1. CLOSED LOOP VOLTAGE CONTROL OF DC DC
BUCK CONVERTER
Submitted in partial fulfillment of the requirement of
BACHELOR OF TECHNOLOGY
by
PARTH MARU [12BEE032]
SHARMA TUSHARKUMAR G. [12BEE049]
GAURANG VADHIYA [12BEE057]
Under the guidance of
Mr. Nitin Prajapati
PANDIT DEENDAYAL PETROLEUM UNIVERSITY
GANDHINAGAR - 382007. GUJARAT - INDIA
JUNE - DECEMBER 2015

2. APPROVAL SHEET
This report entitled Closed Loop Voltage Control of DC-DC Buck Converter by
Parth Maru, Sharma Tusharkumar and Gaurang Vadhiya is recommended for the de-
gree of B.Tech ( 7th semester ) Electrical Engineering.
Examiner:
Supervisor:
Chairman:
Date: November 30, 2015
Place: GANDHINAGAR

3. STUDENT DECLARATION
we hereby declare that this written submission represents our ideas in our own words,
where others idea or words have been included, we have adequately cited and refer-
enced the original sources. We also declare that we have adhered to all principles of
academic honestly and integrity and have not misrepresented or fabricated or falsi-
fied any idea / data / fact / source in my submission. We understand that any viola-
tion of the above will be cause for disciplinary action by the PANDIT DEENDAYAL
PETROLEUM UNIVERSITY and can also evoke penal action from the sources which
have thus not been properly cited or from whom proper permission has not been taken
when needed.
Group Members:
1) Parth Maru [12BEE032]
2) Sharma Tusharkumar [12BEE049]
3) Gaurang Vadhiya [12BEE057]
Signature:
1)
2)
3)
Date: November 30, 2015

4. CERTIFICATE BY SUPERVISOR
This report entitled Closed Loop Voltage Control of DC-DC Buck Converter by
Parth Maru, Sharma Tusharkumar and Gaurang Vadhiya is recommended for the de-
gree of B.Tech - 7th semester Electrical Engineering under the supervision of Mr.
Nitin Prajapati.
Supervisor
Date: November 30, 2015
Place: GANDHINAGAR

5. PREFACE
This report is related to the design of buck convrter and its control. The scope of re-
port work includes the study of a Buck Converter and the issues involved in designing
a Buck Converter. How a Buck Converter works with different types of isolation, the
different kinds of modes available in Buck Converter, the Mode for which it has to be
designed (Current control / Voltage control Mode), the design of the controller and the
most important part of the designing is Compensator.
The system level study has been conducted using MATLAB as well as Powersim
software. It includes the buck converter circuit simulation in both the software and it’s
control using MATLAB software. Compensator circuit for buck converter is simulated
in powersim and parameter calculations are also explained. Basic buck converter cir-
cuit is implemented for the demonstration of the circuit. This circuit converts 12V DC
input voltage into 5V DC output. Closed loop voltage control of this buck converter
is included as the future work for this project.
Chapter 1 explain the basic application and different types of the DC DC converter
circuit. It also explains isolated and non-isolated buck converter circuit difference.
Chapter 2 introduces the basic buck converter circuit. It gives the introduction to the
operation of the buck converter. Different modes of operation of buck converter gives
the basic operation.
Chapter 3 describes the points necessary for the component selection. It gives the
basic idea for selecting buck converter circuit components.
Chapter 4 includes the control circuit development for the buck converter. It includes
PWM waves generation process and compensator design for stable operation. Circuit
for voltage mode control of buck converter is also introduced.
Chapter 5 gives the simulation result for the basic buck converter circuit. It gives the
simulation result of the voltage control of buck converter circuit using PID controller.
voltage mode control of buck converer circuit is also simulated in powersim smartctrl
module to check its stable operation. Open loop buck converter circuit is implemented
but unfortunately execution wasn’t successful.
Chapter 6 covers the points related to future work.
Chapter 7 defines conclusion of our project report.
Chapter 8 covers all the references that have been consulted for the preparation of this
project report.
i

6. ACKNOWLEDGEMENT
We take this opportunity to express our profound gratitude and deep regards to our
guide Mr. Nitin Prajapati, for his exemplary guidance, monitoring and constant en-
couragement throughout the minor project. The blessing, help and guidance given by
him time to time shall carry us a long way in the journey of life. We would also like
to thank Mr. Nilesh Patel, AMES lab Assistant for sharing practical knowledge and
guiding during practical implementation of hardware. We would also like to thank
people who have directly or indirectly supported and guided us.
Lastly, we thank almighty, our parents and friends for their constant encourage-
ment without which this project would not be possible.
ii

7. ABSTRACT
The dc-dc converters are some of the most widely used power electronics circuits for
its high conversion efficiency and flexible output voltage. These converters used for
electronic devices are designed to regulate the output voltage against the changes of
the input voltage and load current.
Various techniques have been developed to meet the requirement of output voltage
and at the same time it is also necessary to get more resolution to increase precision.
The other is to develop new control methods that can utilize the advantages of the
digital controller so as to improve the dynamic performance of the switching power
converters.
The main objective of this project is to study DC-DC Buck converter and to improve
the performance by implementing closed loop voltage control technique. Basically
we design a buck converter circuit to get a stable output of 5V, 0.5A from an input of
maximum 12V.
Main objectives of the project are as following.
• To study and design buck converter and appropriate PWM signal.
• Simulation of buck converter with close loop control system.
• Hardware implementation of the Buck converter circuit.
• To study and implementation of voltage control mode of converter.
• Optimization of whole model of converter.
• Testing and calibration of complete hardware of buck converter to confirm the
actual response with theoretical answers.
iii

8. Contents
1 Introduction to DC DC buck converter 1
1.1 Non-Isolated DC DC converter . . . . . . . . . . . . . . . . . . . . 1
1.2 Isolated DC DC Converter . . . . . . . . . . . . . . . . . . . . . . . 2
2 Buck Converter 3
2.1 States of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1.1 ON State [Switch is Closed] . . . . . . . . . . . . . . . . . . 4
2.1.2 OFF State [Switch is Open] . . . . . . . . . . . . . . . . . . 4
2.1.3 Continuous Mode ⁄Discontinuous Mode . . . . . . . . . . . . 4
3 Component Selection 6
3.1 Inductor Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2 Capacitor Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.3 MOSFET Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.4 Rectifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.5 Loss components in the buck converter circuit . . . . . . . . . . . . . 8
4 Closed loop voltage control of buck converter 9
4.1 PWM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.2 Comparator & Voltage to PWM converter . . . . . . . . . . . . . . . 10
4.3 Feedback control loop . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.4 Compensator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.4.1 Type III Compensation . . . . . . . . . . . . . . . . . . . . . 12
4.5 Voltage mode control of buck converter . . . . . . . . . . . . . . . . 13
5 Simulation Result 14
5.1 Buck converter simulation . . . . . . . . . . . . . . . . . . . . . . . 14
5.2 PID Control of buck converter . . . . . . . . . . . . . . . . . . . . . 15
5.3 Voltage mode control of buck converter using PSIM . . . . . . . . . 16
5.4 Hardware implementation of buck converter . . . . . . . . . . . . . 16
5.5 Converter Specification . . . . . . . . . . . . . . . . . . . . . . . . . 17
6 Future Work 19
7 Conclusion 20
8 References 21
iv

9. List of Figures
2.1 Buck Converter Circuit . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2 ON State of Buck Converter . . . . . . . . . . . . . . . . . . . . . . 4
2.3 OFF State of Buck Converter . . . . . . . . . . . . . . . . . . . . . . 4
2.4 Diode and the load current during the two switch states of operation
of buck converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.5 (a) Continuous Mode (b) Discontinuous Mode . . . . . . . . . . . . 5
4.1 PWM Signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.2 Reference Voltage Comparator . . . . . . . . . . . . . . . . . . . . . 10
4.3 PWM comparator signal . . . . . . . . . . . . . . . . . . . . . . . . 10
4.4 Feedback control system . . . . . . . . . . . . . . . . . . . . . . . . 11
4.5 General Compensator circuit . . . . . . . . . . . . . . . . . . . . . . 11
4.6 Type III Compensation . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.7 Voltage mode control of buck converter circuit . . . . . . . . . . . . . 13
5.1 Buck converter simulation circuit . . . . . . . . . . . . . . . . . . . . 14
5.2 Buck converter simulation result . . . . . . . . . . . . . . . . . . . . 14
5.3 PID control of buck converter simulation circuit . . . . . . . . . . . . 15
5.4 PID control of buck converter simulation result . . . . . . . . . . . . 15
5.5 Voltage mode control of buck converter circuit . . . . . . . . . . . . . 16
5.6 Voltage mode control of buck converter circuit simulation result . . . 17
5.7 Hardware of buck converter circuit . . . . . . . . . . . . . . . . . . . 18
5.8 Open loop buck converter circuit . . . . . . . . . . . . . . . . . . . . 18
v

10. List of Tables
5.1 Buck Converter Specification . . . . . . . . . . . . . . . . . . . . . . 17
vi

11. ABBREVIATION
CMC - Current Mode Control
DC - Direct Current
DCM - Discontinuous Conduction Mode
DCR - Direct Current Resistance
ESL - Equivalent Series Inductance
ESR - Equivalent Series Resistance
MOSFET - Metal Oxide Semiconductor Field Effect Transistor
PWM - Pulse Width Modulation
SMPS - Switch Mode Power Supply
SRBC - Synchronous Rectifier Buck Converter
VMC - Voltage Mode Control
vii

12. CHAPTER 1
INTRODUCTION TO DC DC BUCK CONVERTER
A DC-DC converter is an electronic circuit which converts a source of direct current
(DC) from one voltage level to another. It is also called as Chopper. It is a class of
power converter.
DC to DC converters are important in portable electronic devices such as cellular
phones and laptop computers, which are supplied with power from batteries primarily.
Most DC to DC converters also regulate the output voltage. Some exceptions include
high-efficiency LED power sources, which are a kind of DC to DC converter that reg-
ulates the current through the LEDs, and simple charge pumps which double or triple
the input voltage.
DC- DC converters are the power supply that output a fixed voltage efficiently,
converting the input voltage. There are three types of DC- DC converters.
1. Buck Converter (Step Down Converter)
2. Boost Converter (Step Up Converter)
3. Buck-Boost Converter
DC DC converters can also be devided into two broad categories as following. 1)
Non-Isolated DC-DC Converters, 2) Isolated DC-DC Converters
1.1 Non-Isolated DC DC converter
The non-isolated converter usually employs an inductor, and there is no dc voltage
isolation between the input and the output. The vast majority of applications do not
require dc isolation between input and output voltages. The non-isolated dc-dc con-
verter has a dc path between its input and output. Battery-based systems that don’t
use the ac power line represent a major application for non-isolated dc-dc converters.
Point-of-load dc-dc converters that draw input power from an isolated dc-dc converter,
such as a bus converter, represent another widely used non-isolated application.
Most of these dc-dc converter ICs use either an internal or external synchronous
rectifier. Their only magnetic component is usually an output inductor and thus less
susceptible to generating electromagnetic interference. For the same power and volt-
age levels, it usually has lower cost and fewer components while requiring less pc-
board area than an isolated dc-dc converter. For lower voltages (12V) non-isolated
buck converters can be used.
1

13. 1.2 Isolated DC DC Converter
For safety considerations, there must be isolation between an electronic system’s ac
input and dc output. Isolation requirements cover all systems operating from the ac
power line, which can include an isolated front-end ac-dc power supply followed by
an isolated “brick”dc-dc converter, followed by a non-isolated point of load converter.
Typical isolation voltages for ac-dc and dc-dc power supplies run from 1500 to 4000V,
depending on the application. An isolated converter employs a transformer to pro-
vide dc isolation between the input and output voltage which eliminates the dc path
between the two. Isolated dc-dc converters use a switching transformer whose sec-
ondary is either diode or synchronous rectified to produce a dc output voltage using
an inductor capacitor output filter. This configuration has the advantage of producing
multiple output voltages by adding secondary transformer windings.
Modern electronic devices require efficient, high quality, light weight power sup-
plies. We have linear power regulators, whose principle of operation depends on cur-
rent or voltage division which is inefficient. The main area of application is at low
power levels. When it comes to high power levels switching regulators are used where
switch operates in on and off states. Latest power electronic switches can operate at
high frequencies. Therefore, faster dynamic response to rapid changes is the load
current is possible with high operating frequencies. These High frequency electronic
power processors are used in dc-dc power conversion.The main functions of dc-dc
converters are:
• It can step Up or Step Down Voltage levels.
• It provides isolation between source and load.
• It can regulate the output voltage against load.
• It can reduce the ac voltage ripple on the dc output voltage.
The dc-dc converters can also be divided into two types as:
1. Hard switching pulse width modulated (PWM) converters
2. Resonant and soft switching converters
In this project, we deal with ‘Closed loop control of DC DC Buck converter’which
is very popular for the last few decades and can be used in many applications.
Some applications have additional technical constraints. Consider the power sup-
plies used in battery powered electronics, such as laptop computers or mobile phones
have a requirement of maintaining high efficiency over a wide range of loads. In
desktop computers and servers, the microprocessor supplies must include the capa-
bilities of digitally programmed output voltage. The output must depend on the load
as well the dynamic response must be faster even for large load transients. Voltage
Regulator Modules have multi phase architectures consisting of several buck or sim-
ilar converter modules which operate in parallel to share the load current in order to
improve dynamic response.
2

14. CHAPTER 2
BUCK CONVERTER
In this section we summarized the brief introduction of buck converter and purpose
why we choose the buck converter as well the circuit topology and the brief explana-
tion of the components used in the construction of boost converter.
A buck converter is the most basic SMPS topology. It is widely used throughout
the industry to convert a higher input voltage into a lower output voltage. The buck
converter (voltage step-down converter) is a nonisolated converter. Buck converter is
the most popular topology used to distribute power in complex systems, e.g. server
motherboards, broadband communication boards, etc. The converter itself consists of
one active switch, a rectifier and filter elements. This great simplicity allows for cost
effective high efficient power distribution throughout the application.
Figure 2.1: Buck Converter Circuit
When the switch is closed, the input voltage is connected to the inductor and the
load. Thus the energy will be stored as the current increases in the inductor. And
the capacitor begins to discharge its voltage. When the switch is open, the inductor
discharges its energy to the load resistor, thus decreasing in current will occur in the
system gradually. In this case, the inductor acts as a source to the load to keep the flow
of current without interruption in the circuit.The buck converter has the filter inductor
on the output side, which provides a smooth continuous output current waveform to
the load.
3

15. 2.1 States of Operation
THe buck converter operation can be specified in accordance with switch ON - OFF
state.
2.1.1 ON State [Switch is Closed]
During the ON state, the closed switch results in transferring the energy from the
source voltage [Vs] to the inductor [L], thus the diode becomes reverse bias. In this
state, current through the inductor increases gradually.
Figure 2.2: ON State of Buck Converter
2.1.2 OFF State [Switch is Open]
During the OFF state, the switch is open, now the inductor acts as a source by main-
taining constant energy transfer to the resistor at load. At this stage, the diode starts
conducting and the current through the circuit decreases linearly as the energy in the
inductor discharges.
Figure 2.3: OFF State of Buck Converter
Following graph is obtained when the switch is in ON and OFF states and the
response of the current across the diode and load, where shown accordingly. Here, i0
represents the average current across the load.
2.1.3 Continuous Mode ⁄Discontinuous Mode
During the ON state and than the subsequent OFF state the Buck Converter can operate
in Continuous Mode or Discontinuous Mode. The difference between the two is that
in CCM the current in the inductor does not fall to zero.
4

16. Figure 2.4: Diode and the load current during the two switch states of operation of
buck converter
Figure 2.5: (a) Continuous Mode (b) Discontinuous Mode
Current flows continuously in the inductor during the entire switching cycle in
steady state operation. In most Buck regulator applications, the inductor current never
drops to zero during full-load operation. Overall performance is usually better using
continuous mode, and it allows maximum output power to be obtained from a given
input voltage and switch current rating. Energy from the battery is supplying the load
and is being stored in the inductor L as a magnetic field. The current through the in-
ductor is rising linearly.
In the DCM the current in the inductor falls to zero and remains at zero for some
portion of the switching cycle. It starts at zero, reaches a peak value, and returns to
zero during each switching cycle. In applications where the maximum load current
is fairly low, it can be advantageous to design for discontinuous mode operation. In
these cases, operating in discontinuous mode can result in a smaller overall converter
size (because a smaller inductor can be used). Often the output capacitor must be
large to keep the voltage constant. This phenomenon exists due to the time and energy
required by the load is enough when compared to the complete operating period.
5

17. CHAPTER 3
COMPONENT SELECTION
3.1 Inductor Selection
The fundamental property of an inductor is to oppose the change in the magnitude of
current passing through it. In buck converter the switching action of MOSFET is done
at very high speed. Hence switch produces discontinuous output current, but it is the
inductor, which overcome this problem. During the ON state of MOSFET an electric
current flows in the circuit and energy is stored in the inductor (charging). As soon as
the switch is turned off, there are no current flows to the circuit. At this time inductor
released the entire energy which is stored in ON time. At steady state condition, the
average inductor current is equal to the output current. Duty cycle of the convrter is
given by,
D = Vout+Vd+VL
Vin−Vswitch+Vd
Figure 2.4 shows the inductor current vs. time in CCM where the inductor current
never reached to its zero value. It can be seen from the figure, the inductor current is
not constant, but varies around Iavg between a maximum value Imax and a minimum
value Imin whose difference Il is the peak-to-peak inductor current ripple.
L =
(Vin−Vout)×Vout
Vin×Fsw× IL
Here, IL refers to inductor current. Follwing are the points that need to remember for
inductor selection in buck converter.
• Changing the inductance (L), frequency (f) and duty cycle (D) doesn’t affect the
Iload.
• Inductor is always designed with worst case input voltage.
• Inductance is inversely proportional to frequency.
• Size of the inductor is inversely proportional to frequency.
• Current rating of an inductor is independent of frequency.
• Size of the inductor proportional to load current and is inversely proportional to
inductance.
6

18. 3.2 Capacitor Selection
The function of output capacitance is to store energy. The energy is stored in the
capacitors electric field due to the voltage applied. Thus, qualitatively, the function
of a capacitor is to attempt to maintain a constant voltage. The value of output ca-
pacitance of a Buck converter is generally selected to limit output voltage ripple to
the level required by the specification. Since the ripple current in the output induc-
tor is already determined, the series impedance of the capacitor primarily determines
the output voltage ripple. The three elements of the capacitor that contribute to its
impedance (and output voltage ripple) are equivalent series resistance (ESR), equiva-
lent series inductance (ESL), and capacitance (C). The following gives guidelines for
output capacitor selection.
For continuous inductor current mode operation, to determine the amount of ca-
pacitance needed as a function of inductor current ripple IL, switching frequency
Fsw, and desired output voltage ripple(Vout ripple) , the following equation is used.
C ≥ IL
8×Fsw×Vout ripple
For a certain peak-to-peak output voltage ripple (Vout ripple), the required maxi-
mum ESR of the output capacitor can be calculated by:
ESR ≤
Vout ripple
IL
3.3 MOSFET Selection
In switching power supply power stages, the function of the power switch is to con-
trol the flow of energy from the input power source to the output voltage. In a buck
power stage, the power switch connects the input to the output filter when the switch
is turned on and disconnects when the switch is off. The power switch must conduct
the current in the output inductor while on and block the full input voltage when off.
Also, the power switch must change from one state to the other quickly in order to
avoid excessive power dissipation during the switching transition.
The type of power switch considered in this report is a power MOSFET. Other
power devices are available but in most instances, the MOSFET is the best choice
in terms of cost and performance (when the drive circuits are considered). The two
types of MOSFET available for use are the n-channel and the p-channel. Parameters
to consider while selecting the appropriate MOSFET are the gate drive circuit, maxi-
mum drain-to-source breakdown voltage Vdsmax, and the maximum drain current IDmax.
Points to be considered while selecting MOSFET.
• The MOSFET selected should have a Vdsmax rating greater than the maximum
input voltage, and some margin should be added for transients and spikes.
• The MOSFET selected should have an IDmax rating of at least two times the
maximum converter output current.
7

19. • Losses in the MOSFET are directly proportional to the switching frequency.
• Maximum allowable temperature rise in MOSFET is as follow. where, TJmax is
maximum junction temperature and TAmax is maximum ambient temperature.
TJrise = TJmax −TAmax
3.4 Rectifier
Important criteria for selecting the rectifier include: fast switching, breakdown volt-
age, current rating, lowforward voltage drop to minimize power dissipation, and ap-
propriate packaging. Unless the application justifies the expense and complexity of
a synchronous rectifier, the best solution for lowvoltage outputs is usually a Schottky
rectifier. The breakdown voltage must be greater than the maximum input voltage,
and some margin should be added for transients and spikes. The current rating should
be at least two times the maximum power stage output current (normally the current
rating will be much higher than the output current because power and junction tem-
perature limitations dominate the device selection).
The voltage drop across the diode in a conducting state is primarily responsible
for the losses in the diode. The power dissipated by the diode can be calculated as the
product of the forward voltage and the output load current for the time that the diode
is conducting. The switching losses which occur at the transitions from conducting
to nonconducting states are very small compared to conduction losses and are usually
ignored.
3.5 Loss components in the buck converter circuit
• MOSFET switching & conduction loss Pconduction = IL
2 ×Rdson ×D
• Losses in the diode (Pdiode) = VD ×Iout ×(1−D)
• Losses in the Inductor (PInductor) = IL
2 ×RDCR
• Losses in the Capacitor (PCapacitor) = IL
2 ×RESR
• Controller Losses (Losses in compensation + Losses in control circuit)
8

20. CHAPTER 4
CLOSED LOOP VOLTAGE CONTROL OF BUCK CONVERTER
The heart of a switching power supply is its switch control circuit (controller). One
of the key objective in designing a controller for the power converter is to obtain
tight output voltage regulation under different line and load conditions. Often, the
control circuit is a negative-feedback control loop connected to the switch through a
comparator and a Pulse Width Modulator (PWM). The switch control signal (PWM),
controls the state (on or off) of the switch. This control circuit regulates the output
voltage against changes in the load and the input voltage.
4.1 PWM
PWM is the method of choice to control modern power electronics circuits. The basic
idea is to control the duty cycle of a switch such that a load sees a controllable average
voltage. To achieve this, the switching frequency (repetition frequency for the PWM
signal) is chosen high enough that the load cannot follow the individual switching
events and they appear just a “blur”to the load, which reacts only to the average state
of the switch.
Figure 4.1: PWM Signal
With pulse-width modulation control, the regulation of output voltage is achieved
by varying the duty cycle of the switch, keeping the frequency of operation constant.
Duty cycle refers to the ratio of the period for which the power semiconductor is kept
ON to the cycle period. The Figure shows PWM signals for 10%, 50%, and 90% duty
cycles.
Usually control by PWM is the preferred method since constant frequency opera-
tion leads to optimization of LC filter and the ripple content in output voltage can be
controlled within the set limits.
9

21. 4.2 Comparator & Voltage to PWM converter
Figure 4.2: Reference Voltage Comparator
Switching power supplies rely on negative feedback to maintain the output volt-
ages at their specified value. To accomplish this, a differential amplifier is used to
sense the difference between an ideal voltage (the reference voltage) and the actual
output voltage to establish a small error signal (Vcontrol).
The PWM switching at a constant switching frequency is generated by comparing
a signal-level control voltage (Vcontrol) with a repetitive waveform as shown in the
above figure.
Figure 4.3: PWM comparator signal
The frequency of the repetitive waveform with a constant peak, which is shown
to be a sawtooth, establishes the switching frequency. This frequency is kept constant
in a PWM control and is chosen to be in a few hundred kilohertz range. When the
amplified error signal, which varies very slowly with time relative to the switching
frequency, is greater than the sawtooth waveform, the switch control signal becomes
HIGH, causing the switch to turn on. Otherwise, the switch is off. So when the
circuit output voltage changes, Vcontrol also changes causing the comparator threshold
to change. Consequently, the output pulse width also changes. This duty cycle change
then moves the output voltage to reduce to error signal to zero, thus completing the
control loop.
10

22. 4.3 Feedback control loop
As has been mentioned earlier as well that the output voltages of dc power supplies are
regulated to be within a specified tolerance band (e.g., ±1% around its nominal value)
in response to changes in the output load and the input voltage lines. This process is
accomplished by employing a negative feedback system.
Figure 4.4: Feedback control system
If the power stage of the switch-mode converter in Figure can be linearized, then
the Nyquist stability criterion and the Bode plots can be used to determine the ap-
propriate compensation in the feedback loop for the desired steady-state and transient
response.
4.4 Compensator
After the values for external filter components are chosen than only the power stage
is complete. The original filter of the buck converter by itself has a very low phase
margin which needs to be increased. A better phase margin can be included by adding
a suitable controller in a closed loop configuration.Proper compensation of the system
will allow for a predictable bandwidth with unconditional stability. In most cases, a
Type II or Type III compensated network will properly compensate the system. The
ideal Bode plot for the compensated system would be a gain that rolls off at a slope of
-20dB/decade, crossing 0db at the desired bandwidth and a phase margin greater than
450 for all frequencies below the 0dB crossing.
Figure 4.5: General Compensator circuit
We must compensate the power supply to ensure that the overall loop response is
stable. The purpose of adding compensation to the error amplifier is to counter act
some of the gains and phases contained in the control-to-output transfer function that
11

23. could jeopardize the stability of the power supply. Obviously, the ultimate goal is to
make the overall closed loop transfer function (control-to-output cascaded with the
error amplifier) satisfy the stability criteria. This is to avoid having the closed-loop
phase any closer to 3600. than the desired phase margin anywhere where the gain is
greater than 1 (0 dB).
It is also desirable to have the slope of the gain curve at the crossover point with
a value of -20 dB/decade. The overall frequency-response loop has two parts. The
first includes a power-stage with driver and PWM comparator and the second is the
compensation. The compensation circuit based on an error amplifier with the R and C
external components shapes the required feedback loop frequency response.
4.4.1 Type III Compensation
Figure 4.6: Type III Compensation
Type III network shapes the profile of the gain with respect to frequency and it
utilizes two zeroes to give a phase boost of 1800. This boost is necessary to counteract
the effects of an under damped resonance of the output filter at the double pole. The
Type III compensation circuit has two poles, with two zeros and a pole at its origin
providing an integration function for better DC accuracy. Optimal selection of the
compensation circuit depends on the power-stage frequency response.
GAINTypeIII =
R1+Rz3
R1·Rz3·Cp1
·
(s+ 1
Rz2·Cz2
)·(s+ 1
(R1+Rz3)· Cz3
)
s·(s+
C1+Cz2
Rz2·Cp1·Cz2
)(s+ 1
Rz3·Cz3
)
A traditional type III compensator is sufficient to stabilize the synchronous buck
converter for all three modes subsequently being voltage mode control, current mode
control & mixed mode control. In the analog design process, Type III compensator is
used to compensate a second-order LC filter, usually from a voltage-mode converter.
By using Type III compensation around the VMC error amplifier when the overall
loop performance matches that of a CMC system, albeit at the expense of requiring a
higher-bandwidth amplifier.
12

24. 4.5 Voltage mode control of buck converter
Figure 4.7: Voltage mode control of buck converter circuit
This is a classical control & simple method in which there is only one feedback
from the output voltage. PWM voltage mode controllers have disadvantages. Since
the input voltage is a significant parameter in the loop gain, any changes in the input
voltage will alter the gain and will change the dynamics of the system. The central is-
sue is that a voltage mode controller alone cannot correct any disturbances or changes
until they are detected at the output. In the voltage based controllers the compensation
loop is difficult to implement.
For general purpose single output dc-dc converters, the overall advantage goes
with VMC. The feedback network, even with a Type III compensation network around
the voltage-error amplifier is relatively simple to compensate. In many ways it is sim-
pler than compensating a current loop, plus a voltage loop, plus adding slop compen-
sation. This, along with the improved noise immunity at light loads, makes VMC
attractive from a circuit performance standpoint.
13

25. CHAPTER 5
SIMULATION RESULT
5.1 Buck converter simulation
Figure 5.1: Buck converter simulation circuit
Figure 5.2: Buck converter simulation result
Figure 5.1 & 5.2 shows the simulation circuit and result of the basic buck converter
circuit. Due to the fixed gate pulses (D=45.5%), load voltage reaches to a steady state
(5V) after approximate 1ms. Output current ripple in this case is IL = 0.13A. output
capacitor voltage ripple Vc = 0.025V. Both IL and Vc are within limit with output
voltage constant.
14

26. 5.2 PID Control of buck converter
Figure 5.3: PID control of buck converter simulation circuit
Figure 5.4: PID control of buck converter simulation result
As indicated in the Figure 5.3 & 5.4, step voltage is applied to the buck converter
circuit and it is necessary that output voltage should remain constant. Initially, 5.5V
is applied to the circuit and constant output is achieved with PWM signal generated
by the PID Controller. For PID controller, gain KP = 0.24 , KI = 0.100 and Kd = 0.8.
After the time of 2.5ms, input voltage is switched to the maximum input voltage of
12V. Inductor ripple current before and after changeover in the applied voltage are
approx. 0.05A and 0.17A simultaneouly. Capacitor voltage ripple before and after
changeover in the applied voltage are approx. 0.04V and 0.05V simultaneously. Both
parameter IL and Vc are within limit.
15

27. 5.3 Voltage mode control of buck converter using PSIM
Figure 5.5: Voltage mode control of buck converter circuit
5.4 Hardware implementation of buck converter
Apparatus:
• Constant voltage source, Breadboard, DSO, Arduino Atmega328, Connecting
leads, Function generator, Multimeter.
Component ID:
• MOSFET (IRFZ24N)
• Schottkey diode (1N5817)
• Capacitor (10 µF)
• Inductor ( 330 µH)
• Resistor (10Ω,10W,±5% )
16

28. Figure 5.6: Voltage mode control of buck converter circuit simulation result
5.5 Converter Specification
Input Voltage Range (Vin) 5.5V to 12V
Output Voltage (Vout) 5V
Output Current (Iout) 0.5A
Output power (Pout) 2.5W
Inductor Ripple Current (Iripple) 0.15A (Assumed to be 30% of Iout)
Switching frequency (Fsw) 62kHz
Capcitor Ripple Voltage (Vout ripple) 0.05V (Assumed to be 1% of (Vout) )
Forward voltage drop across Schottkey Diode VF 0.45V
ON resistance of switch at operating point 0.07 Ω
ON resistance of an Inductor 2 Ω
ON resistance of Diode 0.25 Ω
Losses in MOSFET Pswitching +Pconduction 0.6875+0.007825 = 0.6953 W
Pdiode 0.12375W
PInductor 0.5W
Pcapacitor 0.75W
Total Losses 2.06905W
Efficiency (η) 54.71%
Table 5.1: Buck Converter Specification
17

29. Figure 5.7: Hardware of buck converter circuit
Figure 5.8: Open loop buck converter circuit
18

30. CHAPTER 6
FUTURE WORK
Following are the areas of future work.
• Hardware implementation of the Buck converter circuit.
• To study and implementation of voltage control mode of converter.
• Optimization of whole model of converter.
• Testing and calibration of complete hardware of buck converter to confirm the
actual response with theoretical answers.
19

31. CHAPTER 7
CONCLUSION
Designing a voltage-mode controlled buck converter is very challenging. So far the
most difficult part was to determine the simulation for the feedback loop network. The
PWM is a relatively simple concept, but a real world design of this block would be
troublesome. Design and simulation of the circuit is done using PID controller through
MATLAB. Unfortunately auto tuning of PID controller in MATLAB Simulink was not
possible because of some errors. Current values are selected by trial and error method.
Improvement in the response of the converters through the use of a feedback path with
proper controller gain has been achieved by doing the same. Voltage mode control
of buck converter is simulated using powersim and buck converter circuit hardware
implemented. While doing hardware implementation of open loop buck converter,
results are not as per expectation due to failure in gate triggering of MOSFET.
20

32. CHAPTER 8
REFERENCES
1. Marty Brown, ’Power Sources and Supplies: World Class Designs’, Elsevier ,
DEC 2007 , Mumbai (Maharashtra), India.
2. Infineon - Buck converter design Notes, DN2013-01.
3. Muhammad Saad Rahman (2007),Buck Converter design Issue, Master Thesis
in the division of Electronic Devices.
4. Atmel AT04204 - Design a Buck Converter with XMEGA E: Application Note
5. Capacitor selection for buck converter by Texas Instruments Notes.
6. Priyadarshini, Dr. Shantharam Rai, Modelling and Simulation of a PID Con-
troller for Buck Boostand Cuk Converter, International Journal of Science and
Research conference.
7. Muhamad Farhan Bin Umar Baki, A thesis on Modelling and control of buck
converter’, University Malaysia Pahang.
8. Ashish Mondal (May 2014), Digital PID Controller Design for Buck converter,
NIT - Rourkela.
9. Datasheet of different component: IRFZ24N, 1N5817.
10. www.vishay.com 26/11/15.
11. Gaurav Kaushik, ‘Compensator design for DC DC buck converter using fre-
quecy domain specification’ NIT, Rourkela - 2014.
12. Application note, Design of type III compensation network for voltage mode
step down converter, Skyworks 2014.
13. Digi-key Electronics.com Designing compensator networks to improve switch-
ing regulator frequency response 28/11/15.
21