current limit a PWM circuit

Discussion in 'General Electronics Chat' started by Jim_2.0, Aug 27, 2007.

  1. Jim_2.0

    Thread Starter Well-Known Member

    May 14, 2006
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    I have the following PWM driver circuit which steps a 5V PWM IN signal up to a 24V PWM output. The PWM output would actually be preferable to handle 10 - 30V PWM, which I think it will. The voltage that the PWM runs at is applied at +V PWM.

    What I need is some protection to protect the FET if +PWM voltage ( 24V for instance) is applied to the PWM output line. ( PWM XA OUT ). At the moment if this is done, and the circuit driven then a large amount of current will flow from drain to source in the FET, thus killing it :( any ideas?

    This protection needs to be in place if the user wires the circuit incorrectly.

    P.S im an electronics newbie, just trying to modify this existing circuit.
     
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  2. hgmjr

    Moderator

    Jan 28, 2005
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    Is the junction of R10 and R12 connected to the gate of the FET?

    I ask because there is no dot at the junction.

    hgmjr
     
  3. Jim_2.0

    Thread Starter Well-Known Member

    May 14, 2006
    51
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    no it is not connected
     
  4. Jim_2.0

    Thread Starter Well-Known Member

    May 14, 2006
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    im thinking a simple resettable fuse in line with the PWM_XA_OUT line may work.
    this will also protect if too much current is drawn by the external device. I figure it will work as long as the fuse does not exceed the current capacity of the FET.

    or am i totally wrong?
     
  5. nanovate

    Distinguished Member

    May 7, 2007
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    I do not think this circuit would work unless you turn the diode around. What is the purpose of r10 and r12?
    What are you driving with the PWM (what is the load)? As it is if you approach 100% duty cycle you'll put a lot of current through the FET and diode (if it is turn around).
    You could replace the diode with a resistor sized to allow the current to be limited to below the SOA of the MOSFET.
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Turning the diode around will result in smoke from the diode and/or the MOSFET. The diode is probably there to absorb the flyback from an inductive load.
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    You could really use a self-protected MOSFET. Mouser sells them, but they aren't stocked, and the minimum quantity is 2500. If you only need a few, other vendors may sell them in small quantities, or perhaps you could get samples.
    Here is an application note.
     
  8. nomurphy

    AAC Fanatic!

    Aug 8, 2005
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    It appears to me that you have copied the schematic inaccurately, and/or have left out needed circuitry.

    R10 & R12 are pointless, as shown, and D8 is not appropriate given that you have a positive +V_PWM voltage.

    I think you need to double-check what you want and intend to do.

    IF all you are trying to do is convert from a smaller signal to a larger signal (plus an inversion), then change D8 to an appropriately sized drain resistor and delete R10 & R12. Also, adding a 10-100 ohm series gate resistor on Q4 wouldn't hurt.
     
  9. Jim_2.0

    Thread Starter Well-Known Member

    May 14, 2006
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    ok try this, i did leave out some of the cicuit. U7 in this diagram is an optocoupler. with PWM_XB into the opto being the 5V PWM signal from a microcontroller. pin 2 of the opto goes through a 680 ohm resistor to ground. This may explain the operation of the circuit a little better..
     
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  10. nomurphy

    AAC Fanatic!

    Aug 8, 2005
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    D13 prevents any voltage from being at "PWM_XB_OUT" unless it has another source or pull-up or something. If D13 was a resistor, there may be a chance for this to work. But the locations of D13, "+VPWM" and the lack of a drain resistor on Q3, make this circuit very suspect.
     
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Does this circuit drive an inductive load whose other terminal is returned to +V PWM? If so, then the diode is the inductive flyback snubber, and is connected correctly.
     
  12. Jim_2.0

    Thread Starter Well-Known Member

    May 14, 2006
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    yes correct, its is driving a valve coil. Sorry I forgot to mention this :D
     
  13. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Jim,

    What are the specifications on the valve coil?

    The symbol (from the datasheet) indicates there is protection for the MOSFET, even if the Spice symbol doesn't.
     
    Last edited: May 18, 2008
  14. krmason

    New Member

    Nov 17, 2008
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    The output is an "open collector" output with the valve connected to some +V source and the output of his circuit. It would work fine...
     
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