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Need help choosing a Power MOSFET

  1. Feb 4, 2009 #1
    I need help finding a MOSFET to buy that can handle max power of 30W, and that can achieve a resistance of 1-25ohms (using it as a voltage controlled resistor).

    Microsemi originally pointed me to these 2 of theirs:
    http://www.microsemi.com/catalog/part.asp?ID=21208
    http://www.microsemi.com/catalog/part.asp?ID=21223

    Which would work perfectly,
    but the cost was $75 each with minimum of 20, and 16 week leadtime.

    I know most power mosfets arent nearly as expensive, and with much shorter leadtimes.. but I need help picking one that can achieve the above specs.

    Help is greatly appreciated
     
  2. jcsd
  3. Feb 4, 2009 #2

    dlgoff

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  4. Feb 4, 2009 #3
    I guess i need more help with picking out a specific one based on the application (has to handle 30W, and be able to create a resistance across the Drain/Source of 1-25ohms).
     
  5. Feb 4, 2009 #4
    You can ask microsemi for samples.

    Not being an expert on semiconductor, I believe just about any off the self Power MOSFET will fit that criteria you need of 1-25 Ohms. Correct me if I am wrong, the spec that manufacturer gives usually is maximum R(on) resistance, which is in the miliohms. This is the resistance between the Drain and Source when the channel is present for current to flow. When the FET is off, channel is not there, the resistance is infinite (or at least several megaohms). So you can see how the resistance range of 1-25 ohms will fit in the range of milliohms to megaohms.

    Your using the FET for matching impedance to get the most power transfer right? I was thinking about when you first posted your problem. I have a rough idea and want to see what your thoughts are. Others are welcome to chip in of course.
     
    Last edited: Feb 4, 2009
  6. Feb 4, 2009 #5
    yes Corneo that is right (about our application).
     
  7. Feb 4, 2009 #6
    Here's my idea's

    http://www.flickr.com/photos/35159644@N08/3255121034/

    The FET is irrelevant as long as you chose the right one based on the voltage source you got. So if your voltage source is positive, then you need an N channel FET. Since you know that to get maximum power transfer, the load (FET) has to be equal to the source resistance (Rs = 5 ohms in my example). When this occurs you have an even voltage drop as per voltage divider rule. The op amp driving the gate of the FET will automatically find the right gate voltage to set the drain source resistance to match Rs.

    The reason I chose something like this is because I know based on my experience that pinchoff voltage on FETs vary several volts as per manufacturer's data sheet. One date code of FETs can have several volts difference from another date code. To make it work for all FETs, you need an circuit to automatically find the right voltage for you.

    Since I don't know what exactly are your specs to meet your project, you should make sure the FET you chose can handle the power dissipation and maximum voltage on the drain. Make sure the resistors are chose properly as well.
     
  8. Feb 6, 2009 #7
  9. Feb 6, 2009 #8

    dlgoff

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    I believe this diode is due to the MOSFETs construction which prevents spurious latching. It's called the body diode.
     
  10. Feb 6, 2009 #9
    Why one FET when more can share the load? Distributing the load means you don't need an unrealistic heat sink.

    You don't care much about the intergral body diode unless you want to block back-current, but what voltage??

    Voltage plays a major role in price. I'm seeing unit prices around 60 cents per thousand, not 60 dollars.
     
    Last edited: Feb 6, 2009
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