Power Electronics Laboratory Using PSpice

Muhanunad H. Rashid, and Samir A. Al-Biyat Department of Electrical Engineering

King Fahd University of Petroleum & MineralsDhahran 31261

Fax: 966 - 3 - 860 3535

Abstract:

This paper summarizes the usefulness of the circuitsimulator PSpice in understanding the operation ofpower electronic circuit and the control function. Itillustrates through examples the simulation of powerelectronic laboratory by using by PSpice.

Introduction

Power electronics is an application oriented andinterdisciplinary course. It uses power semiconductordevices to perform switching action in order to achieve adesired conversion strategy. The switching slices thevoltage and current waveforms into various intervals,whose beginning and end depend on the boundaryconditions, which are fixed by the circuit parametersand/or control characteristics. The understanding of theoperation of a power electronics circuit requires a clearknowledge of the transient behavior of current andvoltage waveforms for each and every circuit element atevery instant of time. These features make powerelectronics a difficult course for students to understand. A laboratory helps in understanding power electronicsand its control interfacing circuits. The development ofpower electronics laboratory is relatively expensivecompared to other courses in EE curriculum, and as aresult power electronics laboratory facilities are availablein few limited universities. However, the powerelectronics are playing a key role in industrial powercontrol applications. Many universities are recognizingthe importance of power electronics and offer it as alecture course without any laboratory supports.

The student version of PSpice, which is available freeto students, is ideal for class-room use and forassignments requiring computer-aided simulation andanalysis. Probe is like a theoretical oscilloscope and itcan be used as a laboratory bench to view the waveformsof currents, voltages, power, power factor, etc. Thecapability of Probe along with other features to representdata in Table, Value, Function, Polynomial, Laplace,Param, Step makes PSpice versatile simulation tool forpower electronic power courses. Students can designpower versatile electronics circuits, use the PSpice

simulator to verify the design, and make necessarydesign modifications.

Single-Phase PWM Inverter

This is the first example to simulate the operation of asingle-phase PWM inverter. It involves the techniquesfor generating control signals, the use of a DC pulsesource to generate a triangular or square wave, and behavioral modeling in PSpice using VALUE andTABLE descriptions. A half-bridge inverter is shown inFig. 1 which was drawn in PSpice using schematiceditor. The inverter drives an RL-load of R = 10 W andL = 2.5 mH at an output frequency of fo = 1 kHz, andperiod To = 1 ms.

For PWM control, the number of pulses per half-cycle is assumed 10. That is, the switching frequency fs

= 10 kHz and switching period Ts = 100 ms. The carrier voltage vc of triangular wave is represented by aDC pulse wave of 50% duty chyle, 10 V (peak), ton =50 ms, toff = 50 ms, and Ts = 100 ms. Assuming amodulation index of M = 0.7, the reference wave isrepresented as a sine wave of 0.7V (peak), and fo = 1kHz.

The sine wave generator is followed by an ABS valuefunction whose is then compared with the carrier signalto give the error signal which is then multiplied by alimiter with a gain of 10 k.. The output of the limiter isthen multiplied by a DC pulse vg1 of 50% duty cycle, 10V (peak), ton = 0.5 ms, toff = 0.5 ms, and Ts = 1 ms togenerate the gating signals for transistor Q1. The DCpulse vg2 is the inversion of vg1, and generate the gatingsignals for transistor Q2. It has a delay of tdelay = 0.5 ms,50% duty cycle, 10 V (peak), ton = 0.5 ms, toff = 0.5 ms,and Ts = 1 ms. The functional block 'VALUE' is avoltage controlled voltage source and provides theisolation from the ground similar to a pulse transformer.

The SPICE plots of Vg1, Vg2, Vref and Vc areshown in Fig. 2. The instantaneous load current and itsrms value are shown in Fig. 3 which gives a peak loadcurrent of 0.685 A and an rms value of 332 mA.

Figure 2, Gating signals

Thyristor Characteristics

This is the second example in simulating the

characteristics of thyristor which is a commonly useddevice in introducing the concept of power electronics tostudents. It involves derivation of the approximatemodel parameters of the thysristor from themanufacturer's data and simulation of environment. Thethyristor of International Rectifier, type IR18CF is used

Figure 1, Single-phase half-bridge PWM inverter

Figure 3 PSpice plots of the instantaneous and rms load currents

Figure 4 Thyristor test circuit

as a test device as shown in Fig. 4. The modelparameters are derived from the manufacturers data andthe thyristor is modeled as voltage/current controlledswitch [1].

A gate voltage of VG = 5.894 V is applied to the gateand the anode to cathode voltage is swept from 0 to 10 V

in order to capture the turn-on point. This is repeated forVG = 5.898 and 5.91 V. The turn-on characteristics areshown in Fig. 5 which shows the sensitivity of VG on theturning point.

Figure 5 Thyristor turn -on characteristics

Conclusion

PSpice can be used to model and simulated powerelectronic circuits together with the gating control signalsto obtain the desired conversion strategy. Schematiceditorhas many interfacing devices and components bothanalog and digital. Its graphical post-processor Probe islike a theoretical oscilloscope which allows plottingfunctional variables such as power, power factor, Fourier spectrum. Also, it can be used to find the rms,average, and peak voltage and current ratings of devicesand components.

Advanced feature such as PARAM allows find theeffects of parameter variations on the performance of thepower conversions and the WORST CASE analysisallows finding the worst-case due to tolerance in devicesand components values.

The students can observe the effects of changes indesign parameters without actually building the circuit. This enhances their understanding of the circuitoperation and the control function. In the absence of adedicated power electronics laboratory, the laboratoryassignments could be only problems, which are to besimulated and verified by PSpice. Examples of

laboratory experiments on power electronic are given byRashid [2].

PSpice can be used for design verifications of powerelectronics circuits. Also for performance evaluation interms of parameters such as power factor, and totalharmonic factor.

References

1. Rashid, M.H, Power Electronics Laboratory Using Pspice. The IEEE Press, 1996, To bepublished.

2. Rashid. M. H., SPICE For Power Electronics andElectric Power. Prentice - Hall, 1993, Chapter14 - Applications.

3 Rashid, M.H, SPICE For Circuits and ElectronicsUsing PSpice . Prentice - Hall, 2nd Edition,1995.

4. Rashid, M.H., Power Electronics - Circuits, Devicesand Applications Power. Prentice - Hall, 2ndEdition, 1993.