How to choose a MOSFET

Question

I want a MOSFET for my project which is a motor driver. But after going through the datasheet I got confused what to look for.

My current rating for the motor is 21 A. So I want a MOSFET which can handle around 25-30 A. The lithium polymer battery voltage is 11.1 V.

Microcontroller voltage = 5 volts
Brushless motor rating = 21 A
Lithium polymer batteries - 11.1 volts

Basic schematic circuit diagram

There are at least two things you could add to your question, which would help towards formulating an answer: (1) What is your control voltage (i.e. voltage of signal from microcontroller) (2) A schematic of your intended motor driver. — Anindo Ghosh, Dec 20 '13 at 18:40
Reading the datasheet is the right way to approach this problem. A better question would ask for clarification on specifically whatever in the datasheet confused you, rather than broadly asking for a solution to your problem. — Phil Frost, Dec 20 '13 at 19:01
Usually choose Irated >> Iload max. | Power dissipation = I^2.R = I_load_max^2 x Rdson so super low Rdson is needed. | Package chosen must handle Dissipation OK. So say Rdson = 0.010 Ohm then dissipation at say 20A = 40 W. FET must handle40W real dissipation OK and heatsink must allow it to get not too hot so maybe 1C/W rated or just maybe a bit worse. Also Rt-junction_case will affect internal temperature rise. | Lower than 0.010 Ohm Rdson will give lower temperature. | If motor driver is PWM rather than basic on/off then switching losses matter and gate driver impotant and .... ... .. . . . — Russell McMahon, Dec 20 '13 at 20:02
***brushless** motor rating=21A*: Is that a 2-wire brushless motor specifically designed to be a substitute for DC brushed motors? The more commonly available brushless motors cannot be operated the way you have indicated, they are not 2 wire devices and they need more complex BLDC drivers. Also, is that 21 Ampere value the stall current or typical operating current? You need to calculate on the basis of stall current. Please share the motor datasheet. — Anindo Ghosh, Dec 21 '13 at 06:53

score 3 · Answer 1 · edited Sep 30 '17 at 00:09

Basic things:

Supply voltage - is the MOSFET drain-source voltage rated high enough for at least 1.5x your supply voltage plus any normal expected increases?
Gate drive - not enough gate drive will ensure your MOSFET burns on heavy loads - what signal are you supplying to turn on the MOSFET?
MOSFET on-resistance and load current - how much power will be produced by the MOSFET due to its non-ideal zero-ohms on-resistance?
How quickly or how often are you switching the MOSFET on/off - losses in switching can be considerable.

When you've got all this into perspective, one of the most important graphs to look at in the data sheet is how device on-resistance varies with gate drive voltage - you may find you haven't enough gate voltage to efficiently use a MOSFET, and you might need a circuit to interface to the device.

How to choose a MOSFET

1 Answers1