BJT or MOSFET

Discussion in 'General Electronics Chat' started by mahmoud shendy, Jan 2, 2008.

  1. mahmoud shendy

    Thread Starter Member

    Dec 23, 2007
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    0
    Dear all, we have in our lab an inductive load that has an internal esistance of about 25 Ohm, it is required to
    varying the DC current through that coil from 0.5A to 5.5A to perform its operation, and so i think in PWM that switch
    this inductive load ON and OFF from a 150 VDC supply and by varying the DUTY CYCLE of the PWM circuit, we can then adjust
    the average DC current... Now is this right or not, and if this is right what is better to switch this load BJT or MOSFET
    . by the way, the inductance of this load is neglectible..

    I read about the TIPL755 and in its datasheet listed that it is for fast inductive load switching. is it better or using
    power MOS such as BUZ45..

    ( i forgot in this schematic the free wheeling diode, but it is already placed )..

    am waiting for your helpful replies, thanx in advance......... M.Shendy
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    No matter which device you use, some heatsinking is going to be necessary. That is 825 watts you are controlling.

    It would be very helpful to know more about the circuit. If you need a steady field around the coil, using several paralleled BJT's might be a better control solution than a FET and PWM. The average current can be adjusted, but the instantaneous mix of current well above 5.5 amps immediately followed by device cut-off and partial field collapse before the next current spike may make for noise in the experiment.

    What is driving the current passer, and what senses the current/field and then controls current through the coil?
     
  3. mahmoud shendy

    Thread Starter Member

    Dec 23, 2007
    21
    0
    the passer is drivin by simple 555 timer circuit, and the feedback is not necessary, ie i don't need to know the current through the coil ..
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    You can drive the gate of a FET directly from the 555. Use a 100 ohm resistor between pin 3 and the gate. The FET will need a large heat sink and an antiparallel snubber diode to protect the FET. You also might consider using an IGBT. They have better saturation characteristics and drive just like FET's. An Inrenational Rectifier IRG4PC60FPBF only drops 1.8 volts, will handle 120 amps (pulsed), and dissipate 520 watts. And it is rated for 600 volts.
     
  5. mahmoud shendy

    Thread Starter Member

    Dec 23, 2007
    21
    0
    thanx a lot for your help.. bye
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Beenthere, do you really think he needs a heatsink on the MOSFET? Peak current is only 6 amps, not including parasitic capacitance of the load. That's nothing with a low Rds(on) MOSFET. Almost all the power dissipation is during transitions. A 555 can provide a couple hundred milliamps of gate drive, so the transitions can be pretty fast.
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
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    It shouldn't get hot, but some small finned cooler is good insurance.
     
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