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LM5158: Flyback Converter - switching frequency

Gordon Leather
Prodigy 90 points
Part Number: LM5158

Tool/software:

Hello,

I'm working on a 48V to 3x isolated 28V @ 0.2A flyback converter, using LM5158 and a Pulse VPH5-0155-R or Wurth 749196518 1:1:1:1:1:1 transformer with ~23µH primary inductance.

Please could you advise on the practical upper switching frequency limit for this application?

I have seen "2200kHz is way to high for a flyback." on a comment on a different ticket - https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1428045/lm5158-lm5158?tisearch=e2e-sitesearch&keymatch=LM5158%20flyback#

Most flyback reference designs seem to run 150kHz to 300kHz, but I can't find any information on how to select a practical upper switching frequency, or why 2.2MHz is too high....have tried scouring the internet.  If it's due to the limits of the internal FET in the LM5158 (SOA / turn-on - turn-off times), please could you offer some guidance?

Thanks in advance,
Gordon

  • 0
    TI__Genius 16055 points

    Hi Gordon,

    Thanks for reaching out.

    Normally, a typical switching frequency for flybacks is between 50-500kHz. Depending on load and efficiency requirements. Also For EMI reasons, to avoid harmonics on critical frequencies, frequencies in the 100kHz area are selected.

    The reason that 2.2MHz is too high, is that the transformer and the core has to be re-magnetized every cycle. depending on the used magnetic material, there are limitations. It is not impossible to do it, but you need to know which material to choose. please have a look at this article: https://ieeexplore.ieee.org/document/9360294

    Best regards

    Moritz

  • 0 in reply to Moritz
    Prodigy 90 points

    Hi Moritz,

    Thanks for the quick reply.  That is a helpful pointer, thank you....waiting on the details of the materials from the magnetics suppliers now.