A simulation based on MatLab minor project report of Buck converter. DC to DC step down Converter

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Simulation Based Minor Project Report
BUCK CONVERTER
(DC TO DC STEP DOWN CONVERTER)
By
ATIF KHAN (1402921051)
ASHUTOSH SINGH (1402921049)
ANURAG SINGH (1402921037)
Submitted to
Prof. Arun Kumar
Prof. Ameer Faisal
Department of Electrical and Electronics Engineering
KIET Group of Institutions, Ghaziabad
Dr. A.P.J. Abdul Kalam Technical University, U.P., Lucknow
<Nov, 2016>

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CERTIFICATE
This is to certify that Simulation Based Minor Project Report entitled “ Simulation of Buck
Converter ” which is submitted by ATIF KHAN (1402921051), ASHUTOSH SINGH
(1402921049), ANURAG SINGH (1402921037), in partial fulfillment of one of the course
work of B.Tech 5th Semester (2016-17) in Department of Electrical and Electronics
Engineering, KIET Group of Institutions affiliated to Dr. A.P.J. Abdul Kalam Technical
University (APJAKTU), Lucknow.
Date: 10/11/2016 Supervisor
Prof. Arun Kumar
Prof. Ameer Faisal

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ACKNOWLEDGEMENT
It gives us a great senseofpleasure to presentthe reportof the B. TechSimulation
Based Minor project undertaken during B. Tech. 5th Semester. We owe a special
debt of gratitude to our guide Professor Arun Kumar and Prof. Ameer Faisal,
Department of Electrical & Electronics Engineering, Krishna Institute of
Engineering & Technology, Ghaziabad for his constant support and guidance
throughout the courseof our work. His sincerity, thoroughness, and perseverance
have been a constant source of inspiration for us. It is only his cognizant efforts
that our endeavors have seen the light of the day.
We also take the opportunity to acknowledge the contribution of Professor N K
Gupta, Head, Department of Electrical & Electronics Engineering, Krishna
Institute of Engineering & Technology, Ghaziabad for his full support and
assistance to the development of the project.
We also do not like to miss the opportunity to acknowledge the contribution of
all faculty members of the department for their kind assistance and cooperation
during the development of our project. Last but not the least, we acknowledge
our friends for their contribution in the completion of the project.
Signature Signature Signature
Atif Khan Ashutosh Singh Anurag Singh
1402921051 1402921049 1402921037
Date-10/11/16 Date-10/11/16 Date-10/11/16

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Abstract
A Buck Converter is the basic switched-mode power supply topology.The buck
converteris a type of DC to Dc Converter that has an output voltage magnitude
is less than the input voltage magnitude.
The Buck Converter produces voltage ranging from the input voltage to down to
Zero voltage. It is widely used throughout the industry to convert higher DC
input voltage into lower DC output voltage.
Buck Topology:
The Buck Converter is the most popular topology used to distribute power in
complex systems e.g. computer motherboard, broadband communication board,
etc. The output voltage is adjustable based on the Duty cycle of the switching
transistor. One possible drawback of this converter is that the switch does not
have a terminal at ground; this complicates the driving circuitry. However, this
drawback is of no consequenceif the power supply is isolated from the load
circuit (if, for example, the supply is a battery) because the supply and diode
polarity can simply be reversed. When they can be reversed, the switch can be
on either the ground side or the supply side.
A buck (step-down) converter:
The output voltage is typical of the same polarity of the input and is the lower
than the input. Such a non-inverting buck converter use a single inductor for the
buck inductor mode. The buck converter has the filter inductor on the output
side, which provides a smoothcontinuous output current waveform to the load.
This could be considered a qualitative benefit but requires special
considerations for big load transients.

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TABLE OF CONTENTS
1. CERTIFICATE ......................................................................................................................2
2. ACKNOWLEDGEMENT .....................................................................................................3
3. ABSTRACT...........................................................................................................................4
4. INTRODUCTION .................................................................................................................6
5. CIRCUIT DIAGRAM ...........................................................................................................7
6. WORKING ............................................................................................................................7
7. COMPONENT SELECTION................................................................................................9
7.1 INDUCTOR ....................................................................................................................9
7.2 CAPACITOR ...................................................................................................................9
7.3 POWER MOSFET .........................................................................................................11
8. SIMULATION MODEL…………………………………………………………………..12
9. RESULT...............................................................................................................................13
9.1 SIMULATION COMPONENTS SPECIFICATIONS ..................................................14
10. ADVANTAGES ...............................................................................................................15
11. APPLICATIONS..............................................................................................................16
12. REFERENCES ................................................................................................................16

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Introduction
The main applications of this circuit are in regulated dc power supplies, where a
negative polarity output may be desired with respectto the common terminals
of the input voltage and the average output is lower than the dc input voltage.
The output voltage is controlled by controlling the switch duty cycle. The ratio
of output voltage to input voltage is given by:
Vo /Vin= D·(1 /1-D)=Iin/Io ….(1)
Where, Vo and Vin are the output and input voltages, respectively. The term Io
and Iin are the output and input currents, respectively. The term D is the duty
ratio and defined as the ratio of the on time of the switch to the total switching
period. This shows the output voltage to be higher or lower than the input
voltage, based on the duty ratio D. The dc-dc converter inputs an unregulated dc
voltage input and outputs a constant or regulated voltage. The regulators can be
mainly classified into linear and switching regulators. All regulators have a
power transfer stage and a control circuitry to sense the output voltage and
adjust the power transfer stage to maintain the constantoutput voltage. Since a
feedback loop is necessary to maintain regulation, some type of compensation is
required to maintain loop stability.

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Circuitdiagram
The buckconverter is capableof producinga dc outputvoltage which is smaller
in magnitude than the dc input voltage. The arrangement for the basic buck
converter is as shown in figure 1.
Fig.1. Basic Buck Converter
WORKING:
When the Switch S is on, the input voltage is applied across the inductor
and the current in inductor L rises linearly. At this time the capacitor ( C ) supplies
the load current, and it is partially discharged. During the second interval when
the transistor is off, the voltage across the inductor reverses in polarity and the
diode conducts. During this interval, the energy stored in the inductor supplies
the load and, additionally, recharges the capacitor. The steady state inductor
current and voltage waveform is shown in figure 2.

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Using the inductor volt balance principle to find the steady state output voltage
equation yields
VG .TON + VO .TOFF = 0
Vo
=
T
SW
= −
D
VG T 1 − D
OFF
The d varies between 0 and 1 and thus output voltage can belower than the
input voltage in magnitude.
Figure 1. Steady-state inductor voltage and current waveform, buck converter

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Component Selection
1. Inductor
The magnitude of switching ripple in the output voltage in a properly designed
DC supply is much less than the dc component. As a result, the output voltage is
approximated by its dc componentand the value of inductor can be calculated by
using the defining equation of the inductor
V = L
diL
dt
Referring back to figure 2 for the steady state inductor current waveform, it can
be easily deduced the change in inductor current is its slope times the length of
subinterval.
∆ I = (V - V ) ⋅ D ⋅ T
L G O
L sw
The ripple requirement in inductor current sets the inductor value. Typically ∆IL
lies in the range of 10-20% of the full load or maximum value of the dc
component of IO. The peak inductor current which is equal to the DC component
plus the peak to average ripple
∆ I L/2, flows through the semiconductor switches and is necessary when
specifying device ratings. To reduce the peak current a larger value of the
inductor is required. A secondary benefit in lowering the ripple current is that it
reduces core/inductor, ESR and load losses.

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2. Capacitor
The output section of the buck converter is as shownin figure 3.5.The only
steady state component of output capacitor current is that arising from the
inductor current ripple. Hence inductor current cannot be neglected when
calculating the output voltage ripple. The inductor current contains both a
DC and ripple current component. The DC component must flow entirely
through the load resistance R. While the AC switching ripple divides
between the load resistance R and the filter capacitor C.
Figure 2.Output filter section-buck converter
The series impedance of R and C at switching frequency is given by
Z
Rc,C = R C + 1
jω C
1
ZRc,C = Rc2 +
ω2 C2
To ensure minimum ripple at rated output load, the equivalent condition states
that the series R-C branch impedance appear resistive over the frequency band of

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switching component. This is the condition ofminimum ripple and is a reason for
requiring low
ESR
Rc2 + 1 << R
ω2 C 2
C >> 1
2 )ω (R 2 − R C
The output voltage ripple requirement puts an upper bound on capacitor ESR.
Thus the voltage ripple peak magnitude is estimated by
∆V = ∆ IL ⋅ R c +
∆ IL
8⋅ C ⋅ fsw
∆V ≅ ∆ IL ⋅ R c
With the ESR requirement met, the capacitance value can be selected to
achieve adequate filtering. Capacitors are typically paralleled to meet the ESR
requirement. An alternate approachto reduce ∆V is to reduce ∆I but this requires
a larger value of the inductor.
3. PowerMOSFETselection
MOSFETs are used as power switches for their near zero DC gate current and
fast switching times. Its turn-on delay time is proportional to Cgs which is
illustrated as Ciss minus Crss in datasheets. The delay time is equal to the product
of Cgs and impedance of source driving it ignoring any miller effect. It is a
requirement to have delay time much less than switching period. MOSFET's
power dissipation impacts converter efficiency. This includes Rdsonconduction

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losses, leakage losses, turn on-off switching and gate transition losses. Rdsonof
the power MOSFET determines the current it canhandle without excessive power
dissipation. Rdsondirectly affects the converter efficiency. To minimize Rdson,
the applied gate signal should be large enough to maintain operation in the linear,
triode or ohmic region. MOSFET’s positive temperature coefficients mean
conduction loss increases with temperature. A second important consideration
when designing gate drive circuitry is due to Cgd, illustrated as Crss in data sheets.
During turn-on and turn-off, the large swing in Vgd requires extra current
sourcing and sinking capabilities for the gate drive as a direct result of miller
effect.
Simulation Model:

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Result:

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Simuation Components Specification:

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Advantages of Buck & Boost Converter
 Gives the high output voltage
 Low operating duty cycles
 Lower voltage on MOSFET

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Application
 Automotive applications
 Power amplifier applications
 Adaptive control applications
 Battery power systems
 Consumer Electronics
 Communication Applications
References
 P.S. Bhimbhra “Power Electronics”-Fifth Edition
 M.D. Singh “Power Electronics”-Fourth Edition
 Research paper on Buck-Boost Converter