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Old 13th July 2007, 09:41 AM   #1
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Default LM2575 for flyback?

LM2575 is a buck/stepdown SimpleSwitcher.

It has feedback pin, but no "compensation" pin like it's cousin LM2577.

Is it possible to use LM2575 in isolated (optocoupled) flyback SMPS? How to compensate?
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Old 13th July 2007, 03:58 PM   #2
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lumanauw,

Since this is a buck chip (meaning the switch is on the high-side), you're better off using it as an isolated forward converter. Don't forget the reset winding, though. I have never seen a flyback converter with a hi-side switch, though I have to imagine it wouldn't be impossible- just different. Here is a link to a 20W forward converter from National Semiconductor's website: http://www.national.com/an/AN/AN-776.pdf

The'2575 has no compensation pin because it is internally compensated. The nice thing about pinning out the comp pin on the '2577 (& '2587) is that multiple regulators can be paralleled for higher current operation, using the comp pins as the common control point. I have the document showing this (and other SimpleSwitcher tricks), but it's over 100K, so if you want it let me know, and I'll email it to you.

If you're looking for flyback action, just get an LM2577 (or '2587). It is as equally at home in the flyback topology as it is in the non-isolated boost mode.

Steve
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Old 13th July 2007, 04:41 PM   #3
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Hi, Steve,

Thanks for the advice I would like to see the file. My email is lumanauw(at)bdg.centrin.net.id.
Do you have idea how to make an LM2575 chip has constant 50% duty cycle without using the feedback pin? Where should the feedback pin connected to?
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Old 13th July 2007, 04:56 PM   #4
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Originally posted by lumanauw "Do you have idea how to make an LM2575 chip has constant 50% duty cycle without using the feedback pin? Where should the feedback pin connected to?"

Not a clue.

BTW, You've got mail.
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Old 13th July 2007, 05:11 PM   #5
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I've got the mail, thanks

I still curious about making an isolated output with LM2575.
I found this from L4960 datasheet (it is also a buck controller).
Is it possible to make the isolated output working (L4) while we don't load and don't put capacitor 470uf/25 at +12Voutput (L3)?
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Old 13th July 2007, 05:40 PM   #6
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I've seen this before- with the MC33167T. It is simply coupling an "auxiliary" winding to the inductor to derive a second output. Obviously, this output will vary with the duty-cycle seen across it.

If the main output (the +12V one) went unloaded, I think it might make the duty-cycle very low, the pulses very narrow, and hence the auxiliary winding's output proportionally low.
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Old 9th October 2010, 06:02 PM   #7
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Please send me calculat for flyback transformer (smps)
thanks for you
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