above 80V, although for other topologies it does show FETs with 120V.

    If you want to use Webbench Power Designer, you can start with a design that stays below these limits.

    Afterwards you can use Powerstage-Designer to calculate the proper values for the power stage components and the compensation.
    You can download Powerstage-Designer from here:

    Best regards