What is MOSFET
MOSFETs are variable resistors controlled by voltage Depending on the voltage applied between the G and S, the resistance between D and S will vary. When there is zero voltage between G and S, the resistance between D and S is very high, which cuts off the flow of electric current between D and S and turns the MOSFET into an open switch. When the voltage between G and S passes a threshold level (VGS(th)) the resistance between D and S drops very fast and it turns the MOSFET into a close switch.
What to look for in a MOSFET datasheet
1. VGS(th) within the DC 3.3V to DC 5V range
Since the on / off of the MOSFET is to be controlled by Arduino or Raspberry Pi, we need to look for MOSFET with VGS(th) within the normal output voltage range of Arduino or Raspberry Pi. Which in the case of Arduino is around DC 5V, and in the case of Raspberry Pi is around DC 3.3V.
Below are some MOSFETs with VGS(th) within the DC 3.3V to DC 5V range.
2N7000
NTD4906N
2. ID
This is the maximum continuous current the device is allowed to carry under
the conditions described in the datasheet. This value can be related to either package construction, or the maximum current that would result in the maximum Tj. As such it depends on an assumed
mounting base temperature (Tmb), the thermal resistance (Rth) of the device, and its
RDS(on) at maximum Tj.
For any given application, the ID needs to be higher than the current drawn by the device connected to the MOSFET (i.e., for a DC motor that consumes 1 Amp, the ID needs to be higher than 1 Amp).
Note that some suppliers quote the "theoretical" silicon limit, while indicating the package
limited limit in the characteristic curves.
See below the ID of IRF530.
3. Transfer Characteristics Curve
For high current application, aside from knowing that the MOSFET can be turned on by applying +5V (N Channel) or -5V (P Channel), it's very important to know how much ID current is allowed to pass through when VGS is at +5V or -5V.
Below are the Transfer Characteristics Curves of the MOSFET mentioned in this post.
From the curves above, it's clear that none of the MOSFET are fully turned on when VGS is at 5V. Aside from STP36N06 that is fully turned on at VGS = 8V, the other 3 MOSFETs need VGS = 10V to be fully turned on.
As a result, it's important to check whether the ID at VGS=5V is sufficient for the intended application.
P.S. See the end of this post for a list of MOSFETs that are fully turned on at VGS=5V.
4. VDS
This is the maximum drain-source voltage the device is guaranteed to block
between the drain and source terminals in the off-state for the specified temperature
range.
For any given application, the VDS needs to be higher than the voltage of the device connected to the MOSFET (i.e., to control a 12V DC motor, the VDS needs to be higher than 12V).
See above the sample datasheet for the VDS of IRF530.
5. RDS(ON)
MOSFETs are frequently used in high current applications. The RDS(ON) figure is used to calculate power dissipation of the MOSFET so we could tell whether it could handle our intended application and whether a heat sink or other ways of removing heat from the MOSFET are needed.
For the aforementioned MOSFETs, their RDS(ON) could be found in the above photos and the formula for calculating the power dissipation is shown below. Note that "I" represents the current drawn by the load of the intended application.
Assuming an application uses a DC motor that draws 1 Amp of current and the RDS(ON) of the MOSFET is 35m ohm, the calculation below shows that the dissipated power will be 35mW.
The data below are from the datasheet of FQP30N06L.
6. RθJA
Aside from RDS(ON), we also need RθJA and Maximum Junction Temperature Rating for the final calculation. These info. can easily be found in the datasheet of FQP30N06L.
Maximum Junction Temperature Rating
The calculation shows that the maximum power dissipation without using heat sink is 2.4W which is way higher than the 35mW of the intended application. Therefore, no heat sink is needed if the MOSFET is operating in an environment with 25 degree C ambient temperature.
Logic-Level Gate Drive MOSFET - N-Channel
IRL540
IRL540 - Transfer Characteristics Curve
Other N-Channel Logic-Level Gate Drive MOSFETs
Logic-Level Gate Drive MOSFET - P-Channel
NDP6020P
It seems having a P-Channel Logic-Level Gate Drive MOSFET is not so important for most applications because the gate is tied to VCC to turn off the MOSFET and tied to GND to turn it on (perhaps, that's why there are so few on Mouser's website). The reason is well stated below (from Arduino Forum, http://forum.arduino.cc/index.php?topic=184546.0).
Referenec
Using MOSFETS with TTL levels <-- Good read!!
https://arduinodiy.wordpress.com/2012/05/02/using-mosfets-with-ttl-levels/
Understanding power MOSFET data sheet parameters
http://www.nxp.com/documents/application_note/AN11158.pdf
Arduino Power, Current, and Voltage Limitations
http://www.electricrcaircraftguy.com/2014/02/arduino-power-current-and-voltage.html
Selecting a MOSFET for driving load from logic
http://electronics.stackexchange.com/questions/36098/selecting-a-mosfet-for-driving-load-from-logic
MOSFETs and How to Use Them | | AddOhms #11 <----- Very good tutorial. Must watch!!
https://www.youtube.com/watch?v=GrvvkYTW_0k
What is RDS(on) AKA On-Resistance?
https://www.youtube.com/watch?v=hgYgsGzByEE
Switching 12V with an active low 5V signal
http://electronics.stackexchange.com/questions/70214/switching-12v-with-an-active-low-5v-signal
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