Flywheel diode / MOSFET

Discussion in 'General Electronics Chat' started by arranrai, Sep 23, 2015.

  1. arranrai

    Thread Starter New Member

    Sep 22, 2015
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    HI,

    I have two devices that I'm considering buying flywheel diode and MOSFET.

    Flywheel rating
    • Voltage rating : >24V
    • Current rating : >8A
    Also I have MOSFET with ratings as :
    • Voltage rating : >24V
    • Current rating : 16A or higher
    • gate voltage : 10V -/+ 0.1
    Is there anything else that I need to be considering when buying this device. Plus does anyone know of a MOSFET device that can fulfill these requirements.

    Thank you
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Welcome to AAC!
    Your diode rated current should be greater than the maximum current the load switched by the MOSFET draws.
    Use the parametric search engines on any of the major MOSFET sellers' web-sites (e.g. Farnell, Mouser, Digikey) to find a suitable MOSFET. There are many to choose from.
     
  3. TheButtonThief

    Active Member

    Feb 26, 2011
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    If the MOSFET for fast switching or for continuous drain? What's the voltage at the gate? Do you require P or N type MOSFET (or even P.N)? Is 24V a MAX drain-source voltage? And most importantly, do you require depletion or enhancement mode?
     
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    In other words, what is this for?
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,013
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    If this is for a switching regulator circuit then the diode should be a Schottky type to minimize it's dissipation, and the MOSFET should have a low ON resistance.
    If you tell us more about the application, we can give you better advice.
     
  6. alfacliff

    Well-Known Member

    Dec 13, 2013
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    most power mosfets have the diode built in. just like most are N channel, enhancement mode, gate bias to turn on.
     
  7. arranrai

    Thread Starter New Member

    Sep 22, 2015
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    To answer all your questions and give a clear idea I have produced a sketch of what it will be used for. Please see the uploaded MOSFET picture.

    The ADAM module is 4024 that is only able to analogue output -10V to +10V so I cannot directly control the SY3220-5L. So I though of using a voltage controlled switch that would turn on with voltage supplied in the range 0 to +10V and turn off when nothing is applied. When it's on it needs to be able to allow the flow of +24V to travel through. When you see the diagram the Vdd is connected to the supply voltage. If you could please check and see if this design is appropriate for the task that I have explained. If you have a better option or any improvements please let me know.

    Thank You
     
  8. dl324

    Distinguished Member

    Mar 30, 2015
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    Specs for the snubber diode aren't very critical and you could use 1N4001. What is the resistance of your inductive load? Didn't feel like searching for it in the datasheet...

    EDIT: Just noticed that you gave the current requirement in the photo. Just about any NMOS device will work (e.g. 2N7000).
     
    Last edited: Sep 23, 2015
    Roderick Young likes this.
  9. arranrai

    Thread Starter New Member

    Sep 22, 2015
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    Would you say that the circuit I have drawn looks reasonable to work? (drawing was posted Yesterday at 2:36 PM)

    Not sure since I'm applying +10V at the gate to turn on and it allows +24V to travel across it. Would this be possible?

    Please can you just confirm and also the output resistance of the ADAM-4024 is 0.5ohms, so 10V/0.5 = 20A. The current looks too high. So I decided to reduce the voltage I will send to the MOSFET to 2, so I get 4A. What do you think? I'm I right to says this. Or it doesn't matter the output current is this high and since the base current be determined by the base resistance, which in this case is 10K. So 10V/10K = 1mA.

    I'm general, I want to know if the gate current is determined by the gate resistance or the output of the Adam-4024.
     
  10. crutschow

    Expert

    Mar 14, 2008
    13,013
    3,233
    The circuit looks okay but your questions appear to show a lack of understanding about basic circuit function.
    A MOSFET draws no gate current (other then a very tiny leakage current), it's a voltage operated device.
    The current from the ADAM-4024 will be just due to the 10kΩ gate resistor to ground.
    The 0.5Ω output impedance of the ADAM-4024 is in series with its output and it would only deliver 20A (although it likely can't actually deliver that much) if the output were shorted to ground.
     
    Roderick Young likes this.
  11. dl324

    Distinguished Member

    Mar 30, 2015
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    Should work. It's just a simple switch circuit.
    As mentioned earlier, your questions appear to indicate a lack of how the components work and how to spec them.

    Your calculation of the required voltage and current specs for the diode and MOSFET are wrong.

    The snubber diode is there to protect the MOSFET from inductive kickback that could destroy it. It only conducts current when the MOSFET switches off. So you should be concerned about the diode PRV, peak current, and average current ratings. In many cases, a signal diode can be used as a snubber diode and yours is one of them.

    The MOSFET only has to carry the current required to turn the solenoid on. You stated that this was a few tens of mA, so just about any NMOS FET will do. For the MOSFET, you primarily need to be concerned about maximum drain current, S/D breakdown voltage, Vgs to turn on fully, Vgs(max) and on resistance. The minimum on resistance the circuit requires depends on the minimum voltage the solenoid requires to turn on.

    You can't choose an arbitrary gate drive voltage for the MOSFET. You need to provide voltage sufficient for it to turn on completely and quickly. You can see from the attached datasheet that a typical 2N7000 will turn on at around 2V, but you can also see from figure 2 that on resistance is dependent on gate drive voltage. Since the MOSFET is being used as a switch, you want to drive it hard enough to reduce on resistance and, therefore, voltage drop. 2N7000 will tolerate the 10V you had planned to use.

    The calculation you used for determining drive current is wrong. You have a 10K resistor in parallel with the gate capacitance of the MOSFET. So the driver is only charging the gate cap and sending current through the resistor.
     
  12. arranrai

    Thread Starter New Member

    Sep 22, 2015
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    Would you be able to provide assistance with how the calculation need to be done please.

    Thank You
     
  13. dl324

    Distinguished Member

    Mar 30, 2015
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    See below.
    As I mentioned earlier, the snubber diode only conducts current when the MOSFET switches off. The magnitude of that current depends on the inductance and voltage being switched in the circuit. Since this is a transient current, you use the peak current rating (for transients if one is specified). A 1N4184 will tolerate 2A for 1uS. Breakdown voltage needs to be higher than the voltage being switched (24V). PRV for this diode is 100V.
    The MOSFET needs to have a breakdown voltage greater than 24V. Current rating needs to handle current for the load being switched; which you stated to be 17mA (which seems kind of low). You can calculate the current that will be switched from the coil impedance. The device needs to tolerate the gate voltage you plan to apply and that voltage needs to turn on the device completely to minimize on resistance.
     
  14. arranrai

    Thread Starter New Member

    Sep 22, 2015
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    Thank you all the help :)
     
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