MOSFET problem

Discussion in 'General Electronics Chat' started by Tobias, Sep 19, 2009.

  1. Tobias

    Thread Starter Active Member

    May 19, 2008
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    I am using two MOSFETS in parallel to drive four solenoids. The solenoids when turned on take about 40 amps at 16v. Each mosfet/solenoid combination is called a stage. I have four stages I am switching. I have a voltage-in busbar that is 0.080" thick and 0.500" wide that the Source pins on the MOSFET are soldered. I calculate the busbar to handle 465 amps

    When I turn on one stage everything is happy. When I activate the second stage, the solenoids act like they aren't getting full voltage.

    The spec sheet on the MOSFET I am using can be found at this link http://www.vishay.com/doc?73493

    I am pulling the gate directly to ground. Looking at the spec sheet the VGS is -2/-4 min/max. The MOSFETS turn on and off just fine without solenoids connected. Its seems to be a load deal when the second is activated.

    Any ideas?
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
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    Can you post a schematic so that we can better see what the circuit looks like?

    hgmjr
     
  3. Tobias

    Thread Starter Active Member

    May 19, 2008
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    Sure thing, attached are two PDF files. A schematic and board drawing. The MOSFETS are on the top left area of the schematic. The busbars are on the left side of the board drawing. The busbars and MOSFETS are on both sides of the board.
     
  4. Tobias

    Thread Starter Active Member

    May 19, 2008
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    One thing to mention. This board activates nitrous stages. I sent an identical board to a nitrous manufacturer. They put it on their flow bench and activated all four stages simultaneously for fifteen seconds several times.

    I am going to call them monday to inquire about their power supply. I am sure its pretty damn close to the flow bench. In this race car, the battery is atleast ten feet of cable away.

    Could this possibly be a problem with the supply current/wire gage? Do MOSFETS act goofy when current is lower than required by the device needing the power?
     
  5. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    T3 on you schematic has the gate and drain pins swapped. Also you are using FETs and not MOSFETs.
     
  6. Tobias

    Thread Starter Active Member

    May 19, 2008
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    Here is the datasheet
    http://www.vishay.com/doc?73493

    T3 is really unused. I am associating it with the busbar. Its something I had to do with my PCB software to get the busbars on the board.
     
  7. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The threshold voltage of the Vgs of your Mosfets is 2V to 4V. The datasheet shows that they barely turn on and conduct only 0.25mA.
    The datasheet shows that they turn on properly when the Vgs is 10V.

    Your Mosfets look goofy like Jfets and are connected oddly. I don't see anything that will turn them on.
     
  8. Tobias

    Thread Starter Active Member

    May 19, 2008
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    The MOSFETS do turn on and off. Maybe the schematic symbol isn't for MOSFETS but the part number I am using and the spec sheet for the part number says its a MOSFET.

    I am attaching a PDF thats a screen shot of the data from the racecar. The blue trace is the battery voltage. Its a 16v system. The controller activates the first stage right when the car starts to move. The green trace is driveshaft rpm and the red trace is engine rpm.

    When the solenoids are triggered notice the spikes in the battery voltage. The spikes get more frequent as the car goes down the track. The car doesn't have an alternator. The data logger logs fuel pressure at both stages. I won't bore you with details yet with these two channels I can see that the controller is activating the first stage but never actually activates the second stage.

    Today I hooked a scope up to each stage. Without the engine running, fuel pump on and water pump, the outputs are fine and trigger both sets of solenoids just fine. I even recorded a test with the data logger to look at voltage levels the logger was recording. Its fine.

    So I am thinking this is either an electrical noise problem screwing up my PIC or the cable supplying power to the controller isn't big enough gage to handle the load. Tuesday I will be able to fire the engine with my scope hooked up to everything.

    One other thing I noticed looking at the data, about two seconds after the finish line when the car is slowing down, the solenoids activate for about a tenth of a second.
     
    • Data.pdf
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  9. andrewortman

    New Member

    Jul 4, 2009
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    Where's the PIC's power supply come from? A good idea would be to filter the living crap out of the power supply and move the logic units onto another board. Those things are quite sensitive to EMI.
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
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    I don't see a power supply (Vdd?) connected to the sources of your MOSFETs. Am I blind, or is that a schematic error?
     
  11. Tobias

    Thread Starter Active Member

    May 19, 2008
    158
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    PIC power supply is from an AP1117 5v 1a regulator. I have a 0.1uf on the input and 1.uf on the output.

    The source for the MOSFETS isn't vdd, its VIN (16v). I don't tie the VIN going to the AP1117 to the VIN part that is for the sources. So yea its kind of an error. Yet not on the board because the busbars are tied to the VIN trace.

    The collector is pulled high to VIN with a 10k resistor. When the transistor's base goes high, the collector goes to ground. This takes the gate low and the drain outputs the 16v
     
    Last edited: Sep 20, 2009
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