Hi, I'm James, the Bald Engineer.
Both of these are transistors.
One is a BJT and the other is a MOSFET.
Can you tell the difference?
If not, that's OK because in this AddOhms video,
we're going to take a look at MOSFETs, how they work,
and how you can use them in your circuits.
So let's get going.
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This video is part of a two-part series on transistors.
The previous one covered Bipolar Junction Transistors, BJTs.
While this video covers Metal-oxide Semiconductor Field
Effect Transistors, known as MOSFETs.
Generally, you'll use a BJT for small loads,
say less than one amp of current,
while MOSFETs are well-suited for applications
with much higher current.
For the video on BJTs, See addohms.com/ep10.
For now, let's move on MOSFETs.
Moss is a flowerless plant that typically
grows 1 to 10 centimeters.
Some mosses grow up to 50 centimeters
and can be commonly found on trees.
Wait a minute, this is the wrong script!!
Hold on...
The tree sounds right.
OK, Here's the other stuff.
OK, let's try this again.
MOSFETs belong to a family tree of field effect transistors
or known as FETs.
There are JFETs, MOSFETs, and IGBTs.
JFETs actually work a little bit like a BJT, which we've already
talked about.
For this video, we're focused on the MOSFET,
which has two types of modes called
depletion and enhancement.
A depletion mode MOSFET works like a normally closed switch.
Current can flow when no voltage is applied.
Applying a negative voltage actually
causes the current flow to stop.
An Enhancement Mode FET works like a variable resistor.
They come in N channel and P channel types.
Enhancement mode FETs are by far the most common transistor
used today, so let's focus in on them.
Here's the symbol for an N channel enhancement mode
MOSFET, and here's a TO-220 style transistor.
The pins of a MOSFET are identified as the gate,
the drain, and the source.
The field effect part of their name
suggests they work by voltage, compared
to BJT, which works by current.
When voltage is applied between the gate and the source,
current is allowed to flow between the drain
and the source.
Here's the really cool thing about MOSFETs--
they are variable resistors controlled
by voltage, which means depending
on the voltage applied between the gate and source,
the resistance between the drain and source will vary.
With a low voltage at the gate, the resistance
from the drain to source is very high.
It's kind of like an open switch.
As we increase the voltage at the gate,
we pass a threshold voltage, and then
the resistance from the drain to the source drops,
and it drops very quickly.
The key difference between a MOSFET and a BJT
is that the output current isn't a multiplier of the input
because MOSFETs are all about VOLTAGE.
Since the resistance is between the drain and source,
it is known as RDS-on and can always
be found in the MOSFET's datasheet.
For example, this is a FQP30N06L.
Let's take a look at its datasheet from Fairchild.
We can see that RDS is given when
there are two different voltages from gate to source.
At 10 volts, the on-resistance will be about 27 milliohms,
and while at 5 volts, the on-resistance
is only about 35 milliohms.
That's pretty small when you think about it.
We picked this MOSFET on purpose.
It is known as a logic level MOSFET,
because the voltage from gate to source VGS
is lower than 5 volts.
In other words, the threshold to turn the MOSFET on
is low enough to be used by an Arduino or Raspberry Pi.
Not all MOSFETs are logic level compatible,
so it is very important to check to see
what the VGS threshold is before using it in your circuit.
Since you will probably use a MOSFET
in high current applications, it is
important to check how hot it is going to get.
Here's how we calculate if we need a heat sink or not.
The formula to determine how much power
the MOSFET dissipates is resistance
times current squared.
In this case, the resistance is RDS on,
and the current is whatever your load will draw.
Let's use an example of a motor that draws one amp of current.
This means we multiply 35 milliohms
by one amp squared to get 35 milliwatts.
OK, now we need a few more things from the datasheet.
First, we need the junction- to-ambient coefficient, which
is r-theta-ja, and in this case is 62.5 degrees C per watt.
We also need the maximum junction temperature,
which in this case is 175 degrees C. Using this formula,
we can calculate the maximum power
the transistor can dissipate without using a heat sink.
We take the maximum junction temperature
minus the ambient temperature, which
is going to be 25 degrees C, and divide
by the thermal resistance.
This gives a maximum dissipation of 2.4 watts.
In our example, we are only dissipating 35 milliwatts,
so we're safe to operate without a heat sink.
Now you might be wondering, how can the number we calculate
be 2.4 watts when the datasheet clearly said 79 watts?
And that's a really great point.
The 79 watts is if we had the ability
to cool the transistor case to 25 degrees
C, which means you have to be using some kind of heat sink.
But we're going to cover more on that in a later video.
Let's review what you need to know
to use a MOSFET as a switch.
Number one, find out which pin is the gate, drain, and source.
Number two, look at the datasheet
to determine the threshold voltage, which is going
to be shown as VGS or VTH.
Find the drain to source resistance or RDS-on.
Number four, look at R-theta-ja and the maximum junction
temperature to calculate how hot the MOSFET will get.
Visit addohms.com/mosfet-guide, all lowercase,
to download a simple PDF form you can use to calculate these
parameters.
MOSFETs are cool little devices, but they're also a little bit
complex.
So we'll cover them in more detail in future videos.
Make sure you follow us or subscribe
to know when new video tutorials are released.
If you visit addohms.com, you can also
get show notes for this episode, as well as other Addohms
videos.
If you have any questions about MOSFETs or ideas
for future videos, send them our way and keep watching.
Maybe we'll cover them in a future video.