This paper presents an educational kit (simulation & practical electronic circuit) for a PWM inverter, which can be used to educate Electronics Engineering undergraduate students the structure and behavior of a PWM’s inverter pulse generator. This module is taught as part of the “Power Electronics” course of the Department of Electronics Engineering of Piraeus University of Applied Sciences. The concept is to offer our co-students the opportunity to deeply understand how the pulse generator of the inverter works, by virtually and practically experimenting with the pulse generator itself creating the necessary models in the popular platform of MULTISIM (Simulation Tool of National Instruments) and designing/constructing the respective PCB circuits in the also popular platform of ULTIBOARD (Circuit Design Tool of National Instruments). This is important for the success of the learning process of this course. Though it is accompanied with a real-world laboratory, this exercise has not been yet added in the course’s laboratory curriculum.

1. 
Abstract — This paper presents an educational kit (simulation
& practical electronic circuit) for a PWM inverter, which can be
used to educate Electronics Engineering undergraduate students
the structure and behavior of a PWM’s inverter pulse generator.
This module is taught as part of the “Power Electronics” course
of the Department of Electronics Engineering of Piraeus
University of Applied Sciences. The concept is to offer our co-
students the opportunity to deeply understand how the pulse
generator of the inverter works, by virtually and practically
experimenting with the pulse generator itself creating the
necessary models in the popular platform of MULTISIM
(Simulation Tool of National Instruments) and
designing/constructing the respective PCB circuits in the also
popular platform of ULTIBOARD (Circuit Design Tool of
National Instruments).
Keywords — Pulse Width Modulation, Inverter, Simulation, Pulse
Generator, Educational Kit
I. INTRODUCTION
Nowadays, due to the excessive growth of renewable
energy resources the use of inverters finds a lot of applications
in our everyday lives. From a typical U.P.S., that we all might
have in our homes, to a whole Photovoltaic Park that produces
kilowatts of energy. Inverters are devices that convert a DC
source to a variable frequency/RMS value AC source.
Application Examples of an inverter:
 Control Systems of AC Motors
 Renewable Energy Systems (Photovoltaic-PV,
Wind etc.)
 Networks of Electric Energy Transfer Lines
 Production and Storage of Electric Energy Systems
 Reduction of Harmonics Active Filters for
Improvement of Electric Power Quality
 Static VAR compensators(SVCs)
 Uninterruptible Power Supplies (UPSs)
According to our research, although similar kits do
exist, they use different (mainly digital) technology which is
insufficient for undergraduate students to understand the basic
electric and electronic principles of the PWM inverter’s pulse
generator. Thereafter, we decided to construct a kit based in
analog comparators (Op-Amp IC’s) and logic gates, whose
modus operandi is way more understandable.
Firstly, theoretical analysis and methodology of a
PWM inverter is made clear enriched with graphs.
Furthermore, the circuit schematics are explained and
assumptions that were made by using MULTISIM and the
measurements that were made via MULTISIM with the
oscilloscope tool in the simulation process are shown.
Assumptions and all the parts used are listed. In addition, all
the PCB’s made using Multisim/Ultiboard are collocated.
Measurements at the PCB test points were taken with their
respective pictures-graphs taken by a real oscilloscope. Last
but not least, real time measurements and any practical issues
we encountered are explained and their respective solutions
and improvements in the prototype practical electronic circuit
that we made after the first few tests. Any conclusions are
made clear in the last chapter.
II. P.W.M. INVERTER METHODOLOGY.
Generally, by taking advantage of the topology of the
H-Bridge circuit, and by switching on and off the switches S1,
S4 and S2, S3 alternately and every 180o
, we achieve to
transmute our DC source (VIN) to an AC Voltage applied on
the Load Device.
Fig. 1 – An H-Bridge PWM Inverter Circuit demonstrated with parallel pair
of Diode-Switch.
Usually, instead of the parallel combination of diodes
and switches as shown in Fig. 1, controllable semiconductor
devices (Thyristors, GTOs, IGBTs, BJTs and MosFETs) are
used in these circuits (Fig. 2). Thereby, the only way we can
Educational PWM Inverter Pulse Generator
Eleftherios A. Samiotis1
, Dimitrios T. Trigonidis1
, Georgios Fokas1
, Georgios A. Vokas2
Student at dept. of Electronics Engineering, PUAS, samiotislefteris@hotmail.com
Assoc. Professor at Dept. of Electronics Engineering, PUAS, gvokas@puas,gr

2. interact with these circuits is by controlling when and whereas
the semiconductor devices are going to be activated or not.
This can be achieved by low power circuits called Pulse
Generators.
Fig. 2 – An H-Bridge PWM Inverter Circuit demonstrated with parallel pair
of Diode – Semiconductor Device (MosFETs).
A PWM Inverter is a device, as we already mentioned,
that converts a DC Voltage to a variable frequency or/and
variable RMS Voltage AC source like an invert air
conditioner where the 230V/50Hz of the electric power
network is converted to a DC Voltage by a high wattage
Power Supply and then this DC voltage gets inverted to a
variable frequency (not 50Hz) and variable RMS Voltage AC
(not 230V). A PWM inverter can be used to supply with
energy mostly ohmic and capacitive loads or devices while
for inductive loads or devices a similar device with a little bit
of different specs called SPWM Inverter is used. A PWM
inverter cannot be used to supply an inductive load as the
pulse that produces consists of infinite harmonics and so, the
THD factor is massive (over 100%) causing destructive
effects to the load/device. The connection of the load or the
device at the output of the inverter (LOAD R_L_C) is shown
to Fig. 2. The conversion by an H-Bridge PWM Power
Inverter of a DC to an AC Voltage is shown in Fig. 3.
Fig.. 3 – DC to AC Conversion by a PWM H-Bridge Inverter.
III. P.W.M. REAL IMPLEMENTATION (PCBS) AND
SIMULATION (USING NI’S ULTIBOARD & MULTISIM).
P.W.M. Generator Simulation (NI’s Multisim).
A. Assumptions for NI’s Multisim and design and
program results at the Test Points.
To begin with, our pulse generator is line
communicating, which means that it doesn’t have a built in
generator in order to generate the Sinus wave it uses for
comparison, but it uses a relegated sample from the Electric
Power Network with the help of a relegation (230V to 15V)
transformer. This also means that the working frequency of
the Inverter is locked at 50Hz like the frequency of the electric
power network. The entire philosophy of this pulse generator
for the P.W.M Power inverter is based into two simple
comparisons with the AC sample.
In order to make the necessary comparisons two
variable DC sources are needed. One positive, and one
negative. Changing the value of the DC source, the width of
the produced pulse changes too, and by modulating the pulse
width we achieve to affect the RMS voltage that the
load/device will consume. For instance, we can achieve full
(100%) power on an AC light bulb when the lighting
conditions are poor and we can also demand half the power or
less (<50%) when the lighting conditions are better where less
artificial light is required. The circuit that used both for
simulation and the PCBs is shown in Fig. 4.
Fig. 4 – PWM Circuit designed in Multisim.
Firstly, we compare the AC sample (red) with the
positive DC (orange) source, producing a series of Pulses
(blue) as shown in Fig. 5.

3. Fig. 5 – AC and High Positive DC Voltage Comparison and Output Pulse.
The output pulses of this comparator can much wider
if the positive DC Voltage is dropped near 0 Volts (Fig. 6).
Fig. 6 - AC and Near 0V Positive DC Voltage Comparison and Output Pulse.
Also, the same AC sample (red) is compared with the
negative DC source (orange), producing another series of
pulses (blue) as shown in Fig. 7.
Fig. 7 – AC and Negative DC Voltage Comparison and Output Pulse.
The output pulses of this comparator can much wider
if the negative DC Voltage is dropped near 0 Volts (Fig. 8).
Fig. 8-AC and near 0V Neg. DC Voltage Comparison and Output Pulse.
Both (Fig. 5) and (Fig. 7) Pulses (blue) end to the 1st
2-input AND logic gate. The Pulse (blue) produced in the
output of the gate (Fig. 9) is used to trigger the S1, S4
MosFETs (Fig. 2). Various output pulses can occur if the
length of the input pulses at AND1/2 gates change (Fig. 10).
Fig. 9 – AND1 Gate Inputs and Output.
Fig. 10 – A Variation on the AND1/2 output pulses when the compared DC
voltage changes.
Simultaneously, pulses from the comparators also end
to a NOT logic gate, producing pulses that they are inverted
versions from the previous pulses.

4. Both of those inverted pulses end to a 2nd 2-input
AND logic gate. The pulse produced in the output of the gate
(Fig. 11) is used to trigger the S2, S3 MosFETS (Fig. 2).
Fig. 11 – AND2 inputs and output.
The output pulses that will get to all S1, S4 (red) and
S2, S3 (blue) of the pulse generator for the P.W.M. Power
Inverter are shown in the next Fig. (Fig. 12).
Fig. 12 – Outputs to the power inverter S1, S4 (red) and S2, S3 (blue)
Also, if the compared DC voltage changes the total
output of the pulse generator of the inverter changes, as shown
in the next Fig.s (Fig. 13, 14).
Fig.s 13, 14 – Various Output levels of the inverter.
That is the case when different power output in
needed. Fig. 14 is when the inverter works at 100% power and
Fig.s 13 is when the output of the inverter is less.
B. P.W.M. Real Implementation Technique
Assumptions and Parts Used
Due to the availability of our university, instead of
comparator chips, we used operational amplifiers (Op-Amps)
without input and feedback resistors so they can work as
comparators. The op-amps used were two TI’s LF411 IC’s.
An AND gate chip (74LS08) and a NOT gate chip
(74LS04) were used for the logic functions.
For the power supply several parts and regulators were
used. Firstly, a 2x18 V/3 A transformer is used to reduce the
AC voltage from the Electric Power Network, then a full
bridge rectifier(PB1010) is used to rectify the voltage, and
some filter capacitors are used too(Electrolytic of Various
Values at 35V). After this, to supply the logic gates an
LM7805 voltage regulator is used.
For the positive variable voltage an LM317 regulator
IC was used. For the negative variable voltage a LM337
regulator IC is used, and this part of the power supply was
added afterwards. So, instead of making a new PCB we
created a smaller one to be connected with the main power
supply. Last but not least, to supply the op-amps a set of
LM78/7915 ICs are used.
Design of the PCB of the PWM Pulse Generator Circuit.
Using NI’s Multisim and Ultiboard, the circuits and
the printed board circuits were designed. The PCBs were
finally carved with a CNC Router at PUAS Electronics
Engineering Department with the help of our
technician/professor. The designs can be shown in the
pictures below, starting with the Power Supply Main
Multisim Circuit shown in Fig. 15.

5. Fig. 15 – Power Supply circuit by MS.
The Power Supply Main PCB is presented in Fig. 16.
Fig. 16 – Power Supply PCB by Ultiboard.
The Power Supply PS337 (secondary) Multisim Circuit is
presented in Fig. 17.
Fig. 17 – Secondary PS Circuit by MS.
The Power Supply (secondary) PS337 PCB is presented in
Fig. 18.
Fig. 18 – Secondary PS PCB by Ultiboard.
The Pulse Generator Multisim Circuit is presented in Fig. 19
Fig. 19 – Pulse Generator Circuit by Multisim.
The Pulse Generator PCB is presented in Fig. 20.
Fig. 20 – Pulse Generator PCB by Ultiboard.
The Final Boards after the Assembling and Soldering are
presented in Figs 21, 22.

6. Fig.s 21, 22 – The PCBs after the assembling and soldering.
Graphs from oscilloscope at the test points
After assembling and soldering the PCB certain tests
were made to be sure that the circuit works as it should work.
The only electronic measuring instrument used, was an
Agilent Technologies DSO1024A quad channel/200 MHz
oscilloscope.
In the picture below (Fig. 23), the pulses for the S1, S4
MosFETs (Yellow Color) and the pulses for the S2, S3
MosFETs (Green Color) can be shown.
Fig.s 23 – Output pulses at the TPs during the test runs in our Dept.
(Electronics) at University (PUAS).
IV. RESULTS AND DISCUSSION
A. P.W.M. Issues
The most important issue of a PWM is that it cannot
supply an inductive load, but only resistive or capacitive loads
and a combination of them.
Another issue is that in this generator the frequency of
the AC sample cannot be controlled and so the frequency of
the Output AC is locked to this of the electric power network
(50Hz/20ms), as it is line commutating.
Also, both the positive and the negative DC source at
the same time with a stereo potentiometer cannot be
controlled, because the two regulators respond differently to
the input. So, instead of one stereo potentiometer, two mono
potentiometers were used.
Finally, despite the fact that in the simulation perfect
pulses can be seen, into the oscilloscope several spikes due to
noise can be detected. The noise probably occurs due to the
fact that the parts that have been used are not of high grade
(space, military, etc.), they are of commercial grade, and they
are subside to environment conditions (PCB rust, humidity
temperature etc.) and probably have some defects from
production.
B. Improvements/Suggestions
After experimenting with the generator, several
improvements came up to the table making the generator
smaller, more efficient with lower cost of construction. More
specifically:
During the Design: In Multisim/Theoretical Design
Power Supply as presented in the following Fig. 24.
Fig. 24 – The circuit of the improved PS.
During the stage of PCB Power Supply as presented in
the following Fig. 25.
Fig. 25 – The PCB of the improved PS.

7. During the Multisim/Theoretical Design of PWM
stage, as presented in Fig. 26.
Fig. 26 – The Circuit of the improved PWM pulse generator.
Concerning the PCB PWM stage, as presented in Fig. 27.
Fig. 27 – The PCB of the improved PWM pulse generator.
First of all, we realized that using 15 volts to supply
the op-amps is useless when they can be supplied by the same
5 volts as the logic gates.
Also, the sample AC is now suppressed at 5 volts peak,
therefore only a 2x5V, 3A transformer (way cheaper than the
one previously used) is now needed.
In addition, instead of the variable regulators
LM317/LM337 classic regulators LM7905/LM7805 with a
potentiometer were used.
After all these modifications the power supply was
simplified. Also, we observed that under 5 volts power
supply, the circuit works more efficiently.
Regarding to the PWM itself a stereo potentiometer for
both regulators can now be used to control the comparisons.
Taking the above into account, the output of the
improved circuit as simulated in Multisim is presented in the
following Figs 28 and 29.
Fig. 28 – Output pulses (pink – S1, S4), (L. blue – S2, S3) of the improved
PWM generator with high negative (blue) and positive DC voltage (green)
compared with the AC (red) of the network
Fig. 29 – Output pulses (pink – S1, S4), (L. blue – S2, S3) of the improved
PWM generator with high negative (blue) and positive DC voltage (green)
compared with the AC (red) of the network.
V. CONCLUSIONS
A PWM inverter is amongst the most useful devices in
the world of power electronics. The aim of this paper was to
provide an educational PWM design and implementation
procedure as well as the corresponding power part of a dc/ac
inverter. The major parts of this procedure were presented,
while improvements in design and hardware elements are
proposed. The students can observe every part of the pulse
generating procedure in both simulation and real circuit. As
future work, an SPWM pulse generator is to be designed and
implemented in the existing power part of the inverter having
the possibility to verify the capability of the inverter to
eliminate the harmonic components as designed.
REFERENCES
 Microelectronic Circuits 6th Edition, Adel S. Sedra, Kenneth C.
Smith, 2016.
 Power Electronics: Circuits, Devices & Applications 1st Edition,
Muhamad H. Rashid, 2015.
 Power Electronics 4st edition, Stefanos Manias, 2016.
 http://www.ti.com
 http://www.st.com
 http://www.ni.com
 NI Multisim: Circuit Design Suite Pro 12.0.
 NI Ultiboard: Circuit Design Suite Pro 12.0.