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Original question:

WEBENCH® Tools/LM5164: Fly-buck with negative output and coupled inductor selection

LM5164: Fly-buck not working properly with LM5164

Ilya Dontsov
Prodigy 230 points
Part Number: LM5164
Other Parts Discussed in Thread: LM5161

I'm trying to make an LM5164 based fly-buck converter (+/-20V at 400mA each rail) and I can't make the isolated rail to work properly. As soon as I load this rail, the output voltage on it collapses and I have no output. However, if I put a load on the non-isolated rail, I can get some current out of the isolated one. But it's never in 1:1 relationship. For example, in order to get 100mA of current at 20V from the isolated rail I have to load the main rail to 300mA at 20V.

Looking at the switching node waveforms, it looks like the chip tries to keep working in DCM and doesn't "see" the loading on the secondary side of the inductor. Is the LM5164 not suited for fly-buck operation?

  • 0
    TI__Mastermind 39790 points

    Hello IIya,

    The LM5164 runs in DCM mode when the load on the primary is low.  If you are trying to pull current from the secondary while the output current on the primary is low, this will cause the voltage on the secondary to collapse.  I suspect this is what is happening.  The LM5164 is not intended to be used in flybuck topology for this reason.  If this is indeed the case, then you will need to load the primary side, or select a device that has Forced CCM operation such as the LM5161?

    Hope this helps?

  • 0 in reply to David Baba
    Prodigy 230 points

    Hello David,

    Thanks for your answer. Yes, your explanation is close to what I was thinking. It would've been nice if somebody from TI stated this in my original question regarding flybuck configuration for LM5164.

    Have to respin the PCBs and get some more parts to test the LM5161 solution.