Discrete H-bridge with relays and MOSFETs

Discussion in 'The Projects Forum' started by lastRites, Sep 7, 2013.

  1. lastRites

    Thread Starter New Member

    Jul 9, 2011
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    Hello from India :D
    I am designing a cheap motor controller for robots (we usually use DPDT switches and wires:p). My main concerns are cost and reliability. So I have chosen the components accordingly. I plan to get the h-bridge under 4$ for a 10A(stall) motor.
    I am attaching the schematic and would be very grateful if you guys help me improving it and pointing out faults, if any.
     
    Last edited: Sep 7, 2013
  2. lastRites

    Thread Starter New Member

    Jul 9, 2011
    13
    1
    This circuit uses 2 IRFZ44N MOSFETs(50V, 49A, RdsOn:17.5 mohm) in parallel to control the speed using 500Hz PWM. The SPDT relays K1 and K2 (G5LE) are rated 10A at 28V and are used to switch the direction of rotation. What will the MAX current this circuit can handle at 12V?

    I have done one circuit on a perfboard with a single MOSFET with a heatsink and it gets a bit warm(the MOSFET, about 40C) at 5A continuous (a rc car motor at stall).
     
  3. wayneh

    Expert

    Sep 9, 2010
    12,103
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    I think when you parallel MOSFETs you usually want a small resistance between the gates, say a 50Ω resistor, to damp any ringing when they are switched.

    Using R1 at 1kΩ as the source of gate current also concerns me. That's causes a slow turn on and potential heat issue. Maybe it's no concern at 500Hz, but it's kind of bad form. You could reduce R1 and R4 by 2X or more to address this. You just need to limit current thru T1 to stay well within its specs.
     
  4. tubeguy

    Well-Known Member

    Nov 3, 2012
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    +1

    What is the source and the ON voltage of your PWM control signal?
    It would be easier on the relay contacts if you could make sure that the PWM is momentarily off during relay switching.
    Many of the relays available today are, I believe, rather optimistic regarding their current ratings.
     
    Last edited: Sep 7, 2013
  5. ronv

    AAC Fanatic!

    Nov 12, 2008
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    I think D5 wants to be between the FETs and the relays.
     
  6. lastRites

    Thread Starter New Member

    Jul 9, 2011
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    What is the "ON voltage of of PWM" ? I am using a 12V battery to power the circuit. Would it be safe to assume that these relays will be able to handle a motor which takes about 10A at stall at 12V?
     
  7. lastRites

    Thread Starter New Member

    Jul 9, 2011
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    Why? I am using it as a flyback diode to protect the MOSFETs. Please correct me if I am wrong!:confused:
     
  8. ronv

    AAC Fanatic!

    Nov 12, 2008
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    The only diode I see that kind of looks like a flyback diode is the one connected between ground and Vss. I don't see that one doing much, but if it were on the other side of the FETs it would look more like a flyback diode.
     
  9. lastRites

    Thread Starter New Member

    Jul 9, 2011
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    Sorry, the first schematic was flawed. Here is the latest one.
     
  10. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Must be a very small heat sink. 5^2 X .0175 = .4375 watts X 62 degrees C / watt (no heat sink) + 25C room temperature is only 52C. So maybe all is well if it is a tiny heat sink.
     
  11. lastRites

    Thread Starter New Member

    Jul 9, 2011
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    Sorry, I do not understand!
     
  12. ronv

    AAC Fanatic!

    Nov 12, 2008
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    You can calculate the temperature knowing the current and the on resistance and the thermal response of the FET. Based on the 5 amps you talk about yours would be about 50C without a heat sink. So if you have a small heat sink 40C might be expected and should be okay..
     
  13. lastRites

    Thread Starter New Member

    Jul 9, 2011
    13
    1
    Ok thanks! :)
     
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