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Old 7th July 2006, 07:14 PM   #11
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Johnny-

Assuming you are running the '3525's oscillator at 72kHz for a switching frequency of 36kHz (common frequency for almost all car amps with DC-DC converters, AT & ATX power supplies), and your transformer core is a high-permeability ferrite, you should wind your core as follows:

4T + 4T primary
2T + 2T for the +/-5V secondaries
6T + 6T for the +/-12V secondaries.

If you're using regulation with an optoisolator, you will need some powdered-iorn cores for the secondary side filtering, as well.

Amidon Associates has a great datasheet for all their powdered-iron and ferrite cores. The cores I have used are the FT-114-77 (Ferrite Toroid, 1.14" O.D., #77 material). At 20kHz, it can handle about 60W. Extrapolating up to 36kHz, you should easily be able to get 100W through it.

A number of good reference books are available for calculating transformer turns, ratios and the like. The book I refer to the most is Geo. Chryssis' book "High Frequency Switching Power Supplies". The transformer section has all the theory you could ever need/use, but it also has some good, real-world design examples, too. I have taken his design procedure, and boiled it down to a 8-11 (depending on the number of secondaries) step cut-n-dried process to design any double-ended SMPS transformer.

I did an SMPS similar to this about 10yrs ago, using the SG3525, two IRF510s (later IRF540s), a smaller FT-82-77 core and four Schottky rectifiers for the +/-15V secondaries. I got about +/-500mA out of it to power an active crossover for the car. I scrapped the X-over, and cannibalized it's parts out for other projects over the years. It worked out pretty well, too. Sorry no pics of it.

Hope this helps.

Steve
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Old 7th July 2006, 07:46 PM   #12
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Default Mobile supply power conditioning & filtering

FYI: for some interesting tips and designs you might check out this extensive thread as well: Using unregulated AC wall wart

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Old 10th July 2006, 09:13 AM   #13
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Thank you all for replying but my question is this.
If i use only 4t primary then the circuit must draw a lot of current form my car.
because the primary will have zero resistant. and only the fets will prevent a short circuit.

Where am i thinking wrong?

Also i plan to use a ferrite ring of an old pc supply used there as ac filter. Is it good?

Thank you
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Old 10th July 2006, 02:45 PM   #14
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I would NOT use the ferrite from the AC Filter for your trannsformer. Instead, and I can't think of why I didn't mention this in ly last ranting er- posting: Why not just use a modified version of your pc supply? It already puts out +/-5V and +/-12v, so why not? Instead of running it of the AC mains, replace everything on the primary side with components suitable for runing on a +12V supply, and re-wind the main trtansformer's primary fior the 4T + 4T.

That brings me to my other point: you asked why only 4T + 4T for the primary, wouldn't the FETs see a dead short to ground?

The short version to that answer is, no, they will not. At 60Hz, this would almost be a dead short, but as switching frequency goes up, so does impedance. At ~36kHz, the AC impedance of the windings is rather high, lending itself to small size, and rather high efficiency.

There is a thread somewhere in this Forum on modding an old AT or ATX box to run on 12V while keeping the output section intact. Do a search for it.
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