Mosfet switch used to protect a load

Discussion in 'The Projects Forum' started by newbieateverything, Feb 25, 2013.

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  1. newbieateverything

    Thread Starter Member

    Feb 25, 2013
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    hi, i am currently trying to design a Mosfet based switch to protect a load from surge currents, the simulator package i am using is PSPICE.
    I need the mosfet to turn on to protect the load from surge currents and turn off once it has reduced the current to an acceptable level of 5 amps.
    I am aware a mosfet turns off once VDS> VGS-VTH and turns on when VGS>VTH
    however i need to control when it turns off, so far i have only gotten to the point of turning it on, and it turns off by itself after a few ms.
    Please help, also if there are any good links showing what i am talking about that you know of please put them in your reply
     
  2. crutschow

    Expert

    Mar 14, 2008
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    So you want a current limit circuit?

    What is the voltage?

    Post your circuit.
     
  3. #12

    Expert

    Nov 30, 2010
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    He's trying to make a MOV out of a Mosfet.
     
  4. newbieateverything

    Thread Starter Member

    Feb 25, 2013
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    actually i am trying to make a current limiting circuit as the first poster said
    The voltage from the source is 120V, the voltage parallel to the mosfet is 200V, the pulse voltage at the gate is 15V
    i am trying to prevent the current in the load from exceeding 11A, also i want the mosfet to turn off when the current has been reduced to 5A, i dont want its turn off to be determined by VDS>VGS-VTH

    My idea for giving my switch some intelligence is to use an opamp as a comparator to determine when the mosfet turns off.

    Please help
     
  5. crutschow

    Expert

    Mar 14, 2008
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    I'm a little confused. :confused:

    Which device is the load?

    What's the purpose of the circuit?
     
  6. #12

    Expert

    Nov 30, 2010
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    Why would the current reduce to 5 amps?
     
  7. newbieateverything

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    Feb 25, 2013
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    the load is represented by the 10mh inductor and the 10R resistor connected to the 120V source.
    Why do i want to reduce the current to 5A?
    to test it i want to be able to turn off my mosfet when i choose to and not wait for it to turn off by itself

    the purpose of the circuit as of now is to turn on the mosfet and to reduce the current in the load to 0, i have been able to do this much so far.

    however i want to be able to use my mosfet switch to protect the load under surge current conditions.

    however i am not sure about how to simulate these surge conditions, i would however define my surge conditions as 50A going through the load, something i am going to assume it shouldn't take without being compromised.

    i need to configure my mosfet to turn on to reduce the current to 5A and turn off once the current is within the safety region 5A or less.

    I believe i should use an opamp as a comparator to determine when the mosfet turns on and off, however i dont exactly know how to connect it or if this is the best solution available.

    if anything is unclear please let me know so i can explain further, started this out as a hobby so i am very average as of now.
     
    Last edited: Feb 26, 2013
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    What is the purpose of the 200V supply and the diode?
     
  9. newbieateverything

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    Feb 25, 2013
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    to drive the mosfet, the diode is there to prevent reverse voltage from damaging the mosfet when the inductor looses current
     
  10. crutschow

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    That diode and voltage source can be replaced by a zener diode (anode to ground).

    If the voltage is 120V, the current through the 10Ω resistor will be 12A when the MOSFET is on. Do you want to have the MOSFET limit this current to 5A? If so the transistor dissipation will be 350W(!). That will require several large transistors in parallel on a very large (likely fan cooled) heatsink.

    Where is the 500V surge coming from to create the 50A current surge?

    If you told us what this circuit is supposed to do, we could likely offer better suggestions.
     
  11. newbieateverything

    Thread Starter Member

    Feb 25, 2013
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    the circuit ideally is supposed to protect the load(resistor and inductor connected to the 120V) by turning on the mosfet when a surge current is passed through the load, yes i did the calculations and 50A is way to high, the surge current should be 15A and i want the mosfet to turn off when the current across the load drops to 3A.

    I am trying to design a solid state DC circuit breaker for areospace applications ideally, but i cannot test at 600V, my voltage source is limited to 240V.

    The circuit i am currently trying to design is simply the switching mechanism turning the Mosfet on and off to protect the load.
     
  12. Babber

    New Member

    Apr 19, 2011
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    Perhaps your thinking along these lines of the main circuit? You're trying to bypass the current surging through the load?

    I just chose some arbitrary on/off times for the pulsed voltage source to the gate and the pulsed current source. I am also assuming that you expect the drain to rise up due to the inductive kickback, hence the 200V source and diode?

    [​IMG]
     
  13. crutschow

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    Mar 14, 2008
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    I'm still confused. Your original circuit has the MOSFET in series with the load so you would turn the MOSFET OFF, not ON to limit the surge.

    Instead do you want the MOSFET in parallel with the load to shunt away excess current? That would be a transorb type of action.

    How can the current drop to 3A when the normal load current is 12A? Are you anticipating short circuiting the power source?
     
  14. newbieateverything

    Thread Starter Member

    Feb 25, 2013
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    yes exactly, i expect inductive kickback thats why i put the 200V source and the diode, though i realise i can replace that configuration with a zener diode.
    Thanks for the idea, i would need to change the on/off times though.

    so i have tried it based on your suggestion, however i notice something peculiar. The Mosfet takes a really long time to turn on in a few ms, which for a solid state device is alarmingly slow.

    Also i notice some sort of deterioration in the mosfet ability to protect the load from surge currents, if you open the .sch file and put a current marker on the load you might see what i am talking about.

    I am basically just trying to design a mosfet based switch that turns on to protect the load from a surge current and off when the surge current has been dissipated.
     
    Last edited: Feb 26, 2013
  15. newbieateverything

    Thread Starter Member

    Feb 25, 2013
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    to be honest i am quite confused as to how to go about simulating a circuit breaker that is supposed to trip to protect a load from a surge current(eg: lighting strike) The mosfet is my choice of solid state device to use as a switch.

    So to start i tried to design a circuit that can turn on the mosfet and i ensured the mosfet was protected.

    However what i have designed on PSPICE isn't what i want.
     
  16. crutschow

    Expert

    Mar 14, 2008
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    A circuit breaker normally opens to protect a circuit from overcurrent. Are you trying to build such a breaker with solid-state circuits? Or are you trying to protect the circuit during the time it takes the circuit breaker to open?

    Transient protectors normally protect from overvoltage (such as from a lightening strike), not overcurrent, so it's still not clear as to what exactly you are wanting to do.
     
  17. newbieateverything

    Thread Starter Member

    Feb 25, 2013
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    yes i am trying to build a circuit breaker with solid-state circuits, i want the mosfet to turn on to protect the load and off when the current in the load is within its safe operating limit.

    it seems i have been confused longer than i imagined, yes overvoltage is what i need it to protect not overcurrent.
    I got confused because i kept on wanting to measure the current in the load so i somehow assumed i wanted to induce a current pulse in the load and have the mosfet turn on to reduce this current pulse.
     
  18. newbieateverything

    Thread Starter Member

    Feb 25, 2013
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    *Sorry i meant i want the circuit breaker to turn off(i.e open circuit) to protect the load and on when the current in the load is within its operating region.

    This is a little bit confusing really, so my question to you crutschow is how would you go about designing a circuit breaker using a mosfet to protect a load( made up of a 10ohm resistor and a 10mH inductor)?
     
  19. newbieateverything

    Thread Starter Member

    Feb 25, 2013
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    Hi, so this is how far i have gotten based on the discussions thus far

    please remember i am trying to build a solid state circuit breaker to protect the "labeled" load.

    what am i doing wrong? it doesn't seem to work the way i expect it to

    what i want is for my circuit to be able to show that when faced with an overvoltage my circuit is able to protect the load in 1 micro second or less and that the mosfet isn't damaged during this process.
     
  20. crutschow

    Expert

    Mar 14, 2008
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    How does your circuit detect the current?

    V9 in series with U1A which makes no sense. If you want V9 to power the op amp it needs to have the negative side grounded and the positive side going to pin 4. There should be no connection to V3.

    The LM324 is a slow op amp and will not rapidly turn-on the MOSFET in response to a signal. Better to use a comparator such as the LM339 which has a much faster response.
     
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