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Old 31st December 2005, 06:38 AM   #1
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Default Using ATX PSU trans for 12V SMPS?

I've taken apart quite a few computer PSU, and I want to make use of the EI Ferrite power transformer in them for a SMPS project. My largest transformer came from a 550W ATX PSU.

I know in a normal computer PSU, 120V is dropped to 12V and 5V by use of different Center Tapped windings on that transformer, switched at a really HF. They are wound similar to Car Amp SMPS toroids, using multiple paralleled turns of wire for each winding.

I was thinking of using the transformer backwards, driving one of the secondaries as a primary with switched 12V push pull and use the HV primary as a secondary.

Also a few of the transformers I have even have the HV primary center tapped (or have 2 seperate primaries) even though the CT was not used in the computer power supply, I could make use of is when driving an amp. I was thinking of getting +/- 50V or so.

I was also thinking that a 120V power inverter could be made from these as well.

Any thoughts?
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Old 31st December 2005, 10:32 AM   #2
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Taking into account the turn ratios that I've seen, you are probably going to get +-35V to +-50V right with the center tapped secondaries, however, note that nor the secondaries neither the primaries are very symmetrical, so this approach should be used with care in any push-pull arrangement.

If I remember properly, usual turn ratios are 20+20:7+7+3+3. This means two 20 turn primaries connected in series and two 7 turn secondaries for +12V output that are tapped at 3 turns in order to get +5V and +3.3V outputs. Modern higher power units will probably employ more primary turns in order to increase turn ratio and decrease the required primary current level for a given output power, just at the expense of a higher minimum operating voltage.
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Old 31st December 2005, 04:30 PM   #3
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It will work no problem..i remember using the same AT pcb of SMPS..removing high volt transistor drivers with mosfets..had to add a pair of pull down transistors for each arm..by modifying the circuit around drive transistors..
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Old 1st January 2006, 03:02 AM   #4
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Well, I'm going to be trying that soon.

I'm just trying to figure out if I should use large bipolars, or go to MOSFET to drive the transformer windings...........

I have lots of high power bipolars, but few MOSFETs. I'll try to go bpolar if I can, but if I *have* to use MOSFETS instead, then I will.
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Old 1st January 2006, 04:53 PM   #5
Eva is offline Eva  Spain
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MOSFETs are superior for 12V switching applications. Don't even try if you don't have any specific low-voltage switching bipolar transistor.
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