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Thread: Anyone try a Forward Converter?

  1. #1
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    Wink Anyone try a Forward Converter?

    Last weekend I modified a line powered ATX supply to work off 12V. I wound a transformer, and built up the PWM control circuit. So basically half of the supply is kept intact, and the other half (AC side) is replaced. It works great to 8V, but below that it shuts down. I think I've got that problem solved, so it can go down to about 6.5V for a brief moment. (Starting the car, I measured my 2000 VW to drop the battery voltage to 7.5V. )

    Anyway has anyone seen plans for such a supply? If there was enough interest I could draw up some schematics and post them here.

    Presslab

  2. #2
    Variable Bitrate
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    oeh oeh me me I want!

    if it is really that easy.... but shure, I'd like to see how you did this
    If at first you don't succeed.........
    destroy all evidence you tried

    for info on t6369c LCD or Presslab's powersupply check:
    http://www.namms.tk <=updated!!

  3. #3
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    PressLab,
    We are always looking for new, better, and cheaper powering solutions. I do not know much about power supplies, but your solution sounds very interesting. I, as I'm sure many others, would like to know more
    Athlon 500, 128 MB, ATI Xpert 4MB AGP TV-Out, Sound Blaster Live! Value, 8.4 GB, 2x20 Backlit LCD, Keypower 250 Watt ATX DC-DC, DSchmitd Startup/Shutdown Controller, IRMan + Backlit IR Remote, Custom Keypad, 5.6" NTSC LCD, In-Dash Slot Loading DVD, all installed in an '87 Honda Accord LX-i

  4. #4
    mpt
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    Question

    Hey this is great! Someone really did it! I tried to modify my AT supply about a year ago so it worked with a 14.4V - 10V supply, but i found out i nedded a very large transformer to transform those 14.4 to 48V so it could work on the PMW circuit. But it didnīt work out and i give in. Maybe i fried something before trying it. I guess ill never know...

    Please post that circuit here on the forum (if you want to of course).
    mpt

  5. #5
    FLAC
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    Cool

    PressLab, we are all waiting and can't wait no MORE.....
    abcd-1
    Author of CobraI,II,III and now CobraIV.
    You can contact me on AOL instant messenger....nick is cenwesi or cenwesi3

  6. #6
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    looks like you could solve alot of problems PressLab. I would be very interested to see this too....
    Project - GAME OVER :(

  7. #7
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    Okay, here is a little info.

    The transformer is a large E-Core that I stripped from a 300W AC-DC supply I bought at the surplus store for $7. I need the big ferrite core so it doesn't saturate at the 30A+ peak primary current. The Forward Converter topology helps with this too. I put the transformer in the oven to loosen the glue and carefully disassembled it. I counted the turns ratio and did some quick inductance calculations and I changed the secondary turns by a factor of 2. Meaning for each seconday turn that I unwound, I re-wound it with twice the turns. I also used a slightly smaller gauge wire but I doubled it up. Less eddy currents in the wire this way. I noted which windings were on which layers so as to rewind it in the same fashion.

    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. It is VERY important to keep the turns ratio, and have exactly 2 turns for example. (Bend the wire at 90deg when done with the winding back to the bobbin pin.) Also the layer of each turn is important too. I will post all this info when I make it pretty.

    The PWM chip I used is the SG3525A, it is only $0.75, but now that I see the 7.5V dip I think I will use another chip. I'll post the current design for now.

    It operates at 125kHz which seems to be pretty efficient, but I will play with it to see if I can get the duty cycle lower. (i.e. more efficient.)

    I used two IRF540N switch transistors, they are excellent, with Rds(on) of only 0.04 ohm! They are also popular and cheap. Their Vgs should be good at 6.5V. I will try the IRL540 to see if it help it to go lower in input voltage.

    I used a National Semi. LM2671M-ADJ simple switcher to power the +5V standby, as I had this available and the current demands are low. (720mA) I will probably design some simple circuit instead of the LM2671, but for now it works fine.

    That's all for now, I'll post more when I get it ready.

    Presslab

  8. #8
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    sounds good!!!!

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


    If you have a camera, some pics would be great!! and a circuit diagram...


    ... hey, we dont ask for much!!!!
    Project - GAME OVER :(

  9. #9
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    I thought about doing that too! Where did you learn about transformer design? That’s my biggest stumbling point. Can you recommend any books? As for your 7.5-volt problem, I used a Schottky Diode and a Capacitor to filter the input of the supply, and worked just fine for me with the 10.5 - 14 volt laptop supplies I was using. I'm not sure how long your battery stays below the threshold, but I used an MBR3045 and a 3300 uF Cap and it solved my problems. (I know how it feels to want to avoid that though..)

    Jeff_
    MPEGBOX - Plexiglass Computer
    www.mpegbox.com

  10. #10
    FLAC
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    [QUOTE]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
    MPEGBOX - Plexiglass Computer
    www.mpegbox.com

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