12VDC to 24VDC flyback converter(multiple output)

Discussion in 'The Projects Forum' started by ahtat, Aug 7, 2011.

  1. ahtat

    ahtat Thread Starter New Member

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    Hi,

    I'm trying to design a flyback DC-DC converter which can provide up to 24V DC from a normal 12V DC lead-acid battery that every car has. It can be used to run radios, small lights, relays, horns and other 24V accessories(multiple output) from a 12V vehicle.

    I have done research on flyback topology and design the circuit diagram. I couldn't get the 24V DC from the output, after I assembly all the components. Is my design having problem or I selected the wrong flyback transformer?

    Attach are my design and the info of flyback transformer.
    http://images.elektroda.net/11_1312757763.gif
    http://www.farnell.com/datasheets/323239.pdf
    IC chip is UC3843AN(current mode) and flyback transformer is myrra 74014.

    PLS HELP and advice!!! Thank you.

    Attached Files:

  2. praondevou

    praondevou Well-Known Member

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    First thing but this has nothing to do with your problem:
    Isn't the feedback comparator comparing the FB voltage to 1/2 of Vref, i.e. 2.5V? That means if the circuit was working it wouldn't regulate the output voltage to 24V but to about 49V.

    Then, which of the transformer pins did you use? The one u posted doesn't have a ratio 1:2 even if I put the two secondaries in series. It also would have to be mounted the other way around since it's primary has much more turns than it's secondary. Can u name the terminals u used please?

    When u powered the circuit up, did u measure the pulse width at the 3823's output?
  3. ahtat

    ahtat Thread Starter New Member

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    Thank you for your reply.

    Then what i should do to regulate the output voltage to 24V?
    i use pin 6 and pin 4(+E and drain) for the primary and pin7 &8 and pin9&10 for secondaries.

    which pin should tested for measure the pulse width?use output pin and GND pin?
    Sorry. I am newbie in this area.please give some guide and advice.
    thank you very much.
  4. SgtWookie

    SgtWookie Expert

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    So instead of a 1:2 transformer, you have it wired as a 2:1 transformer.

    You could turn it around and use the secondary as the primary. The secondary of that transformer is only rated for 0.4A. You need to set your sense resistor high enough so that you don't exceed the transformer's current rating; or you will have a ruined transformer. If you exceed the peak rating, the transformer core will saturate, with resulting heavy current and high power dissipation.

    Replace that 300k resistor with a 140k Ohm resistor; with the lower resistor being 16.2k, you should wind up with the correct output voltage.

    Since the secondary winding is limited to 400mA, your output will be less than 200mA @ 24v.

    The pulse width can be measured on the OUTPUT pin, using GND for a reference.
  5. ahtat

    ahtat Thread Starter New Member

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    hi all, thank you for all reply and advice.

    I was able to generate first output(24VDC) with S1 and 0V,but further add on S2 and 0V,the circuit showed error and can't generate both outputs(24VDC). For the second output, I only connect the zener diode(1N5818) and capacitor. Should I add the feedback loop for second output or add opto-coupler to isolate the output voltage?
    Please advice.Thank you.
  6. SgtWookie

    SgtWookie Expert

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    You need to update your schematic and post versions to show what worked, and what did not work.

    Otherwise, it will be difficult to suggest a course of action.
  7. ahtat

    ahtat Thread Starter New Member

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    Attached is my schematic diagram.
    S1 is pin 9 from transformer 74014, and S2 is pin7.


    I was able to generate first output(24VDC) with S1 and 0V,which connect 2 resistor(39k&2k),zener diode(1N5818) and capacitor(560uF) but further add on S2 and 0V,the circuit showed error and can't generate both outputs(24VDC). For the second output, I only connect the zener diode(1N5818) and capacitor. Should I add the feedback loop for second output or add opto-coupler to isolate the output voltage?
    Please advice.Thank you.

    Attached Files:

  8. SgtWookie

    SgtWookie Expert

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    Best as I can figure out, it seems like you are STILL using pins 7 and 9 on the secondary side.

    That's just not going to work.

    Pins 7,8,9, and 10 need to be on your primary side. Connect pins 8 & 9 together. Connect 7 to +12v, and pin 10 to the switch input (pin 4) on the regulator.
    Pins 6 and 4 are your secondary side.
    Ground pin 4. Connect pin 6 to D1's anode. Remove D2, Cout2.

    Leave pins 1 and 2 of the flyback transformer disconnected.
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