MOSFET selection

[AGCB]

I've been searching for a tutorial or guide in mosfet selection. What are basic guidelines to selecting a MOSFET for a 1-10 amp load? Is there a disadvantage to choosing one with much greater curent or voltage capability? I know there is a difference with the logic level devices. I'm just looking for rules of thumb. Thanks. Aaron

 

[Jaguarjoe]

As Vds gets larger, Rdson grows with it because the slab of silicon in the MOSFET is " long and narrow".
As Id gets larger, the time to turn the MOSFET on and off grows with it because the slab of silicon is "wide and short" which increases the gate capacitance.

 

[Meatball]

So if you're just driving a silly LED or something like that, you can jump beyond the called for specifications and only gain thermal stability and whatnot?

 

[Jaguarjoe]

If you're only doing small stuff like LED's or relays under 1 amp or so, just about anything will work provided you have enough gate voltage to turn the MOSFET on. Many MOSFETs require 10 volts. Logic MOSFETs, generally with an "L" in their designation only need 5 volts which is ideal for use with microcontrollers like the Picaxe. IRL530 is a very common MOSFET for everyday use.

 

[AGCB]

As Vds gets larger, Rdson grows with it because the slab of silicon in the MOSFET is " long and narrow".
As Id gets larger, the time to turn the MOSFET on and off grows with it because the slab of silicon is "wide and short" which increases the gate capacitance.

Thanks. This is basically what I was looking for. I want to drive some automotive motors which draw 3-4 amps under maximum load.

 

[Jaguarjoe]

Be sure you put in a 3 or 4 amp commutating (or flywheel, or back emf) diode across the motor to protect the MOSFET and put a 10k resistor from the gate to ground to make sure it doesn't float..

 

[smanches]

Be sure you put in a 3 or 4 amp commutating (or flywheel, or back emf) diode across the motor to protect the MOSFET and put a 10k resistor from the gate to ground to make sure it doesn't float..

Just a small clarification in case you're doing a high side mosfet, but place the resistor from gate to source.

 

[Jaguarjoe]

Ah, good catch!

 

[AGCB]

Thanks all. Just what wanted. Aaron

 

[shortbus=]

Here is a good explanation of what things on a Mosfet data sheet mean and how to figure what you need to know;

FET Current Ratings -- Chuck's Robotics Notebook

 

[AGCB]

A very enlightening artical!!! Thanks