MOSFETs and How to Use Them | AddOhms #11
MOSFETs and How to Use Them | AddOhms #11
MOSFETs are the most common transistors used today.
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They are switches that can be used an Arduino, BeagleBone, or Raspberry Pi. This is part 2 of a 2 part series, which covers MOSFETs. (Part 1 Covered BJTs).
When it comes down to it, MOSFETs are pretty complicated devices. This video will cover the basics of what you need to use them in your circuit, including calculating if you need a heat sink or not.
Learn what they are and how to use them.
Part 1 (BJTs):
• BJTs as Transistor Switches | AddOhms…
Show Notes:
http://www.addohms.com/ep11
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#mosfet #transistor #
Content
1.67 -> Hi, I'm James,
the Bald Engineer.
4.25 -> Both of these are transistors.
6.08 -> One is a BJT and the
other is a MOSFET.
8.75 -> Can you tell the difference?
10.16 -> If not, that's OK because
in this AddOhms video,
12.89 -> we're going to take a look
at MOSFETs, how they work,
15.14 -> and how you can use
them in your circuits.
17.369 -> So let's get going.
18.16 -> [MUSIC PLAYING]
30.6 -> This video is part of a
two-part series on transistors.
33.48 -> The previous one covered Bipolar
Junction Transistors, BJTs.
38.33 -> While this video covers
Metal-oxide Semiconductor Field
42.86 -> Effect Transistors,
known as MOSFETs.
47.36 -> Generally, you'll use
a BJT for small loads,
50.18 -> say less than one
amp of current,
52.4 -> while MOSFETs are
well-suited for applications
54.65 -> with much higher current.
56.21 -> For the video on BJTs,
See addohms.com/ep10.
60.59 -> For now, let's move on MOSFETs.
64.18 -> Moss is a flowerless
plant that typically
66.26 -> grows 1 to 10 centimeters.
68 -> Some mosses grow up
to 50 centimeters
69.92 -> and can be commonly
found on trees.
73.15 -> Wait a minute, this
is the wrong script!!
76.135 -> Hold on...
77.86 -> The tree sounds right.
80.14 -> OK, Here's the other stuff.
81.39 -> OK, let's try this again.
84.75 -> MOSFETs belong to a family tree
of field effect transistors
88.3 -> or known as FETs.
90.25 -> There are JFETs,
MOSFETs, and IGBTs.
93.55 -> JFETs actually work a little bit
like a BJT, which we've already
97.39 -> talked about.
99.14 -> For this video, we're
focused on the MOSFET,
101.51 -> which has two types
of modes called
103.52 -> depletion and enhancement.
106.93 -> A depletion mode MOSFET works
like a normally closed switch.
111.33 -> Current can flow when
no voltage is applied.
116.18 -> Applying a negative
voltage actually
118.16 -> causes the current flow to stop.
121.01 -> An Enhancement Mode FET works
like a variable resistor.
124.94 -> They come in N channel
and P channel types.
129.03 -> Enhancement mode FETs are by
far the most common transistor
132.24 -> used today, so let's
focus in on them.
136.67 -> Here's the symbol for an
N channel enhancement mode
139.61 -> MOSFET, and here's a
TO-220 style transistor.
144.8 -> The pins of a MOSFET are
identified as the gate,
148.22 -> the drain, and the source.
151.34 -> The field effect
part of their name
153.77 -> suggests they work
by voltage, compared
156.98 -> to BJT, which works by current.
161.85 -> When voltage is applied between
the gate and the source,
166.33 -> current is allowed to
flow between the drain
168.436 -> and the source.
171.63 -> Here's the really cool
thing about MOSFETs--
174.47 -> they are variable
resistors controlled
176.45 -> by voltage, which
means depending
179.15 -> on the voltage applied
between the gate and source,
181.59 -> the resistance between the
drain and source will vary.
185.83 -> With a low voltage at
the gate, the resistance
188.32 -> from the drain to
source is very high.
191.27 -> It's kind of like
an open switch.
194.66 -> As we increase the
voltage at the gate,
196.67 -> we pass a threshold
voltage, and then
199.37 -> the resistance from the
drain to the source drops,
202.31 -> and it drops very quickly.
205.76 -> The key difference
between a MOSFET and a BJT
208.24 -> is that the output current
isn't a multiplier of the input
212.54 -> because MOSFETs are
all about VOLTAGE.
218.44 -> Since the resistance is
between the drain and source,
221.11 -> it is known as RDS-on
and can always
224.38 -> be found in the
MOSFET's datasheet.
227.23 -> For example, this
is a FQP30N06L.
233.2 -> Let's take a look at its
datasheet from Fairchild.
236.08 -> We can see that
RDS is given when
238.69 -> there are two different
voltages from gate to source.
241.57 -> At 10 volts, the on-resistance
will be about 27 milliohms,
246.44 -> and while at 5 volts,
the on-resistance
248.99 -> is only about 35 milliohms.
251.33 -> That's pretty small
when you think about it.
254.78 -> We picked this
MOSFET on purpose.
257.29 -> It is known as a
logic level MOSFET,
260.86 -> because the voltage
from gate to source VGS
263.98 -> is lower than 5 volts.
266.47 -> In other words, the threshold
to turn the MOSFET on
269.14 -> is low enough to be used by
an Arduino or Raspberry Pi.
273.01 -> Not all MOSFETs are
logic level compatible,
276.22 -> so it is very important
to check to see
278.14 -> what the VGS threshold is
before using it in your circuit.
282.77 -> Since you will
probably use a MOSFET
284.69 -> in high current
applications, it is
286.37 -> important to check how
hot it is going to get.
289.7 -> Here's how we calculate if
we need a heat sink or not.
293.68 -> The formula to
determine how much power
295.54 -> the MOSFET dissipates
is resistance
297.61 -> times current squared.
300.98 -> In this case, the
resistance is RDS on,
303.76 -> and the current is whatever
your load will draw.
307.44 -> Let's use an example of a motor
that draws one amp of current.
311.83 -> This means we
multiply 35 milliohms
314.34 -> by one amp squared
to get 35 milliwatts.
318.46 -> OK, now we need a few more
things from the datasheet.
322.34 -> First, we need the junction-
to-ambient coefficient, which
325.04 -> is r-theta-ja, and in this case
is 62.5 degrees C per watt.
331.93 -> We also need the maximum
junction temperature,
334.87 -> which in this case is 175
degrees C. Using this formula,
340.83 -> we can calculate
the maximum power
342.48 -> the transistor can dissipate
without using a heat sink.
347.78 -> We take the maximum
junction temperature
349.91 -> minus the ambient
temperature, which
352.07 -> is going to be 25
degrees C, and divide
354.53 -> by the thermal resistance.
356.78 -> This gives a maximum
dissipation of 2.4 watts.
361.19 -> In our example, we are only
dissipating 35 milliwatts,
364.46 -> so we're safe to operate
without a heat sink.
368.53 -> Now you might be wondering,
how can the number we calculate
371.95 -> be 2.4 watts when the datasheet
clearly said 79 watts?
377.26 -> And that's a really great point.
380.04 -> The 79 watts is if
we had the ability
382.86 -> to cool the transistor
case to 25 degrees
386.16 -> C, which means you have to be
using some kind of heat sink.
390.84 -> But we're going to cover more
on that in a later video.
394.65 -> Let's review what
you need to know
396.14 -> to use a MOSFET as a switch.
398.03 -> Number one, find out which pin
is the gate, drain, and source.
401.93 -> Number two, look
at the datasheet
403.88 -> to determine the threshold
voltage, which is going
406.37 -> to be shown as VGS or VTH.
410.38 -> Find the drain to source
resistance or RDS-on.
414.7 -> Number four, look at R-theta-ja and the maximum junction
419.11 -> temperature to calculate
how hot the MOSFET will get.
423.18 -> Visit addohms.com/mosfet-guide,
all lowercase,
427.72 -> to download a simple PDF form
you can use to calculate these
431.1 -> parameters.
433.877 -> MOSFETs are cool little devices,
but they're also a little bit
436.46 -> complex.
437.58 -> So we'll cover them in more
detail in future videos.
440.31 -> Make sure you follow
us or subscribe
442.64 -> to know when new video
tutorials are released.
445.31 -> If you visit
addohms.com, you can also
447.56 -> get show notes for this episode,
as well as other Addohms
450.63 -> videos.
452.12 -> If you have any questions
about MOSFETs or ideas
454.28 -> for future videos, send them
our way and keep watching.
457.28 -> Maybe we'll cover them
in a future video.
Source: https://www.youtube.com/watch?v=GrvvkYTW_0k