The 1st Engineering Education Conference, University of Duhok, 17-19 April, 2012 (Special Isue) Journal of University of Duhok, vol. 17, No. 1 (Pure and Eng. Sciences), pp 61-70, 2014
Switching Mode Regulators using MATLAB/SIMULINK as a Teaching Tool for Undergraduate Power Electronics Courses
Aslan Sabahaldeen Jalal Abdi University of Baghdad Al-Khwarizmi College of Engineering Department of Mechatronics Engineering Email:
[email protected] [email protected]
Layth Jameel Buni Qaseer University of Baghdad Al-Khwarizmi College of Engineering Department of Mechatronics Engineering Email:
[email protected]
Abstract: This paper describes a new course structure in teaching undergraduate power electronics courses, namely Switching Mode Regulators (SMRs) course. The main modification adopted to a previous work [1] is the addition of MATLAB/SIMULINK models that are used to enhance students assimilation to theoretical steady – state and dynamic analysis and prepare them to the laboratory power electronics experiments. This new approach is deemed to be time saving with the benefits of raising the students understanding and hence percenatge of success. Also, large number of students can be taught using this approach. The accuracy of this approach is verified through analytical examples and comparison with existing experimental work. Keywords: power electronics, education, DC motor drives, DC – DC converters, MATLAB/SIMULINK. 1. Introduction: Teaching any technical engineering subject in electrical engineering is often complicated due to its complexity in explaining the analysis of any system regarding both its transient and steady – state running conditions. The Power Electronics Course is one of these subjects in which the instructor usually finds difficulties in explaining these running principles for the students using standard theoretical lecture notes. On the other hand, teaching technical subjects are usually accompanied by laboratory experiments in which some of the running condition performances can be viewed/obtained directly using measuring instruments, while those performances that need an addition of extra elements to the considered circuitry can not be measured using the nowadays widely used experimental hardware kits, due to the kit's hardware power limitations. Besides the fact that most of the students consider that the laboratory sessions are boring and that they can not make a suitable connection between the theoretical analysis and the experimental results leading to further need of laboratory time with further complications for the laboratory instructors. Choosing the MATLAB/SIMULINK package, and its SimPowerSystems (SPS) tools, was due to the fact that many researchers and educators in different electrical engineering branches gain benefits, for teaching purposes, through using this tool in describing and investigating the performances of different electrical systems. This tool was helpful in studying the transient stability of AC/DC interconnected system, [2]. However, the described work with an appropriate development may extend its use for teaching power system courses to enhance theoretical lectures. Different electrical machines works had been illustrated regarding the use of this tool to implement tests, speed control and investigating steady – state performances of these machines, [3-6].
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The 1st Engineering Education Conference, University of Duhok, 17-19 April, 2012 (Special Isue) Journal of University of Duhok, vol. 17, No. 1 (Pure and Eng. Sciences), pp 61-70, 2014
An illustration of the application of this tool in simulating and learning feedback controller design of dc–dc switching converters has been given in [7]. However, the work demonstrated the application of this tool in constructing the converter model in its analytical form rather than implementing the circuit of the converter under investigation. Other works directed towards teaching idealized dc – dc converters had been demonstrated in [8 – 10]. Appreciating the efforts spent in these works, all were presented to students as a ready tool programmed with MATLAB language [7], and with JAVA language linked with the PSPICE simulator package [9, 10]. Although these works have been used to improve learning power electronics subjects, it should be appreciated that supplying students with tools that provide avoidance or reduction of the lengthy calculations time where the students have to spend in their study is out of the pedagogical issues. Irrespective of the work considered, the reported results in [2 – 7] clarify that the MATLAB /Simulink package and its SPS tools is a well teaching tool including its ability of predicting and investigating the performance of the electrical system under consideration. The main objective of this work is to introduce the use of MATLAB /SIMULINK package and its SimPowerSystem (SPS) tools to implement SMRs circuits, namely the Buck and the Boost regulators, for investigating the steady – state performances of these regulators. In addition to a descriptive explanation of using this tool in teaching undergraduate power electronics course regarding the considered subject is introduced.
2. Course Description The aim of this course is to teach third year undergraduate students of the Mechatronics Eng. Dept. at the University of Baghdad, the principles of steady – state operation of Buck and Boost SMRs, and how to investigate their performances experimentally. In order to achieve this objective, the students must understand the theoretical operation of such regulators’ topologies and the problems that they may face when designing a practical converter in their career. Basically, the new course structure is divided into three main parts that will be explained and discussed separately in the following sections. 2.1 Theoretical Lectures Description In general, this part involves ten lectures that are introduced via two credit hours lecture notes. The first four lectures cover the theoretical analysis and the appropriate circuit knowledge required for understanding the principles of operation of DC chopping. The second four lectures are devoted to the explanation of the theoretical analysis of both the Buck and Boost regulators. Each regulator with its steady – state analysis is covered through two sets of lecture notes for each type independently. All lecture notes presented to the students are prepared with stringent textbooks dependence [11 – 13]. The ninth lecture is devoted to solve numerous theoretical problems. Also, two theoretical design questions are solved to students with another two which are given to students as homework. Through these solved problems the students gain some experience in calculating different circuit's component values and investigating, theoretically, the regulator performance to meet a desired specification. The final lecture deals with introducing the MATLAB/SIMULINK package and SPS toolboxes to the students at the computer laboratory. The instructor introduces the library of the SPS that is accompanied with the SIMULINK package [14]. It is worth mentioning that the library included in the SIMULINK package is abundant, and hence the instructor focuses on some parts that are used in constructing the proposed SIMULINK models. The introduction of the package includes demonstrations of some useful examples, which are
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The 1st Engineering Education Conference, University of Duhok, 17-19 April, 2012 (Special Isue) Journal of University of Duhok, vol. 17, No. 1 (Pure and Eng. Sciences), pp 61-70, 2014
accompanied with it. Eventually, students become familiar with this package's tools and can easily deal with the models that are given to them regarding the aforementioned regulators under investigation. 2.2 SIMULINK models, integration with Theoretical analysis The switching device used in both models is the Metal-Oxide Semiconductor Field-Effect Transistor (MOSFET). It is worth mentioning that different switching devices may be used such as the Silicon Controlled Rectifier (Thyristor), the Insulated Gate Bipolar Transistor (IGBT), or the Gate Turn OFF Thyristor (GTO). However, this work is focused on demonstrating the principles of operation of SMRs rather than demonstrating the switching characteristics of the device used. Appreciating that the internal characteristics of the used device can be specified using its dialog box. Figure (1) shows the dialog boxes of the MOSFET and the Thyristor as examples.
(a) (b) Figure (1): Switching device dialog box (default) (a) MOSFET, (b) Thyristor After explaining the basic principal of using this package to the students, the SIMULINK models of the Buck and Boost regulators are given to them, as shown in Figures (2) and (3) respectively. The basic circuit diagram of a Buck regulator is shown in Figure (2 – a), and the corresponding SIMULINK model is shown in Figure (2 – b). Figure (3 – a) shows the circuit diagram of a Boost regulator, whereas Figure (3 – b) shows the SIMULINK model used to simulate the Boost regulator. The device control signal is implemented using a pulse generator in which the ON time duration and the system switching period (T) can be varied from the dialog box. Hence, controlling the ON time duration assures the control technique used to control the circuit duty cycle (D), and satisfies the switching technique used for controlling the SMRs output voltage. The circuit filter inductor, capacitor and load resistor are represented with a series RLC branch/load blocks by setting the unwanted parameters to zero, or infinity as required. All other circuit measuring units are drawn from the measuring units' branch of the SPS toolbox. The SIMULINK models' graphical results which, in a user – friendly manner, are used to explain to students the most common operating principles that are extremely useful for steady – state analysis of both regulators given in theoretical classes.
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The 1st Engineering Education Conference, University of Duhok, 17-19 April, 2012 (Special Isue) Journal of University of Duhok, vol. 17, No. 1 (Pure and Eng. Sciences), pp 61-70, 2014 iS
Lf Chopping Device
+
iL
idm
+
iC
Dm
VS
iO
RL V O
Cf
(a)
(b) Figure (2): Buck regulator (a) Basic circuit diagram (b) SIMULINK model
iS = iL
+ VS
Lf
Dm Chopping Device
idm
iO iC
Cf
+ RL
VO
(a)
-
(b) Figure (3): Boost regulator (a) Basic circuit diagram (b) SIMULINK model
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The 1st Engineering Education Conference, University of Duhok, 17-19 April, 2012 (Special Isue) Journal of University of Duhok, vol. 17, No. 1 (Pure and Eng. Sciences), pp 61-70, 2014
Samples of these graphs are shown in Figures (4), (5) and (6). They are used to increase the students' assimilation to the conventional differential equation which relates the inductor voltage ( vL ) to its current ( i L ). This relation is i L t t diL 1 (1) vL L diL t vL t .dt dt L0 i L 0 where iL = iL(0) at t = 0, and hence
iL t iL 0
t
1 vL t .dt L 0
(2)
Irrespective of the mode of operation, Continuous Figure (4) or Discontinuous Figure (5), the waveforms of all circuit variables must be repeated over a complete steady – state time – period Ts, then via graphs, the students get a better understanding for the following conclusions: 1- The steady – state inductor current is the same at the two ends of one complete switching period Ts, i.e. T
1 s iL Ts iL 0 vL t .dt 0 Lo
(3)
2- Integrating (2) over one switching time – period Ts and using (3) shows that the inductor voltage averaged over one switching period Ts is zero: DT Ts DTs Ts 1 s VL vL t .dt vL t .dt 0 vL t .dt vL t .dt (4) Ts o DTs o s DT area A
area B
A t
0
B
IL2
IL1 0 iL(0) Where:
t
DTs Ts
iL(Ts)
I L1 iL 0 iL TS ; I L 2 iL DTS
Figure (4): Inductor voltage and current waveforms during steady – state operation (Continuous Conduction Mode CCM).
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The 1st Engineering Education Conference, University of Duhok, 17-19 April, 2012 (Special Isue) Journal of University of Duhok, vol. 17, No. 1 (Pure and Eng. Sciences), pp 61-70, 2014
These conclusions are explained using the inductor voltage and current waveforms, with duty ratio (D) for both modes of operation shown in Figures (4) and (5). The students will figure out that the area A in (volt–sec) is responsible for the inductor current rise, from its initial value iL(0), and equals in magnitude to the area B that causes the inductor current to fall to its final value iL(Ts). From Figure (5), the students can get a better understanding for the DCM operation of the regulator with variation of the load resistor. As clear in the figure, the effect of increasing the load resistor increases the current discontinuity period and hence affecting the output voltage.
A
0
t B DTs
RL Increase
Ts
IL2 t
0 IL1 = 0
iL(0) iL(DTs) iL(Ts)
Figure (5): Inductor voltage and current waveforms during steady – state operation (Discontinuous Conduction Mode DCM).
By analogy, Figure (6) shows SIMULINK graphs that are used to explain a similar analysis regarding the capacitor voltage and current, in any switching mode regulator circuit operating in steady – state conditions, to enhance the students' assimilation for the following: 1- The steady – state capacitor voltage is the same at the two ends of one complete switching period Ts, i.e. (5) vC Ts vC 0 2- The capacitor current, averaged over one switching period Ts, is zero: T
1 s IC iC t dt 0 Ts o
(6)
Also, the simulation models can be used to show waveforms of different parameters in the model under investigation. Figure (7) shows the inductor, capacitor and load currents in addition to the output voltage waveforms for both regulators under steady – state operation. Eventually, the students become more familiar with this simulation package tool and can easily deal with the models that are given to them regarding the two regulators under investigation.
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The 1st Engineering Education Conference, University of Duhok, 17-19 April, 2012 (Special Isue) Journal of University of Duhok, vol. 17, No. 1 (Pure and Eng. Sciences), pp 61-70, 2014
vC (0)
vC (Ts)
DTs
t
Ts
A t B
Figure (6): Capacitor voltage and current waveforms during steady – state operation
t
0
0
t
0
t
0
t
t 0
0
t
t
0
0
(a)
t
(b)
Figure (7): Current variation in inductor, capacitor and load in addition to output voltage waveforms variation during steady – state operation (a) Buck regulator (b) Boost regulator
2.3 Laboratory Experiments After introducing the first and second parts of the course to the students, they will interact effectively with laboratory experiments sessions. The experimental kit used for conducting the laboratory experiments is the POWER BOARD, Power Electronics Kit type 5125 from hps System Technik shown in Figure (8). It is worth mentioning that the operating frequency range for this kit is (0 – 1000 Hz). The students are given the experimental note book which contains 10 experiments that they must conduct during the whole course. Two experiments deals with the aforementioned
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The 1st Engineering Education Conference, University of Duhok, 17-19 April, 2012 (Special Isue) Journal of University of Duhok, vol. 17, No. 1 (Pure and Eng. Sciences), pp 61-70, 2014
SMRs. Connection circuit diagrams of Buck and Boost regulators using the experimental kit are shown in Figure (9). All the parameters of each regulator are shown in Figure (9). To obtain simulation results, students have to vary the parameters of the SIMULINK model according to those in the circuit implemented with the experimental kit. The procedure to run these SIMULINK models is listed in Appendix I.
(a)
(b)
Figure (8): (a) POWER BOARD Type 5125 (b) Buck regulator experimental set up with POWER BOARD Type 5125
(a)
(b)
Figure (9): Experimental connection circuit diagram (a) Buck regulator (b) Boost regulator One of the steady – state operating performances that the students must understand is how to obtain the ripple in the output voltage (ΔV0). This value can be calculated theoretically for both regulators [11]. For the Buck regulator, it may be expressed in terms of the circuit parameters and voltages as:
V0
V0 VS V0 8 L C f 2 VS
(7)
and for the Boost regulator it can be expressed as:
V0
I 0 V0 VS V0 C f
(8)
Where: 68
The 1st Engineering Education Conference, University of Duhok, 17-19 April, 2012 (Special Isue) Journal of University of Duhok, vol. 17, No. 1 (Pure and Eng. Sciences), pp 61-70, 2014
ΔV0 V0 I0 VS L C f
: : : : : : :
Ripple in output voltage, (V) Average output voltage, (V) Average output current, (A) Supply voltage, (V) Inductance, (H) Capacitance, (F) Switching frequency, (Hz)
Equations (7) and (8) clarify how the output voltage ripple is affected by varying different circuit parameters listed in these equations. However, effects of varying the input voltage, the inductor and the capacitor can not be implemented experimentally with the experimental kit used in laboratory experiments due to their fixed values. These parameters variations may be examined using the SIMULINK models to show their effects on the ripple in the output voltages. Effects of varying the duty ratio and the frequency (up to 1000 Hz) may only be implemented experimentally. Graphs of the comparisons that students are requested to obtain and draw between experimental and SIMULINK models’ results are shown in Figures (10) and (11). It can be seen that some differences between the experimental and the SIMULINK models results occur. However, the percentage deviation does not exceed a maximum amount of 7%. This is considered as fair justification for the use of MATLAB/SIMULINK package to implement practical SMRs circuits. 0.16 SIMULINK
Output Voltage Ripple (V)
0.14
Experimental
0.12 0.1 0.08 0.06 0.04 0.02 0 0
20
40
60
80
100
Duty Cycle (%)
Figure (10): Buck regulator output voltage ripple variation with duty ratio f = 1000Hz, VS = 12.8V. 450 400
Output Volatge Ripple (mV)
Simulink 350
Experimental 300 250 200 150 100 50 0 0
10
20
30
40
50
60
70
80
90
Duty Cycle (%)
Figure (11): Boost regulator output voltage ripple variation with duty ratio f = 1000Hz, VS = 13.2V. 69
The 1st Engineering Education Conference, University of Duhok, 17-19 April, 2012 (Special Isue) Journal of University of Duhok, vol. 17, No. 1 (Pure and Eng. Sciences), pp 61-70, 2014
3. Conclusions The main scope of this work is to present a new teaching technique based on MATLAB/SIMULINK package and its SimPowerSystems tools. The presented models built with this tool were used as a supporting tool for teaching undergraduate power electronics courses. The two regulator types covered in this paper are the Buck and the Boost SMRs under steady – state operating conditions. For each regulator type, the SIMULINK model has been used to assist learning students the most common regulators' principles besides learning experimental results verifications. Moreover, the presented models are easy to use by students and the considered results indicate that these SIMULINK models are successfully modeled to obtain the steady – state characteristics of the regulators under investigation. Regarding pedagogical issues, the presented approach was effective in saving laboratory time, reducing the students' failure percentage and makes them gain a valuable experience in designing and investigating the performance of such regulators. This is achieved by the possibility of the "Stop and Repeat" property that the SIMULINK package has. Other converters/regulators topics, as the inverting mode regulator and the flyback converter, may be studied with this tool as an extension to the present work to prepare a useful virtual laboratory for teaching undergraduate power electronics courses. Furthermore, undergraduate power electronics course that integrates up-to-date computer hardware and software tools in both lecture and laboratory sessions also meets the expectations of today’s students who are eager to use computers and simulation tools in every aspect, and this effectively attracts more students [2].
4. Appendix I: SIMULINK models run procedure of both regulators implemented with the Experimental Kit 1) For both regulators, set the values of different circuit components according to those values implemented in the experimental kit. Make sure to add any component that is used in the experimental circuit and was not included in the circuits represented by the SIMULINK models. 2) Set the switching frequency (duty period) for the pulse generator to 1000Hz. This holds for both regulators. 3) For the Buck circuit, set the MOSFET snubber resistance (RS=1M Ω) and snubber capacitance (CS=0 F). Also, set these values for the freewheeling diode as (RS=10M Ω) and (CS=1μ F)). Finally, set the simulation type to discrete with a sampling time of (5μ seconds). 4) For the Boost circuit, set the MOSFET snubber resistance (RS=100 Ω) and snubber capacitance (CS=1μ F). Also, set these values for the freewheeling diode as (RS=100 Ω) and (CS = 0 F). ). Finally, set the simulation type to discrete with a sampling time of (5μ seconds). 5) Set the frequency attributes of all the mean value blocks to the switching frequency used with the pulse generator. 6) Specify the stop time of the simulation. 7) Run the simulation with the pulse generator duty ratio (D) staring from (0.1) to (1.0) in steps of (0.05). For each step, read the output voltage (V0) from its measuring display box, and from the scope that displays the output voltage, read the output voltage ripple (ΔV0). Record all data in a table to be drawn on a graph paper with those obtained experimentally. Discuss your results with a brief description of each component effect on these results.
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5. References [1] S. A. Shirsavar, ''Teaching practical design of switch – mode power supplies'', IEEE Trans. Edu., Vol. 47, No. 4 pp. 467 – 473, Nov. 2004. [2] KA Folly, B. S. Limbo, ''Experience in Using MATLAB Power System Toolbox (PST) for Transient Stability Study of an AC/DC Interconnected Power System'', Proc. Africon, pp1-4, 2007. [3] S. Ayasun and C. O. Nwankpa, ''Induction motor tests using MATLAB/SIMULINK and their integration into undergraduate electric machinery courses''; IEEE Tran. Edu., Vol. 48, pp 37 – 46, 2005. [4] S. Ayasun and C. O. Nwankpa, ''Transformer tests Using MATLAB/SIMULINK their integration into undergraduate electric machinery courses'', Comp. App. Eng. Educ., 142150, 14 (2006). [5] S. Ayasun and K. Gultekin, '' DC motor speed control methods using MATLAB/SIMULINK and their integration into undergraduate electric machinery courses''; Comp. App. Eng. Educ.,347-354, 15 (2007). [6] A. Sa. J. Abdi, ''Steady state performance investigation of a three phase induction motor running off unbalanced supply voltages'', Al-Khwarizmi Eng. J., Vol.7, No.3, pp 1-12, Jun. 2011. [7] J. H. Su, J. Chen, and D. Wu, “Learning feedback controller design of switching converters via MATLAB/SIMULINK,” IEEE Trans. Educ., vol. 45, pp. 307–315, Nov. 2002. [8] P. F. Miaja, D. G. Lamar, M. A. Azpeitia, A. Rodríguez, M. Rodríguez, and M. M. Hernando. ''A Switching-Mode Power Supply Design Tool to Improve Learning in a Power Electronics Course''; IEEE Trans. on Edu. Vol. 54, No. 1, pp. 104-113, Feb. 2011. [9] F. A. S. Gonçalves, L. P. Sampaio and C. A. Canesin, ''Interactive DC-DC converters tools for instant design and education'', in Proc. Brazilian PE Conf., pp 1074 – 1081, 2009. [10] C. A. Canesin, F. A. S. Gonçalves, and L. P. Sampaio, ''Simulation tools of DC-DC converters for power electronics education'', in Proc. EPE Conf., 2009. [11] M. H. Rashid, Power Electronics: Circuits, Devices and Applications, 3rd ed., Pearson Prentice-Hall, Upper Saddle River, NJ – USA, 2004. [12] N. Mohan, T. M. Undeland, and W. P. Robbins, Power Electronics: Converters, Applications and Design, 2nd ed., John Wiley & Sons, New York, 1995. [13] A. Pressman, Switching Power Supply Design, New York, McGraw-Hill, 1998. [14] Natick, SimPowerSystems for use with SIMULINK, User's Guide, Math Works Inc., MA, 2002.
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