Power MOSFET N-Ch 30V/62A IRLB8721

Handles up to 30V @ 62A, very low Rds(on) of 8.7mΩ and is 5V logic compatible..

DESCRIPTION

The Power MOSFET  N-Channel IRLB8721 is an Enhancement Mode MOSFET that can handle up to 30V @ 62A and is 5V logic compatible.

PACKAGE INCLUDES:

• Power MOSFET N-Channel IRLB8721

KEY FEATURES OF POWER MOSFET N-CHANNEL IRLB8721:

• N-Channel Power Enhancement Mode MOSFET
• Up to 30V and 62A power handling capability
• R_{DS(on) =} 16mΩ at 4.5Vgs  / 8.7mΩ  @ 10V
• 5V Logic compatible with current derating

MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor which is why we just call it a MOSFET for short.  Enhancement mode means that when the device has zero Gate voltage, the device is off.  This is denoted by the schematic symbol with the broken line which indicates that it does not conduct when there is no Gate voltage.

Power MOSFETs are most often used a switches where they are turned fully ON or OFF to control a load such as a motor or high power LEDs.  They are ideally suited for this because when the MOSFET is turned fully ON (Saturation Region), it has a very low resistance and can pass a lot of current without much power being dissipated in the device similar to a mechanical switch.  When they are turned OFF (Cut-Off Region), they act as an open circuit much like a mechanical switch would when it is off.

For some applications MOSFETs are also used in their Linear Region where they are are partially conducting, such as for an amplifier, analog fan speed controller or battery charger.

Example N-Channel MOSFET 5V Logic Drive Circuit

The very low ‘ON’ resistance means that there is very little voltage drop through the device and that also helps to keep power dissipation down.

If using this device with a 5V uC, it is possible to drive the MOSFET gate directly from an digital output pin.  When driving it this way, the MOSFET is not being driven to be quite fully ON and so the maximum resistance through the device increases from about 8mΩ to about 16mΩ.  While still very small, it does mean that the device will drop double the amount of voltage through it and dissipate twice as much heat, so it’s maximum current handling capability will be cut to about half of the stated amount which is usually fine for most applications.

N-Channel MOSFET Theory of Operation

MOSFET spec sheets can look pretty complicated, but for many applications we just need to pay attention to a few key parameters that are explained here.

V_DS : Drain-To-Source Voltage is the maximum voltage that the device can be used to switch. If you’re switching 12V, you need a device with a V_DS > 12V and usually you want something with a fair amount of safety margin.

I_D : Continuous Drain Current is the maximum current that the device can handle. this will often be specified under several conditions such as at 25C room temperature and at 100C or similar high operating temperature. Achieving the maximum current through the device assumes that you are driving it fully on and that appropriate heat sinking is applied. If you have a device that draws 10A, you need an I_D > 10A. Generally the higher the I_D rating of the device compared to the amount of current you need to pass though it, the easier it will be to manage thermals.

V_GS : Gate Voltage is the voltage differential between the Gate and the Source which is how hard the MOSFET is being driven.

V_GS(th) : Gate Threshold Voltage is the voltage at which the MOSFET starts to conduct. Any voltage less than this will drive the MOSFET to the OFF state known as the Cut-Off Region.  To possibly be logic compatible, the V_GS(th) needs to be well under the logic high voltage level.

R_DS(on) : Static Drain-to-Source On-Resistance is the minimum resistance of the MOSFET when it is driven to the fully ON state known as the Saturation Region. The key to look for here is that R_DS(on) may be specified at one or a couple of V_GS voltages.

If it is spec’d at 10V only, the part is not logic compatible and needs something close to 10V to drive it into saturation. This means a MOSFET driver, transistor or some other means is required to drive the gate with something close to 10V.

If there are two voltages listed, the highest voltage will be the voltage at which the device is fully saturated and show the lowest resistance, often 10V. The lower voltage is often around 4.5V and shows the resistance if you were to drive it directly off of 5V logic. Having this specified implies that the device is at least partially 5V logic compatible even if it isn’t being driven to full saturation.

Looking at the example spec sheet below, this is telling us that to drive the MOSFET to full saturation requires a V_GS voltage of 10V where max resistance is 8.7mOhm. If we were to drive it instead at 4.5V directly off of 5V logic, the resistance basically is twice as high, but is still only about 16mOhm.

A device like this can be driven directly off of 5V logic, but because its internal resistance is twice as high, it will drop twice the amount of voltage and dissipate twice as much power/heat in the device for the same current.  From a practical standpoint, this means it can handle about 1/2 the full rated current than it could handle if it was driven at 10V.  I would consider this as being partially 5V logic compatible.  If you are using a 60A device to control a 20A load for instance, this will generally be fine.  If you need 40A out of it, then you will need to drive it harder.

Notes: 

1. None

TECHNICAL SPECIFICATIONS

FURTHER READING:

Nick Gammon tutorial on driving motors or other loads with power transistors.