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Flyback output cap and compensation for LM2586

Ed Marquette
Prodigy 60 points
Other Parts Discussed in Thread: LM2586, LM3478, LM2577

I'd like to use the LM2586 to make a 300 V supply at about 10-15 mA, with a 24 V input.
I've built it and it seems to work well.

I have some questions though:

1) Size is critical, and the webbench circuit calls out 100 uF output cap (350 V) which it going to be a problem.

I built it with the 22 uF cap I had on hand and it seems ok using the 47uF/1.78K values suggested by webbench.

Will there be a stability problem if I use a small output cap?

I haven't seen any information on design of the compensation network and its relation to the output cap size.

Thanks

  • 0
    TI__Guru 57555 points

     

    The WeBench design may be conservative.  The best way to check stability is to do a load transient on the output and look for wringing.  Or you could observe the output voltage as the regulator is starting-up and look for any overshoot or wringing.  The compensation components can be tweaked based on the results.  Since the compensation for a current mode boost can be a little tricky, we prefer to let WeBench do the calcs, rather than put equations in the data sheet.  However, the LM2577 and LM3478 data sheets have equations for boost and flyback.  The internal parameters of the LM2586 will be different, but maybe those equations can give you a guide.