628 Journal of Power Electronics, Vol. 10, No. 6, November 2010

JPE 10-6-7

Distance Education in Soft-Switching Inverters
Dan Lascu†, Pavol Bauer∗, Mircea Băbăiţă∗∗,

Mihaela Lascu∗, Viorel Popescu∗, Adrian Popovici∗, and Dan Negoiţescu∗

†∗ Dept. of Applied Electronics, Politehnica University Timişoara, Romania
∗∗ Dept. of Power Electronics and Electrical Drives, Delft University of Technology, The Netherlands

Abstract

The paper describes aspects regarding an E-learning approach of resonant ac inverters. The learning process is based on
“Learning by Doing” paradigm supported by several learning tools: electronic course materials, interactive simulation, laboratory
plants and real experiments accessed by Web Publishing Tools under LabVIEW. Built on LabVIEW and accompanied by a
robust, flexible and versatile hardware, the experiment allows a comprehensive study by remote controlling and performing real
measurements on the inverters. The study is offered in a gradual manner, according to the Leonardo da Vinci project EDIPE
(E-learning Distance Interactive Practical Education) philosophy: theoretical aspects followed by simulations, while in the end
the real experiments are investigated. Studying and experimenting access is opened for 24 hours a day, 7 days a week under the
Moodle booking system.

Key Words: Engineering education, Remote controlled experiments, Internet-based distance measurement, Courseware, Resonant
inverters.

I. INTRODUCTION
The rapid changes in society and technology have also

generated a demand for more flexible engineers having more
qualifications than just a high level of technical or scientific
specialization. Distance learning has been promoted across
the entire education sector due to the increasing number of
people that educate themselves as part of their professional
development. The drawback of a pure theoretical approach in
a curriculum is that less attention is paid to the phenomena
looming around laboratory experiments and real components.
The results of this, corroborated with the rapid development of
computer simulations, were that hands-on laboratory experi-
ence was vanishing and that computer simulations gained more
and more attention. However, it is of crucial importance for
the student to gain practical experience. Physical experiments
help the students in practical testing and allow them to see
the influence of second and higher order effects or parasitics
that are often difficult to simulate as in reality. Hence it is
of great importance to make the student familiar with the real
world experience. Although classical hands-on laboratories are
very useful, they may have limitations regarding space, time
and staff costs. These problems can be significantly alleviated
by using remote experiments and remote laboratories, when
the students operate with real systems, although they are not
present in the laboratory. For engineering related distance ed-
ucation the use of a web-based delivery mechanism is the only

Manuscript received May 3, 2010; revised Aug. 19, 2010
† Corresponding Author: dan.lascu@etc.pt.ro

Tel: +40-256-403343, Fax: +40-256-403295
∗ Dept. of Power Electronics and Electrical Drives, Delft University of

Technology, The Netherlands
∗∗ Dept. of Applied Electronics, Politehnica University Romania

realistic method for providing hands-on experience, allowing
remotely located students to complete laboratory assignments
unconstrained by time or geographical considerations. They
can change parameters, perform experiments, observe results
in graphical or numerical form and download them [1], [2].
Modern education shifts emphasis from teaching to learning,
assuming that knowledge is not transferred, but the learner
himself constructs knowledge on the basis of prior knowledge
and additionally acquired information. While in the teaching-
oriented approach the student is rather passive, in the vision of
learning-oriented the student plays an active role, constructing
knowledge on the basis of prior knowledge and additionally
acquired information, with teaching as a facilitating precondi-
tion. This approach is referred to as constructivism and neo-
constructivism [3], [4]. In this spirit, in order for the “Learning
by doing” or “Learning by experimenting” approaches to be
successful, clear learning objectives have to be formulated.
The course module “Resonant AC Circuits” presented in the
paper is integrated in the Leonardo da Vinci EDIPE program
framework. It is dedicated to bachelor students, particularly
to the students in Politehnica Univesity Timişoara enrolled
in Telecommunications Systems and Technologies studies but
also to anyone interested in understanding resonant inverters
operation.

II. LEONARDO DA VINCI PROJECT AND ITS
PHILOSOPHY

The Leonardo da Vinci project EDIPE was approved to
create a full set of distance experiments called PEMCWebLab
in an integrated learning platform, providing the user with a
practical experience in Power Electronics and Electrical Drives



Distance Education in Soft-Switching Inverters 629

Fig. 1. Structure of the distance laboratory for E-learning. practical
teaching of resonant inverters.

Fig. 2. The booking window

education. The participants are twelve universities with the
span across the EU. The results were:

• Elaboration of clear learning objectives for distance ex-
perimental education,

• Guidelines for project oriented measurements,
• Synthesis oriented experimental work,
• Technology and technical documentation for distance

practical education and measurements via Internet,
• Different experiments, each with its own specificity.
The learning issues addressed in the PEMCWebLab inte-

grated learning platform are:
• Learning objectives;
• Education;
• Animation and Simulation;
• Experiment.
The order of these issues is important. For safety reasons no

student will be allowed to perform any experiment until he or
she has proofed adequate knowledge regarding the requested
experiment. First the learning objectives are defined, while a
theoretical background of each individual experiment is given
in part Education. For the “Resonant AC Circuits” module the
learning objectives are summarized below:
1. Prerequisites: Minimum knowledge on sinusoidal circuits

analysis, including phasors. Elementary transfer functions
calculation and interpretation.

Fig. 3. Hardware architecture of the “Resonant AC Circuits” module.

2. Series resonant inverter (SRC) behavior. Resonant fre-
quency, characteristic impedance and quality factor def-
initions and significance.

3. Understanding switch implementation in a SRC loaded
bridge topology. Applying and handling with sinusoidal
approximations in high quality factor resonant circuits.
Elementary zero current and zero voltage switching phe-
nomenon understanding.

4. LCC resonant inverter behavior. Resonant frequency, and
quality factor definitions and significance. Open circuit
output voltage and short circuit current values. Elliptical
nature of the output characteristic of a resonant circuit.

5. Other resonant inverters: parallel resonant inverter (PRC),
LLC inverter and dual of the SRC.

Interactive animation and/or Simulation are the steps pre-
ceding the real Experiment.

After completion of the real experiment the students are
given a questionnaire and they have to submit their report for
the final evaluation. All learning procedures are recorded for
future reference and analysis.

Studying and experimenting access is opened for 24 hours
a day, 7 days a week.

III. SYSTEM CONFIGURATION
Each experiment has its own server because it is located in a

different location, as Fig.1 shows. Remote users first log onto
a main booking server, located at the University of Maribor,
after which they will be directed to the specific server for
actually performing the experiment into a separate individual
page. The booking system is based on the Moodle course
management system [5]. In addition to the original booking
system, some modifications have been made [6], enabling
easy booking creation for those remote experiments that were
developed under the LabVIEW Web Publishing Tools. The
layout of Moodle pages for all experiments is uniform. The
Moodle window for booking the experiment is presented in



630 Journal of Power Electronics, Vol. 10, No. 6, November 2010

Fig. 4. The Switching Board Interface block diagram.

Fig. 2. The users are divided in two groups: authorized users
and guests. Only authorized users can control the experiment.
The guests can only observe measured values and waveforms
but they have no control of any part of the system.

The local server uses LabVIEW [7] for controlling the
experiment. The Web Publishing Tools that come with Lab-
VIEW are used both for interfacing and remote controlling
the experiment, namely to choose the converter type, select
the desired waveforms and perform different measurements.
The proposed hardware solution depicted in Fig. 3 is based
on a so called “Power Board” manufactured in Politehnica
University Timişoara that offers good modularity and simple
connectivity. The PCI-MIO-16E-1 DAQ card together with
the SCB-68 shielded I/O connector block for DAQ devices
are used for acquisition and measurements. The Switching
Board Interface (SBI) is controlled from LabVIEW and its
purpose is for configuring the desired inverter topology and
to select the signals for data acquisition. Its block diagram is
presented in Fig. 4. It contains a 32 bit shift register, each
output being buffered in order to be able to switch on and off
a relay matrix. As the NI PCI-MIO-16E-1 acquisition board
is used, the three inputs I1, I2 and I3 in the register area
are connected to the digital outputs O1, O2 and O3 of the
acquisition board respectively. The registers are 8-bit serial-in
parallel-out D-type SN74HC594 [8] shift registers. The storage
clock SRCLK of each register is connected to digital output O1
of the acquisition board, while output O2 is connected to each
shift register clock RCLK input. Both clocks are positive edge
triggered. Serial input data is provided by the digital output
O3. The parallel outputs of the registers form a 32-bit word,
each bit addressing a relay in the relay matrix.

For buffering purposes, four ULN2804 high-voltage high-
current Darlington arrays are used, allowing up to 500mA for
driving the inductor coil of the relay. The relays play a double
role: they contribute in the configuration of the desired inverter
topology while other relays provide the signals of interest
to the acquisition board as the number of analog inputs is
limited. In order to ensure safe operation, the order the relays

are switched is important, namely the configuration is settled
without connecting the power supply. Only after that the power
supply is connected. When finishing an experiment the power
supply is disconnected first, then the rest of the power circuit
and measured signals.

IV. DESCRIPTION OF THE LEARNING PROCESS

LabVIEW main program operates according to the flow
chart depicted in Fig. 5. Functionally, it consists of two main
WHILE loops: one for selecting the experiment from the list
and one for performing measurements when an experiment
is activated. In both loops, for security reasons, the program
monitors the user’s connection state such that if the remote
panel window is left without normally exiting the program,
the experiment is automatically closed and a warning message
is displayed. Otherwise, the experiment would remain perma-
nently connected, which could be dangerous for the power
circuit. All LabVIEW classical buttons are disabled except
for the START button, such that only the program buttons
are available for the user. This allows that as long as the
power circuit configuration is in establishment or the program
is switching from one experiment to another, no control is
available for the user, again for safety reasons.

Simulations are performed before any experiment in order
to understand the basic operating principles of the resonant
converter under study. The component values are the same as
those that will be used later for breadboard. Simulations are
performed using the Caspoc package [9]. The user friendly
interface allows one to build and change the power circuit
using drag and drop procedures. Another interesting feature of
Caspoc is the animation capability which is extremely useful
in switching circuits, as it reveals the on and off devices at
a certain time moment and the currents paths as well. Of
course, the simulated waveforms are obtained without parasitic
elements or measurement noise, which is quite an advantage
as it offers the possibility to compare them to the real ones and
notice the differences. For example, the simulation results for



Distance Education in Soft-Switching Inverters 631

Fig. 5. Flow Chart of the main LabVIEW program.

the LCC resonant inverter below resonance are depicted in Fig.
6. State plane trajectories are also possible. The hard turn on
and soft (zero-current) turn off transitions of the transistors are
obvious. The student will have to decide whether it is possible
to replace the transistors with naturally-commutated thyristors
and to justify the answer. From the simulation results it is
also clear that diode turn-off transition is not lossless. This is
a good starting point to put the student to anticipate what will
happen when real diodes will be used. Then he or she will
be asked to provide a solution to assist the transistors turn on
process and finally the small inductors introduced into the legs
of the bridge will be the expected answer. Each of these will
be verified by simulation, which is very simple as they imply
minor modifications of the circuit.

Corresponding to the two mail WHILE loops in the main
program two panels are handled by the user. When entering the
experiment the Selection Front Panel (SFP) is displayed. It is a
simple panel where the user only selects the desired topology
for investigation: series resonant inverter (SRC), LCC inverter,
parallel inverter (PRC), LLC inverter or the dual of the series

Fig. 6. Simulated waveforms in the LCC resonant inverter: resonant
current, transister current and transistor corresponding freewheel diode, this

up to down order.

inverter. An example of SFP is presented in Fig. 7 for the LCC
inverter. The panel dedicated for measurements, visualizing
waveforms and for changing parameters is the Measurement
Front Panel (MFP). For the LCC resonant converter the MFP
is shown in Fig. 8. In the main program each experiment
is assigned its own DAQ Assistant. This individualization is
necessary because each experiment has its specific signals
set required to be analyzed. Any signal from the list can be
displayed with a selectable gain and analyzed separately or
together with any other signal or removed from the displayed
signal list. As two or more signals can be displayed in a
scope, phase shift between them can be revealed which is
important in studying zero voltage or zero current switching.
Acquisition can be frozen any time thus facilitating to perform
measurements on the displayed waveforms using cursors. The
user can measure time, dc, amplitude or rms of harmonics,
overall rms, phase shift and total harmonic distortion (THD).
The spectrum is also available in the dedicated spectrum
analyzer.



632 Journal of Power Electronics, Vol. 10, No. 6, November 2010

Fig. 7. Selection front paid

As resonant inverters operation strongly depends on the
switching frequency that is the main control parameter, the
switching frequency can be adjusted from the MFP with some
upper and lower default limits for safety reasons.

In each experiment the equivalent quality factor is estimated
and the student is asked to draw a conclusion in what circum-
stances the sinusoidal approximation [10] is valid and how
the real measurements match or not with the results provided
by the sinusoidal approximation. Emphasis is devoted also
to device stresses such as peak resonant currents and peak
resonant voltages. For the LCC and LLC inverters the other
important frequencies such fm and f∞ that result from the Bode
diagrams [10] are put in evidence.

As an example, the same LCC inverter simulated in Fig.
6 is practically investigated. The waveforms are those in Fig.
8 when the switching frequency is chosen to be 4 kHz. Soft

zero current switching turn off can be observed. Compared to
the simulation, it can be seen that the hard turn on transistor,
similar to PWM, is accompanied by high current spikes.
The high current spikes are due to the freewheel diode in
antiparallel with the transistor in the same leg, corroborated
with the energy stored in the drain-to-source capacitances and
in the depletion layer capacitance of diodes, that is also lost
when the transistor turns on. This is a typical phenomenon
when the LCC converter is operated below resonance.

Figure 9 presents the same converter when operated at
7kHz, that is above resonance. It is obvious that soft zero
voltage switching turn on is achieved since the transistors
conduct after their respective antiparallel diodes.

On the other side, it can be seen that transistor turn off
transitions are not lossless as neither the transistor current nor
the transistor voltage is zero. Different other tasks have to be
performed by the students, such as operation above resonance,
operation at fm and f∞, in each situation examining the soft
switching condition. Then, for a fixed switching frequency,
the student will be asked to measure the amplitude of output
voltage and current fundamentals, in open circuit, short circuit
and for different resistive loads. Finally he has to come out to
the elliptical nature of the output characteristic.

V. CONCLUSIONS

The PEMCWebLab was conceived as a complete educa-
tional solution in the field of Power Electronics and Electrical
Drives. The “Resonant AC Circuits” module described in the
paper offers five types of resonant inverters to be studied.
The simulation and animation before any experiment help the
student to understand the basic operation principles of these
topologies. Although simulation is useful, the real distance
experiments allow the user to see the differences between ideal
and real waveforms affected by nonidealities and parasitics.
The students can both visualize the main waveforms and to
perform a lot of measurements on the acquired signals. In
Politehnica University Timişoara a Distance Learning Cen-
trum (see http://www.csid.upt.ro/) exists where students from
different companies of electronic profile do their studies in
electronics. We also carried out an investigation focused on
students’ opinions and views about this modern teaching tool
and how they perceive the E-learning concept. They were
asked to fill in a questionnaire in which an evaluation of
the remote laboratory was performed. Their opinions were
favourable and they really got it as an educational benefit.

ACKNOWLEDGMENT

This work has been performed within the project ”Elearn-
ing Distance Interactive Practical Education (EDIPE)”. The
project was supported by the European Community within
framework of Leonardo da Vinci II programme (project No
CZ/06/B/F/PP-168022). The opinions expressed by the authors
do not necessarily reflect the position of the European Com-
munity, nor does it involve any responsibility on its part.



Distance Education in Soft-Switching Inverters 633

Fig. 8. Measurement front panel – LCC resonant converter experiment, fS = 4kHz.

Fig. 9. Measurement front panel – LCC resonant converter experiment fS = 7kHz



634 Journal of Power Electronics, Vol. 10, No. 6, November 2010

REFERENCES
[1] P. Bauer, J. Dudak, D. Maga, V. Hajek, “Distance practical education

for power electronics,” International Journal of Engineering Education,
Vol. 23, Issue 6, pp. 1210-1218, 2007.

[2] P. Bauer, V. Fedak, V. Hajek, “Survey of distance laboratories in power
electronics,” 39th IEEE Power Electronics Specialists Conference (PESC
08) Records, Vol. 1-10, pp. 430-436, Jun. 2008.

[3] O. Rompelman and E. de Graaff, “The engineering of engineering
education: curriculum development from a designer’s point of View,”
European Journal of Engineering Education, Vol. 31, No 2, pp 215-
226, 2006.

[4] D. H. Jonassen, K.L. Peck and B.G. Wilson, Learning with Technology:
A Constructivist Perspective, pp. 2-18, 1999.

[5] Moodle - www.moodle.org
[6] D. Hercog, B. Villaca, B. Gergič and M. Terbuc, “Moodle booking

system for remote experiments,” Proc. Int. Symp. Remote Eng. Virtual
Instrumentation, pp. 1-8, 2006.

[7] National Instruments, LabVIEW Measurements Manual, 2007.
[8] Texas Instruments - www.ti.com, Apr. 11th, 210.
[9] Caspoc, Simulation Research - User Manual - http://www.simulation-

research.com, Apr. 11th, 2010.
[10] R. W. Erickson, D. Maksimović, Fundamentals of Power Electronics –

second edition, Kluwer Academic Publishers, 2001.
[11] S. Ang, A. Oliva, Power-Switching Converters, Second Edition, CRC

Press, Taylor and Francis Group, 2005.

Dan Lascu was born in Timişoara, Romania, on June
30, 1961. He received the B.Sc. and M.Sc. degrees in
electrical engineering and the PhD. degree in electron-
ics from Politehnica University Timişoara, Romania.
Since 1990 he has been with the Politehnica University
Timişoara, Applied Electronics Department, and since
2007 he has been a full professor in the Power Electron-
ics Group. His current research is in the field of switch-
ing converter synthesis, converter modelling, converter

simulation, active power factor correction and soft-switching techniques. Dr.
Lascu is a member of the Romanian Electronics Engineer Association (AIE)
and IEEE member.

Pavol Bauer received his Masters in Electrical Engi-
neering at the Technical University of Kosice (‘85) and
Ph.D. from Delft University of Technology (’95). Since
1990 he is with the Delft University of Technology,
currently as an Associate Professor at the Dept. of.
Electrical Power Processing. He has published over
210 journal papers and papers at the international
conferences in his field, holds an international patent
and organized several tutorials. He is teaching Power

Electronics, Electrical Drives and related subjects. Dr. Bauer is a senior
member of the IEEE member of EPE and also member of international
steering and scientific committees of numerous international conferences.

Mircea Băbăiţă was born in Orastie, Hunedoara, Ro-
mania on July 18, 1964. He received the degree in
electrical engineering from the Politehnica University
of Timişoara in 1989. Since 1991 he has been with
the Politehnica University Timişoara, Faculty of Elec-
tronics and Telecommunications, Applied Electronics
Department. He had 6 weeks scholarship at University
of Strathclyde, Glasgow, Scotland – 2000, 2 weeks
scholarship at Uninersity of Mannheim, Germany –

2002. He has published more than 50 technical papers, 6 books and 20 grants
in the field of digital logic, motor drives and power electronics. M. Băbăiţă
is a member of the Romanian Electronics Engineer Association (AIE) and a
member of the Romanian Robotics Engineer Association (AIR).

Mihaela Lascu was born in Timişoara, Romania, on
May 7, 1962. She received the degree in electrical engi-
neering from the Politehnica University of Timişoara in
1986 and the PhD. degree in electromagnetic compat-
ibility and measurement techniques in 1998. Now she
is associate professor with the University of Timişoara.
Her current research is the field of graphical program-
ming, biomedical signal processing and numerical tech-
niques applied in electromagnetic compatibility. Dr. M.

Lascu is a member of the Romanian Electronics Engineer Association (AIE).
and a member of the Romanian Association of Electromagnetic Compatibility
(ACER).

Viorel Popescu was born in Hitiaş, Romania, on April
15, 1947. He received the M.Sc. degree in electrical
engineering and the Ph.D. degree in computer engi-
neering from Politehnica University Timişoara in 1969
and 1981 respectively. Since 1970 he has been coped
in the Power Research Team at the Electronics and
Telecommunications Faculty from Politehnica Univer-
sity Timişoara, where he is currently a full professor of
Power electronics since 1993. He has published more

than 100 technical papers and 7 books in the field of switched-mode power
supplies, monitoring, control and diagnostics of drives and robotics. His
currently research interests include harmonic pollution produced by electronic
power systems, PWM techniques for power converters, active power filters
and intelligent control of power electronics equipment. Prof. V. Popescu is
a member of Romanian Electronics Engineer Association (AIE) and IEEE
member.

Adrian Popovici was born in Timişoara, Romania on
February 5 1964. He received the B.S. and PhD degrees
in electrical engineering from Politehnica University of
Timişoara in 1989 and 2004 respectively. Since 1991
he has been with Politehnica University Timişoara,
Faculty of Electronics and Telecommunications, Ap-
plied Electronics Department. His research interest is
in matrix converters, low power systems and materials
for electronics.
Dan Negoiţescu was born in Timişoara in 1966. He
received the B.S. and PhD degrees from Politehnica
University of Timişoara in. 1992 and 2005 respec-
tively. Since 1994 he is with Politehnica University of
Timişoara presently being an associate professor in the
Power Electronics Group. His main research concerns
active power filters, switched-mode power supplies and
modern control in power electronics