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Thread: Anyone try a Forward Converter?

  1. #11
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    [quote]Originally posted by Jeff Mucha:
    <STRONG>
    Originally posted by magnetik:
    [QB]sounds good!!!!

    So what does the transformer windings do? raise the input voltage higher??? like an inverter???? or am I missing something.???
    QUOTE]

    Basically what a ATX AC-DC power supply does is takes the 120 in, converts it to 170 with a bridge rectifier and caps. It is then switched on off into the primary of a high frequency transformer. The secondarys have a turn ratio with the primary such that 170 goes to 12, 5, 3.3 etc..

    If I'm not mistaken, and correct me if I'm wrong, all that Presslab did was rewind the transformer so that the secondary windings look the same as they did before but the primary has a lot less turns so that an input of 12 volts can be used instead of the 170 volts. He scrapped the old transformer because the current on the primary previously was fairly low at 170 volts but now to get the same kind of power rating, he needed a lot more current and the old transformer would saturate (cause problems under heavy load). He scrapped the pwm circuit for controlling the 170 volt transistor switcher and used his own for the new 12 volt one. Is that more or less it?

    Jeff</STRONG>
    That's pretty much it. The secondary windings are actually all doubled in the number of turns. Otherwise I'd need 1.5 turns on the primary. Talk about a peak current! The output cross regulation is within 5%, so doubling the windings didn't droop the output.

    I don't really want to go the schottky diode route, at the best it's lost efficiency (heat), at the worst it's little fluffy clouds of black smoke (surge current). (120W / 0.80%) / 8V = 18.75A. 18.75A * 0.5Vd = 9.4W diode loss

    I planned on getting a book from Amazon for $90, but I gave it a shot first. I guess I got lucky, I never bought the book. I downloaded all the app notes on flyback and forward converters I could find. I'll post references to them when I get time.

    If you have a camera, some pics would be great!! and a circuit diagram...
    I have a camera, and I'll probably just throw up a web page to stick all the stuff on it.

    Presslab

  2. #12
    FLAC
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    Jeff, PressLab thanks very much for your explinations....

    And since we are ****ing off the original tranny, the same winding ratios would work on ATX supplies designed for 240Volt Australian usage too, would it not?

    How small is the final powersupply. Can you fit the re-designed transformer back into the ATX case shell..... maybe this is how the ARISE guys build their powersupply units!! hehehehe!

    I guess if some schematics were posted, and the ferrite core could be purchased of the size required (it is a core, not a rod?) im guessing you would have the problem solved for us guys like myself still using a PS/inverter setup.....

    this is exciting stuff...
    Project - GAME OVER :(

  3. #13
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    Originally posted by PressLab:
    <STRONG>The transformer has three primary windings, each 3 turns, 18GA doubled up. The secondary windings are all 22GA, they are 8 turns doubled up, 4 turns tripled up, and 2 turns tripled up. </STRONG>
    another thing... does this double/tripple windings explain the multiple 12volt/5volt/3.3volt outputs needed?
    Project - GAME OVER :(

  4. #14
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    Originally posted by magnetik:
    <STRONG>

    another thing... does this double/tripple windings explain the multiple 12volt/5volt/3.3volt outputs needed?</STRONG>
    No, the double/tripple windings just ups the ammount of current that can be induced. Plus it lowers the eddy currents, (which I don't understand, brain dumped that from my undergrad education, is it something to do with skin effect or the way current flows through a round wire, or hysterisis?? Arg, too much digital design knoledge pushing out the analog stuff.) :-)

    The number of turns dictates the voltages.

    Jeff_
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  5. #15
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    Lightbulb

    Originally posted by magnetik:
    <STRONG>Jeff, PressLab thanks very much for your explinations....

    And since we are ****ing off the original tranny, the same winding ratios would work on ATX supplies designed for 240Volt Australian usage too, would it not?

    How small is the final powersupply. Can you fit the re-designed transformer back into the ATX case shell..... maybe this is how the ARISE guys build their powersupply units!! hehehehe!

    I guess if some schematics were posted, and the ferrite core could be purchased of the size required (it is a core, not a rod?) im guessing you would have the problem solved for us guys like myself still using a PS/inverter setup.....

    this is exciting stuff... </STRONG>

    Yeah, the transformer will work for the 240VAC ones as well. If your power supply is set up like mine, then the same transformer could theoretically work without any winding differences. For example, the old transformer had a primary winding of 41 turns (120VAC). Yours would probably be the same, if you have the switch in the back for 120/240, or if it says 'autoswitch'. If it is 240VAC only, it might be 82 turns on the primary. They basically just do a full-wave or half-wave rectification based on what the switch is set to.

    The modified power supply is small enough to fit into the same microATX power supply case, although the fan needs to be moved up to allow the bigger transformer to fit. I just took out the funny looking fan, and replaced it with a regular case fan, if fit fine. When it is done a good dose of hot-glue will keep the pieces from rattling and breaking in the car.

    The ferrite core is called the "E" core. It is two E's back to back like this "E3". There is a bobbin, like the sewing kind but square, that you wind the wire onto. The two E's then slide over the bobbin, and they touch in the center and outer edges. Sometimes the E core center leg has been gapped, so it doesn't touch in the middle. This allows a magnetic flux bias in the core without saturation. We don't need that with the Forward Converter, it just hurts the maximum flux capacity of the core. In a continous-mode flyback converter, there is a magnetic bias, therefore the core is probably gapped. Any power supply over about 200W is not a flyback, as those don't work well with that much power.

    So to scavenge parts go with a nice beefy supply. I bought two, good thing because I broke one of the E cores getting it off! Good thing I had a spare.

    I just did some testing, and at 7V I can do 60W (probably more, my 12V supply only does 11A) It is about 60-80% efficient, based on load (best efficiency is at about 50W load). I played with the frequency a little to get it better, but I think I can get more out of it.

    I am thinking more and more on getting a better PWM chip that goes down to 6V or so and using the IRL540s. Any suggestions on a cheap flexible PWM chip with mosfet sink/source drivers?

    Presslab

  6. #16
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    Presslab,
    Can't believe your battery voltage drops that low. Working on cars through the years I've always read that your battery voltage should not drop below 10V during cranking otherwise this indicates a bad battery.

    Is this voltage measured across the battery terminals itself?

  7. #17
    Maximum Bitrate Raas's Avatar
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    Thumbs up

    Whoaaa...

    Reading this makes me shiver from excitement !!
    (Maybe)
    Finally the sollution to the power issues..

    Makes me wanna crawl into my computer.. travel the internet to you and make you modify like 2 of my psu's....

    Although.. A DIY-webpage would be fine also

    Keep up the good work man !!!
    Looking forward to the website when you worked it out and tested it !!!

    /me happye 'cause off GOOD news.

    Greetz
    Raas - The Netherlands
    ME: VIA epia m10000, lilliput 7', opus 150w, 80gb<br>
    GF: IBM Thinkpad 380, ext. 3.5 80gb, 40x4, PB-IR

  8. #18
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    If this really does work, and i get get it to work, i may have to worship the ground that you walk on.

    Few questions. will it work for an AT (i cant see why not)

    and what is the total cost for the transformation? (and the unit of currency )

    cheer mate
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  9. #19
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    Originally posted by Probedude_2000:
    <STRONG>Presslab,
    Can't believe your battery voltage drops that low. Working on cars through the years I've always read that your battery voltage should not drop below 10V during cranking otherwise this indicates a bad battery.

    Is this voltage measured across the battery terminals itself?</STRONG>
    It is measured at the PSU with an o'scope. It has ~15ft of #10 wire to it. The 7.5V transient is only some microseconds in duration, the resistance of the starter is very low during the initial moment you start it. That means high currents. After the initial moment, while still starting, the voltage looks to be about 11.5V or so.

    Presslab

  10. #20
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    Originally posted by hornet:
    <STRONG>If this really does work, and i get get it to work, i may have to worship the ground that you walk on.

    Few questions. will it work for an AT (i cant see why not)

    and what is the total cost for the transformation? (and the unit of currency )

    cheer mate</STRONG>
    It will work for the AT as long as you follow the turns of the original PSU. I bought a surplus power supply for my transformer, USD$7, but if you bought one new (2 E cores, and a bobbin) it should be about USD$20 at the most I'd think. The rest of the electronics might run another $15-20 or so.

    Presslab

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