driving mosfets

Discussion in 'General Electronics Chat' started by hemgaurav, Jan 31, 2009.

  1. hemgaurav

    Thread Starter New Member

    Jan 30, 2009
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    0
    I am making a small electric vehicle for polio afflicted physically challenged.
    The mechanical details have already been worked out but I need help in the speed control of the DC series wound starter motor (carbon brush type) which I am using to power the vehicle.
    The total wattage to be used is only 250 watts with 36 volts battery power. The drain current would there fore be limited to about 10 amps only, though at high torque it may draw up to 60 amps for short durations, which a couple of mosfets in parallel could manage easily.
    I want to use an IC SG 3524 for the gate drive.
    I have used this IC earlier to make inverters which have worked well, but this speed control circuit is blowing up the mosfets and the drive transistors. Presently the entire circuit fitted on the test bench operates well for a minute or two at low speeds and then the mosfets burn out (without getting hot).
    I have attached the schematic. Kindly check this out and Please help. It's Urgent and for a good cause.
     
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,693
    904
    What size of main capacitor (the one across the battery) are you using? 4QD has a good discussion of that item. Second, have you considered a dedicated mosfet driver? I have used the LM1158 for a high-power, series-wound motor (old Ford starter). It includes feedback from both top and bottom mosfets. There are certainly many other drivers out there. Of course, you will/may need to generate your PWM separately, but the LM1158 only needs one signal. It generates the top and bottom gate controls from that.

    Finally, with a series-wound motor and that controller, what are you going to do for reverse?

    John
     
  3. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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    I think the MOSFET in parallel with the motor needs to be a snubber diode instead...
     
  4. hemgaurav

    Thread Starter New Member

    Jan 30, 2009
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    0
    Hello John, thanks for your prompt reply.
    I have not used any main capacitor as yet because it was not shown in the circuit . 4QD talks about its importance but has not explained in any circuit how its value can be calculated or what value and type it should be. Kindly enlighten me on this aspect...or simply advise what capacitance value and the type of capacitor should be used. SG 3524 is a dedicated mosfet driver and I used it because it has a shut down and sense facility too at pins 10 and 4/5 respectively. I have also used it earlier in inverters and therefore I feel comfortable using it. I will use a DPDT switch for reversing the motor , which could be achieved manually or with relays.

    Gaurav
     
  5. hemgaurav

    Thread Starter New Member

    Jan 30, 2009
    4
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    Hello Alberto,
    There is a time delay provided in the circuit by way of the 150 pf capacitor so that both mosfet operations do not overlap in ON position which would short the battery. I have also tried disconnecting the parallel mosfet which shorts the motor but the main mosfet still blows out. Today I will put a diode across each of the mosfets to assist the intrinsic diodes of the mosfets, as advised by you. But I am waiting for John to tell me what capacitor I can use as the main capacitor. As I said I will use a DPDT switch for reverse, I also dont want any braking or regenerative charging through the circuit, because I want to keep the circuit portion simple and reliable.

    Gaurav
     
  6. hemgaurav

    Thread Starter New Member

    Jan 30, 2009
    4
    0
    Hello,
    I did try diode 6A4 and IN5408 putting them in parallel with a .22uf across the two terminals of the motor, but the diode was heating up considerably. I have also tried BY229 but again it was heating up too. This was the reason I had to go in for the mosfet operating on the High side so that I could despense with the diode. Though I can try using the snubber cct (diode + .22uf) combination along with the mosfet across the motor ...if you advise.

    Gaurav
     
  7. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    904
    I used a 2400 uF /50 V capacitor with a series-wound motor that was somewhat more than three times the power of the motor you are using and ran it at 12V.

    John
     
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