Need help with H-Bridge

Discussion in 'General Electronics Chat' started by ahur, Apr 4, 2013.

  1. ahur

    Thread Starter New Member

    Apr 4, 2013
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    Hello, I'm trying to make a 30amp (motor stalls @ ~25) h-bridge using 2 p-channel mosfets and 2 n-channel mosfets and 2 npn transistors acting as drivers. I've attached a similar schematic except it is using darlington transistors instead of mosfets.

    The n-channel mosfets I'm using are the FQP30N06L.
    http://www.fairchildsemi.com/ds/FQ/FQP30N06L.pdf

    p-channels are FQPF47P06.
    http://www.fairchildsemi.com/ds/FQ/FQPF47P06.pdf

    The bjts are p2n2222a.
    http://www.2n2222datasheet.com/pdf/on_semiconductor_2n2222_datasheet.pdf

    When I activate the left side of the circuit, the right high side mosfet gets warm. Did I mess up in the selection of mosfets? Or am I an idiot who wired this thing up incorrectly?
     
  2. antonv

    Member

    Nov 27, 2012
    149
    27
    So Q1-4 are now the FETs you mention. For them to turn on properly you need to apply a gate voltage that is referenced to their sources. Right now your gate voltages are floating so your results should be weird.

    Are you just using this H-bridge as a forward/reverse control or are you going to use PWM for speed? How you use it will determine the circuit you need.
     
  3. ahur

    Thread Starter New Member

    Apr 4, 2013
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    I'm sorry, I should have elaborated more on the purpose. The gates getting 5V from a 16F1937. I'll be using PWM for speed control and I also need the circuit to be able to brake.

    Also, I'll be heatsinking these FETs with cut up northbridge heatsinks from archaic mobos and some thermal adhesive.
     
    Last edited: Apr 4, 2013
  4. antonv

    Member

    Nov 27, 2012
    149
    27
    If you are going to use PWM then you will have to use MOSFET driver ICs that can put enough current into the transistor gates to turn them on and off fast so that they don't have time to overheat.

    MOSFETs are different enough from BJTs that you cannot replace the BJTs with MOSFETs and hope it will work.

    What frequency will the PWM be at?
     
  5. codeboy2k

    New Member

    Jun 18, 2012
    7
    2
    further to what antonv says here, you should not start with a BJT based H-bridge, there are plenty of examples of MOSFET H-bridges on the net. Start with a known good one, and make it work for your 30A.

    A quick google image search for MOSFET H-bridge turns up many useful circuits.

    Find a good diode protected circuit.
     
  6. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,026
    3,789
    If you are using a PIC, search the microchip website for the following application notes.
    • AN847–“RCModelAircraftMotorControl” (DS00847)
    • AN893–“Low-costBidirectionalBrushedDC Motor Control Using the PIC16F684” (DS00893)
    • AN905–“BrushedDCMotorFundamentals” (DS00905)

    Good luck
     
    spark8217 likes this.
  7. ahur

    Thread Starter New Member

    Apr 4, 2013
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    It'll be at a very low speed .5Hz, so I'll probably have to implement some sort of software PWM. I have some darlington transistors coming in, I was trying to work with what I had lying around.
     
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