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16-4 A PRACTlCAL AND LOW COST PWM BAlTERY CHARGER USING FUZZY LOGIC CONTROL FOR UPS APPLICATION YU QIN AND SHANSHAN DU MEMBER, IEEE CONTROLLED POWER COMPANY 7955 STEPHENSON HWY TROY, MICHIGAN 48083 (810) 528-3700ABSTRACT One successful approach for theIn this paper a practical and low cost realization of a single phase low costPWM battery charger for UPS UPS systems can be shown in Fig.1.application is proposed. For this type ofPWM battery charger system, the powertransistors used for PWM inverter areutilized to charge the battery in batterycharger mode, thus fewer systemcomponents are required for the UPSsystem and higher overall systemefficiency is achieved. By using Fig. 1 FERRORESONANTTRANSFORMER UPSadvanced FUZZY LOGIC technique forthe battery system feedback control, thebattery charger system is able to This is a stand-by UPS using aachieve a better dynamic performance FERRORESONANT transformer, whereand easier implementation. the transformer is used as a constant voltage regulator. Normally the1. INTRODUCTION commercial line voltage is regulated in amplitude by the FERRORESONANTThe growing sophistication in modern transformer, and the battery and PWMtechnologies in the fields of inverter is in stand-by mode. Oncommunication, computer, networks, identifying failure of the AC input line,process control systems and automatic the line side static switch will be openedproduction lines have increased the and the inverter and battery will bedemand for Uninterruptible power brought on the system, continuouslysystem (UPS). Recently in particular, supplying the UPSS load. The batteriesdemand for single phase small capacity, that are used to supply DC power to thehigh efficiency, high performance, low PWM inverter during power failure mustcost UPS is increasing incidental to be recharged at regular timedecrease in size and sophistication in intervals(1). In order to charge theperformance of data processing battery, normally a separate batteryequipment. charger system has to be provided, as seen in Fig.1. Note that the addition of a power conversion stage (the battery charger) results in lower over-all system 443 W833-2U344W $4.00 1994 IEEE
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16-4 efficiency and more system component source, VB is the battery power source, counts. However, it is possible to use L is the inductor added to the system, inverter for this purpose. When (note that in a practical system, this configureated as push-pull system inductor L is embedded in the power shown in Fig.2, an inverter of the pulse transformer, for instance, it could be width modulation (PWM) type can leakage inductance between windings function as an AC to DC converter, of the transformer), power transistors transferring power from the utility to the Q1 and Q2 along with their antiparallel battery. body diodes are used in either a inverter mode or a battery charger mode. the system is configurated as a push-pull circuit where a transformer winding with a center tap is connected to the power transistors Q1 and Q2 as well as the battery power VB, the polarities of the transformer are shown in the figure. When in a inverter mode, switches Q1 Fig. 2 BIDIRECTIONAL PWM CONVERTER and Q2 are alternately turned on and off every half cycle of the fundamental frequency and the width of the pulses is Instead of using classical output dependent on a battery voltage and a feedback control technique to design output voltage. When in a battery the control section for the battery charger mode, a train of high frequency charger, which normally employs PI PWM signals are applied to switches Q1 control technique, a advanced FUZZY and Q2, so that the circuit now acts as a LOGIC technique is adopted to boost AC to DC converter. The inductor implement feedback control. By using L in the circuit is used as a energy FUZZY LOGIC technique, it is possible exchange element to transmit the to design the control system using energy to the battery. human experience without going through tedious control design method, 2.2 BASIC THEORY FOR BATTERY such as model battery charger as a CHARGING MODE time-invariant linear plant and based on approximation of the linearized model to Fig.3 is a simplified circuit for the battery determine all the control parameters, so charger mode. The circuit now looks a easier implementation is obtained and very much like a DC-DC boost a better dynamic performance is converter. The switch S is in position A achieved. during period DT and in position B during period (1-D)T. During period DT 2. SYSTEM DESCRIPTION AND the battery VB and the utility voltage BASIC THEORY VSA are connected so the voltage drop acrossed the inductor L is given by: 2.1 SYSTEM DESCRIPTION VL=VSA+VB. The current in the inductor L is then derived as: Fig. 2 shows a bi-directional PWM converter used in a UPS proposed in this paper. Here VSA is the utility power 444
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16-4 In practical system as shown in Fig.2, the battery charger system transmits the energy which is stored in inductor L by cycling through the four modes listed in Table 1, to the battery VB. When the current flowing through the inductor L, 11, is positive, Q1 is tumed on, the I I antiparallel body diode D2 is forward biased, L is directly applied with AC input voltage, therefore the inductor current I1 is increased linearly. In this mode, the inductor L is charged through Q1 and D2 by AC input voltage VSA (this is referred as a charge mode). When Q1 is turned off, the energy stored in the inductor L is released through diode 0 2 to charge the battery VB (This is referred as a transfer mode).Fig 3 (a) SIMPLIFIED CIRCUIT FOR BAlTERY Furthermore, when inductor current I1 is MODE negative, Q2 is turned on so that the (b) VOLTAGE WAVEFORM OF VL inductor L is charged through Q2 and (c) CURRENT WAVEFORM OF 1I the antiparalled body diode D1 by ACAt time t = DT, the switch S reverses the input voltage VSA. When Q2 is turnedbattery connection. Since VB > VSA, off, the energy stored in the inductor L isthe peak inductor current is reached at transferred to the battery VB.this point, and it will now begin to Therefore, by proper adjusting time ratiodecline. During the remainder of the between the charge and transfercycle, the inductor current is given by: modes, the charging current to the battery is controlled. 3. CONTROL METHOD FOR THE BAlTERY CHARGERThe current I1 (t) from (1) and (2) forms There are number of ways to charge athe charging current ID. The charge battery. It is most popular in theinto battery VB is the time integral o the f industry to use a constant-voltagecurrent ID over the whole period. charge with current limited method. Fig.4 shows the operating principle of this method.Table 1 FOUR MODES OF BAlTERY CHARGER 445
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16-4 t Vr *a - I Fig 5 CONVENTIONAL CONTROL FOR BATTERY CHARGER In this paper, a advanced FUZZY LOGIC technique is employed. With the FUZZY LOGIC control design, it is not required to model a battery charger Fig. 4 CONSTANT VOLTAGE CHARGE WITH system as a linear time-invariant CURRENT LIMITED system, instead, only design parameters are specified such as charging rate, It is essential to provide feedback charging current, charging time and so control for the battery charger system so on. Fig.6 shows a excellent alternative that the battery wont be overcharged method of controlling battery charger and a optimum charging rate is system. It is clear to see that a PI obtained. Furthermore, the feedback controller along with MAX circuit are control system provides a appropriate replaced by a FUZZY LOGIC controller. gain for the battery charger system so that it will operate in a stable condition for all situations. Traditionally, a classical feedback control technique is employed such as proportional integral (PI) control. For this, a battery charger system is first modeled as a linear time- invariant systems for its operating Fig. 6 FUZZY INFERENCE CONTROLLER FOR region, then a classical control design BATTERY CHARGER method such as BODE plot is used to determine the system parameters such 4. FUZZY LOGIC IMPLEMENTATION as gain margin and phase margin and so on (2), based on all these information To design a FUZZY LOGIC controller for along with design specifications, the battery charger system, a input gains for the PI controller are membership function relating to battery determined. Fig.5 shows a typical voltage (VB), a input membership battery charger control system function relating to battery charging employing a PI controller. In the current (ID), a output membership system, the MAX function is used to function of a value relating to accommodate two feedback inputs, modulation index (M) are constructed in namely, battery voltage (VB) and battery the way that they are in trapezoidal charging current (ID) in order to perform shape and between two membership a constant voltage charge with current functions there i a overlap region. s limited. Fig.7 shows input membership functions for this application. Note that unlike 446
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16-4conventional BOOLEAN LOGIC, theboundaries of these ranges are not Examples of FUZZY inference rules setcutoff points where the label applied in the FUZZY LOGIC controller are asfully on one side of the cutoff and does follows:not apply at all on the other side of thecutoff. Instead, there is a region where Rule (1): If VB is VERY-LARGEinput values gradually change from and ID is VERY-HIGHbeing fully applicable to completely then M is VERY-LOW-Minapplicable. These input membershipfunctions have several labels, for Rule (2): If VB is VERY-LARGEexample, VERY-SMALL, SMALL, and ID is HIGH then M isVERY-LARGE, for battery voltage (VB) LOW-Mand VERY-LOW, LOW, VERY-HIGHfor battery charging current (ID). Also The FUZZY LOGIC inference involvesoutput membership functions have three primary processes:several labels VERY-LOW-M, LOW-M. FUZZIFICATION, RULE EVALUATION,VERY-HI-M and so on for modulation and DEFUZZIFlCATlON. Fuzzificationindex (M). Membership functions are takes battery voltage (VB) and batteryprovided for microcontroller which charging current (ID) values andperforms FUZZY LOGIC inference to combines them with stored membershiphave numerical meaning to each label. function information to produce theEach membership function identifies the grade of membership. Once grade ofrange of battery voltage (VB), battery membership is produced, The FUZZYcharging current (ID) and modulation LOGIC controller will evaluate rules. Allindex (M) that correspond to a label. fuzzy outputs are cleared before ruleAlso a set of rules using battery voltage evaluation. The truth value for each rule(VB), battery charging current (ID) as is the minimum of the fuzzy inputs forinputs and modulation index (M) as a that rule, and this truth value is stored tooutput are generated based on design each fuzzy output for that rule unless aspecifications and past design larger value is already stored in theexperience. fuzzy output. When all fuzzy outputs are derived, the DEFUZZIFICATION is performed. DEFUZZlFlCATlON is the process of combining all fuzzy outputs into a specific composite result (modulation index M) to the UPS system. The CENTER - 0F-GRAVITY method is used in the DEFUZZ1FICAT1ON process. The whole FUZZY LOGIC control algorithm is carried out by software using A I 2 3 4 5 6 7 R 9 Io microcontroller. The same microcontroller is also used for other system control functions such as:Fig. 7 (a) INPUT MEMBERSHIP FUNCTION FOR VB (b) INPUT MEMBERSHIP FUNCTION FOR ID 447
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16-4 1. Inverter PWM Control 2. Digital filter function 3. True RMS calculation for system -- parameters 4. RS-232communication for status ,.Am .O.¶S.Y report and system configuration 5. Adaptive line voltage control 6. Display and key interface function 7. Dynamically modify input switching point based on characteristic of FERRORESONANTtransformer and output load of UPS using FUZZY LOGIC technique (3). and so on. all functions are under the control of a REAL-TIME KERNEL. -m. ~~ ~ ~ ~~ ~~~ 0 0 0 9 5. SIMULATION AND EXPERIMENTAL Fig. 8 (a) INSTANTANEAOUSBAlTERY VOLTAGE (b) INSTANTANEAOUSCHARGING RESULTS CURRENT The proposed battery charger system Fig.9 shows simulation result for the was simulated by EMTP simulation average battery voltage and average software. Table 2 shows the circuit battery charging current. parameters used for the simulation. >- ____... ...... ................. ... ..... .............. ............. . .......................................................... _.... Table 2 CIRCUIT PARAMETERS FOR SIMULATION Fig.8 shows simulation result for the instantaneous battery charging current and battery voltage waveforms. Fig. 9 (a) AVERAGE BAlTERY VOLTAGE
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16-4 U.. I I i-_ Fig. 11 WAVEFORM OF VOLTAGE ACROSS POWER DEVICES Fig. 9 (b) AVERAGE EATERY CHARGING CURRENT The proposed battery charger system was tested in a 3KVA UPS system. Fig. 10 shows the waveform of inductor current taken from a operating battery charger system. and Fig.11 is the waveform of the voltage acrossed power devices, Fig. 12 is the waveform of the battery charging current. Fig. 12 WAVEFORM OF EATERY CHARGING CURRENT 6. CONCLUSION A practical and low cost battery charger Fig. 10 WAVEFORM OF INDUCTOR CURRENT using advanced FUZZY LOGIC for UPS application is proposed in this paper. This type of battery charger employs the same power devices used for PWM inverter to charge the battery, so that higher system efficiency and lower system components count is achieved. Instead of using a classical control technique, this type of battery charger uses FUZZY LOGIC control technique to control battery charging process, easier implementation and better 449
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16-4 dynamic system performance are obtained. ACKNOWLEDGMENT The authors would like to express their appreciation to Mr. James Rigney and Mr. Gordon Middler for their valuable advises and support effort. REFERENCES Mohan, Undeland and Robbin, Power Electronic: Converters, Application and Design. JOHN WILEY & SONS, 1989 Kuo, Digital Control System, second edition, SANDERS COLLEGE PUBLISHING, 1992 Yu Qin, S. S. Du, "How FUZZY LOGIC can improve the performance of Uninterruptible power system", IEEE APEC Conference in San Diego, CA March 6-12, 1993, p.p 540-542 450
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