Paralleling mosfets vs. 1 big mosfet

[Bill French]

Hello, i'm trying to learn about mosfets for motor control with an arduino, and my google searching is coming up a little short.

Are there technical pros/cons to using say 4 x 20 amp mosfets vs. 1 x 80 amp mosfet?

What are the general thoughts on the matter?  Seems most motor controllers parallel their mosfets but maybe it's just a cost thing?  Thanks!

[Dan Lavin]

Unlike bipolar transistors, mosfets can be paralleled. Because Rds goes
up with increasing temperature mosfets automatically balance current
among them themselves. It is my understanding that power mosfets are
actually constructed of paralleled internal arrays.

As for one large vs multiple smaller mosfets, economic rather than
technical considerations predominate. There is are tradeoffs of board
space, heat sink requirements, and component cost to be considered. At
higher frequencies and larger sizes gate capacitance is an issue to be
considered as well. Large gate capacitance means a larger gate resistor
is required to keep inrush current within limits and that means slower
turn on times. Specs may differ between one large and several small mosfets

When you look at motor controllers, you may be seeing H-bridges rather
than paralleled mosfets as an H-bridge requires 4 mosfets. Check.

Generally, to control a mosfet from an arduino, select one with logic
level gate drive and use a small resistor between the arduino and the
mosfet gate. Calculate the resistor value to keep the gate capacitor
current inrush within the limit the arduino can source or sink, but it
never needs to be more than about 100ohms

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[Bill French]

Awesome, thank you .. any opinions on mosfet driver ics?  I've been looking at the Allegro A3941 (full bridge) and A3946 (half bridge) as they seem to solve a lot of the things that confuse me, like "synchronous rectification" and "charge pumping".

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[kfia...@tampabay.rr.com]

Bill
On one controller I have taken apart the (2) parallel mosfets were rated at 94a ea (IRFB4410) with the controller rated at 40 amps. A motor under stall can pull 4 to 6 times rated current. Which I replicated triggering them with a pricey MICROCHIP TC4426MJA duel channel mosfet driver until I figured that if you add a capacitor they run way cooler and you can use the cheaper version less that $2 version of the driver.
Hope this helps.
ken

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[Nick Vermeer]

Mosfets are not universally safe to parallel. It is still possible to get thermal runaway, even if it is less common than in bipolars. Also, if you parallel Mosfets, they should be all thermally connected to the same heat sink.

Also when you have high gate capacitance you actually want to be able to shoot more current into the gate so you can switch on the FET more quickly. The problem is that some micro controllers will be destroyed by the high current pulse, so when driving them directly you need a resistor. The better solution is an external driver to switch the gate.

I recommend you try starting with a one MOSFET switch circuit and a small motor for your initial testing. The arduino PWM output can be directly connected to a fairly large N channel MOSFET. You can then use this as a speed control in one direction for a motor. In high power circuits you will need a gate driver since almost all FETs want 10v to fully saturate their gate.

What kind of motor are you looking to drive, and what's the application?

[Bill French]

I've played successfully with little motors and testing, but this is for the power racing series, so I am looking at controlling 4+ horsepower motors.

[Michael Shiloh]

By the way, if you're more interested in a quick solution than learning
(sometimes it's the right choice) then Robot Power makes a nice high
current motor shield called Mega Motor. I've used it in a couple of
projects and never had any trouble:

up to 40A of current and up to 28V.

http://www.robotpower.com/products/MegaMotoPlus_info.html