Help selecting a MOSFET

Discussion in 'General Electronics Chat' started by HyGear, Jul 22, 2011.

  1. HyGear

    Thread Starter New Member

    Jul 21, 2011
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    I am a mechanical engineer with very little electrical background, but I was recently asked by my boss to design a simple controller board for a piece of agricultural equipment because we don't have any electrical engineers. I have already had prototypes of the PCB made, but I'm having trouble with getting them to work properly. Basically, I have input from a sensor that feeds into a series of logic gates. The logic gates decide which of two MOSFETs to activate and the MOSFETs activate a set of relays. All of the logic on the board is working correctly and it is sending a 3.6V signal (from a SN74ls08 AND gate) to the MOSFETs, but the MOSFETs won't activate.

    Unfortunately I can not share my schematic on this website, but I can tell you that I used this MOSFET:
    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=FDD6N50TM_WSCT-ND

    Also, I connected the relay similar to this (minus the LED):
    http://honeyrunapiaries.com/blog/2006/blog/honey-sticks/honey-stick-machine-part-2a/

    Can anyone tell me what I am doing wrong? I really don't know the proper way to choose a MOSFET so I have a feeling I may have picked the wrong type or specifications.

    I appreciate any feedback you guys can give me!
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The main problem is that the gate voltage is to low to turn the mosfet on.
    Can you post the schematic?

    Bertus
     
  3. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Yes, Gate Threshold Voltage of this MOSFET is between 3V and 5V.

    You may use other gates that give you a higher voltage signal, or you put a transistor at the output to increase the voltage (if you have another power supply available)
    Why is the voltage 3.6V? The 74LS08 should give you a 5V output (can still be too low)
     
  4. HyGear

    Thread Starter New Member

    Jul 21, 2011
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    @bertus
    Sorry, but I can not post the schematic because it has proprietary information on it.

    @praondevou
    If I am reading the datasheet correctly, the SN74LS08 has a "typical" output voltage of 3.4V so I think the 3.6V is ok.
    http://focus.ti.com/lit/ds/symlink/sn74ls08.pdf

    As far as the MOSFET goes, I'm still not clear on the difference between the Vgs(th) and Vgss. In the spec sheet for the MOSFET I chose, it says Vgs(th) is between 3-5V but for Vgss it gives a value of 30V. Does this mean that the minimum turn on voltage is beteween 3-5V and the maximum is around 30V? If so, should I choose a MOSFET with a Vgs(th) of say 2-3V?

    Thanks!
     
  5. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    You're right. I hadn't seen this. Can be as low as 2V for minimum Vcc.

    Is 5V all you have available as a power supply for your circuit?
    Yes, the threshold can be anything between 3V and 5V, so your input signal has to be bigger than 5V. Vgss is the absolute maximum rating. If you exceed this, the transistor gets damaged.
    Yes, you could also use a MOSFET with less gate voltage requirements, I've seen some with thresholds of 2V max. but they were no power mosfets and tiny SMD.

    If you want to find another mosfet, what's the ratings you need? SMD or through-hole?
     
    Last edited: Jul 22, 2011
  6. HyGear

    Thread Starter New Member

    Jul 21, 2011
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    In terms of power supply, I have 12V from the battery and 5V from the onboard voltage regulator.

    I'm looking for a surface mount MOSFET in a D-PAK package that can switch on either a relay (500mA) or a solenoid (3A). The main reason for this is that I'm not sure if my board can handle driving the solenoids directly due to the heat it will generate. If all else fails I will drop back to using the relays.
     
  7. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    You could also use a CMOS IC like the 4081, output voltage goes almost rail-to-rail, but it's not the same pinout, and in worst case it would still not work, if you had a mosfet whose gate threshold voltage is 5V.

    Either you find another mosfet or you increase your power supply voltage.
     
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  8. HyGear

    Thread Starter New Member

    Jul 21, 2011
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    Thanks for the info. You guys have been a great help!
     
  9. praondevou

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    Jul 9, 2011
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    Last edited: Jul 22, 2011
  10. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Here is another one: NID9N05CL from ONSEMI. Actually if you google for "low gate voltage mosfet d-pac" you will find several of them.
     
  11. HyGear

    Thread Starter New Member

    Jul 21, 2011
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    I didn't see your last two posts, but I ended up going with this MOSFET:

    http://www.fairchildsemi.com/ds/FD/FDD8451.pdf

    My problem is that the MOSFETs are still not activating even with the lower threshold voltage. On one of my prototype boards I even connected a jumper wire from my 5V (500mA) supply to gate and it still doesn't activate.

    What am I doing wrong? Is it possible that my board manufacturer is frying these things by accident?
     
  12. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    @HyGear - You chose another mosfet with a 10V GS voltage. The Voltage GSth is where the mosfet just barely turns on. Ignore that voltage in the data sheet for most cases. The one you should be concerned with is on the first page of most data sheets.

    What you should be looking for is called a "logic level" mosfet.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    What you really look for is the voltages specified with Rds(on), that MOSFET does have a specification for Vgs=4.5v of 30m Ohms.

    Are these MOSFETs being pick and placed, or manually soldered in?
    Are they being handled using proper ESD protection measures? If they are being assembled in a rather dry environment, static electricity can zap them without a person even noticing. It only takes ±30v across the gate and source terminals to kill most MOSFETs, and you don't really notice a shock from static until it's up around 3kv.
     
  14. HyGear

    Thread Starter New Member

    Jul 21, 2011
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    Thanks for the info guys. I really wish I hadn't been stuck with this little project because of my lack of electronics experience, but on the bright side I'm learning quite a bit.

    @SgtWookie: The original boards were picked and placed but the new MOSFETs appear to have been soldered by hand since they are slightly skewed. Also, as far as the Vgs voltage goes, it should turn on at 4.5V right? I'm sorry if this is a stupid question but I'm still trying to fully understand all of these MOSFET specifications.

    Another issue that I am having is that I'm getting really low voltages out of my AND logic gate (7408 chip), but I'm not sure if that is because of the wrong MOSFET or if something else is wrong on my board. My Vcc voltage is exactly 5V and I am using 3 of the 4 AND gates. On 2 of the gates the output voltage is around 3.6V but the 3rd gate (which is connected to the MOSFET) is only putting out 1.8-2V. Would this low voltage be due to wrong MOSFET?
     
  15. SgtWookie

    Expert

    Jul 17, 2007
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    Well, if a day went by without learning new things, it would get rather boring in a hurry. ;)

    I see. If it's a decent house, they should have been following ESD procedures.

    That's a tough one. They are supposed to follow closely what's in the datasheet. If you look at the datasheet again:
    http://www.fairchildsemi.com/ds/FD/FDD8451.pdf
    Page 3, figure 2 on the upper right. I don't know offhand how much current your relays/solenoids need in order to energize. However, looking at the Vgs=3v line, the normalized drain-to-source resistance should be about 2.3 times the specification with a drain current of 5A. However, 3v is also the maximum threshold voltage; where Vgs=Vds and Id=250uA. The situation improves when Vgs gets above 3.5v.

    Since you are attempting to use these parts when Vgs will be less than 4.5v, you really need to either test each MOSFET prior to installation to see if they will work, or go with yet another MOSFET that has an Rds(on) specification given with the Vgs your driving IC can output, or else use a MOSFET gate driver IC.

    That's not good. It sounds like the MOSFET may be shorted from gate to drain.

    You never mentioned how long these MOSFETs were going to be turned on for. If the cycle time is fairly slow (say, operating at less than 100Hz) then the drive current is not critical.

    Also, you didn't mention if the output driving the MOSFET gate is driving anything else. The original 74nn's had a rather limited fan-out due to the high current requirements. The 74Lnn's reduced the power requirements while improving the fan-out if they were driving other 74Lnn IC's If you're mixing 74nn and 74Lnn's, you could be having problems.

    Not being able to see the schematic certainly makes things more difficult.
     
  16. HyGear

    Thread Starter New Member

    Jul 21, 2011
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    The solenoids draw about 3A and if I drive the solenoids with relays it will draw about 0.5A. Also, the switching time should be no more than about 0.5 s because of the sensor I am using.

    I really don't want to screw this up again so I was wondering if you could recommend a MOSFET that will work for my situation. I'm thinking something like this one:

    http://www.vishay.com/docs/91324/91324.pdf

    but I'm not sure because I'm still getting confused by all of the parameters.
     
  17. SgtWookie

    Expert

    Jul 17, 2007
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    I'm sorry, I'm fresh out of time for the moment.

    Perhaps later.
     
  18. Adjuster

    Well-Known Member

    Dec 26, 2010
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    This is a professional job, not hobby or school work, but for security reasons you cannot reveal full details of the task to would-be advisers on this forum. You say that you have little knowledge of electronics, and require guidance on selecting suitable components.
    It would therefore seem quite possible that there is some basic error in the circuit configuration, such as not having the FET in common source, not having back-emf protection...or many other possibilities. If this were the case, then even the most ideally specified device might not give you the required results

    Unfortunately, nobody can know this if you have to keep the details secret. Given these circumstances, I would strongly suggest that you obtain help from a competent local electrical engineer, who can be made to agree to non-disclosure of your proprietary information.
     
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