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Old 21st January 2006, 10:48 PM   #11
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Old 21st January 2006, 11:23 PM   #12
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gearheadgene:

In order to learn a bit about self oscillating flybacks, I recommend you studying the standby +5V section of seveal low-cost ATX PC power supplies. You will find a lot of ideas there.

Concerning the +5V supply for the big PWM, you can get it from the primary side of the oscillating flyback, as it creates a 10..20V low current supply for free (optocoupler supply).

I had some self-oscillating schematics that I took from ATX PSUs to study them some years ago, but they are in another computer that suffered a hard drive crash and it can no longer connect to internet, so I can't publish them until I repair it.

Check also MC44608 flyback control IC from On-Semi, it's routinely used in the PSUs of low-cost TVs as it has a whole lot of features packed in a DIP-8 package, while requiring very few external components. It even has a standby mode featuring very low losses.
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Old 22nd January 2006, 12:26 AM   #13
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Old 22nd January 2006, 12:31 AM   #14
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Old 22nd January 2006, 01:36 AM   #15
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Eva,

Thanks for the lead to ON semi, I'll check on it. In the mean time, just had a few questions regarding the bifilar windings you mentioned earlier. Say I go with the method you suggested - sandwich the bifilar +/-40V secondary between 2 half windings of the eprimary. Intuitivly, I believe this method will make both the + and - side match well. I'm not sure what splitting the primary does for you, though. At a minimum, it requires the mylar tape in 2 places and requires extra margins - making the construction a bit tougher, yes? In a garage operation, maybe this is ok, but what about production. Seems like a big adder to the cost. What's your feeling for a more traditional stack, e.g. all the primary windings, then the secondary (bifilar or otherwise).

btw, you can call me gene.
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Old 22nd January 2006, 03:04 AM   #16
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Sandwitching halves the leakage inductance and reduces the stray magnetic fields radiated by the transformer. Sorry, I thought you knew that fact already. It's a common practice in mass production, altough you may just decide not to do so
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Old 22nd January 2006, 01:44 PM   #17
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Originally posted by gearheadgene
Eva,

Thanks for the lead to ON semi, I'll check on it. In the mean time, just had a few questions regarding the bifilar windings you mentioned earlier. Say I go with the method you suggested - sandwich the bifilar +/-40V secondary between 2 half windings of the eprimary. Intuitivly, I believe this method will make both the + and - side match well. I'm not sure what splitting the primary does for you, though. At a minimum, it requires the mylar tape in 2 places and requires extra margins - making the construction a bit tougher, yes? In a garage operation, maybe this is ok, but what about production. Seems like a big adder to the cost. What's your feeling for a more traditional stack, e.g. all the primary windings, then the secondary (bifilar or otherwise).

btw, you can call me gene.

you can get the mylar tape through most distributors, and a lot of surplus outlets carry it -- how close are you to Newington btw?
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Old 22nd January 2006, 07:36 PM   #18
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Newington is about 1 hour from here, why?
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Old 22nd January 2006, 08:13 PM   #19
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The ARRL headquarters is in Newington -- they have a lab there, probably a bunch of folks who can point you in the direction of obtaining the stuff you need for your SMPS. (There is also a "how to" in the ARRL handbook and you can look over the trafo winding section while you are there.)

the ARRL website www.arrl.org also has a page devoted to swap meets in every state -- great sources for parts.

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Old 24th January 2006, 09:29 PM   #20
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What's your opinion on deriving power for the digital logic from the + or - 40VDC supply? Actually, now that I think about it, even if that doesn't load down the supply too much, it could add switching noise to the rails and that feeds my audio amps (yuck). Maybe not the best idea. But tempting, nonetheless (easier to go from +40 to 5 non-isolated, than it is from 400 to 5 isolated from a magnetics and compliance perspective)

Know of any reference designs for +5 and +3.3 VDC offline smps that uses PFC front end (+400VDC input)?

gene
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