Power MOSFETs

[audioguru2]

Power MOSFETs have a very low resistance from source to drain when switched on. But they also have a high gate capacitance. They can swich very quickly if their drive charges their gate capacitance fast enough.

Is it true that the chip is simply thousands of little MOSFETs connected all in parallel?

 

[Kevin Weddle]

A high gate capacitance can be a misleading topic. I don't think that it actually charges like a capacitor, but exhibits the frequency response. I may be wrong. What is true is that the gate capacitance will keep the VGS from developing at a higher frequency. By the way, isn't the transfer characteristic of a MOSFET just nice. I like the way it keeps some predictability to the signal.

 

[Cuckoo1]

Hi everyone.
Power MOSFETS do consist of many little MOSFETS connected in parallel. These are called "cells". For given die dimensions, companies will try to fit as many cells as possible (they connect in parallel so as to drop the drain - source resistance), while keeping a reasonable gate capacitance. What changes between companies is the cell structure (almost every company has patented their own - VMOS, SIPMOS, HEXFET etc), which will finally determine the gate capacitance as well.
As for how the "gate capacitance" charges, it does so pretty much like a capacitor (the structure metal/polysilicon - SiO2 - Si, IS a capacitor), but does not follow exactly the curve. That's because there are other parasitic capacitances (gate - source, gate - drain, gate - body etc.) which also charge giving a slightly different curve.

You might want to check out here:
http://www.powerdesigners.com/InfoWeb/design_center/articles/MOSFETs/mosfets.shtm
for a more descriptive presentation on the topic.

 

[audioguru2]

Hi Cuckoo,
Thanks for confirming my theory.
Good article.

 

[Kevin Weddle]

What do you suppose the gate capacitance amounts to. If they say it's in picofarad range, that is real low. I have seen picofarad capacitors and they seem much larger than the would be gate capacitance. So I wonder if the specification is intended for calculation purposes only. That is the voltage loss and behavior at higher frequencies.

 

[audioguru2]

Kevin,
Look at the datasheet for any power Mosfet and you'll see that its gate capacitance is thousands of picoFarads. Such a high capacitance requires a high current gate driver to charge and discharge the capacitance quickly, so that the Mosfet doesn't heat too much during its switching transitions.

 

[Yevgenip]

I read an article (Also in Scientific American) about Super-Transistors. It talked about super-quick transistors and extremly high-voltage resisotrs. 8)
I'll post the article if someone is interested.

 

[trigger]

Hi all,

Check this out....... may be useful for this topics

http://www.edacafe.com/books/ASIC/Book/CH03/CH03.2.php

(or the whole free ebook at http://www.edacafe.com/books/ASIC/ASICs.php)

 

[Dazza]

Hi Yevgeni P, yes I would be very interested in reading that article, please do post .

 

[Yevgenip]

I have a few trouble uploading, I'll post it soon then.

 

[Yevgenip]

Man, Why won't it send it??? >

 

[Yevgenip]

Lets try this step by step:
-One:

Facts_about_the_Transistor.pdf

 

[Yevgenip]

Two:

From_Sand_to_Silicon_-_Manufacturing_an_Integrated_Circuit.pdf

 

[Yevgenip]

Three:

How_the_Super_Transistor_Works.pdf

 

[Yevgenip]

Four:

Microprocessors_in_2020.pdf

 

[Yevgenip]

Five:

The_Future_of_The_Transistor.pdf

 

[Yevgenip]

Six!!!:

The_Transistor.pdf

 

[Yevgenip]

At Last!!!! I am victor!!!! ;D

 

[Dazza]

Thanks Yevgeni P ,

very interesting, good stuff .