MOSFET Heating and Shorting in Solenoid Controller

Discussion in 'The Projects Forum' started by dansh0, Mar 3, 2016.

  1. dansh0

    Thread Starter New Member

    Feb 29, 2016
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    I recently updated a circuit to control the magnitude and direction of a solenoid using 4 MOSFETs and an Arduino. The original was designed by someone else, prototyped, and used with 12 V and about 1A successfully. My upgrade now uses up to 48V and 6A since it is used with different solenoids which require higher power. I changed necessary resistances and components to be able to handle the higher power and made sure that the PCBs were able to handle the power as well.

    We are now seeing problems where the MOSFETs will overheat and short (not sure in what order) when used higher than about 3A. Any idea of why this could be? I have been reviewing this for two weeks with no success.

    I attached the schematic and datasheets. Inputs 1 and 2 are PWM signals (5V max) attached to the NPN transistors to control how strong the current will be through the P-Type MOSFETs. Inputs 3 and 4 are logic on/off signals (0V/5V) connected directly to the N-Type MOSFETs to allow or stop flow. When one set of diagonal MOSFETs (left P and right N) are open, the flow through the 8 ohm solenoid is positive and negative when the other diagonal set is open. I am fairly certain that both sets of MOSFETs are being fed full gate-source voltage.

    Additional questions include:
    -Are the diodes across the MOSFETs redundant if the MOSFETs have a diode rated for the voltage/current
    -Are the N-Type MOSFETs necessary. They are close to ground and overheat more often than the P-Type.
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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    It may be it is just a poor choice of FETs. You will need a fairly large heat sink with the ones you have. P=I^2R so at 6 amps the FETs are dissipating about 7 watts.
    You could eliminate the diodes.
    You need the low side FETs.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    Yes, you should get MOSFETs with an Ron of ≤25mΩ (at Vgs = 5V for the N-MOSFETs) so they don't overheat without a heatsink a 6A (dissipating <1W).

    Note that the Vgs maximum is usually 20V or less for most MOSFETs so you need to change the gate resistance values for the P-MOSFETs to meet this requirement if you are operating from a 48V supply.
    For example changing the Rgs resistors to 2kΩ and the gate-collector resistors to 6kΩ for the PMOSFETs will reduce the maximum Vgs to 12V when they are ON (NPN transistors conducting).
     
    Last edited: Mar 3, 2016
  4. ScottWang

    Moderator

    Aug 23, 2012
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    Fore the 48V application, I did some modified, you can see them with blue color square frame.

    Hibridge-48V_Solenoid_dansh0_scottWang.gif
     
  5. dansh0

    Thread Starter New Member

    Feb 29, 2016
    3
    0
    This is all great feedback. Regarding the resultant power, why would the 7 watts overheat the N-channel FET if it is rated to 60W power dissipation at room temp?

    I made a poor mistake and vastly misunderstood the necessary Vgs term for the P-channel FETs. That will be an easy fix but I'm curious why this did not overheat those FETs instead of the N-channel ones that are usually the culprit. Is there a common symptom of exceeding max Vgs?

    Thanks!
     
  6. ScottWang

    Moderator

    Aug 23, 2012
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    When a voltage want to adding to the Vgs, if the Vgs exceed 20V then you have to using a zener to protect it , I will pick up a 12~15V zener to limiting the Vgs.

    Modified the values of resistors just make the circuit have a properly working voltages and current, the important thing is to choose a better mosfet which has a low Rds(on), you could get MOSFETs with a Rds(on) ≤20 mΩ or ≤10 mΩ and then it will reducing the chance to generate the heat, so you may no need the heatsink or just need a small one.

    You might choose an Ids rating current 3 times of what load or device needed and choose a Vds rating voltage 1.5 times of what load or device needed.

    If you can't get a logical level Vgs with a low Rds(0n) then you may try a Vgs(on) = 10V, some of them are working with Vgs(on)=5V still can get a great Ids current.
     
  7. ronv

    AAC Fanatic!

    Nov 12, 2008
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    It's a trick spec..:D The 60 watts is if you keep the case of the FET at 25C. But it heats up as time goes by. There is another spec for thermal resistance junction to ambient of 62.5 C per watt. So with 7 watts it would be 430C. Maximum operating temperature is 150C.

    To be honest, I'm surprised it didn't pop right away. FETs in an H-bridge almost always fail shorted unless the power supply is big enough for them to blow up.

    Thanks!
     
  8. crutschow

    Expert

    Mar 14, 2008
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    3,232
    It has nothing to do with whether the MOSFETs are P-channel or N-channel. Don't understand why you think N-MOSFETs fail more readily than P-MOSFETs. :confused:
    It's just a function of their Ron as specified in the data sheet.
    It's the current through that resistance that generates the heat and possible failure from overheating.

    The only effect of exceeding the Vgs maximum rating is break-down of the gate insulating oxide layer which causes transistor failure.
     
  9. dansh0

    Thread Starter New Member

    Feb 29, 2016
    3
    0
    I bought new MOSFETS and redid the voltage divider and the circuit works great now! Thanks so much for the help!
     
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