1. INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 1, January- February (2013), © IAEME
& TECHNOLOGY (IJEET)
ISSN 0976 – 6545(Print)
ISSN 0976 – 6553(Online)
Volume 4, Issue 1, January- February (2013), pp. 131-138 IJEET
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TRANSIENT RESPONSE IMPROVEMENT OF BUCK CONVERTER
# * #
Arun Kumar Pandey , Prof. S. K. Misra , Sumit Kumar Misra
#
P.G Student (Power Electronics & Control)
(Department of Electrical Engineering, HarCourt Butler Teconological Institute
Kanpur-208002, India)
*
Professor
(Department of Electrical Engineering, HarCourt Butler Teconological Institute
Kanpur-208002, India
ABSTRACT
Sliding mode (SM) and Proportional-integral-derivative (PID) controller are
implemented for Buck converter in a continuous conduction mode. A closed-loop controller
system is implemented and goes through the transient time response of buck converter.
Sliding mode controller is worked as robust controller and can be used to reduce the Delay
time and settling time of the system. In power drives controller are required to be of faster
transient response to increase the synchronization of the system. The closed-loop system and
the transients of the closed loop system are analyzed for various circuit parameters. Under a
large range of operating points, the buck converter under the SM controller has a output
voltage accuracy upto ± 0.02V and operating frequency of 5 MHz (approx). The merits of the
SM controller are compared with some other controller.
Keywords: Transient response, sliding mode control, buck converter, settling time,
simulation.
1. INTRODUCTION
Direct current converters are used to convert source voltage to other voltage level by
varying the duty cycle of the switches in circuit. As they are non linear systems it is difficult
to implement their control design. Since traditional control methods are designed by taking
one reference operating point, they did not give the proper transient response and parameter
variation responses.
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2. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 1, January- February (2013), © IAEME
The buck dc-dc converters are used were the required output voltage signal should be lower
than the source voltage signal. In buck converter, an extremely high speed switching devices
are placed and the better efficiency of power conversion with the steady state is to be achieve.
This is achieved by a appropriate switching process of the circuit. Hence the motive of the
switching control in buck converter is to implement high power transfer efficiency and sharp
tracking of output voltage. In this paper transient performance of buck converter is analyzed.
For better transient response in DC-DC converter settling time and overshoot should be
minimum. For this different types of controllers are used. Like as linear controller PI, PID
and in nonlinear controllers SMC (sliding mode controller), neural network and fuzzy
control. To design the switching characteristic of converters, any nonlinear control methods
can be used, like neural network, fuzzy control and slide mode control. The implementation
of a PID controller is based on the Bode plot or tuned with Ziegler-Nichols techniques. Due
to the highly nonlinear characteristics of the DC-DC converters, the PID control does not
allow disturbances rejection and fast transient response time. Hence it is very interesting as
well as difficult to develop more nonlinear and advanced non-conventional robust control
structures to improve the performance of the DC-DC converters. Sliding mode controllers are
well known for their robustness and stability but this kind of controller operate at very large
(infinite) switching frequency so-called chattering phenomenon [2].
PWM DC-to-DC converters are very popular for the last three decades, and that are widely
used at all power levels. Since switching converters constitute a case of variable structure
systems, the sliding mode (SM) control technique can be a possible option to control this kind
of circuits [5]. Sliding Mode (SM) controllers are well known for their robustness and
stability. The nature of the controller is to ideally operate at an infinite switching frequency
such that the controlled variables can track a certain reference path to achieve the desired
dynamic response and steady-state operation [3]. This requirement for operation at high
switching frequency, create challenges in the feasibility of applying SM controllers in power
converters. This is because extreme high speed switching in power converters results in
excessive switching losses, inductor and transformer core losses, and electromagnetic
interference (EMI) noise issues.
Different control programs are applied to design the DC-DC converters for getting a robust
output voltage. As DC-DC converters are time variant and nonlinear systems. The use of
linear control techniques to control these converters is not appropriate. A multi-loop control
technique, like current mode control, will greatly improve the dynamic behaviour, but the
current mode control is not fully non linear control. So it does not support the large signal
disturbances and it remains difficult for the implementation of higher order converter
topologies.
In this paper we had implemented the buck converter using non linear controller i.e. sliding
mode controller for the faster transient response of the system. We had also implemented
buck converter using conventional linear controller and provide the simulated results for the
comparison of the transient responses of both the converters with change in different
parameters.
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3. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 1, January- February (2013), © IAEME
2. BUCK CONVERTER
The buck converter in Fig. 1 is powered from a constant DC source through a diode
rectifier and uses a controlled switch to govern unidirectional power flow from input to
output. The converter includes one capacitor and one inductor to store and transfer energy
from the input to output. A filter is used to smooth the voltage and current waveforms. The
circuit is assumed to be operating in the steady state continuous conduction mode. The
capacitor is large enough to filter the AC component and provide a constant output voltage.
Fig.1. DC-DC buck converter
Fig.2. Turn on switch
Fig.3. Turn off switch
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4. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 1, January- February (2013), © IAEME
When the switch S1 is on and D is reverse biased, the dynamics of inductor current iL and the
capacitor voltage Vc are
(1)
When the switch S1 is off and D is forward biased, the dynamics of the are
(2)
When the switch S1 is off and D is also not conducting,
(3)
The state space representation for converter circuit configuration can be expressed as
(4)
Where
(5)
Here x is the state vector and A’s and B’s are system matrices. The state matrices and the
input vectors for the ON and OFF periods are
(6)
2.1 Controller Design
In SM control, the switching function is employed to decide the input states u to the system
(7)
where α is the sliding coefficient to be designed. For stability concern, the value of α should
be greater than zero [4]. The basic control law is expressed as
(8)
The basic control law in (8) determines the switching state u and directs the phase trajectory
toward the sliding surface (S= 0) in phase plane.
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5. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 1, January- February (2013), © IAEME
(9)
However, to maintain the phase trajectory on the sliding surface and force it toward the origin,
the Lyapunov's second method must be obeyed [4].
3. BUCK CONVERTER USING PID CONTROLLER
After studying the following results we can that the conventional controllers may be
used in power circuits where sharp parameter changes does not govern the system stability as
well or lesser settling time doesn’t effect the performance of the system.
Fig.4. SIMULINK Model of buck converter using PID Controller
Fig.5. SIMULINK Result with Vin = 24V, Vref. = 12V and Settling Time = 1.25 X 10-3 sec
(PID Controller).
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6. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 1, January- February (2013), © IAEME
Fig.6. SIMULINK Result with Vin = 24V, Vref. = 12V and Settling Time = 1.65 X 10-3 sec
(PID Controller).
4. BUCK CONVERTER USING SM CONTROLLER
Here some experimental results are presented to explain the lesser settling time of
siliding mode controller with variable input-output conditions.
Fig.7. SIMULINK Model of buck converter using SM Controller
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7. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 1, January- February (2013), © IAEME
Fig.8. SIMULINK Result with Vin = 50V, Vref. = 24V and Settling Time = 1.5 X 10-4 sec
(SM Controller).
Fig.9. SIMULINK Result with Vin = 24V, Vref. = 12V and Settling Time = 1.4 X 10-4 sec
(SM Controller).
5. TRANSIENT RESPONSE COMPARISON BETWEEN PID AND SM CONTROLLER
TABLE I
SIMULATION RESULTS FOR DIFFERENT CONTROLLERS
Controller Settling Input Output
Type time(sec) voltage(V) voltage(V)
SMC 1.4 X 10-4 24 12
SMC 1.5 X 10-4 50 24
PID 0.75 X 10-3 200 120
PID 1.6 X 10-3 24 12
PID 1.25 X 10-3 50 24
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8. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 1, January- February (2013), © IAEME
By comparing the results of fig.5,6 (output of PID controller) and fig.8,9 (output of SM
controller), now it can be concluded that the transient response of SM controller is faster
than PID controller and the SM controller output is more stable than PID controller output.
Hence for getting greater voltage accuracy in short duration we have to use SM controller.
6. CONCLUSIONS
After analyzing the performance comparison result of the paper it can be concluded
that the settling time of sliding mode controller is lesser than conventional PID controller.
The simulation results shows high accuracy of the SM controller for achieving a reference
voltage with fast transient responses and strong system robustness. But when we have to
reduce the cost of a system by compromising with less accuracy and complexity, than PID
controller can be used.
7. ACKNOWLEDGMENT
The authors would like to acknowledge the contribution by Department of Electrical
Engineering, HBTI Kanpur for their support and guidance.
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