A Novel Buck-Boost Topology For Battery Chargers
Remya V
PG Scholar, Dept. of EEE
VVIT, Karadparamba, Malappuram, India
remyavnair89@gmail.com
Abstract--- The demand for rechargeable batteries increases,
and thus the demand for battery chargers. There are different
kinds of design solutions available for implementing battery
chargers. Some of them are hardware based solutions and some
are microcontroller based solutions. In a microcontroller based
solution, there is flexibility of using the same hardware for
charging different batteries and making only slight changes in
the software. Generally the Buck converter topology is used as a
DC- DC conve
erter to provide the controlled output power supply
to the batteries. But in this case a problem may arise, for
example, if you want to charge a 4.2V Li-ion batteries from a 5V
supply due to the presence of the protection diode and other
small drops across other components. This drop is generally
about 1V which makes it very difficult to provide 4.2V to the Liion batteries using the buck converter topology. This proposed
circuit describes a simple technique for implementing a noninverting buck-boost converter which requires only one inductor.
This converter is basically the result of cascading a Buck
converter with a Boost converter. This converter can be
controlled by two PWM signals from the microcontroller and can
be used as a Buck converter or Boost converter whenever
required. So this solution combined with the flexibility of the
microcontroller can be used to charge a wide range of the
batteries using the same hardware. By "tuning" the three
constants in the PID controller algorithm, the controller can
provide control action designed for specific process
requirements.
Index Terms— Buck Boost Controller, PID controller,
Ziegler Nichols Tuning.
I.
INTRODUCTION
THERE are many different types of battery systems. Of
these the conventional lead acid batteries are commonly used
for UPS applications. In the normal mode when the line
voltage present, the battery is trickle charged to offset the selfdischarge by the battery. This requires that a constant trickle
charge voltage be applied across the battery, and the battery
continuously draws small amount of current, thus maintaining
itself in a fully charged state. In the event of line outage, the
battery supplies the load. The capacity of battery expressed in
ampere-hours, which is the product of a constant discharge
current and the duration beyond which the battery voltage falls
below a voltage level called final discharge voltage. The
battery voltage should not be allowed to fall below the final
Kavya K
Asst. Professor, EEE Dept
VVIT, Karadparamba
Malappuram, India
discharge voltage; otherwise battery life shortened. Once the
line voltage restored, the battery bank in UPS is brought back
to its fully charged state.
BUCK BOOST TOPOLOGY
Generally, the Buck converter topology is used as a
DC-DC converter to provide the controlled output power
supply to a resource. The system operates as a buck converter
for duty cycles between 0-50% in the microcontroller, and as a
boost converter for duty cycles between 50-100%. The system
is therefore also advantageous because the microcontroller
algorithm prevents a dead short from occurring during
operation in the boost converter mode. The PID controller
calculation (algorithm) involves three separate parameters; the
Proportional, the Integral and Derivative values. The
Proportional value determines the reaction to the current error,
the Integral value determines the reaction based on the sum of
recent errors, and the Derivative value determines the reaction
based on the rate at which the error has been changing. The
weighted sum of these three actions is used to adjust the
process via a control element. By "tuning" the three constants
in the PID controller algorithm[1], the controller can provide
control action designed for specific process requirements. This
is implemented in the microcontroller based embedded system.
This converter is basically the result of cascading a Buck
converter with a Boost converter. This converter can be
controlled by two PWM signals from the microcontroller and
can be used as a Buck converter or Boost converter whenever
required.
II.
MODES OF OPERATION
The modes of operation is explained below using the circuit
shown in fig. 1
Fig. 1. circuit showing the modes of operation
We can use this converter as buck-boost converter in fig .1, as
a buck converter or as a boost converter by selecting different
combinations of switches SW1 and SW2 driven by the PWM1
and PWM2 signals output by the microcontroller. This converter
can be used as a non inverting buck-boost converter by selecting the operating mode from table which briefly describes the
convert modes. The different modes are shown in table.1
Table .1
IV.
CIRCUIT DIAGRAM
The circuit diagram of the proposed design is shown
in fig. 2,
Tabular column showing the modes.
If we look at phase 2 in table .1, here the switch SW1
(PWM1) is OFF and switch SW2 (PWM2) is ON . This
condition never occurs either in a buck converter or in boost
converter.
So you should always take care in your software that this
condition must not happen. To avoid this, if we assume that
initially both switches are in OFF condition then you should
use the following guidelines to manage the PWM signals
driving the two switches.
Fig. 2. The overall circuit diagram
BUCK SECTION
The circuit diagram of the buck section is shown I fig. 3,
1. Keep the frequency of both PWM signals the same, to
better control when synchronizing the two PWM signals
using the next three guidelines.
2. The duty cycle D1 of control signal PWM1, must be greater
than the duty cycle D2 of control signal PWM2.
3. PWM1 should be enabled before the PWM2 signal.
4. PWM1 should be disabled after the PWM2 signal
III.
THEORY OF OPERATION
The DC-DC converter uses a combination of buckboost converter and boost converter mode to charge the Li-ion
battery. In case of Li-ion, the constant current constant voltage
charging algorithm is used to charge the battery. Here we have
chosen the input voltage just enough to show the functionality
of the converter in buck-boost mode and boost mode.Initially,
the converter works in Buck-Boost converter mode to charge
the battery in constant current mode by keeping the duty cycle
of PWM2 constant and varying the duty cycle of PWM1. As
soon as there is an overflow condition for the duty cycle of
PWM1. The converter switches from buck-boost converter
mode to boost converter mode. And then duty cycle of PWM2
is varied to follow the algorithm while SW1 remains in ON
condition. Following section shows the software flow chart for
this combination.
Fig. 3. Circuit diagram of the basic buck section
The step-down dc-dc converter, commonly known as
a buck converter, is shown above . It consists of dc input
voltage source VS, controlled switch S, diode D, filter inductor
L, filter capacitor C, and load resistance R. Typical waveforms
in the converter are shown in Fig. 4 under the assumption that
the inductor current is always positive. The state of the
converter in which the inductor current is never zero for any
period of time is called the continuous conduction mode
(CCM). It can be seen from the circuit that when the switch S
is commanded to the on state, the diode D is reverse-biased.
When the switch S is off, the diode conducts to support an
uninterrupted current in the inductor. The relationship among
the input voltage, output voltage, and the switch duty ratio D
can be derived, for instance, from the inductor voltage Vl
waveform. According to Faraday’s law, the inductor voltsecond product over a period of steady-state operation is zero.
For the buck converter (VS – VO) DT = -VO(1 – D)T Hence,
the dc voltage transfer function, defined as the ratio of the
output voltage to the input voltage, is MV =(V0/VS )=D
voltage it produces during the discharge phase is related to the
rate of change of current, and not to the original charging
voltage, thus allowing different input and output voltages.
The basic principle of a Boost converter consists of 2 distinct
states:
CHARACTERISTICS OF BUCK CONVERTER
•
•
In the On-state, the switch S is closed, resulting in an
increase in the inductor current;
In the Off-state, the switch is open and the only path
offered to inductor current is through the fly back
diode D, the capacitor C and the load R. This results
in transferring the energy accumulated during the Onstate into the capacitor.
----------------------(1)
From the above expression it can be seen that the output
voltage is always higher than the input voltage (as the duty
cycle goes from 0 to 1), and that it increases with D. The
characteristics of the boost converter is shown in the
waveforms.
CHARACTERISTICS OF BOOST CONVERTER
Fig. 4. Characteristics of the buck section
BOOST SECTION
A boost converter (step-up converter) of fig. 5 is a power
converter with an output DC voltage greater than its input DC
voltage.
Fig. 6. Characteristics of the boost converter
CASCADE BUCK-BOOST CONVERTER
Fig. 5. Circuit showing the basic boost section
It is a class of switching-mode power supply (SMPS)
containing at least two semiconductor switches (a diode and a
transistor) and at least one energy storage element. Filters
made of capacitors (sometimes in combination with inductors)
are normally added to the output of the converter to reduce
output voltage ripple. The key principle that drives the boost
converter is the tendency of an inductor to resist changes in
current. When being charged it acts as a load and absorbs
energy (somewhat like a resistor), when being discharged, it
acts as an energy source (somewhat like a battery). The
The cascaded form of buck-boost converter is shown in the
fig. 7
Fig. 7. Cascaded buck boost converter
The CBB converter has four valid operating states
corresponding tothe status of S1 and S2.When operated as a
dc-dc converter, the inductor current under cyclic steady- state
has three intervals of operation, namely ‘boost’ interval (Db T)
{both S1 and S2 ON}, ‘free-wheel’ interval (Df T) {S2 ON
and S1 OFF}, and ‘capacitor charge’ interval (Do T) {both S1
and S2 OFF}. The fourth state (S1 ON and S2 OFF) is invalid.
Also, in any switching cycle, Db +Df + Do =1.
reference variable is often called the set point. The integral,
proportional and derivative part can be interpreted as control
actions based on the past, the present and the future.
ZIEGLER NICHOLS TUNING
PID controllers are probably the most commonly
used controller structures in industry. They do, however,
present some challenges to control and instrumentation
engineers in the aspect of tuning of the gains required for
stability and good transient performance. There are several
prescriptive rules used in PID tuning. An example is that
proposed by Ziegler and Nichols in the 1940's and described
in Section 3 of this note. These rules are by and large based on
certain assumed models
.
PID CONTROLLER STRUCTURE
Fig. 8. Inductor (IL) and switch ‘S1’ (IS1) waveform
V.
PID controller encapsulates three of the most
important controller structures in a single package. The
parallel form of a PID controller has transfer function:
PID CONTROLLER
The PID controller is the most common form of
feedback. It was an essential element of early governors and it
became the standard tool when process control emerged in the
1940s. In process control today, more than 95% of the control
loops are of PID type, most loops are actually PI control. PID
controllers are today found in all areas where control is used.
The controllers come in many different forms. There are
standalone systems in boxes for one or a few loops, which are
manufactured by the hundred thousand yearly. PID control is
an important ingredient of a distributed control system. The
controllers are also embedded in many special purpose control
systems. PID control is often combined with logic, sequential
functions, selectors, and simple function blocks to build the
complicated automation systems used for energy production,
transportation, and manufacturing. Many sophisticated control
strategies, such as model predictive control, are also organized
hierarchically. PID control is used at the lowest level; the
multivariable controller gives the set points to the controllers
at the lower level. The PID controller can thus be said to be
the “bread and butter’s’ of control engineering. It is an
important component in every control engineer’s tool box.
PID controllers have survived many changes in technology,
from mechanics and pneumatics to microprocessors via
electronic tubes, transistors, integrated circuits. The
microprocessor has had a dramatic influence on the PID
controller. Practically all PID controllers made today are
based on microprocessors. This has given opportunities to
provide additional features We will start by summarizing the
key features of the PID controller. The “textbook” version of
the PID algorithm is described by:
----------(2)
Where, y is the measured process variable, r the reference
variable, u is the control signal and e is the control error. The
------------------------(3)
where: Kp := Proportional Gain
Ki := Integral Gain Ti := Reset Time =Kp/Ki
Kd :=Derivative gain Td
Rate time or derivative time =Kd/Kp
The proportional term in the controller generally
helps in establishing system stability and improving the
transient response while the derivative term is often used
when it is necessary to improve the closed loop response
speed even further. Conceptually the effect of the derivative
term is to feed information on the rate of change of the
measured variable into the controller action. The most
important term in the controller is the integrator term that
introduces a pole at s = 0 in the forward loop of the process.
This makes the compensated1 open loop system (i.e. original
system plus PID controller) a type 1 system at least; our
knowledge of steady state errors tells us that such systems are
required for perfect steady state set point tracking. This is
more formally stated in the following theorem as in fig. 9
Fig. 9. The basic control loop Theorem
For the unity feedback system of fig shown above, perfect set
point control can only be achieved for the controller, C(s), if
and only if; . The open loop forward gain has a steady state
gain of infinity.
VI.
FLOW CHART
Fig. 13. Simulated result of buckboost section
VIII.
APPLICATIONS
Modern portable applications require batteries with
higher power densities. Battery chemistries such as nickel
metal hydride (Ni MH), Lithium-ion (Li-ion), and Lithiumpolymer (Li-poly) have adequately served the needs of these
applications. However, in the practical world, there's often a
need to maintain system voltage at a constant value, whether
the battery is charged or discharged; therefore, a single
converter is needed to provide both buck and boost operations.
Fig. 10. The flow chart of the PWM generation
VII.
SIMULATION RESULTS
The simulation results are shown ,
Fig. 11. Simulated result of buck section
Fig. 12. Simulated result of boost section
IX.
CONCLUSION
This application note describes a simple but very
useful technique for implementing a microcontroller
controlled low cost non-inverting buck-boost converter. This
converter can be used as a buck converter or as a boost
converter or as a buck-boost converter. The example shows
the application of this converter in a battery charger. However
it can also be used in other portable applications or any
application getting its power from a rechargeable battery. Or
you could use this technique to make a USB charger to charge
Li-ion batteries or 3 or more Ni-MH Cells in series.
REFERENCES
[1]. Zhong Wu, Jianhui Zhao and Jiyang Zhang “Cascade
PID
Control of Buck-Boost-Type DC/DC Power
Converters*”,Proceedings of the 6th World Congress on
Intelligent Control and Automation, June 21 - 23, 2006,
Dalian, China.
[2]. Thomas Nussbaumer, Guanghai Gong, Marcelo Lobo
Heldwein, and Johann W. Kolar, “Modeling and Robust
Control of a Three-Phase Buck+Boost PWM Rectifier (VRX4)”, Ieee Transactions On Industry Applications, VOL. 44,
NO. 2, MARCH/APRIL 2008.
[3]. Zhong Wu, Jianhui Zhao and Jiyang Zhang, “Cascade
PID Control of Buck-Boost-Type DC/DC Power
Converters*”, Proceedings of the 6th World Congress on
Intelligent Control and Automation, June 21 - 23, 2006,
Dalian, China.
[4]. Richard Morrison, and Michael G. Egan, “A New Modulation Strategy for a Buck-Boost Input AC/DC Converter”, Ieee
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