1. 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) 528-3700
ABSTRACT
One successful approach for the
In this paper a practical and low cost realization of a single phase low cost
PWM battery charger for UPS UPS systems can be shown in Fig.1.
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
Fig. 1 FERRORESONANTTRANSFORMER UPS
advanced FUZZY LOGIC technique for
the battery system feedback control, the
battery charger system is able to This is a stand-by UPS using a
achieve a better dynamic performance FERRORESONANT transformer, where
and easier implementation. the transformer is used as a constant
voltage regulator. Normally the
1. INTRODUCTION commercial line voltage is regulated in
amplitude by the FERRORESONANT
The growing sophistication in modern transformer, and the battery and PWM
technologies in the fields of inverter is in stand-by mode. On
communication, computer, networks, identifying failure of the AC input line,
process control systems and automatic the line side static switch will be opened
production lines have increased the and the inverter and battery will be
demand for Uninterruptible power brought on the system, continuously
system (UPS). Recently in particular, supplying the UPS'S load. The batteries
demand for single phase small capacity, that are used to supply DC power to the
high efficiency, high performance, low PWM inverter during power failure must
cost UPS is increasing incidental to be recharged at regular time
decrease in size and sophistication in intervals(1). In order to charge the
performance of data processing battery, normally a separate battery
equipment. 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 W8'33-2U344W $4.00 1994 IEEE

2. 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

3. 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 AC
At time t = DT, the switch S reverses the input voltage VSA. When Q2 is turned
battery connection. Since VB > VSA, off, the energy stored in the inductor L is
the peak inductor current is reached at transferred to the battery VB.
this point, and it will now begin to Therefore, by proper adjusting time ratio
decline. During the remainder of the between the charge and transfer
cycle, the inductor current is given by: modes, the charging current to the
battery is controlled.
3. CONTROL METHOD FOR THE
BAlTERY CHARGER
The current I1 (t) from (1) and (2) forms There are number of ways to charge a
the charging current ID. The charge battery. It is most popular in the
into battery VB is the time integral o the
f industry to use a constant-voltage
current 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

4. 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 won't 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 it's 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

5. 16-4
conventional BOOLEAN LOGIC, the
boundaries of these ranges are not Examples of FUZZY inference rules set
cutoff points where the label applied in the FUZZY LOGIC controller are as
fully on one side of the cutoff and does follows:
not apply at all on the other side of the
cutoff. Instead, there is a region where Rule (1): If VB is VERY-LARGE
input values gradually change from and ID is VERY-HIGH
being fully applicable to completely then M is VERY-LOW-M
inapplicable. These input membership
functions have several labels, for Rule (2): If VB is VERY-LARGE
example, VERY-SMALL, SMALL, and ID is HIGH then M is
VERY-LARGE, for battery voltage (VB) LOW-M
and VERY-LOW, LOW, VERY-HIGH
for battery charging current (ID). Also The FUZZY LOGIC inference involves
output 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. Fuzzification
index (M). Membership functions are takes battery voltage (VB) and battery
provided for microcontroller which charging current (ID) values and
performs FUZZY LOGIC inference to combines them with stored membership
have numerical meaning to each label. function information to produce the
Each membership function identifies the grade of membership. Once grade of
range of battery voltage (VB), battery membership is produced, The FUZZY
charging current (ID) and modulation LOGIC controller will evaluate rules. All
index (M) that correspond to a label. fuzzy outputs are cleared before rule
Also 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 for
inputs and modulation index (M) as a that rule, and this truth value is stored to
output are generated based on design each fuzzy output for that rule unless a
specifications and past design larger value is already stored in the
experience. 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

6. 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

7. 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

8. 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