Download Study of Switching Characteristics

American International University- Bangladesh
Faculty of Engineering (EEE)
Lab exp-09:
of MOSFET
:Study of
Switching Characteristics and Switching Loss
Course teacher: MOHAMMAD ISMAIL HOSSAIN
Course Title: Electronic Devices Lab
Course code: 00717
Section: Q
Semester: Spring (2015-2016)
Date of submission: 04-04-2016
Group No: 01
NO
NAME
ID
1
HASAN MEHRAB
15-28369-1
2
ISLAM RAKIBUL
15-29297-1
3
SARKER APURBO KUMAR
15-28261-1
4
LISAN SAKHWAT HOSSAIN
15-28256-1
5
ALAM, CHOUDHURY ASBHA
15-28518-1
6
MAJUMDAR ANIK KUMAR
15-28306-1
Title:Study of Switching Characteristics and Switching Loss of MOSFET
abstract:
The most common transistor types are the Metal Oxide Semiconductor Field Effect
Transistors (MOSFETs) and the Bipolar Junction Transistors (BJT). BJTs based circuits
dominated the electronics market in the 1960's and 1970's. Nowadays most electronic
circuits, particularly integrated circuits (ICs), are made of MOSFETs. The BJTs are mainly
used for specific applications like analog circuits (e.g. amplifiers), high-speed circuits or
power electronics.
There are two main differences between BJTs and FETs. The first is that FETs are chargecontrolled devices while BJTs are current or voltage controlled devices. The second
difference is that the input impedance of the FETs is very high while that of BJT is relatively
low.
As for the FET transistors, there are two main types: the junction field-effect transistor
(JFET) and the metal oxide semiconductor field effect transistor (MOSFET). The power
dissipation of a JFET is high in comparison to MOSFETs. Therefore, JFETs are less
important if it comes to the realization of ICs, where transistors are densely packed. The
power dissipation of a JFET based circuit would be simply too high. The combination of ntype and p-type MOSFETs allow for the realization of the Complementary Metal Oxide
Semiconductor (CMOS) technology, which is nowadays the most important technology in
electronics. All microprocessors and memory products are based on CMOS technology. The
very low power dissipation of CMOS circuits allows for the integration of millions of
transistors on a single chip. In this experiment, we will concentrate on the MOSFET
transistor. We will investigate its characteristics and study its behavior when used as a switch.
The objective of this experiment is to study the MOSFET as
1. Switching characteristics and
2. Switching loss
Theory and Methodology:
MOSFETs Structure and Physical Operation
The MOSFETs are the most widely used FETs. Strictly speaking, MOSFET devices belong
to the group of Insulated Gate Field Effect Transistor (IGFETs). As the name implies, the
gate is insulated from the channel by an insulator. In most of the cases, the insulator is
formed by a silicon dioxide (SiO2), which leads to the term MOSFET. MOSET like all other
IGFETs has three terminals, which are called Gate (G), Source (S), and Drain (D). In certain
cases, the transistors have a fourth terminal, which is called the bulk or the body terminal. In
PMOS, the body terminal is held at the most positive voltage in the circuit and in NMOS, it is
held at the most negative voltage in the circuit.
There are four types of MOSFETs: enhancement n-type MOSFET, enhancement p-type
MOSFET, depletion n-type MOSFET, and depletion p-type MOSFET. The type depends
whether the channel between the drain and source is an induced channel or the channel is
physically implemented and whether the current owing in the channel is an electron current or
a hole current.
The cross section of an enhancement NMOS transistor is shown in figure below. If we put the
drain and source on ground potential and apply a positive voltage to the gate, the free holes
(positive charges) are repelled from the
Schematic cross section of an enhancement type NMOS transistor
Symbols for Enhancement NMOS Transistor
Symbols for Enhancement PMOS Transistor
region of the substrate under the gate (channel region) due to the positive voltage applied to
the gate. The holes are pushed away downwards into the substrate leaving behind a depletion
region. At the same time, the positive gate voltage attracts electrons into the channel region.
When the concentration of electrons near the surface of the substrate under the gate is higher
than the concentration of holes, an n region is created, connecting the source and the drain
regions. The induced n-region thus forms the channel for current flow from drain to source.
The channel is only a few nanometers wide. Nevertheless, the entire current transport occurs
in this thin channel between drain and source.
A common application of MOSFETs is switches in analog and digital circuits.
Switches in analog circuits can be used for example in data acquisition systems, where they
serve as analog multiplexors, which allow the selection of one of several data inputs.
A simple example of a switching circuit based on an n-type enhancement transistor and a
resistor is shown below. The voltage applied to the gate controls the conductance of the
channel. A zero or low value of VGS, the conductance is very low so that is the transistor acts
like an open circuit and no current flows through the load resistor RL. When V GS exceeds the
threshold, the channel conductance becomes higher and the transistor acts like a closed
switch. The channel resistance is not getting zero but the resistance is getting small so that the
output voltage Vout is getting small. Fig.(a) below shows an NMOS switching FET and its
models for Vin = 0 (Fig. (b)) and Vin = +5V (Fig. (c)). In each case, the FET is modeled as a
mechanical switch.
Fig: NMOS transistor switch
As for PMOS, a negative value of VGS has to be applied to turn the transistor on. The
operation can be described using the curves shown in figure below. When the input voltage,
VGS, of the transistor shown is zero, the MOSFET conducts virtually no current, and the
output voltage, Vout, is equal to VDD. When VGS is equal to 5V , the MOSFET Q-point
moves from point A to point B along the load line, with VDS = 0.5V . Thus, the circuit acts as
an inverter. The inverter forms the basis of all MOS logic gates.
Fig: MOSFET switching characteristics
Pre-Lab Homework:
Apparatus:
1) Trainer Board
:
2) MOSFET
:
3) Resistors
4) DC Power Supply
5)
6)
7)
8)
Power Supply Cable
Multimeter
Signal Generator
Oscilloscope
1pc
IRF540 [ ]
1pc
: 1KΩ
[ ]
:
[2
pcs]
Precautions:
Have your instructor check all your connections after you are done setting up the circuit and
make sure that you apply only enough voltage to turn on the MOSFET, otherwise it may get
damaged.
Experimental Procedure:
Problem 1:
Study of Switching Characteristics of MOSFET
Circuit Diagram:
RD = 1K
IRF540
VD = 20V
V
GS
Experimental Procedures:
1.
2.
3.
4.
5.
Set VGS to zero and recorded the VDS, VL and ID.
Increased the gate voltage VGS gradually and recorded the readings.
reading was talking until ID = 20mA (or the saturation current of the MOSFET).
Note the condition of VDS and ID of steps 1 and 3.
Repeated the experiment for VDD = 15 Volts.
Problem 2:
Study of Switching Losses of MOSFET:
Circuit Diagram:
Experimental Procedure:
1. Construct the circuit as shown in the above figure.
2. With square/sinusoidal wave gate signal observe the VDS and ID simultaneously.
3. Drawn both the wave shapes.
Data Table: 1
VDD = 15V
VDD = 20V
VGS
0
0.5
VDS
15.00
15.02
VL
0
0
ID
0
0
1
15.02
0.1 mv
0
1.5
2
2.5
15.02
14.03
15.05
0.2 mv
0.2mv
8.2mv
0
0.03 mA
0.43mA
VGS
0
0.5
VDS
20.03
20.03
VL
0
0
ID
0
0
1
19.99
0
0
1.5
2
2.5
20.01
20.03
20.04
0
0
0
0
0.06 mA
1.83 mA
Question and Answers:
1.Plot VGS Vs VDS and VGS Vs ID.
25
20
15
10
Series1
5
0
0
-5
1
2
3
4
5
6
200
180
160
140
120
100
Series1
80
60
40
20
0
-20 0
1
2
3
4
5
6
2. What is the gate voltage that turns on the MOSFET?
snA: ta bloThd ohA al ng Tl v hT3 V.
3. From the curve of ID and VDS plot PD = VDS * ID, Power loss during ON time of the switch.
450
400
350
300
250
Series1
200
150
100
50
0
0
0.5
1
1.5
2
2.5
3
3.5
4. What will happen if frequency of the gate signal is increased to a very high level?
snA: ta dh n bloThd g ia hA A oa a hA Ta aa e nio nia hA A.
Discussion and Conclusion:
All the apparatus were checked before the start of the experiment. The oscilloscope was
calibrated before the start of the experiment. Care should be taken to avoid short connections.
Short connections can produce heat (due to high current flow) which can be harmful for the
components and damage the component.
ta A rm a e nT ohA m aalae g Tlstudy about Ta Awitching iharacteristics and Awitching ooss
of MOSFET .ta ghTh hng Ta molT g i ab A lvTh n g aale Ta rm a e nT malb g TahT MOSFET
bloThd A malb g g hng Ta mlo a olAA nia hA A haT a gl A nlT hiT bhT nT o h i aTh n dhT
ialAA nd Ta dhT bloThd .
Reference(s):
1. American International University–Bangladesh (AIUB) Electronic Devices
Lab Manual.
2. A.S. Sedra, K.C.fahm Smith, Microelectronic Circuits, Oxford University Press (1998)
.