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Old 6th March 2013, 02:30 AM   #1
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Default smps forward - need help with bias voltage

I am looking for some help. The attached schematic is a partial of the whole thing, but I hope this is enough to give you the idea (not that there's a whole lot more, though). What you are looking at is the transformer, the driver, and some magnetics. On the right hand side of the transformer is the isolated secondaries. They all share the same filter coil, L202. I am still working on it, but for now let's just say it is working about as expected. But, on the upper side of line-side of the transformer, L201 pins 1 and 2 (top center of the picture), is supposed to be bias voltage to run the PWM chip. This part isn't working so well. The winding polarity was dorked up, but I got that all patched up now. As you may notice, L204 is its own inductor.

Well, although I can get the V50AP winding to regulate correctly, the bias voltage does not cross regulate for beans. I think it is because it doesn't share L202.

So that's the problem - I need to get that to run at around 13 volts or so. Any ideas? I have seen some designs that take couple onto L201 as a flyback design. Maybe I can consider that. Can anyone think of a way to get the bias winding to regulate better? An alternatives? I am open to suggestions

thanks
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Old 6th March 2013, 02:56 AM   #2
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Add an extra turn or two and add a LDO linear regulator.
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Old 6th March 2013, 04:07 PM   #3
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Hey that sounds pretty good, thanks!
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Old 6th March 2013, 04:12 PM   #4
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Perhaps the inductor is lesser than needed, and it runs dry, and lost regulation.
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Old 6th March 2013, 04:53 PM   #5
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Actually, I am having a tough time with the output inductor. Although initially followed some of the Unitrode notes, I remain stuck knowing the proper value.

What's hanging me up is setting up for minimum load - which might be really low, while max load could be very high. Then the windings go out of regulation when the loads are fubar.
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