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smartsarath2003
Joined: 12 Feb 2004
Posts: 75
Helped: 2
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 03 Oct 2004 18:17 Help with Mosfet Biasing |
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Hai,
I had a problem using mosfets. I am new to them and I tried use N-channel Enhancement mode mosfet(IRF540). I am attaching a rough circuit blocks as well. When i connected the gate to drain, i had approximately Vds=1-2V @5V Vin. I tried to connect the gate in voltage devider bias by using another 10k between ground and gate. But no effect. Rds(on) is 0.044R from data sheet.
Also The reason for me using the mosfet instead of Transistor is faster switching action. I red somewhere that all the terminals have some cepacitance between each other. if they have some cepacitance, won't this slow down switching  . Also does the current direction reverses in case of Depletion mosfets(S--> D) when compared to Enhancement mosfets (D-->S).Can anyone please clarify my doubts plz
Thanks
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onemilimeter
Joined: 23 Sep 2004
Posts: 65
Helped: 2
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| 04 Oct 2004 4:04 Help with Mosfet Biasing |
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| The Vgs (voltage difference between gate and source) should be above a voltage threshold to turn on the mosfet. Remember, it's the voltage at gate referred to the voltage at source, NOT GROUND/COMMON. If you drive your mosfet for low-side application (i.e. load at high-side), then the source is usually the ground/common of the circuit. However, as shown by your circuit, your mosfet is at high-side, so your driving circuit needs to be modified, in my opinion.
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flatulent
Joined: 19 Jul 2002
Posts: 4856
Helped: 292
Location: Middle Earth
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| 04 Oct 2004 5:29 Re: Help with Mosfet Biasing |
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It looks like you are making a constant current source or current limiter. You will need to use a P type MOSFET for the series pass transistor and put the source to the input and the drain to the output.
This is a good situation where SPICE type simulators are very good because you can examine the simulated currents. PSpice has a free student version for downloading that is good for this.
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smartsarath2003
Joined: 12 Feb 2004
Posts: 75
Helped: 2
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| 05 Oct 2004 23:26 Re: Help with Mosfet Biasing |
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| I intially thinking about using a power transistor(npn) like BD677. But I used mosfet because switching is bit faster and don't need any biasing resistance. If use Pchannel MOSFET, it may goes off for a small leakage current or off always .
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