PWM transformer

Discussion in 'The Projects Forum' started by Andyph, May 6, 2009.

  1. Andyph

    Thread Starter New Member

    May 6, 2009
    Just wondering if anyone can help me -
    I have a PWM design which switch 24 volts via a TL494 and two IR 2100s into a push pull FET bridge and a large step up transformer which gives an output of 600 volts - which is then recified and smoothed into a small bank of caps and then pulsed into load of about 500 ohms.
    What I am wanting to do is to vary the output voltage down. I have done this by presenting a PWM signal (of about 5 Khz) to the 494 - obviously a low mark space ratio only turns the push/pull on for a small while and generates a smallish voltage in the caps - the bigger the PWN mark space the more volts output . The output regualtes well any any voltage between 0 and 600 can be acheived - success - so far.
    The FET bridge stays cool at 600 volts output - even though the power output is high. But as the output voltage is reduced - suddenly the voltage trace accross the transformer changes ( from a quite nice squareish wave ) and the heat developed in the FET (and transformer ) increases dramtically - infact at any output voltage below about 500 the system produces lots of heat (in the FETS and transformer) even though the system should be working less hard.
    I have tried playing with the pulses into the FET - increasing the Deadtime and also reducing the pulse width but without joy . Also changing the freq of the converter which is about 60 khz seems to do nothing
    I am guessing the transformer just does not like dumping (even a few pulses 600 volts) into a capacitor load that is a few hundred volts lower - this somehow gets reflected into the primary of the transformer and "upsets" the FETS
    Can anyone shed more light on the matter - and hopfully come up with a Solution?????
  2. bertus


    Apr 5, 2008

    Do you have a schematic of what you have upto now?
    What type of transformer are you using now?
    Transformers usualy do not like squarewaves.