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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

(81 0) 528-3700

ABSTRACT

In this paper a practical and low cost PWM battery charger for UPS application is proposed. For this type of PWM battery charger system, the power transistors used for PWM inverter are utilized to charge the battery in battery charger mode, thus fewer system components are required for the UPS system and higher overall system efficiency is achieved. By using advanced FUZZY LOGIC technique for the battery system feedback control, the battery charger system is able to achieve a better dynamic performance and easier implementation.

1. INTRODUCTION

The growing sophistication in modern technologies in the fields of communication, computer, networks, process control systems and automatic production lines have increased the demand for Uninterruptible power system (UPS). Recently in particular, demand for single phase small capacity, high efficiency, high performance, low cost UPS is increasing incidental to decrease in size and sophistication in performance of data processing equipment.

One successful approach for the realization of a single phase low cost UPS systems can be shown in Fig.1.

Fig. 1 FERRORESONANT TRANSFORMER UPS

This is a stand-by UPS using a FERRORESONANT transformer, where the transformer is used as a constant voltage regulator. Normally the commercial line voltage is regulated in amplitude by the FERRORESONANT transformer, and the battery and PWM inverter is in stand-by mode. On identifying failure of the AC input line, the line side static switch will be opened and the inverter and battery will be brought on the system, continuously supplying the UPS'S load. The batteries that are used to supply DC power to the PWM inverter during power failure must be recharged at regular time intervals(1). In order to charge the battery, normally a separate battery 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

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efficiency and more system component counts. However, it is possible to use inverter for this purpose. When configureated as push-pull system shown in Fig.2, an inverter of the pulse width modulation (PWM) type can function as an AC to DC converter, transferring power from the utility to the battery.

Fig. 2 BIDIRECTIONAL PWM CONVERTER

Instead of using classical output feedback control technique to design the control section for the battery charger, which normally employs PI control technique, a advanced FUZZY LOGIC technique is adopted to implement feedback control. By using FUZZY LOGIC technique, it is possible to design the control system using human experience without going through tedious control design method, such as model battery charger as a time-invariant linear plant and based on approximation of the linearized model to determine all the control parameters, so a easier implementation is obtained and a better dynamic performance is achieved.

2. SYSTEM DESCRIPTION AND BASIC THEORY

2.1 SYSTEM DESCRIPTION

source, VB is the battery power source, L is the inductor added to the system, (note that in a practical system, this inductor L is embedded in the power transformer, for instance, it could be leakage inductance between windings of the transformer), power transistors Q1 and Q2 along with their antiparallel 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 and Q2 are alternately turned on and off every half cycle of the fundamental frequency and the width of the pulses is dependent on a battery voltage and a output voltage. When in a battery charger mode, a train of high frequency PWM signals are applied to switches Q1 and Q2, so that the circuit now acts as a boost AC to DC converter. The inductor L in the circuit is used as a energy exchange element to transmit the energy to the battery.

2.2 BASIC THEORY FOR BATTERY CHARGING MODE

Fig.3 is a simplified circuit for the battery charger mode. The circuit now looks very much like a DC-DC boost converter. The switch S is in position A during period DT and in position B during period (1-D)T. During period DT the battery VB and the utility voltage VSA are connected so the voltage drop acrossed the inductor L is given by: 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

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I I

Fig 3 (a) SIMPLIFIED CIRCUIT FOR BAlTERY MODE (b) VOLTAGE WAVEFORM OF VL (c) CURRENT WAVEFORM OF 1 I

At time t = DT, the switch S reverses the battery connection. Since VB > VSA, the peak inductor current is reached at this point, and it will now begin to decline. During the remainder of the cycle, the inductor current is given by:

The current I1 (t) from (1) and (2) forms the charging current ID. The charge into battery VB is the time integral of the current ID over the whole period.

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 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 02 to charge the battery VB (This is referred as a transfer mode). Furthermore, when inductor current I1 is negative, Q2 is turned on so that the inductor L is charged through Q2 and the antiparalled body diode D1 by AC input voltage VSA. When Q2 is turned off, the energy stored in the inductor L is transferred to the battery VB. Therefore, by proper adjusting time ratio between the charge and transfer modes, the charging current to the battery is controlled.

3. CONTROL METHOD FOR THE BAlTERY CHARGER

There are number of ways to charge a battery. It is most popular in the industry to use a constant-voltage charge with current limited method. Fig.4 shows the operating principle of this method.

Table 1 FOUR MODES OF BAlTERY CHARGER

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V r *a - t

Fig. 4 CONSTANT VOLTAGE CHARGE WITH CURRENT LIMITED

It is essential to provide feedback control for the battery charger system so that the battery won't be overcharged and a optimum charging rate is obtained. Furthermore, the feedback control system provides a appropriate 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 it's operating region, then a classical control design method such as BODE plot is used to determine the system parameters such as gain margin and phase margin and so on (2), based on all these information along with design specifications, the gains for the PI controller are determined. Fig.5 shows a typical battery charger control system employing a PI controller. In the system, the MAX function is used to accommodate two feedback inputs, namely, battery voltage (VB) and battery charging current (ID) in order to perform a constant voltage charge with current limited.

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 system as a linear time-invariant system, instead, only design parameters are specified such as charging rate, charging current, charging time and so on. Fig.6 shows a excellent alternative method of controlling battery charger system. It is clear to see that a PI controller along with MAX circuit are replaced by a FUZZY LOGIC controller.

Fig. 6 FUZZY INFERENCE CONTROLLER FOR BATTERY CHARGER

4. FUZZY LOGIC IMPLEMENTATION

To design a FUZZY LOGIC controller for battery charger system, a input membership function relating to battery voltage (VB), a input membership function relating to battery charging current (ID), a output membership function of a value relating to modulation index (M) are constructed in the way that they are in trapezoidal shape and between two membership functions there is a overlap region. Fig.7 shows input membership functions for this application. Note that unlike

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conventional BOOLEAN LOGIC, the boundaries of these ranges are not cutoff points where the label applied fully on one side of the cutoff and does not apply at all on the other side of the cutoff. Instead, there is a region where input values gradually change from being fully applicable to completely inapplicable. These input membership functions have several labels, for example, VERY-SMALL, SMALL, VERY-LARGE, for battery voltage (VB) and VERY-LOW, LOW, VERY-HIGH for battery charging current (ID). Also output membership functions have several labels VERY-LOW-M, LOW-M. VERY-HI-M and so on for modulation index (M). Membership functions are provided for microcontroller which performs FUZZY LOGIC inference to have numerical meaning to each label. Each membership function identifies the range of battery voltage (VB), battery charging current (ID) and modulation index (M) that correspond to a label. Also a set of rules using battery voltage (VB), battery charging current (ID) as inputs and modulation index (M) as a output are generated based on design specifications and past design experience.

A I 2 3 4 5 6 7 R 9 Io

Examples of FUZZY inference rules set in the FUZZY LOGIC controller are as follows:

Rule (1): If VB is VERY-LARGE and ID is VERY-HIGH then M is VERY-LOW-M

Rule (2): If VB is VERY-LARGE and ID is HIGH then M is LOW-M

The FUZZY LOGIC inference involves three primary processes: FUZZIFICATION, RULE EVALUATION, and DEFUZZI FlCATlON. Fuzzification takes battery voltage (VB) and battery charging current (ID) values and combines them with stored membership function information to produce the grade of membership. Once grade of membership is produced, The FUZZY LOGIC controller will evaluate rules. All fuzzy outputs are cleared before rule evaluation. The truth value for each rule is the minimum of the fuzzy inputs for that rule, and this truth value is stored to each fuzzy output for that rule unless a larger value is already stored in the 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 C E NTE R - 0 F-G RAV ITY method is used in the DE FUZZ1 FI CAT1 ON process. The whole FUZZY LOGIC control algorithm is carried out by software using 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

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1. 2. 3.

4.

5. 6. 7.

Inverter PWM Control Digital filter function True RMS calculation for system parameters RS-232 communication for status report and system configuration Adaptive line voltage control Display and key interface function Dynamically modify input switching point based on characteristic of FERRORESONANT transformer and output load of UPS using FUZZY LOGIC technique (3).

and so on. all functions are under the control of a REAL-TIME KERNEL.

5. SIMULATION AND EXPERIMENTAL RESULTS

The proposed battery charger system was simulated by EMTP simulation software. Table 2 shows the circuit 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.

- -

,.Am .O.¶S.Y

-m. ~ ~~ ~ ~~ ~~~

0 0 0 9

Fig. 8 (a) INSTANTANEAOUS BAlTERY VOLTAGE (b) INSTANTANEAOUS CHARGING

CURRENT

Fig.9 shows simulation result for the average battery voltage and average battery charging current.

>- ____... _.. .. ...... ................. ... ..... .............. ............. . ..........................................................

Fig. 9 (a) AVERAGE BAlTERY VOLTAGE

- _

Fig. 9 (b) AVERAGE EATERY CHARGING CURRENT

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Fig. 11 WAVEFORM OF VOLTAGE ACROSS POWER DEVICES

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

Fig. 10 WAVEFORM OF INDUCTOR CURRENT

A practical and low cost battery charger 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

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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 6-1 2, 1993, p.p 540-542

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