Gate drive problems with parallel IRF740's

Discussion in 'The Projects Forum' started by Jimmeh30, Jun 3, 2011.

  1. Jimmeh30

    Thread Starter New Member

    Mar 4, 2011
    22
    1
    G'day folks.
    I've been struggling with this for months now and not getting very far. I'm trying to build a plasma speaker from an old tv flyback (ac disc type)

    Initially I was getting good results using 2x IRF250's driven from a 555 timer ic with a bc639/640 totem pole driver. Eventually it fried and I bought a bunch of IRF740's.

    Since the current handling of the 740's is lower I decided to run 3 in parallel and found that the gate drive wasn't sufficient.

    Now I'm running three totem pole drivers (639/640) and the 740's are running extremely hot. After 10 or so seconds, one invariably blows follwed closely by the remaining two.

    The gate drive waveform is clean and it seems to run well at around 22Khz accept for the heating.

    Totempole drivers are running at 12v and the gate resistors are 33ohm with a anti-parallel diode for gate discharge, back emf protection is a 12Kv diode from drain to source of all three fets.

    My question is, could I be having problems with timing given the three seperate totem poles? Nothing else has changed since I had it running with "less robust" mosfets. My scope isn't a dso so how would I go about measuring and comparing the slew rates?

    I haven't got a circuit diagram atm, but if it'll help I'll draw one up.
    Thanks in advance, Any help would be greatly appriciated

    sorry, also, I only have paint to draw a diagram atm, could someone give me some direction as to how to post the image? thanks
     
    Last edited: Jun 3, 2011
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Around here, if a schematic is not on display, the circuit does not exist!
    It's mighty tough to give suggestions on fixing a nonexistent circuit.

    Schematics are the language; they eliminate so many questions. Threads that don't start off with good schematics tend to drag on forever, and never get resolved; it's pretty much a waste of time and server storage space.

    I'm not sure why you are using IRF740's unless you actually need that high of a Vdss; you pay a big penalty for Rds(on) at 0.55 Ohms and a rather large gate charge at 65nC's. They're rated for Id=10A @ 25°C, but that drops quite a bit when they get hot. If you are not using (a) heat sink(s) on them with heat sink compound, they will have a rather poor thermal resistance from the junction to ambient.
     
  3. Jimmeh30

    Thread Starter New Member

    Mar 4, 2011
    22
    1
    I decided on 740's because I thought they'd handle the back emf better than the 250's and also because there was a lot of 50 on ebay for 20$ au delivered :)

    As far as schematics... just finished em, here they are!

    I know this isn't how it should be done, but I'm trying to work with what's in my parts bins. I can see however it's probable that I'm gonna have to bite the bullet and buy more bits.:rolleyes:


    oh, and yeah, atm they're on a 300Lx100Wx5mm thick piece of ally plate that's bolted to two 300x50mm heatsinks, all with copper grease as "thermal paste". With a 24v 80mm fan, they stay around 30 deg C. Overkill I know but it's the only way I've managed so far.
     
    Last edited: Jun 4, 2011
  4. Jimmeh30

    Thread Starter New Member

    Mar 4, 2011
    22
    1
    My computer exploded the same night of my last post.... ya win some, ya lose some :p

    Just a quick update, I tested the "proposed new setup" and it works extremely well. With the addition of a quick and dirty diode/resistor/capacitor snubber, no more exploding fets, no more heat problems.

    Gives a good 80mm long silent arc using two antiparalled flybacks.
    Insulation becomes a problem at 32v and 18amps input :D

    Cheers to all who took an interest.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Sorry I'd missed your posting of schematics last week; I've not been on-list much lately.

    The 393 dual comparator has open-collector outputs; they can sink current but cannot source it.

    Also, they have a practical limit of 6mA current sink capability.

    I see you have a 1k resistor from the driver bases to the output of the 393, but no pull-up resistor. That would only serve to turn the MOSFET gates OFF, but not ON.

    If your source is really 12v (not ~14v for a lead-acid battery) then you would need to use a 12v/6mA = 2k pull-up resistor from the bases to +12v, and no resistor between the bases and the 393 out.

    If you're actually using lead-acid batteries as a source, then you'd need to use a 2.3k pull-up resistor.
     
  6. Jimmeh30

    Thread Starter New Member

    Mar 4, 2011
    22
    1
    Cheers for the reply, and no need for apologies, this project is keeping me busy, but only cause I'm keen to get it rawing! :D

    My brand new second hand ebay computer hasn't arrived yet(bloody public holidays) so I'm prolly gonna be a bit slow to respond also, (before mid next week)

    I just spent half an hour or so responding to ur reply, but optus (ISP) wigged out and it got lost so to summarise it...

    It's blown up again cause I pushed it too hard and the back emf defeated the snubber.

    The 393 was taken out before my last post, it's being driven directly from the 555 atm with conciderable loss of control over both duty cycle and freq :(

    Thanks for the comment about the 393 and it's sink source capability... sink/source I figured I understood and that confirmed it for me, open collectors however... still n.i. lol

    ATM the psu is a 120amp battery charger, max output is 36V
    By next week I'll have the rectifier diodes for my 20.5v 20.5v AC dual secondary 30amp xfmr and will have it wired and ready to go for a full blown 60vDC test

    I'm lookin for more critique on this circuit, I know they're hard to get running and it's close to what I'm looking for as far as output, but to get more from it from here is going to need a much less "brute force" approach

    Any and all input will be met with gratitude.

    At this stage, cya's all on thursday, assuming the new(2nd hand) MB arrives by then :D
     
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