MOSFETs and How to Use Them | AddOhms #11

MOSFETs and How to Use Them |  AddOhms #11


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. [MUSIC PLAYING] 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.

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