Interactive JAVA Applets for Power Electronics E-Learning* Shadi Harb, Kalaid Kalaldiah*, Ahmad Harb and Issa Batarseh Department of Electrical and Computer Engineering College of Engineering and Computer Science University of Central Florida Orlando, Florida 32816, USA
[email protected]
Abstract—Due to the high nonlinearity and the mathematical complexity of power electronics circuits, their analysis has become more infeasible to describe. Power Electronics Education is a daunting, challenging task and many undergraduate students find it very difficult to grasp the basic concepts of many power electronics circuits and understand their physical and operational modes. In this paper, an interactive power electronics teaching Web site is propose and, three main Web pages are developed for the students, the instructors and the interactive Java Applets. Interactive online materials are posted for both students and instructors including lecture nodes, simulation problems, examples and exercises, textbook presentations, and online exams. A feedback form is used to get the input of students and instructors on the Web site. Interactive applets are developed for 29 power electronics circuits to help students better understand the circuit behavior under certain circuit parameters variations. Exact steady state waveforms are plotted with flexibility to have students change different circuit parameters and observe the circuit response in an interactive manner. This project represents the first phase in the development of interactive sites in power electronics education. The current Web site is located at www.batarseh.org. Phase two will focus on closed loop response and dynamic modeling, and phase three will emphasize the system design of power electronic circuits. Keywords: Power Electronics Teaching, Power Electronics Education, Interactive Web site, Student Web page, Instructor Web page, Interactive Applets.
I.
INTRODUCTION
The recent advances of technologies for the Internet and for telecommunications has put online education at the forefront and given it the key role in leading the modern education methodologies. The on hand technology allows the Internet to be used as ideal medium in E-Learning, which has a place in both academia and industry for the following reasons. First, E-Learning is an effective and flexible way of delivering a formative content in terms of time and cost due to the rapid dissemination of information technology and the lake of constraints of physical class attendance. The second advantage of E-Learning is finding an educational standardized template for online courses. Third, E-Learning *Partially funded by APECOR Corp, 3259 Progress Drive, Ste A Orlando, FL 32826, USA, and NSF Int-0423645. ** Ketab Scientific Solutions, Inc, Amman, Jordan
provides new methodologies of assessing students objectively and maintaining academic standards such as online exams, bulletin boards, chatting rooms, Web conferencing and so on. Finally, although E-Learning can’t stand alone education, E-Learning is at least a way of identifying and hosting only content for fields that may not require active teacher participation. Over the last 10 years, several national efforts have been initiated to develop multi-media and Web-based education material. Most of these efforts focused on developing Graphical User Interfaces (GUI) for the purpose of “display,” but with limited interactivity. For example, some of the Web-based simulation sites are predefined and only limited to specific engineering design examples. It is shown that the engineering course Web pages display specific engineering problems, and students are asked to address these problems by following step-by-step instructions. The next section discusses the importance of E-Learning in power electronics education and how interactive applets help describe highly nonlinear and multi-disciplinary fields in power electronics. In Section III, a general overview of the interactive Web site has been presented. The detailed Web components are described in Section IV. Section V presents some DC-DC design examples in Java applets with all detailed design steps. Future plans and modifications are proposed in Section VI, and the conclusion is given in Section VII. II.
E-LEARNING IN POWER ELECTRONICS
Education in electrical engineering and specifically in the Power Electronics area is made difficult by the limitation of the topics that can be realistically learned by students within traditional courses. In the industry of power electronic, only limited time and resources are allowed for transferring up-to-date knowledge to develop new competitive products. Hence, the use of multimedia tools in delivering power electronics education to students and engineers is a very important topic to explore. Due to the multi-disciplinary and highly nonlinearity of the power electronics field, the field has evolved rapidly with advances in technologies and the introduction of many new applications areas. As a result, there are pressing needs to get the knowledge and skills more effectively to students and to apply the knowledge and capabilities on system design problems.
Design engineers today become overwhelmed or often more confused due to the almost infinite number of online resources and information related to power electronics and its applications that companies, manufacturers and individuals put on the Web claiming support for professionals. Therefore, there is a rising need to develop online computing materials that are capable of performing a comprehensive electrical circuit analysis and designing and modeling power electronic circuits. Such a solution would greatly improve s student’s learning capabilities and enhance the utilization of the Web’s resources in today’s educational system. Because of the nature of the field, teaching power electronic courses is a daunting challenge for electrical engineering faculty. Many of the engineering courses are being taught and delivered through multi-media means, such as the Internet, through video streaming and the Web or through other means such as video, CD and email. As a result, power electronics courses should be delivered similarly. The interactive teaching of power electronics will no doubt simplify the topics and improve the design capabilities. The proposed interactive applets, with their instructional design, have shown great advantages to better depict and understand the power electronics circuits operational modes, which are more infeasible to be described by bunch of highly nonlinear complex equations. Interactive applets have the capability to design and trace circuit modes by varying the input parameters and carrying out a real time simulation with the capability to visualize the circuit parameters and observe the impact of their variations on the design. III.
WEBSITE COMPONENTS DESCRIPTION
The online Web site, “Teaching Power Electronics,” located at www.batarseh.org, was organized into the following three main Web pages: the student, the instructor and the JAVA applets. The student and the instructor Web pages have access to many online materials in power electronics education. Some of the materials are shared for both students and instructors, and some of them are restricted only for instructors. Such shared online materials are Lecture Notes, Simulation Problems, Examples and Exercises, Web site-Based Courses, Textbook Corrections, and Feedback and Interactive Applets. Special materials are added only for instructors such as Power Point Presentations from Textbook, Textbook and Problem Figure Presentations, Textbook Corrections and Feedback, Solution Manual and Sample Exams. Useful links are posted also for both students and instructors for more online references in power electronics teaching. The main homepage for the Web site is shown in Figure 1.
Figure 1: Overview of the Web Site Homepage IV.
DETAILED ONLINE WEB MATERIALS
The Web site has posted a large amount of online materials in power electronics education, some materials are shared by both students and instructors, and some of them are only for instructors for use as described below: 1) Lecture Notes: The site has lecture material based on the “Power Electronics Circuits” textbook, and the lecture notes cover the textbook chapters in addition to the practical exams taken from the preliminary course (EEL 5245 – Introduction to Power Electronics), which is taught at University of Central Florida in power electronics. 2) Simulation Problems, Examples, Exercises: This material provides both students and instructors with diversity of problems, examples and exercises in power electronics circuits. Several suggested textbook problems are posted with their solutions under the following links: Solved Examples Using PSPICE & MathCAD, Solved Problems using PSPICE & MathCAD and Solved Exercises using PSPICE & MathCAD. Some problems are associated with real simulation files, which are made by either PSPICE or MathCAD or both. Other problems are described only by their textual and graphical final results. 3) Website Based Courses: Few Web-based online courses are posted online for power electronics education. The site posts courses, such as Power Electronics I-EEL 5245, which is taught on the WebCT platform at University of Central Florida and the Interactive Power Electronics Seminar (IPES). 4) Textbook Corrections and Feedback: The site contains the recent textbook corrections for each chapter. Each correction provides the page number, the error and the correction. A feedback form is provided to get input for the online material and the interactive applets. 5) Useful Links: Several useful links to academic and research agencies are posted for both students and instructors, such as the UCF Florida Power Electronic Center (FPEC), IEEE, IEEE Power Electronics Society, NSF (National Science Foundation) and NASA.
6) Power Point Presentations from Textbook: Textbook presentations are prepared for each chapter based on the “Power Electronics Circuits” textbook. 7) Textbook and Problem Figures Presentations: A List of textbook figures and problem figures are posted for each chapter in the textbook. 8) Solution Manual: A solution manual for the “Power Electronics Circuits” textbook can be obtained by contacting John Wiley & Sons, Inc. Web site (www.wiley.com/college). 9) Sample Exams: The site provides sample exams and quizzes with interactive student self-assessment capabilities, which are used by the instructor in the power electronics class. Several questions are suggested for each main topic in power electronics. 10) Interactive Applets: The interactive applets show real time simulations for several basic power electronics topologies with the flexibility to change the input circuit parameters and observe the real output waveforms correspondingly. Such power electronics topologies are classified as following; DC-DC converters, Rectifiers and Inverters. Applets Development Process: Two main simultaneous processes are cooperated together to develop the java applets software: the Matlab modeling and the instructional design process. 1. Matlab Modeling: Matlab modeling is a mathematical modeling (M-Model) constructed from preprocessed data that contains reduced equations with their labeling and other metadata. These MModels are capable of mapping the concept to a real-world manipulative environment. The mathematical modeling generation is described in the Figure 2.
simulation engines to support both instructor lead and selfpaced learning. Development Stages of an Applet: Each applet has the following four development stages: circuit analysis, circuit modeling, checking fidelity of generated results and Java outputs, as shown in Figure 4 below.
Figure 4: Development Stages of an Applet Graphical Design (GUI): The GUI is presented in a more informative way and is easy to use with more user-friendly buttons and graphical panels, as shown in Figure 5.
Figure 2: Mathematical Modeling In the next development phase, the M-Models and their descriptive contents are coordinated together and assembled into a uniform programming language, namely JAVA. Circuit modules are developed and tested by Matlab and then transformed into JAVA as shown in Figure 3.
Figure 5: GUI Overview GUI Online Tutorial: An interactive online tutorial and help topics have been developed to show how to use java applets on the circuit design development. Table 1 shows the GUI toolbar description. Table 1: GUI Toolbar Description
Figure 3: M-Model Transformed into JAVA 2.
Instructional Design: Instructional Design illustrates how to better present the concepts, convey the objectives of the course in a pedagogical way and appropriate it to suit the targeted audience. Interactive tutorials are accompanied with the
Name Add Graph Position Pointer
Functionality Adds graph chosen from menu.
How to use Click the graph title in the menu.
Icon
Zoom Box Hand
Show Interval Lines
Change the magnification level in the graph for specific area. Control which part of the graph is visible.
Click and drag to select the zoom area then click in the selected area. Click and drag.
Shows the interval Lines and time interval between them.
Button up: lines are invisible. Button down: lines are visible, to show interval between the two lines Drag first line to initial point and the other in the end point, the interval is shown at the status bar below the graphs. Button up: grid is invisible. Button down: grid is visible. Button up: graphs are controlled* separately. Button down: graphs are controlled* simultaneously. *controlled: (zoom, zoom box, hand and interval lines) Note: when Lock graph is enabled all graphs will be scaled automatically.
Show Grid
Show/hide the grid lines.
Lock Graph
Unify the tools (zoom, zoom box, hand and interval lines) functionality though all graphs simultaneously.
Undo
Go back to the previous view (zoom, zoom box and hand). Go forward to next view (zoom, zoom box and hand).
Redo
Reset
Reset the circuit parameters into its initial
Scale Auto Scale
Help
conditions and scale the graphs. Display fit view for all graphs. Display fit view for all graphs while changing the circuit parameters. Displays help window. V.
Button up: Auto Scale is on. Button down: Auto scale is off.
DESIGN EXAMPLES
Resonant Zero Current Switching Buck Converter (Fig.6) A. Design Aspects: The rationale behind designing ZCS converters for educational purposes is slightly different than that of industrial design matters. The main purpose of the applet for this circuit is to show how the system figures out the switching time to switch on or off the Zero volt/current. Hence, the clock frequency (f) and its duty cycle (D) should be determined to match a certain value that will make the charging waveform IL (Inductor current) reach zero, at which point the clock and the resonant switch implement a ZCS design by switching from a high/low then low/high state periodically. For the purpose of argument, the gain of this converter was made available as a variable parameter from (0.1-0) and the load R was initially set to 100ohm. Remembering that this converter is a zero-current switching converter, its operation is quite simple in view of charging and discharging behaviors of both C and L. The components of the resonant switch are also variable parameters.
Figure 6: Circuit Diagrams of (a) Buck and (b) Resonant Buck
B. Power of Applet Tool: The real power of the applet tools, besides solving power electronics I-V equations, is the ability to use this tool as an animation/simulation engine. The current direction and magnitude is animated in correlation with parameter values and circuit operation. Changing the parameter value is made easy while viewing the immediate effect on the waveforms generated. The waveforms are real value oscilloscope-like waveforms, but also track the location of the red ball on the waveform, which can be moved along the waveforms manipulated empowers the user to see all the relations between current value direction, path with real values of voltages and currents on the waveforms. Refer to the figures' explanations below for more insights into this experience.
3.
Figure 9.0 (C): The circuit has just came out of resonance t=t2. Two things will happen: (1) the switch will open at IL(t2) =0, and L is disconnected from the buck network and (2) charging is now handed over to C after it had received an abundance of it from L. The buck converter is now dependant on C to maintain its I0 current (notice the direction of current in the capacitor in comparison with Figure 7). 4. Figure 9.0 (D): At t=t3, the buck equivalent current source — for a period (T-t3) — is in a standalone position. No charging is fed either from L or C. The buck converter with the switch off is illustrated in the next design example.
Figure 8: ZCS Buck Converter at Resonance
Figure 7: Applet GUI Snap Shot 1 1.
Figure 7.0 (A): The ZCS at the start is t=0, and the source vss starts charging L linearly such that IL(t) =Vss(t)/L. This charging mode continues until t=t1, where the diode starts to conduct. This charge is retained in the buck network (current source equivalent I0) to maintain the buck load current on an average I0. 2. Figure 7.0 (B): After feeding the buck circuit (current source equivalent) from the current of IL, the converter is just at the threshold of entering resonance t=t1, and the diode will be open circuit.
Figure 9: ZCS Buck Snap Shot 2
C. Calculation Procedure (Double-Checking Against Design Equations): The following procedure verifies and shows how the applet successfully reduces cumbersome calculations and focuses on the principle operation of the circuit and the meaning of the figures and parameter values more than on getting correct numbers.
D. Design Algorithm in Focus: The design algorithm presented here is an abstract code segment that resembles how to:
(I) Specify design waveforms and observe changing the variables on following waveforms.
(II) Calculate capacitor:
(I) Achieve zero current switching (ZCS) by synchronizing the off switch timing with the end of the charging inductor current waveform.
Assumptions into consideration (II) Assign parameter values by entering them into the applet GUI. Range values of all parameters are: % V= 1 100Volts % L= 0.001 H- 0.1 H % C= 0.000000001 F- 0.00001 F % R= 1- ohm % M= 0.1 – 1 Testing parameter values were chosen as follows: Vss=10 V; source voltage L=0.001 Henry; C=1nF R=100; Initial Load Value M=1; Variable gain set to 1
Parameter Value
Vcmax 20 V
F= 89.9 KHz t1
0.99 µs
Resonant frequency Linear Time Charging
% (i) Synchronize at off times at zero inductor current (ZCS).
if Z0*I0/vss >1
Table 2: Calculated Design Parameter for ZCS Buck Circuit Applet Parameter Duty Switching Load ILmax Cycle frequency 0.5
50 % duty cycle Characteristic Impedance Normalized load
vout= vss*M; Calculate output voltage I0=vout/R; Calculate output current t1=L*M/R; time of linear Charging from IL
(III) Calculate all values from the accompanying equation set (see section D: Design Algorithm in focus).
Value
D=0.5; Z0=sqrt (L/C); Q=R/Z0; w=sqrt (1/ (L*C)); f0=w/ (2*pi); t1=L*M/R;
1010
t2
5.5 µs
0.02 A
t3
6.7 µs
(IV) Observe load change effects due to changing L, C. The following summarizes load changes effects. Table 3: Effect of Extreme Value of L and C ZCS Buck Circuit Applet Waveforms Value Extremes Cmin Cmax Lmin RL ↑ VC ↑ RL ↓ VC ↑ IL↓ Fs↑ IL↓ Fs↑ Lmax RL ↑ VC ↑ RL ↓ VC ↑ IL↑ Fs↓ IL↓ Fs↓ (V) Observe waveforms generated by the applet in Figure 2 and compare with those in Table 2.
for i=1:100 R=R+10; I0=vout/R; if Z0*I0/vss T T1= 2*t3; D2=T*D/T1; D=D2;
T=T1; else end % (ii) Calculate IL (t) & Vc (t) in perspective for the SCZ buck for: 0 < t < t1 IL (t) = ((vss)*t)/L; % inductor current for: t1 < t < (DT=t2) IL(t)2= I0+ (vss/Z0)*sin (w*(t-t1)); for: t1 < t < t2 vc (t)=vss(1-cosw(t-t1)) ; % capacitor voltage for: t2 < t < t3 vc (t)=-I0/C(t-t2)+Vc(t2) vc (t2) = vss*(1-cosw(t2-t1)) t3-t2 = C/I0 *(vc (t2)) E. Fast Testing (Normal Buck Applet Circuit): This section is about the normal Non-isolated buck circuit applet. It was added for the purpose of completeness, since it is related to the previous discussion of the ZCS buck applet. All the way through the latter ZCS case, the buck network was considered as a current source, and all the focus was on the soft-switching mechanism. Presented here is a fast routine for viewing and manipulating this circuit’s parameters. Hopefully, this exercise will give the user the feel of integration and usefulness of the power electronics applets. Input parameters: Fs=50 KHz (fixed); D= 0.6 (Duty Cycle) L=0.1mH; R =10ohm, Vin =33V Expected waveforms values are: a. In continuous conduction mode Gain: Vout=Vin*D =0.6*33=19.8 V ILmax= D*Vin*(1/R + ((1-D)*T)/2L) = 2.7720 A ILmin= D*Vin*(1/R - ((1-D)*T)/2L) = 1.1880 A VLmax= Vin-Vout =13.2 V VLmin= -Vout =-19.8 V I-average = (ILmax+ ILmin)/2=1.98A IL (critical) = ((1-D)/2)*T*R = 0.4 microH R-critical=25ohm Fs-critical=20 KHz @ R=10ohm, D-0.6 b.
In discontinuous conduction mode
Set R= 60ohm (Discontinuous Conduction Mode) D1= (-D+ sqrt (D2+ ((8*L)/(R*T))))/2; = 0.2066 Vout=vss*(D/ (D+D1)) = 24.5468 V
I0 =vout*D1*T/L = 1.0144 A ILmax = D*vss*((1/R) + ((1-D)*T/ (2*L))) = 1.1220 A Icmax=ILmax/2 = 0.5610 A Icmin= - ILmax/2 = -0.5610 A Table 4: Effect of Load Resistance Changes on Waveforms of Buck Circuit Applet Value Extremes Towards increasing R(load) Towards decreasing R(load) VI.
Continuous Mode IL ↓ IC ↓ Vout ↑ IL ↑
IC ↑ Vout ↓
FUTURE PLANS
As the ongoing project is aimed at developing an interactive Web-based applet for complex power electronics circuits, students have the ability to dynamically change and visualize the circuit parameters and conduct a real time simulation for different operational circuit modes and observe the impact of changing circuit parameters on design oriented problems. The first phase of the research project was conducted at University of Central Florida to module steady state response of basic DC-DC converters. The systematic functions on the mathematical equations of the steady state solution of DC-DC converters has been created into a dynamically fit form displayable on the Web that meanwhile allow steady-state solution manipulation through instant circuit parameter variations. Designing DC-DC converter modules, which was developed at University of Salerno, is focusing on an integrated Design-Oriented Power Electronics Education framework for DC-DC converter investigations to be used as a training tool in power electronics education and as a verification procedure tool for the steady state solution developed in the first phase. Due to the complex mathematical modeling of the operational and configurations of DC-DC converters, and while DC-DC converters should respond to the variations in the input and output voltages with a smooth and stable response through a feedback control system, it is not enough to understand the time domain operation of the converter to design the controller. Therefore, the closed loop response and the dynamic modeling should also be considered when teaching the power converter through the Small-Signal Modeling of PWM Converters, which can be considered the second phase of the proposed E-Learning methodology. The proposed phase should deal with the dynamic modeling of DC-DC converters and the frequency plots of converters, which allow engineers and students to design and obtain the small signal models in closed loop forms with certain dynamic performance requirements. The Design-Oriented Analysis should be applied to the switching converters learning process. The basic principle relies in putting equations and the expressions of their solutions in a way that highlights the impact of the system’s parameters on the solution and that emphasizes the “inverse-
of-analysis” nature of design-oriented problems, where roles of physical parameters and electrical variables are exchanged. This will be the third phase to complete the picture of the E-Learning tools. VII.
CONCLUSIONS
Online interactive materials were developed for the “Power Electronics Teaching” Web site. The E-Learning materials are available for both students and instructors. Lecture notes and power point presentations are created based on the textbook material “Power Electronics Circuits.” E-exams, which provide several textbook and suggested problems, are also available for student selfassessment. Interactive applets are introduced as an innovative and interactive E-Learning tool in power electronics education as well. Applets focus on developing a new learning module for the DC-DC converters and the ACDC rectifiers, for educational as well as industrial use. With the increasing industrial requirements for providing handson graduates, new teaching methods need to be developed to meet today’s challenging market demands. The interactive applets provide the user with the ability to dynamically change and visualize the circuit parameters. The applets will allow the student and designer to optimize the design and realize the impact on the solution through a series of dynamically updated curves that relate all the parameters in a graphical form. REFERENCES [1] [2] [3] [4] [5]
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