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WEBENCH® Tools/LM5160: LM5160 web bench

Scott Morrill
Prodigy 60 points
Part Number: LM5160

Tool/software: WEBENCH® Design Tools

I have been unable to get proper regulation from this flybuck design from web-bench.  I've tried numerous custom solutions, and every time it is the same thing, horrible regulation over load, 24volt out drops to 20volts even with just .5amp load.  Running out of ideas of what it could be, my transformer is 32uH and using a turns ratio of 2.5, therefore my primary Vout is 9.6 to 9.65volts.  The Ton period looks okay at roughly 40%.  Is it reasonable to think I can get proper operation from a flybuck at 24Vout and 1Amp(24watts).  Is 24volts to high for an isolated voltage of a flybuck design?webench_design_LM5160 24V-1AMP.pdf  

  • 0
    TI__Mastermind 26350 points

    Hello Scott,

    It depends on how much load drop you can tolerate.  At first glance achieving anything close to 5% is very unlikely for a open loop design that implements a step up transformer 1:2.5.  You can improve the load regulation by implementing a flybuck boost scheme.  But if your tolerance requirement needs to be below +/-5% I suggest look at a closed loop design using a flyback?  Alternatively you can use an opto coupler with the flyBuck as shown in the link below.

    Hope this helps?

    David.

  • 0 in reply to David Baba
    Prodigy 60 points

    Hi David,

    Thanks for your inputs.

    I talked to Richard Elmquist(of TI at 512-434-1560) today and he suggested I could expect as good a 2.5% regulation.  Would you mind talking to Richard and confirming if I can expect such results.  BTW- WEBENCH suggests I should be in the neighborhood of 1%.

    I guess I'd hope for confirmation of about 2.5% regulation with 35uH-1:2.5 ratio transformer and 24Vouts in and 24 volts isolated output at 1Amp.  If not I'll consider adding the opto, but I'm getting very conflicting information from you guys.

    Thanks in advance,

    Scott Morrill

    can we talk my cell is 585-545-8802

  • 0 in reply to Scott Morrill
    TI__Mastermind 26350 points

    Hi Scott,

    The reference alone for the LM5160 is +/-1%. the biggest error is due to the fact that you are regulating the primary no information about the secondary side voltage is fed back to the primary side so variation will be significant relatively speaking. As mentioned the leakage inductance is the biggest cause of error, followed by the variation in the diode forward voltage drop and the DCR of the transformer. From experience using a 1:2.5 turns ratio you will not get +/-5% over line and load. Please consider an opto implementation to improve your vout accuracy.

    Hope this helps?

    David.