Hi guys! In this lesson, I will explain the basic
structure and working principle of MOSFETs used in switching, boosting or power balancing tasks in
DC-DC converter circuits, motor driver circuits and many more power electronics circuits. MOSFETs
are the most widely used field-effect transistors that operate with voltage control. We can
examine FETs in two groups as you can see here. We had a lesson in which I explained JFET
before. I will explain the MOSFET in this trouble. The abbreviation of MOSFET comes from
the initials of the Metal Oxide Semiconductor Field Effect Transistor words.
You see the shape of a MOSFET transistor on the left and the symbol on the
right. MOSFET also has three pins. As in JFET, one of them is Gate, the other is Drain, and the
last is Source. Pin names are the same in MOSFET as in JFET. Their order is not always like
this. Locations may vary depending on the model. G, D and S abbreviations are used for these.
In the symbol, the pin naming is like this. We can compare the structure of the MOSFET to
a faucet, as in all transistors. We can think of the Gate pin, where the trigger is made, as the
valve of the faucet, and between the Drain-Source, where the current passes, as the direction in
which the water flows in the faucet. When a trigger voltage is applied between Gate-Source,
a current between Drain-Source is controlled. MOSFETs are produced in two ways, N-Channel and P-Channel. In the symbol, the outward arrow on the Gate pin is N-Channel, and the inward one is
P-Channel. If you remember, it was the opposite in JFET. So what's the difference between
them? The working principle of both is the same. The direction of the controlled current
in N-Channel MOSFET is from Drain to Source, while the direction of controlled current in
P-Channel MOSFET is from Source to Drain. In other words, while the positive(+) pole of the
voltage source is connected to the Drain pin of the MOSFET in N-Channel MOSFET, the positive(+)
pole of the voltage source is connected to the Source pin of the MOSFET in P-Channel MOSFET.
Let's compare MOSFET, BJT and JFET transistors according to the current they can withstand.
While currents close to 1A and under 1A can be controlled with BJT and JFET transistors, much
larger currents can be controlled with MOSFET transistors. We can see this by looking at the
datasheet of some MOSFET transistors. E.g; When we look at the datasheet information of the very
widely used IRFZ44 MOSFET, there is information that current can be controlled up to a maximum
current of 49A. This current value is really a great value. In addition, a maximum voltage of
55V can be controlled with this MOSFET. When we look at the datasheet of IRF540 MOSFET, another
widely used MOSFET, there is information that a maximum current of 23A can be controlled and a
maximum voltage of 100V can be controlled with it. The fact that a wide range and much greater
current and voltage control can be achieved with MOSFET increases the attractiveness
of MOSFET compared to other transistors. For example, you see a DC-DC Boost Converter here.
The circuit diagram for this is basically the same as here. The amplification process is according
to the switching speed of the MOSFET located here. Triggers are made at high frequency
values at the KHz level. In this way, MOSFET and shockly diodes are preferred in order
to obtain healthier results instead of using normal silicon diodes in order to respond to high
frequency switching speeds in converter circuits. DC-DC converters and motor drivers using MOSFETs naturally get hot because very high current
flows through them. As such, they need coolers. We usually see MOSFETs in circuits as fixed
to heatsinks that will dissipate the heat like you see here. For example, in the motor
speed control circuit you see on the right here, there is a heatsink fixed to the MOSFET, which
I show with the red arrow. We can see that there are heatsinks connected to MOSFETs in this
DC-DC Boost Converter Amplifier converter circuit you see on the left. Well, let's look
at the basic working principle of a MOSFET now. Here you see a simple motor speed control circuit
with the circuit diagram in the upper left corner. There is a small DC motor connected to
the Drain pin of the N-Channel MOSFET. There is a DC source to drive this motor.
Here, there is a potentiometer connected between the Gate and Source pins of the MOSFET
to provide the voltage to trigger the MOSFET. The reason for connecting a resistor between
the potentiometer and the MOSFET is that a reverse current in the MOSFETs does not damage
the circuit elements connected to the Gate pin. Thanks to the resistor here, the intensity of
that current is reduced. Thus, the circuit devices connected to the Gate pin are not damaged.
If we talk about this circuit, since there is no damage to the potentiometer, this circuit
will work smoothly even if there is no resistor. Now, when we change the resistance
value of the potentiometer here, a voltage will be applied between the Gate-Source
of the MOSFET. With this voltage, the MOSFET will be triggered and the current coming out of
the positive (+) pole of our power supply will flow between the Drain-Source pins of the
MOSFET by following the path we have shown with the arrows, and our motor will rotate with this
current. As the trigger voltage will change by increasing or decreasing the resistance value of
the potentiometer, the Drain current will change and the speed of the motor will change according
to this current. We can also control the motor by connecting a power supply that provides a
constant 5V voltage instead of a potentiometer. While the MOSFET is triggered above 5V,
it is not triggered at the voltage below. For a better understanding of
this motor speed control circuit, I built the circuit on a breadboard. With a 9V
battery, we can control the speed of a small DC motor with a potentiometer thanks to the MOSFET
in this way. As here, the working principle of systems with big electric motor fans is the same.
With MOSFET, we can control the speed of a motor not only with a potentiometer, but also with a
microcontroller as here. Since the resistance value on the Gate pin of the MOSFET transistors
is very large, there is no current flow. That is, the trigger part and the part connected to the
load are isolated from each other. As such, MOSFET can be easily used with a microcontroller such
as Arduino. Thanks to the 5V square wave received from the PWM signal output of the Arduino, speed
control can be done easily. According to the duty value of this signal, that is, by adjusting
the expansion and contraction of the signal, we can make the motor rotate fast or slow. Here,
the resistor connected to the Source pin of the MOSFET provides protection for the reverse current
that may come to the Arduino microcontroller. Since MOSFETs are easily affected by static
electricity and due to the resistor used with it, they are generally used as driver modules, not
alone in the circuit. Thanks to these modules, MOSFET connections can be made easily in the
circuit. Thus, ease of use is also ensured. For example, Microcontroller,
motor and power supply connections can be made easily when the MOSFET is not
a stand-alone MOSFET Driver Circuit module, as is the case here. When we look inside this
module, we see the circuit here. The MOSFET Driver Circuit module is formed by combining the
required resistor and MOSFET connections. Thus, the Digital Signal Input, Load connection
and power connection to be connected to the Microcontroller are gathered on
a module to provide ease of use. Now let's simulate an example circuit in
the Proteus program and finish our lesson. This is the basic structure and
working principle of MOSFET, which is used in power electronics circuits and
many other electrical and electronics circuits, friends. I hope it was helpful and you liked
it. Hope to see you in our next lesson. Goodbye.
