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Old 13th April 2005, 02:34 PM   #1
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Default switchmode powersupply winding ratio

alright, I'm a little confused (thats normal though)

BUT!

with a switchmode powersupply.... if I have a centre tapped 5 + 5 winding on the primary (its low voltage input) does my secondary winding have to be a ratio of half the total primary?

SO!!! if I have 10 volts in, and 5 + 5 on the primary, and want 12volts out, do I need a single 6 volt winding on the secondary... yes?

yeah, thats right, isn't it.. ok.. thanx

5 + 5 on the primary it is.... is that ok?

and then a 6 turn winding for 12volts, and a... damn...

alright... basically, I want to make a computer powersupply that will run off a car battery.... even while the engine is cranking... so it would be good if it could keep the voltages up even with 10 volts in... any ideas for the winding ratios? expecially seeing as I need 3.3 volts too...
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Old 13th April 2005, 11:26 PM   #2
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Follow the peak voltage.

You have 10-0-10V and 10T CT = 20V peak across the length of 10T. That's 2V/T. So yes for 12V you need 6T, call it 7T with losses (core, winding, rectification etc.).

If 10V is the minimum voltage you expect, then that should do it right there, right?

If you need more then just add another turn or two, and "cruise" at a lower duty cycle.

Oh, and as for the other voltages - you could buck regulate each one (as done on the motherboard for the processor's 1.7 to 2.5V these days), or of course use other windings. You'll just need to control the turns fractionally speaking... build it but don't varnish the transformer. Add or remove turns until the ratio between the voltages is right (ratio since voltage depends on input and duty cycle, assuming you're using choke input rectification and filtering on this thing..).

Tim
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Old 14th April 2005, 05:27 AM   #3
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the question is HOW do I get the right winding rations for the other voltages.... if I have 12 windings for 12 volts (which is ALOT of windows for a switchmode powersupply, then I need 5 windings for the 5 volts, and CAN"T get 3.3 volts.... its either 3 volts.... or 4 volts..
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Old 14th April 2005, 06:55 AM   #4
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Nope, just bend the wire square away from the center of the core at whatever point to get the fractional turns. Like I said, move it around until you get it right, then (I forgot to mention before) cement it in place.

FWIW, 3.3, 5 and 12 share few primes so your first integer combo would be 33, 50 and 120 turns, just slightly inconvienient. (Heck, you'd use fewer turns on a 60Hz iron core transformer!)

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Old 14th April 2005, 03:01 PM   #5
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Fractional turns require a split core, it doesn't work otherwise. Anyway, remember that the output voltage in such an unregulated topology is proportional to the input voltage so regulation is mandatory

Standard ATX power supplies have only 5V and 12V windings with 3:7 ratio that actually provide about 8..10V and 20..24V peak, then these voltages are rectified and feed to a coupled buck inductor with multiple windings. Both transformer and inductor turn ratios are critical and have to be adjusted to compensate for diode and copper voltage drops

Regulation is achieved by adjusting continuously the duty cycle to compensate for input and load changes. The 3.3V output is derived from the 5V transformer winding and has its own regulation achieved by shortening the duty cycle by means of a saturable reactor core

It would be easier to start from a working ATX PSU and redesign the primary side since it would have already a lot of things working in the secondary side
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