help with H-bridge Design P and N channel

Discussion in 'The Projects Forum' started by thekaiser11234, May 8, 2012.

  1. thekaiser11234

    Thread Starter New Member

    Apr 29, 2012
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    iam new to H-Bridge design so i need help ,
    im making H-Bridge High side P low side is N

    im using 24 volt motor 250 watt rated current
    i had to hook up a zener diode to limit the vgs voltage to 15 v
    but im not sure if im doing it right how to choose series and parallel resistors ?
    or im doing just good or i need little edit ??

    also are those transistors good for this circuit or shall i change ??
    also how to decide ?
    how to choose gate resistance ? * please note im new to this *

    sometimes i see Mosfets turn orange ( why ) ?

    -- can i hook inputs to in1 and in2 to controller directly or is it dangerous ?
    im using bc547 bjts

    also shall i add flyback diodes or not needed ?how to spec them ??

    [​IMG]
     
  2. thekaiser11234

    Thread Starter New Member

    Apr 29, 2012
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  3. thekaiser11234

    Thread Starter New Member

    Apr 29, 2012
    25
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    [​IMG]

    sorry for double reply
     
  4. thekaiser11234

    Thread Starter New Member

    Apr 29, 2012
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    also if simulation files needed i can post them

    thank you :)
     
  5. thekaiser11234

    Thread Starter New Member

    Apr 29, 2012
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    nothing? :S :(
     
  6. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    When do you see that?
     
  7. thekaiser11234

    Thread Starter New Member

    Apr 29, 2012
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    if i use irf540 as low side
     
  8. thekaiser11234

    Thread Starter New Member

    Apr 29, 2012
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    shal i add resistances to the gate and between gate and source ??

    What shall be there values depending on wat ? :(
     
  9. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,055
    By "orange" you mean glowing hot? It's failed to a short if you've seen that. And released its magic smoke.
     
  10. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,055
    The resistor on the gate is to limit current inrush to the fet and/or from the driver. At low frequency, it's not much of a concern. But as frequency increases, you need to move the gate charge in and out rapidly and the current rises. Without a limiting resistor, it could lead to damage.

    A resistor from gate to source is always a good idea, to turn off the fet whenever the input floats. The gate capacitance is low, so a 10k or so will ensure turn-off even at high frequency. But you'll need to do the math. The RC constant of the resistor-gate should be well less than 1/3 or so of the off time, to ensure the gate goes fully off between cycles.
     
  11. thekaiser11234

    Thread Starter New Member

    Apr 29, 2012
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    yes there was smoke as i remember
     
  12. thekaiser11234

    Thread Starter New Member

    Apr 29, 2012
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    this was super helpful :)

    but i want to know what do u mean with low frequency ???

    im using arduino so my frequency is 500 hz
     
  13. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,055
    Excellent, you caught me being vague. Your 500Hz is "low" but it comes down to a calculation of average current over time. You have to move that gate charge onto the gate 500 times and remove it 500 times each second.

    Your fet is likely a dead short and needs to be replaced. I built myself a little MOSFET test rig using a 9v battery and an LED with a current-limiting resistor. I can test whether a MOSFET still switches the LED on and off when I touch the gate to + on the battery. Your's won't go off, I'll bet.
     
  14. thekaiser11234

    Thread Starter New Member

    Apr 29, 2012
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    thank you for replying again but it seems i dont seem to get u totally kinda hard for a newbie ? im still trying

    any way i know my questions are dump but cant move to better place without them so ..

    wat do u mean by dead short ,like how did u know based on wat readings of the datasheet ?
    what i get i shall add resistance between bc547 collector and gate , i'd give it a value of 330 ohm ??
     
  15. thekaiser11234

    Thread Starter New Member

    Apr 29, 2012
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    the low side im changing to irf3205 cuz its wat in stock is it a good choice ?
     
  16. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,055
    I mean it was destroyed and now is like a piece of wire connecting all 3 pins.
    I have done no calculation, but I think that is much smaller than it needs to be. Calculate your time constant, RC, where R is your resistor and C is your gate capacitance. Multiply by 3 to allow full discharge during an "off" interval. At 500Hz, that's 1ms. If the 3RC is bigger than 1ms, you need to use a smaller value resistor. But you don't want any more load (too small a resistor) on the gate than is needed.
     
  17. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,055
    That looks like a fine MOSFET, but it may not survive any better than the IRF540 did. You need to fully understand why the circuit failed before risking more parts. I don't fully follow your circuit and cannot comment on why it failed, but I do know that "shoot through" where both MOSFETS are on at the same time, is a common H-bridge problem. How big is your motor?
     
  18. thekaiser11234

    Thread Starter New Member

    Apr 29, 2012
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    MY1016 motor 250 watt 24 volt / rated 14 ampere
     
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