Need help choosing the right FET?

Discussion in 'The Projects Forum' started by electronice123, Jun 3, 2009.

  1. electronice123

    Thread Starter Senior Member

    Oct 10, 2008
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    I am trying to figure out how to pick the right FET. After reading and searching around a little it seems there are many to choose from and I don't know where to start.

    The circuit I am building uses a FET to switch 2 relays (12V coils with 100 ohms each) at a speed of 100kHZ-1.5MHZ.

    The FET gate voltage is 10V.

    Any help here?

    What is threshold?

    What do I need to look for when looking at datasheets?
     
    Last edited: Jun 3, 2009
  2. DC_Kid

    Distinguished Member

    Feb 25, 2008
    638
    9
    "coil" relays at 1.5MHz ???

    you need a FET that meets the specs of your circuit. P or N channel drive (from the sounds of it you need a N channel).

    Rds(on) is the drain-to-source resistence when the FET channel is conducting fully. the current Ids will determine how beefy a FET you need and with Rds will give you idea as to how hot it will get.

    look at the Vgs vs Ids graphs (http://www.irf.com/product-info/datasheets/data/irf1404.pdf), gives you a idea as to the "on" profile.

    for freq response take a look at Td and Tr.

    you'll also need to be careful driving the FET at that freq. the gates have capacitance which will yield a impedance, so make sure the driver can handle that impedance, etc.

    the coils are inductive, which means you need to protect the FET from the flyback voltage. a FET that is avalanche rated is good. recovery time of the internal zener might become important as freq gets higher, etc.
     
    Last edited: Jun 3, 2009
  3. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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    I agree with DC Kid, a mechanical relay is not going to switch at 1.5MHz? What type of part are you looking at? Can you use a transistor or FET as the switch itself?
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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  5. electronice123

    Thread Starter Senior Member

    Oct 10, 2008
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    0
    The relay I'm purchasing can switch up to 10kV at 1MHz, which might be higher than I even need to go. I'll probably be closer to 5kV but I won't know for sure until I get it all working.

    Thanks for the comments and help everyone, I really appreciate them!

    WOW, those relays are expensive!!!
     
  6. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
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    Just for our own education, would you mind posting the manufacturer and part number?
     
  7. DC_Kid

    Distinguished Member

    Feb 25, 2008
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    perhaps its a solid state relay??
     
  8. electronice123

    Thread Starter Senior Member

    Oct 10, 2008
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    It's a high voltage high dpeed vacuum relagy made by gigavac...

    http://gigavac.com/products/relays/datasheets/g20/index.htm

    I don't plan on driving the realy over 10kV as I don't want to get anywhere near X ray emission!

    I just need to find a FET than can switch a 12V signal at 1A and 1.1MHz.
     
  9. hgmjr

    Moderator

    Jan 28, 2005
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    I think you are confusing the frequency of the signal that the relay can switch/pass with the rate at which the relay can be switched on and off. The activate and release times are on the order of tens of milliseconds. It is a mechanical relay afterall.

    Pretty much any MOSFET should be up to the task as long as it handles the coil current and has a decent RDSon.

    hgmjr
     
  10. electronice123

    Thread Starter Senior Member

    Oct 10, 2008
    302
    0
    Yep your right. I thought the switching frequency was the speed the relay could switch at, not the frequency traveling through the relay.


    The acutal frequency is going to be under 10kHz. What I need is to find a way to switch a 10kV signal on and off very fast?

    One of the relays I was looking at had an operate time of 15ms, and a release time of 9ms. Which only turns out to be 20Hz.

    Now I'm wondering how this could be done? What I am trying to do here is switch the output voltage of a high voltage transformer very fast. Positive then negative then positive etc to the load (capacitor). Just for a physics expermient.

    Now I'm thinking maybe a spark gap???
     
    Last edited: Jun 6, 2009
  11. DC_Kid

    Distinguished Member

    Feb 25, 2008
    638
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    if there is load on that 10kV (which there will be with a cap there) the relay contacts will not go open as fast as you think they will. your best bet is to go solid state. this is 10kV AC 60Hz ??
     
  12. electronice123

    Thread Starter Senior Member

    Oct 10, 2008
    302
    0
    The transformer actutally supplies 10kV DC. (It's a special type of transformer) The positive and negative 10kV DC output is what I want to pulse on and off alternately. The reason being is I need to use the positive and negative voltage at different times at high speed, not both together.

    How can this be done solid state?
     
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