full bridge induction heater problem

Discussion in 'The Projects Forum' started by ajrenzetti, Oct 14, 2011.

  1. ajrenzetti

    Thread Starter Member

    Dec 20, 2010
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    ive been trying to figure this out for awhile, but with no success. I built a full bridge circuit for an induction heater using 4 irfp064n mosfets. When i turn on the power, but not the signal, everything seems fine: the mosfets dont conduct. when i do turn on the signal, however, the mosfets seem to work the way they should for about 5 seconds and then the signal displayed on the oscilloscope on the tank circuit stops. when i turn off the signal, the mosfets still seem to be conducting and start smoking in a matter of seconds if i dont turn off the power supply too. when i turn the supply back on, but not the signal, the mosfets still continue to conduct. I know mosfets dont work well with inductive currents, but thats why they have antiparallel diodes built in them. when i check the driver signal, it works fine. is there something i am missing? why wont it work?
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    I bet the driver is wrong.
    First, what are the voltages? A mosfet can only handle +/-20V of Vgs and I think you are getting beyond that. Second, why is there a separate supply called Vcc? The driver chip is supposed to be operating from the "main" voltage, but is limited to 18V. You need to use some other driver, for example the IR2111 or some other similar.

    Also, do you realize that the voltage on the top transistors needs to be 20V more than "main voltage" to turn them on?
    Last, what is your target operating frequency?
     
  3. ajrenzetti

    Thread Starter Member

    Dec 20, 2010
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    i had to put the vcc there to form a net for the hidden pins of the drivers for the pcb design part. the vcc itself is 12v from a computer psu. the signal inputs are inverted of each other like they should be. the drivers im using are each supposed to turn 1 mosfet fully on and off. lastly, im pretty sure the operating frequency is anything from 50-100khz.

    do you think it may be the antiparallel diodes not working well enough?

    also, i did try using the ir2111, but i could never get it to drive the mosfets. out of the 20 that i bought, only 1 worked for only a few minutes. i dont know why this happened, but i gave up on using that ic. is there anything that might explain why it never drove the mosfets?
     
    Last edited: Oct 14, 2011
  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    I would have to see your circuit to be able to tell anything.

    You seem to have missed this part:
    "Also, do you realize that the voltage on the top transistors needs to be 20V more than "main voltage" to turn them on?"
    You cannot turn an N-fet ON on the positive rail without having at least 12V above that rail. The circuit you have would work if you had P-fets in place of Q1 and Q3.
     
  5. ajrenzetti

    Thread Starter Member

    Dec 20, 2010
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    dont p-fet's turn on when the signal voltage is low? im not sure how that would work, as it would simply short one side of the bridge.
     
  6. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    P-fets turn on when Gate is at cca -10V against Source. Source goes to +V, drain to load. You will obviously have to invert your drive signal too.
     
  7. ajrenzetti

    Thread Starter Member

    Dec 20, 2010
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    ok i think i understand what you are saying. here is the revised schematic. is this what you mean by replacing Q1 and Q2?

    Also, would this prevent the mosfets from doing what they did in the first design?
     
  8. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Yes that looks better. Only big mistake is that Q1 and Q3 are upside down, see that little diode in the symbol? It should not be conducting in normal state :)

    Also I don´t know why C2 and C3 are there, I would swap them for some ~50K resistors to prevent the inputs from floating when you disconnect the signal.
     
  9. kubeek

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    Sep 20, 2005
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    Also, you should add a fast diode across each of the 10ohm resistors. Anode goes to the gate side for the N transistors, the other way for the P transistors. This is needed because your drivers have no dead-time, so this connection allows for faster turn-off than turn-on, so that the transistors do not cross conduct.
     
  10. ajrenzetti

    Thread Starter Member

    Dec 20, 2010
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    i wonder why ISIS draws the mosfets out upside-down. are you sure they are upside-down? or is it just the diodes. also, C2 and C3 are there because the datasheet says it helps with the signal.

    ive just revised the schematic. does this look right?
     
  11. ajrenzetti

    Thread Starter Member

    Dec 20, 2010
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    the picture i just posted seems too small, so heres a larger version of it(i hope).
     
  12. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    I am sure they were upside down, the line that connects one pin with the arrow says which pin is source and which is drain.

    This way it looks ok, but still you should add the resistors in parallel to c2 and c3, better safe than sorry.
     
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