Guide to MOSFET

A Beginner's Guide to MOSFET by Moser

IRFP260N image from Pins are Gate, Drain, Source from left to right.
If you need to switch high current and or high voltage loads with a micro controller you’ll need to use some type of transistor. I’m going to be covering how to use a MOSFET since it’s a better option for high power loads. This guide will be just a brief introduction that will discuss how to drive a mosfet in a simple manner with the ultimate goal of making it act like an ideal switch.
Refer to the N or P channel basic wiring schematics and remember the three pins: Gate, Drain, and Source. When I mention something like Gate-Source potential difference, I’m talking about the difference in voltage between the two pins.
N channel MOSFET
How to think of a MOSFET
A MOSFET may be thought of as a variable resistor whose Drain-Source resistance (typically Rds) is a function of the voltage difference on the Gate-Source pin. If there is no potential difference between the Gate-Source, then the Drain-Source resistance is very high and may be thought of as an open switch — so no current may flow through the Drain-Source pins. When there is a large Gate-Source potential difference, the Drain-Source resistance is very low and may be thought of as a closed switch — current may flow through the Drain-Source pins.
P channel MOSFET
N channel: For an N channel MOSFET, the source is connected to ground. If we want to let current flow, we can easily raise the voltage on the gate allowing current to flow.
P channel: Looking at the P channel MOSFET, the source is connected to the power rail V2. In order to allow current to flow the Gate needs to be pulled to ground. To stop the current flow, the gate needs to be pulled to V2. A potential problem is if V2 is a very high voltage it can be difficult raising the gate to the V2 voltage. Also note that logic is inverted for a P type mosfet!
Drain-Source resistance: Ideally we want Drain-Source resistance to be very high when no current is flowing, and very low when current is flowing. The main issue using MOSFETS with micro controllers is that the MOSFET may need 10-15 Gate-Source potential difference to get near its lowest Drain-Source resistance.
IRFP260N gate capacitance
IRFP260N current curves.
Gate-Source Capacitance:There is also a capacitance on the Gate-Source pins which prevents the mosfet from switching states quickly.
Mosfet Driver
A half bridge is needed to quickly turn a mosfet on and off and raise the Gate to a high voltage. There are many ICs available which can do this. Here’s a list of just a few that I’ve tested. Schematics are also provided!
Fet driver is a Half Bridge
  • MIC4422YN – Max of 18v, 9Amps peak, 2 Amps continuous.
  • MCP1407 - Max of 18v, 6Amps peak, 1.3 Amps continuous.
  • UCC27424  – Can drive two MOSFETs, Max of 15v, 4Amps typical.
All of these drivers performed nearly identically (~20ns rise, ~30ns fall).
Mosfets I tested
It was originally part of the plan to get some data about these guys, but I have been very busy with school. The MOSFETs have plenty of graphs inside the datasheets!
MIC4422YN, and MCP1407
An Important Reminder: Don’t forget that typically the heat sink on the back of a mosfet is connected to the Drain! If you mount multiple MOSFETs on a heat sink, the MOSFET must be electrically isolated from the heat sink! It’s good practice to isolate regardless in case the heat sink is bolted to a grounding frame.
Schematic Diode: If the load is somewhat inductive, you’ll need to put a diode to discharge the inductor. If you want more detail, look at the International Rectifier pdf at the end.
Mosfet Example
Arduino Mosfet Example
Without the driver, the Gate takes longer to charge, and it peaks at 5v.