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Old 23rd April 2007, 11:39 AM   #1
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Default MOSFET input capacitance

Some while ago I did a few interesting measurements on the input capacitance of a couple of different MOSFETs as a function of Vgs. Following a discussion of the matter in another thread

http://www.diyaudio.com/forums/showt...54#post1190954

I post here my measurements results and copy some of my comments I've made in that thread.

What I did was to measure the Cgs capacitance with a capacitance meter (1MHz 10mV max) sweeping the Vgs from 0 to Vth and above, leaving the drain connected to the source through a resistor (Vsd=0, Id=0) . Tomorrow I will show my data for a small VMOS from Zetex ZVH0545A and a logic-level power VMOS IRL530N. I've found that the static gate capacitance measured this way is indeed increases over the threshold region by 2 to 3 times for a very small increase in Vgs.

Apparently this change has a very simple explanation - when the channel of the MOSFET is off, the area of the channel itself under the gate is essentially isolated and Cgs+Cgd consists only from a capacitance between the gate and the metallised source/drain connections. When Vgs reaches the threshold level the channel begins to conduct and the area beneath the gate connects to the circuit, markedly (2 to 5 times) increasing the input capacitance even without the Miller effect - in fact even in the abcence of any drain current! If you put some voltage on the drain it somewhat changes the shape of the curve, but not the character of it.

As you can imagine in an audio amplifier there are many cases where the Miller effect is not really present - i.e. in a common drain follower configuration of the O/P stage. However in a class AB amplifier the Vgs of the output devices goes through the threshold region and that kind of a change in the input capacitance of a MOSFET could be quite significant in understanding of unlinearities in such a stage, especially at high frequencies.

Cheers

Alex

First picture is for ZVN0545A @ Vds=0
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Old 23rd April 2007, 11:41 AM   #2
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Here is ZVN0545A @ Vds=5V
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Old 23rd April 2007, 11:43 AM   #3
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ZVN0545A drain current corresponding to the graph above @ Vds=5V
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Old 23rd April 2007, 11:44 AM   #4
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And here is the graph for IRL530N
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Old 23rd April 2007, 11:46 AM   #5
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Good work, but if you are applying a voltage across gate-source, how can you be sure that driving equipment is not affecting the measured capacitance which is also across gate-source?
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Old 23rd April 2007, 11:52 AM   #6
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Quote:
Originally posted by richie00boy
Good work, but if you are applying a voltage across gate-source, how can you be sure that driving equipment is not affecting the measured capacitance which is also across gate-source?
Here is the schematics for my setup for 0V Vds:

Cheers

Alex
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Old 23rd April 2007, 12:03 PM   #7
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Here is one of very few similar measurements out of a manufacurer's datasheet:

this one for 2SJ162

Cheers

Alex
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Old 23rd April 2007, 12:13 PM   #8
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Alex,

London city is too far to throw an IRFP140N which is in my hand Have you power IRFs?
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Old 23rd April 2007, 12:18 PM   #9
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Quote:
Originally posted by anli
Alex,

London city is too far to throw an IRFP140N which is in my hand Have you power IRFs?
Unfortunately, no. I may order few just for fun but it would not be very soon. I do have a number of other MOSFETs but all these are small to medium power and not IRF

Alex

P.S. However I think that this kind of the input capacitance behaviour is likely to be generic for all MOSFETs.
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Old 23rd April 2007, 03:01 PM   #10
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International Rectifier has an application note AN-957 "Measuring HexFet Characteristics" which you should take a look at -- you don't need a Tek 576 to do this.
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