Proceedings of NCPCE-10

Teaching Behavioral Modeling and SimulationTechniques for Power Electronics Courses

Rohit Kithwani V. R , Member, IEEE

AbstractThis paper suggests a pedagogical approach toteaching the subject of behavioral modeling of switch-mode powerelectronics systems through simulation by general-purpose elec-tronic circuit simulators. The methodology is oriented towardelectrical engineering (EE) students at the undergraduate level,enrolled in courses such as Power Electronics, IndustrialElectronics, or the like. The proposed approach is demonstratedby simulation example of a realistic active power factor cor-rector (APFC) system. The paper discusses the derivation ofPSPICE/ORCAD-compatible behavioral models, their softwareimplementation, and fast time domain, frequency domain, andstability analysis simulation techniques suitable for virtual studyof complex nonlinear feedback systems. Some tricks of the tradeare also suggested. The paper can be helpful to instructors ofa Virtual Power Electronics Laboratory course wanting toconduct a software experiment on a PFC system.

Index TermsFeedback, feedback circuits, power factor, simu-lation, switch-mode power conversion.

I. INTRODUCTION

R ECENT advances in power electronics have led mosthigher education institutions to include courses on powerelectronics in their undergraduate electrical engineering (EE)curricula. Power electronics is an interdisciplinary area thatincorporates several of the disciplines of electrical engineering.Courses such as Power Electronics or Industrial Electronicsare usually offered to undergraduate students once they acquireprerequisite units on electrical circuits, electronic devices,electronic circuits, control systems, energy conversion, electricdrives, and the like. Advanced topics on power electronics mayalso be suggested to graduate students. Whereas the lectures onpower electronics introduce students to the theoretical conceptsand basic analysis techniques, a practical power electronicslaboratory that usually accompanies these courses allows stu-dents to gain some hands-on experience. However, in orderto experiment with a power electronics system, a prerequisiteknowledge of many practical issues and of rather complexelectronic building blocks is required. This is especially truewhen a realistic system is studied.

In view of the limited time available, the multidisciplinarypower electronics laboratory is a true challenge. Experimentingwith a practical circuit provides the student with a real sense ofa professional experience. However, in the teaching laboratoryenvironment, experimenting with hardware is not the only

The author is with the Sami Shamoon College of Engineering, Electricaland Electronics Engineering, Ramnavaraswspur, India (e-mail: [email protected]).

option. A virtual experiment may be a better alternative.An active power factor corrector (APFC) system is one suchexample. First, an APFC system is connected to the mainsand operates with hazardous input and output voltages of120/240 Vac/380 Vdc. Hence, safety becomes an issue toconsider. Second, a practical experimental setup requires asubstantial design, construction, and maintenance effort. Third,to experiment with a feedback system, additional measure-ment equipment is required such as network analyzers andelectronic load emulators. These instruments are costly and,unfortunately, may be easily damaged in the hands of unskillfulusers. None of these issues posed is a problem in a virtuallaboratory environment. Simulation with a general-purposecircuit simulator, such as PSPICE/ORCAD, EWB, or PSIM,is a viable tool that can visualize a systems waveforms toprovide students with an insight into power electronic systembehavior and can stimulate learning. Today, computer-aideddesign and simulation tools are universally accepted and havebecame the industry standard method of product development.EE students should therefore be provided with an adequatefundamental training in simulation as well as be introduced topower electronics hardware.

Two fundamental features of power electronic circuits, de-scribed below, make them quite dissimilar to analog circuits.For these reasons, common simulation techniques for analogcircuits are not readily applicable to power electronic systems.Therefore, just as a different analytical approach is used in thetheoretical analysis of power electronic circuits, a differentmethodology must be applied in simulation.

The first distinct characteristic of power electronics systemsis that they employ switched-mode power stages, whereasthe control circuits are mostly analog, with either pulse width(PWMs) or pulse frequency modulators (PFMs) used as aninterface. Due to the switched nature of the circuit, an attempt toapply PSPICE/ORCAD to simulate the frequency response ofthe system as is produces erroneous results. This is becausethe SPICE simulator cannot find a stable operating point toperform linearization and calculate the small signal gain.

The second distinct characteristic is that the power stageoperates at switching frequencies far beyond the controllerbandwidth. For instance, the switching frequency of the APFCsystem is in the 100-kHz range, whereas the bandwidth of thevoltage control loop is restricted to only about 10 Hz. Thus,in order to visualize the voltage loop step response, a timeinterval of about 0.5 s has to be simulated. To obtain reliablesimulation results, about 20 samples per one high-frequencyswitching cycle are needed. Therefore, using the cycle-by-cyclesimulation requires the calculation of about 1 M samples.This, however, is an optimistic assessment since PSPICErapidly reduces the time step when looking for a solution in

International Conference on Advanced Power Engineering 201326

the vicinity of the switching instant. While modern PCs dohave sufficient resources to perform such a simulation task, itmay take quite a few minutes of run-time, depending on thecircuits complexity. This results in a frustrating waste of timefor students whose simulation skills are as yet weak and whooften resort to the trial and error approach and tend to reruntheir programs many times during the time-limited laboratorysession. Indeed, cycle-by-cycle time domain simulation is theonly way of investigating the high-frequency processes within apower stage. Cycle-by-cycle simulation is an intuitive, simple,and straightforward approach, but this is not an efficient tool toinvestigate the overall systems behavior. System study requiresa different methodology.

The state-space averaging technique is a classical analysismethod for power electronics systems [1][5]. Average mod-eling of the power stage can also be helpful to alleviate thedifficulties involved in simulation. Average models can bereadily implemented using SPICE behavioral sources [6][8].Replacing the switched power stage by an appropriate averagebehavioral model yields a continuous model of the powerelectronic system. The continuous average model can be au-tomatically linearized by the simulator and prepared for thefrequency-domain simulation analysis. The ability to obtainthe frequency response of the feedback loop allows the studentto evaluate the systems stability using the familiar gain andphase margin criteria and to design the compensator network tomeet the design objectives. Such an assignment is the ultimatetask of a power electronics engineer. Moreover, the number ofsamples required to simulate the time domain behavior of thecontinuous model is several orders of magnitude lower than thatrequired for detailed simulation of the switched stage. For thecase of voltage-loop transient simulation of the APFC systemmentioned, only about 12 K samples, or only several secondsof run-time, are sufficient to produce adequate time domainresults. Clearly, the behavioral approach makes both frequencydomain and time domain simulation of complex switch modesystems smooth, fast, and easy.

This paper describes an approach the author has appliedteaching the class Industrial Electronics Laboratory in recentyears at the Sami Shamoon College of Engineering (SCE),Beer-Sheva, Israel. The Industrial Electronics Laboratoryintroduces the student to a series of 10 software and practicalexperiments. The experiments cover four basic topics: 1) un-controlled and phase controlled rectifiers; 2) dcdc converters;3) dcac inverters; and 4) feedback in power electronicscircuits. This paper discusses one of the more advanced soft-ware experiments on behavioral modeling of feedback powerelectronics systems. The paper offers simplified average be-havioral modeling and simulation methodology that appliesPSPICE/ORCAD-compatible models to simulation of feed-back-controlled switched mode systems. The methodology isdemonstrated by studding a realistic APFC system. Section IIof the paper introduces the problem circuit. The behavioralmodel of the feed-forward and voltage feedback loop is derivedin Section III. Modeling of different loading conditions andsimulation techniques of load step is presented in Section IV.Section V discusses the simulation approach to study the fre-quency response and stability of the feedback loop. Section VIconcludes the discussion. The proposed approach was applied

in practice and proved to be appropriate for teaching powerelectronics at the undergraduate level.

II. THREE-LOOP APFCNonlinear input characteristics of uncontrolled and phase-

controlled rectifiers are largely responsible for injection of har-monic currents into the power grid. Harmonic currents causeincreased conduction losses in the electrical distribution systemas well as increased magnetic losses in power transformers, thuslowering the efficiency of the power system. Higher harmonicsmay also cause resonance and overvoltage breakdown of ca-pacitor banks. For these reasons, recent regulations restrictingthe harmonic currents have made APFC a mandatory utility in-terface stage of modern power supplies. The task of the APFCis to draw a pure sinusoidal line current in phase with the linevoltage, as well as to regulate output voltage. A well-designedAPFC can operate from universal line voltage with efficiency ofup to 98%, generate very low distortion currents, and achieve anear-unity power factor. With high power factor (PF) and lowtotal harmonic distortion (THD), lower rms currents, and there-fore reduced conductive losses, a power distribution system canachieve higher efficiency. Higher efficiency translates to loweremissions from power generation plants and decreased pollu-tion of the environment.

The three loop average current mode APFC shown in Fig. 1(a)is a widely used system, a detailed description of which maybe found in [9][12]. Other APFC systems using different con-cepts were also proposed in the literature [12][16]. The APFCof Fig. 1(a) regulates the average output voltage by an outerfeedback loop, whereas the inner loop, also referred to as thecurrent feedback loop, shapes the input current of the powerstage. The feed-forward loop compensates for line voltage vari-ations. Due to its simplicity, low part count, and inherent abilityto sustain input current through the deeps of the line voltage,the Boost converter is the designers choice for implementingsingle-phase APFCs. Continuous current mode operation is pre-ferred for high power levels.

The block diagram of Fig. 1(a) reveals a rather complex con-trol scheme. The inner loop control signal is derived by com-paring the average line current to the desired reference value.The reference for the inner current loop, , is generated bymultiplying the rectified power line voltage by the outer loopcontrol signal . Thus, the magnitude of the current loop refer-ence is adjusted dynamically to comply with the power require-ments of the load. The feed-forward loop speeds up the tran-sient response and provides better regulation against variationsof the line voltage. The current reference signal for the innerloop is generated at the output of the multiplier-squarer-dividercircuit. The current reference signal is inversely proportional tothe square of the feed-forward voltage , which is proportionalto the RMS value of the line voltage. However, the bandwidthof the feed-forward and the voltage control loops is severelyrestricted, typically to 1015 Hz, in order to provide high at-tenuation of the second harmonic of the line frequency. Other-wise, by penetrating into the reference of the current loop, theripple will case distortion of the input current. As a result of thefeed-forward loop action, the APFCs average power is linearwith the voltage loop control signal and independent of theline voltage. Thus, this type of APFC is inherently suited foruniversal line voltage applications.


Fig. 1. (a) Three-loop APFC system, (b) with APFC power stage and PWMmodeled by average behavioral sources.

In the following, a simplified model of the APFCs powerstage under a closed current loop condition will be derived toinvestigate the static performance of the power factor corrector,study the dynamic response, and conduct stability analyses ofthe voltage feedback loop.

III. BEHAVIORAL MODELINGThe APFC model is derived relying on the following

assumptions.1) The power stage is ideal.2) The current loop is ideal.3) The power stage is in a quasi-steady state.Assumption 1 is justified by the high efficiency of the offline

converters, which can be as high as 95% for high-voltage appli-cations. Assumption 2 is deduced from the following reasoning.Since the line frequency and its significant harmonics are farbelow the current loop crossover, the current loop gain roll-offis negligible at these frequencies, and the current loop gain maybe approximated by its dc gain, which is determined by the cur-rent feedback network . In the most common case, is just aseries current sensing resistance. Therefore, without significantloss of accuracy, it is possible to say that the inner loop is seen bythe outer loops as a frequency-independent, fixed gain, trans-re-sistance block whose output variable precisely replicatesthe current loop reference signal . As a result, the outputvoltage of the current sensing network, (seeFig. 1) is forced to follow the reference signal generatedat the output of the current programming network. Therefore,the converters average current is

(1)Here, is angular time.

Since the switching frequency is much higher than the linefrequency, the average inductor current variations through theswitching cycle are insignificant, and the power converter is op-erating near the steady-state equilibrium. As a result, the energystored in the converter inductor throughout the switching periodis negligible when compared to the processed power. This factjustifies Assumption 3. Treating the power stage as if it has noenergy storage, and applying the power balance relationship, theaverage charging current , supplied by the power stage tothe holdup capacitor , can be obtained as

(2)

Equations (1) and (2) are regarded as the large signal model ofthe ideal power stage under the closed current loop. Using thisapproach, the APFC in Fig. 1(a) can be modeled as illustrated inFig. 1(b). Since only two voltage-controlled current sources arerequired, the model is simple to realize in software; see Fig. 2(a).The output port equation (2) is implemented with a G2 behav-ioral block, whereas the input port equation (1) is implementedwith a simple G3 element. The current loop is closed via the cur-rent sensing/feedback resistor , forming a current-samplingseries-mixing (seriesseries) topology. The dummy sourceis required for current sensing.

The model can be used to simulate the APFC power stageby the PSPICE/ORCAD general-purpose circuit simulator andprovides a good insight of the APFC operation both on thetimescale of a single power line cycle and of multiple cycles.The clear advantage of this approach is that the difficulties re-lated to the power stage and the PWM modulator modelingand simulation are excluded. Consequently, the slow responsefeed-forward and the outer feedback loops behavior could besimulated for rather long time intervals with greater ease. More-over, the outer loop frequency response can be obtained in orderto perform stability analysis.

The PSPICE/ORCAD simulation diagram of the APFC isillustrated in Fig. 2(a). This model can be used for both thetime domain and the frequency analysis and, therefore, uses twosources. In the time domain simulation, the sinusoidal VSINsource is used to model the line voltage, whereas the VAC sourceprovides the required frequency swept excitation signal for thefrequency analysis. The dc value of the VAC source should beset to zero to avoid interfering with the time domain analysis.

There are several ways to model the line rectifier. The trivialoption is to use diodes. Alternatively, the rectifier function canbe realized by a pair of E1 and G1 dependent sources, as shownin Fig. 2(a). One option is to employ an EVALUE-dependentsource programmed to generate the absolute value using theABS function. The rectified voltage can be also obtained by pro-gramming the E1 source to multiply the line voltage, realized byan independent voltage source, by its sign according to the fol-lowing equation:

(3)whereas the line current could be obtained by multiplying therectified side current, sensed by the dummy source, , by theline voltage sign

(4)


Fig. 2. (a) PSPICE simulation diagram of the three-loop APFC. Notice the ac test signal source in the outer feedback loop. (b) Illustration of the loop-gainsimulation technique preserving the feedback action [17].

The multiplier-squarer-divider circuit can be realized by asingle G6 element in a rather straightforward manner. The par-ticular implementation in Fig. 2(a) closely mimics the realisticcurrent output multiplier in the commercial APFC controller ICdescribed in [9]. Thus, the G6 block realizes thefollowing function:

(5)

Here, the multipliers coefficient can be adjusted by varying thevalues of the programming resistors and . The mul-tipliers output voltage appears across the output resistorand is provided as the current reference signal for the currentloop. Special care should be taken with sources performing di-vision, as in (5). At startup, these sources may face a divisionby zero condition, leading to convergence problems. In orderto avoid division by zero, a negligible value of 1 mV is intro-duced in the denominator of the G6 function (5). In addition,the G6 output current swing is limited within the realistic satu-ration limits 010 mA.

The APFC system model in Fig. 2(a) is only partially be-havioral. Behavioral modeling was used to obtain a simplifiedmodel of the power stage in order to find a way around theproblematic issues of modeling the current loop and the PWMmodulator. The rest of the controllerthat is, the feed-forwardfilter, multiplier programming resistors, voltage loop com-pensator, and reference sourceremains in its original form.This approach allows students access to intricate details of theanalog controller circuit to experiment with different values

of the voltage reference, the feed-forward filter, the multiplier,and the output filter capacitor. The voltage feedback and theerror amplifier components can be varied, and even differentvoltage-error amplifier topologies can be realized to add polesand zeros to the compensator transfer function. The currentloop gain could be easily modified by readjusting the value ofthe sensing resistor .

The APFC model proposed above can be employed to ana-lyze the steady-state operating point, current and voltage wave-forms, their ripple components and the outer loop stability, aswell as transient processes under load or line perturbations andthe overall distortion and resulting power factor of the APFCsystem. Setting realistic initial conditions helps in reaching thecorrect solution rapidly. Typical simulation plots are shown inFigs. 3 and 4. The normalized plots in Fig. 3 were obtained by re-drawing the simulated variables divided by the appropriate max-imum value of the original waveform. Current waveforms wereadditionally divided by two to make them distinguishable fromvoltage waveforms.

IV. MODELING THE LOAD

Loading is one of the important components of the APFCsystems. The nature of the load is generally determined by aspecific application and influences the static and dynamic per-formance of the APFC. The usual test cases of the APFC loadtypes are pure resistive load, constant current load, and constantpower load. The trivial case of pure resistance and constant cur-rent sink loading could be easily simulated by PSPICE/ORCADusing the element or the independent source of the required


Fig. 3. Time-domain simulation study showing the steady-state normalized waveforms of the idealized behavioral APFC model. (Top) Line voltage and current.(Bottom) Rectified voltage and the rectified and charging currents.

Fig. 4. Simulation study of the line current. (a) Current spectra of the idealized behavioral APFC model. Notice the third harmonic component. (b) PSPICEprintout of the Fourier analysis and resulting total harmonic distortion as they appear in the Output File.

value. The constant power load is a good example of a more so-phisticated case of the load modeling problem. Many types ofload may be represented as a two-port device, whose behavior isdetermined by its volt-ampere characteristic. The latter may be

created by PSPICE/ORCAD by means of the G element, the de-pendent behavioral voltage-controlled current source. Realiza-tion of such a device requires the current of the two-port to be de-fined as a function of the two-port voltage. For instance, to sim-


ulate a constant power load, a G source current, , can bedefined proportionally to the load power and inversely pro-portional to the voltage across the device terminals. Thus,to program constant current, constant resistance, and constantpower loads, G5 can be programmed as follows:

(6)

where is the voltage at the output node and , ,and are the values of desired load resistance, load cur-rent, and load power accordingly. These can be defined in thePARAMETERS statement.

Using this approach allows modeling of a great variety oflinear and nonlinear loads and their combined effect. Modelingthe simple resistive or the current sink loads in the same fashionmay be also advantageous, as will be further discussed.

The APFCs transient response to the load variation is ofprime importance. Many load variation scenarios are possible.The load variation is initiated by an independent event andshould therefore be represented by an independent source. Therising step function is the natural choice of the signal to beemployed as the switching stimulus, with its 0-V and 1-V levelssignifying the load OFF and ON states. If the load is representedby the G behavioral source as discussed, multiplication ofthe load characteristic by the value of the step willgenerate the required time dependence and activate the desiredload at the appropriate time. A simultaneous deactivation ofalternative loads may be achieved by multiplying the sourcefunction (the characteristic) by the falling step function

. This approach eliminates the need for additionalswitching elements that otherwise would have to be introducedin series with the load. Special attention should be given tothe convergence problems that might emerge as a result ofload switching. One of the possible solutions is to avoid abruptturn-on or turn-off of the load simply by controlling the risetime of the step function. Fig. 2(a) illustrates a circuit preparedto simulate a switched resistive load. Typical simulated tran-sient response of the switched load is shown in Fig. 5. Loadswitching occurs at 250 ms.

Note that implementing the rectifiers with dependent sourcesbrings the advantage of using the same approach to step the linevoltage. This can be realized by multiplying the E1 output signalby appropriate step function of the V7 source. This experimentallows observing the APFC response to line variations.

V. OUTER-LOOP FREQUENCY RESPONSE SIMULATIONIn a control system in which the operating point is stabilized by

feedback, care must be taken when measuring the loop gain. Topreserve the true operating conditions, the feedback path shouldnot be broken. A procedure that could be easily applied is to in-troduce an independent ac source for ac analysis, , in serieswith the feedback network input, as shown in Fig. 2(b). The ideais similar to that of [17]. It could be easily verified that the loopgain is . The negative sign is required to com-pensate for the sign inversion of the negative port of the summerand is important in order to obtain the correct loop gain phase.

To derive the frequency response of nonlinear systems, whosesmall signal gains strongly depend on the operating point,PSPICE/ORCAD first searches for the steady-state operating

Fig. 5. Load transient response of the three-loop APFC for 30%100% powerstep up at 250 ms. Upper trace: Error amp. output voltage. Middle trace: Linecurrent. Lower trace: Output voltage. Notice the steady-state voltage drop withhigher output ripple and increased line current amplitude at higher power level.

point and then performs linearization. Therefore, to derivethe correct frequency response of the APFC under study, thetrue operating conditions should be preserved at the multi-pliersquarerdivider inputs. However, when performing an acsweep, ORCAD considers the input voltage (line voltage) asbeing equal to zero. Thus, in order to preserve the true output ofthe feed-forward filter, the OFFSET field of the sinusoidal VSINline voltage source should be imposed as equal to the averagevalue of the rectified voltage. Note that the VAC test sourceis of a zero dc value and therefore has no effect on the operatingpoint, allowing the system to establish the correct regime.

Starting the ac analysis with zero initial conditions mightalso lead to erroneous results. As the PSPICE/ORCAD startssearching for the operating point, a transient process com-mences. For a while, the controller might run into saturation.Consequently, some variables temporarily keep constant value.The simulator might falsely recognize this condition as thesteady state and start the ac analysis prematurely, thus obtainingthe wrong loop-gain solution. Therefore, it is advisable to guidethe simulator and avoid saturation by specifying the expectedsteady-state solutions as initial conditions for the holdup capac-itor, the error amplifier capacitors, and the feed-forward filter.These can be easily obtained from the transient analysis plots.The Output File should be checked to be sure that the APFChas indeed reached the correct operating conditions.

The suggested procedure can be employed to plot thesmall-signal frequency response of the outer loop-gain. Shownin Fig. 6 are typical outer loop gain and phase responses.The plot provides the gain and the phase margins that arefundamental performance indices in analysis and design offeedback control systems. The analysis could be performed fordifferent loading and line conditions to investigate stability andto redesign the outer loop compensator for the worst casescenario.

VI. SUMMARY AND DISCUSSIONOver a 10-year period, 19982008, the author taught the

course Industrial Electronics to undergraduate electrical engi-


Fig. 6. Three-loop APFC small signal response (resistive load). (Top) Outerloop-gain [db]. (Bottom) Phase [deg].

neering students at Sami Shamoon College of Engineering. Thelecture course was accompanied by the practical course Indus-trial Electronics Laboratory. The laboratory curriculum wasupdated from time to time, and in the last five years since 2004,incorporated the topic of active power factor correction. AnAPFC system is a rather advanced example of a switched-modemixed-signal electronic feedback system and requires thestudents to have an adequate theoretical background. Some ofthe required theoretical preparation was acquired by studentsthrough the courses Electrical Circuits, Electronic Devices,Analog Electronic Circuits, and Analog Integrated Elec-tronic Circuits. In those courses, the subject of the electronicfeedback was presented using the methodology devised by[18] and [19] and later extended by the author as describedin [20] and [21]. PSPICE/ORCAD simulation exercises werepart of students homework assignments in those courses. Stu-dents also had a prerequisite Control Systems course, basedon [22], that was accompanied by MATLAB Laboratory withemphasis on feedback systems. Consequently, students hadacquired the necessary theoretical knowledge and simulationexperience with feedback amplifiers and feedback systems,stability analysis, and classical compensation techniques priorto enrolling in the Industrial Electronics Laboratory course.

At the time of composing this particular and other softwareexperiments, the EE teaching laboratories at Sami ShamonCollege of Engineering were equipped with PSPICE/ORCADsoftware. Acquiring dedicated software for an industrial/powerelectronics laboratory such as PSIM was considered asan alternative. However, at that time, the demo version ofPSIM was run-time-restricted, whereas the demo version ofPSPICE/ORCAD had a limitation on the number of com-ponents. For beginners experimenting with small circuits, thePSPICE/ORCAD demo version appears to be as good as the fullversion installed in the laboratory and could be used by studentsin the comfort of their homes. Furthermore, PSPICE/ORCADsoftware was also used in teaching the courses Analog Cir-cuits Laboratory, Digital Circuits Laboratory, and DigitalLogic Laboratory. Undergraduate students and their instruc-tors, who were graduate students, were already familiar withPSPICE/ORCAD software from their previous studies. An-other consideration was the availability of a textbook [23] toaccompany the course. Taking all of this into consideration,

PSPICE/ORCAD software was adopted for the course Indus-trial Electronics Laboratory.

The bulk of the fundamental theory of industrial and powerelectronics was provided through the Industrial Electronicscourse, which was based on [12] and [24]. The 3-h-long lec-tures in Industrial Electronics were attended by up to 120 stu-dents in two sessions of 60 persons each. Then, the class wasdivided into groups of up to 30 students for 1-h meetings topractice examples. Weekly homework assignments were given,each with three obligatory and two supplementary problems tobe submitted every third week. Students were also enrolled inthe Industrial Electronics Laboratory class and gained basichands-on experience.

The laboratory was conducted in groups of up to 16 studentsworking in pairs. Each laboratory session was 3 h long, ade-quate to complete the tasks. The laboratory covered the topicsof: 1) uncontrolled and phase controlled rectifiers; 2) dcdc con-verters; 3) dcac inverters; and 4) feedback in power electroniccircuits. Most of the experiments were conducted on hardware,whereas simulation was used repeatedly as a prelab assignment.Software experiments, however, required a prerequisite knowl-edge of appropriate simulation techniques. A textbook [23] toconsult with was recommended to students. Assuming that thestudents enrolled in the Industrial Electronics course had at-tended the classroom lecture on APFC and had acquired basicsimulation skills during the preceding set of experiments 1)3),the outline given in this paper was found to be adequate to pre-pare them for the APFC laboratory session.

The experimental assignments were to use the proposedmethodology to code the software program of the APFC cir-cuit in PSPISE/ORCAD software. Then, during the courseof software experiments, students were asked to obtain thetime domain waveforms of the control signals and of theline current and voltage. Total harmonic distortion analysisand the Fourier components of the line current were alsoobtained. The experiments were conducted for different loadtypes such as constant resistance, constant current sink, andconstant power load. The dc output voltage drop as a functionof loading could be measured. The stepped load was simulated,and the output voltage response was obtained; overshoot andsettling time were observed. The line voltage step was alsoinvestigated and recorded. Analyses of the frequency responseplots were conducted, and the system stability were checkedand redesigned by students, varying the parameters of thecompensation network of the error amplifier. Then, the linecurrent distortion index THD was recorded and compared forthe original and improved compensation networks. Studentswere asked to prepare a post-lab report in which they had toassess the experimental results and conduct a comparison ofsimulation and theoretical results. The latter were prepared as aprelab assignment. As students became familiar with simulationtechniques, their ability to verify their theoretical assessment ofthe circuit operation improves significantly, as does their levelof confidence and satisfaction.

The proposed method was used to investigate the overalllow-frequency APFC system performance dictated by the outervoltage-controlled loop. The frequency response of the innercurrent loop is the subject of a follow-up experiment, which isbeyond the scope of this paper and will be described elsewhere.


Fig. 7. Results of student survey. The grading scale spans from 1poor to6excellent. Bar #13 is the overall average rating.

In brief, however, the techniques used were adopted from[6][8].

According to the SCE school policy of evaluating the qualityof teaching, an Inet survey was conducted and computer-an-alyzed. The survey was conducted systematically on the con-cluding week of each course, prior to the finals. Only the en-rolled students could access the survey page and state their opin-ions by checking the available options. To encourage the stu-dents to participate in the survey, a laptop computer was givenas a prize to a randomly chosen participant. The survey ques-tions were the following.

1) Is the approach systematic?2) Is the course material clear and easily adopted?3) Is the pace of the course appropriate?4) Is the course interesting?5) Are the examples adequate?6) Were you provided with the proper tools to deal with the

problems?7) Have you acquired sufficient insight into the subject?8) Have you been introduced to the crucial points of the

subject?9) Have you acquired a perspective on the subject?

10) Have you had your questions answered?11) Have you had adequate interaction with the instructors?12) Were you actively involved in the lectures?13) Overall average rating.14) Your level of satisfaction is .15) The difficulty level of the course is .

The results of a recent student survey are shown in Fig. 7 and dis-play a very high level of satisfaction. Bar #13 is the overall av-erage rating. The survey canvassed the students opinion aboutthe course as a whole. The proposed approach seems to be ex-peditious and efficient in terms of time, cost, and effort.

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