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Paralleling transistors...

Discussion in 'Electronic Basics' started by Tim Williams, Dec 1, 2004.

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  1. Tim Williams

    Tim Williams Guest

    I'm working on an induction heater circuit, since my function generator died
    I have to make it self resonant and since I don't feel like op-amps right
    now that means self-excited class C BJT's, which I've done successfully
    before on a smaller scale (namely, two 2SC3519's in parallel with 0.15 ohm
    RE's[1]). Having already blown a few MOSFETs before the function gen. died,
    I'm guessing I can't very well make those work in the same class just yet...

    First of all:
    This was upgraded to a 200W (40V 5A as measured) model:
    (Heating a strip of transformer iron; you can see the heatsink and emitter
    resistors below my hand.)
    The power source is a rewound MOT. I rewound it again for 100V 10A output,
    which gets +130VDC at the capacitors. I found six 2SC2625's laying around
    so paralleled them all in a nice bank and tried the exact same circuit, but
    alas, one blew (shorted, fun) after another. It was always the one on the
    end, so I'm wondering if it had parasitic oscillations?

    Yes, long post, I'm being thurough... Comments?

  2. Tim Williams wrote:
    In order to get reasonable sharing and good behavior you need 3 heavy
    busses for the transistors with the signals connected to the middle of
    the busses. Better yet would be something like ring busses connected
    to their signals with three parallel wires like spokes to the center.
    The devices can be connected all around like flower petals. Keep in
    mind that your induction coil sprays out a lot of stray field, and the
    transistors should be several diameters away from the coil to reduce
    stray coupling that varies device to device. Connect the coil to the
    transistor and supply busses through a transmission line.
  3. Tim Williams

    Tim Williams Guest

    Just emitter resistors, right? Aside from the wiring.
    How about not blowing the MOSFETs?
    I don't think "heavy buss" is quite applicable here, I intend on having up
    to 200A reactive flowing, maybe more, but not necessarily that much in the
    transistors. At least what I recieved today from Digi-Key, 10 x MJE18008,
    rated for 16A peak (so 160A total) and 1000Vcbo. Meh. In the final version
    I intend on using a strip of 1/4" tubing for it. This is only supposed to
    be 1kW or so, nothing too big.
    Last setup I had was two devices per (small, only for intermittent testing)
    heatsink, with the leads going out to buss wires (18ga.), so three units
    strung together.
    Heh... first model I made, I could actually feel the magnetic pull. :)
    Ok. Shouldn't be a problem...
    TL? We're at 50kHz here.

  4. I think wire wound source resistors would be a good idea, also. The
    inductance helps as much as the resistance.
    Not heavy (low resistance) to carry hugh current without overheating,
    but heavy (low inductance) to keep differences between devices small.
    I am not concerned with impedance matching, just field containment. A
    big wire inside a copper pipe would work. Two big wires twisted
    would, also. This will make up part of the tank capacitance so teflon
    insulated wire would keep the losses from melting the insulation.
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