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ron
Joined: 24 Dec 2001
Posts: 30
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 10 Oct 2004 17:55 FET Gate Protection ??........ |
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Hi All,
I've got to drive a BSS138 FET Gate from an external source, via 2-3 feet of wiring, and inside a machine consisting of a couple of noisy mains motors and their mains wiring etc.
Originally I didnt bother with any Gate protection but found spikes were blowing them whenever the mains motors started up. I just had a 1k pullup resistor to 12V at the FET end at this stage.
I've have now added a 10k series gate resistor, a single zener diode from Gate to Source at the FET end, a 100nf cap to ground where my external cable comes into the FET end pcb, and all seems OK so far.
My question is though, is a standard zener diode sufficiently fast enough for both positive and negative spike protection on my Gate ??? Can I be rest assured that my FET is now safe ! ????
Any FET experience appreciated
Thanks in Advance
Ron. 
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VSWR
Joined: 07 Feb 2002
Posts: 640
Helped: 53
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| 10 Oct 2004 19:30 Re: FET Gate Protection ??........ |
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An RC-type lowpass filter in combination with a zener diode is often a sufficient FET gate transient protection. In your case you have a time constant of at least 10 kohm * 0.1 uF which gives 1 ms. Your zener diode has probably a switching time of less than 1 us (depending on type). This gives a a safty factor of at least 1000, as the RC-network slows down the rise time of any transients to a low value which the zener easily can handle.
To verify the effect of your FET gate protection, I suggest you connect a memory oscilloscope across the zener. Then start up and shut down the machine a few times while watching the oscilloscope.
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ArchAngel
Joined: 01 Jul 2004
Posts: 49
Helped: 14
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| 06 Dec 2004 11:01 FET Gate Protection ??........ |
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| one zener diode is not enough for both positive and negative spike protection, u need another one from vdd to gate
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moda
Joined: 05 Dec 2004
Posts: 67
Helped: 4
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| 06 Dec 2004 13:24 Re: FET Gate Protection ??........ |
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another option is:
2 diode (like 1n4148) will do the work for you.
One from Gnd (A) to the gate.
2nd from the gate (A) to the Vdd
add the RC network and you are safe now.
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yaxazaa
Joined: 13 Nov 2004
Posts: 116
Helped: 3
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| 07 Dec 2004 8:05 Re: FET Gate Protection ??........ |
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| It will be safer if you use four diode as ESD prection devices on both sides, two to GND, two to Supply
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