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Increasing output of flyback transformer?

Discussion in 'General Electronics Discussion' started by carebare47, Dec 3, 2010.

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  1. carebare47


    Oct 21, 2010

    I have recently build a flyback transformer driver consisting of a 555 timer (@12v) at about 25KHz driving four MOSFETs (IRF960) in parallel (MOSFETs aren't great, so I stuck four of them in parallel. It seemed to help a lot) which in turn drives a primary coil of a flyback transformer (The negative of the coil goes to minus 12v, so there is 24v across the coil). I can get sparks of about 4/5cm(ish). This is my transformer and driver working: . I have also audio modulated it (from computer sound card) via a 10nF capacitor on pin 5 of the 555 (voltage control I think), but the arc is too small and thus too quiet. Anyone got any ideas of how I could increase the output of the flyback?

    Many thanks,


    EDIT: Also, the MOSFETs get quite hot (too hot to touch after about 30 seconds). I haven't ran it for more than a minute because I don't want to fry them. What is it exactly that causes the MOSFETs to heat up so much, and is there anything I can do about it? And what is it in the MOSFET that gives a large arc from the transformer? Voltage rating, amperage etc... Someone explaining that would help a lot also ^^
    Thanks again :)
    Last edited: Dec 3, 2010
  2. carebare47


    Oct 21, 2010
    IRF9630, not IRF960.
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Show us the circuit (the video told me nothing of use (other than you should be careful that you don't get flashover to earth at places you don't want it).

    The mosfets could get hot because:

    1) insufficient base current available.
    2) insufficient base voltage available.
    3) Miller effect issues

    See here for a reasonable list of things that can go wrong.

    Are you using external diodes to provide protection for the mosfet from the inductive spike that occurs each time the current to the primary is disrupted?
  4. bob_rich


    Nov 10, 2010

    the output from a 555 timer would not provide a enough drive current for charging/discharging the fets input and miller charge. this may make them switch slowly and thus get hot. I would put a driver chip specifically designed to drive mosfets (such as for example MAX4424 max4420). For a flyback you need to turn off very sharply as this is where you want all the energy in the coil to charge up the stray capacitance to get the maximum possible voltage. also check the the current level in the coil is not so high that the fets come out of saturation

    best of luck

  5. carebare47


    Oct 21, 2010
    The circuit consists of a 555 timer with a 10nF capacitor and two 1.5k resistors in astable mode. The output of the timer goes to the gates of four parallel MOSFETs (whose sources go to +12v and drains go to the primary). The primary coil then goes to -12v. The primary coil consists of 8 turns of copper wire (cannot remember its diameter, it was bought a while ago).

    The current through the primary coil is 1.6A max and the base voltage is 6.5v (as far as I can tell). The 555 is at a 66% duty cycle (I think) at about 32,000Hz. Will look into that driver chip.



    EDIT:: I cannot seem to get a reading for base current, whenever I try the circuit doesn't turn on. But the 555 max output current is 200mA.
    Last edited: Dec 5, 2010
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