Paralleling MOSFET's?

Can you parallel 3 MOSFET's by wiring Q1s gate to Q2s source then Q2s gate to Q3s source, parallel the sources and drains of all 3 MOSFET's together then add some diodes (the diagram is below)?

They will be oscillating so no balancing resistors are required.
 

Picture of Paralleling MOSFET's?
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YOu need LOTS, like potentially amps of gate drive to get the things saturated and ON, HARD AND FAST. Only once the gate is saturated will the things play nice in parallel.

Likewise you need to suck the charge out very quickly when they want to turn off - this is why you can buy specific gate drive ICs

Steve
The MadScientist (author)  steveastrouk5 years ago
I thought by wiring the gates in series I would avoid large gate drive current due to lower capacitance but higher voltage.
No, you can't do that !
The MadScientist (author)  steveastrouk5 years ago
What's wrong with it?
You can't wire them in series, you haven't now. Work it out. What happens when the top transistor turns on ?
The MadScientist (author)  steveastrouk5 years ago
Well if it turns on individually then it would take all the current but it shouldn't so I can't see anything wrong.
Well look, Vgs for M1 is 30V. Vgs for M2 is ?????????

Yes, that's right, I know you looked, its 0.6V, because as you've seen the diodes clamp Vgs to 0.6V.

So, dumb idea. Its can't be done like that.

Steve
The MadScientist (author)  steveastrouk5 years ago
The +30V square wave pulse would be from a different power source from the source drain power so the +30V would be across all three MOSFET's.
Give it up: it won't work. Just do it the way that works. The way you have it doesn't put the gates in series. D1 guarantees the "separate" supply is tied 0.6 volts from the main supply.

What have you got against doing it right ? You can't eliminate that actual gate capacitance: charge has to be moved in and out at speed for a good switch.
The MadScientist (author)  steveastrouk5 years ago
Ok.
It won't help. You need CHARGE in the gates of each device.
iceng6 years ago
Mosfets are inherently stable in parallel as Andy points out here is why ;

Unlike a bipolar transistor if a fet draws a bit more current it heats a bit more
and its resistance increases a bit reducing its current automatically.

Just like placing series resistors with paralleled bipolar NPN or PNPs which as
they heat up try to hog more current until cascade destruction.

A
No, no, no. MOSFETS Are ONLY "stable " if they are fully saturated !!!!!!!

They DO NOT play nice in linear modes, only in switching modes.

Steve
I do NOT have experience paralleling mosfets in a linear (unsaturated) mode.
So I am building a test on a load greater then each device capacity to see
if they will hog  and die or balance and live. 

I want to believe but need the confirmation before my eyes.

The Test will observe the current in each fet where the gates and suorce
pins are combined and adjusted to half such that if one hogs it will die in
a cascade fail mode.

Any thoughts may be useful before I begin.

A
Yes, don't. You'll kill your mosfets.....
This is from bitter experience of being told that Mosfets work fine in parallel.....
I am well stocked :-)
NO MOSFETS ARE NOT INHERENTLY STABLE !!!

They are ONLY stable if switched in full satyuration.

Steve
True, but I was thinking at the 2K amps kind of level. For piddly loads like 20, I agree with you, its not worth the hassle.
AndyGadget6 years ago
 
The usual way of connecting MOSFETs in parallel is to just connect the G, S and D of each to the corresponding pin on the others.  This will triple your overall gate capacitance so the driving signal should be meaty enough to get them switching hard on and hard off as rapidly as possible.