Gate-source voltage needed to switch a FET
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 8th January 2007, 06:27 PM #1 rtarbell   diyAudio Member   Join Date: Sep 2005 Gate-source voltage needed to switch a FET According to most datasheets of powerFETS, the suggested gate-source voltage to switch a FET into full "ON" mode is at least 10V, while the max gate-source voltage is 20V (as listed under absolute maximum ratings). ==> Is there much of a difference between switching a FET using a gate-source voltage of 12V, as opposed to 15V, as opposed to 18V? I know the higher the Vgs, the faster the FET switches (and thus switch losses can be reduced), but is there a "law of diminishing return" here? I don't have much practical experience with switching FETs; I wanted to ask before I try anything dangerous...
 8th January 2007, 06:35 PM #2 N-Channel   diyAudio Member     Join Date: Mar 2003 Location: Sol System I would say a good Gate-Source voltage would be 15V. This way, no problems.
jan.didden
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Join Date: May 2002
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Re: Gate-source voltage needed to switch a FET

Quote:
 Originally posted by rtarbell According to most datasheets of powerFETS, the suggested gate-source voltage to switch a FET into full "ON" mode is at least 10V, while the max gate-source voltage is 20V (as listed under absolute maximum ratings). ==> Is there much of a difference between switching a FET using a gate-source voltage of 12V, as opposed to 15V, as opposed to 18V? I know the higher the Vgs, the faster the FET switches (and thus switch losses can be reduced), but is there a "law of diminishing return" here? I don't have much practical experience with switching FETs; I wanted to ask before I try anything dangerous...

The data sheet also shows you characteristics of Id vs. Vgs for a given Vds. You can see that the Vgs actually controls the Id. So, you really have to be sure you have enough gate drive for the expected Id you want to switch. Err on the plus side: too high Vgs doesn't harm (as long as you stay below the max; better use a zener shunt here!).
If the 10V Vgs is enough to turn it hard on, there's not much point in going higher. Going higher does NOT turn it on faster: it turns on faster if you can sink more current into the gate capacitance, so use a driver with (relatively) high currrent capacity, at 10 V.

Jan Didden
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 8th January 2007, 07:21 PM #4 N-Channel   diyAudio Member     Join Date: Mar 2003 Location: Sol System Yeah, that's what I meant!
jan.didden
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Join Date: May 2002
Location: The great city of Turnhout, BE
Quote:
 Originally posted by N-Channel Yeah, that's what I meant!

... but it's not what you said

Jan Didden
__________________
Music is dither to the brain; lets me think below the usual chaos - me
Get more Linear Audio for less! Check out my Autoranger and SilentSwitcher

 8th January 2007, 08:43 PM #6 batee   diyAudio Member     Join Date: Mar 2004 Location: Rolla, MO High Side FET drive... Here's what I did for a FET switching in a motor control project, but it's pretty generic. Works for 0-100% PWM duty cycle. http://www.diyaudio.com/forums/showt...81#post1088581
 8th January 2007, 09:28 PM #7 batee   diyAudio Member     Join Date: Mar 2004 Location: Rolla, MO Schematic and board here: http://www.diyaudio.com/forums/showt...90#post1088590
 8th January 2007, 09:48 PM #8 sawreyrw   diyAudio Member   Join Date: Apr 2006 Location: Minnesota A Vgs higher than 10 volts will turn the MOSFET on faster (with a given gate resistance) because the effective gate capacitance will be charged faster. However, it will take longer to discharge this same capacitance, so the total turn off time will be longer. In addition, the power to drive the gate will increase. All in all, it's best to use the vendors spec. which is typically 10 volts.
 8th January 2007, 11:25 PM #9 Eva   diyAudio Member     Join Date: Oct 2003 Location: Near the sea A higher Vgs usually produces lower Rds (or lower Vce for IGBT) and thus lower conduction losses for any given current, however, the improvement is not always worth the effort or the risk of going near the 20V limit (datasheet has the last word on that). I routinely use 19V gate driver supplies in most of my circuits. __________________ I use to feel like the small child in The Emperor's New Clothes tale
 9th January 2007, 10:27 AM #10 VEC7OR   diyAudio Member     Join Date: Jul 2003 Location: Vilnius Yes, and reduces useful life of fets, also you have to suck more charge out of gate. IMO anything over 15V is overkill, but again depends on fets. __________________ Profanity took a very interesting form in Russia where there exists a language of sorts, most of its words based on four basic profane roots - nouns p...is, wh.0.e, c..t and verb f..k. It is possible to sensibly communicate using just these four basic roots.

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