ANALYSIS AND SIMULATION OF BUCK SWITCH MODE DC TO DC POWER REGULATOR

International Journal of Technical Research and Applications e-ISSN: 2320-8163,
www.ijtra.com Volume 3, Issue 1 (Jan-Feb 2015), PP. 97-103
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ANALYSIS AND SIMULATION OF BUCK SWITCH
MODE DC TO DC POWER REGULATOR
G. C. Diyoke
Department of Electrical and Electronics Engineering
Michael Okpara University of Agriculture, Umudike Umuahia, Abia State Nigeria.
geraldiyoke@gmail.com
Abstract-This project envisages a Buck dc – dc
converter mathematical analysis and simulation. This power
regulator is made up of some vital circuit elements such as
inductor, freewheeling diode, filter capacitor and electronics
power switch. The circuit is analysed based on two modes of
operation namely: continuous current conduction mode and
discontinuous current mode. Ansoft Simplorer software is
used to carry out the circuit simulation under the two modes
of operation which aided in verifying the calculated results.
Both calculated and simulated waveforms are displayed. The
results obtained are very similar.
Keywords- Buck converter, Continuous current
conduction mode, Discontinuous current conduction mode
Pulse Width Modulation, Power regulator.
I. INTRODUCTION
DC to DC power converters are applied in a variety of
applications, including power supplies for personal computers,
office equipment, spacecraft power systems, laptop computers,
and telecommunication equipment, trolley cars, track motor
control, as well as dc motor drives. The converter input is usually
an unregulated dc voltage / current sources derived from any of
such sources like electromechanical dc generator, a dc battery, a
rectified ac source, a solar photovoltaic panel, hydrogen based
fuel cell etc as shown in fig. 1.
Fig.1. A Block Diagram of dc-dc Converter System
When the converter load is a dc motor or an
electromechanical process load (such as a battery on charge), the
load is usually generalized as a series combination of a
resistance, an inductance and a load electromotive force (emf).
On the other hand, especially for the most applications with
regulated dc output, the converter load is usually a resistance in
parallel with a filter capacitance. The converter produces a
regulated output voltage, having a magnitude (and possibly
polarity) that differs from the input [1]. The regulation is
normally achieved by pulse width modulation at a fixed
frequency. The converter switch can be implemented by using a
(1) power bipolar junction transistor (BJT), (2) metal oxide
semiconductor field effect transistor (MOSFET), (3) gate turn off
thyristor or (4) insulated gate bipolar transistor (IGBT) [2].
In the main, there are six types of the basic dc to dc with
each type having performance characteristics suitable for a
particular application. These basic types are the step down or
Buck converter, the step up or Boost converter, the convectional
Buck-Boost converter, the Cuk’s converter, the sepic converter,
and the Zeta converter. The performance of buck converter has
been analysed in many papers amongst them are [3][4]. In this
paper a detailed analysis of Buck dc to dc converter will be
analysed.
II. THE GENERALIZED BASIC HARD-SWITCHED DC
TO DC CONVERTER
The basic hard-switched dc to dc converter has one main
unidirectional active semiconductor switch, one main diode and
one main inductance of which performs the dual function of a
filter and a current limiter. In the hard switching of the active
switch, both the switch current and voltage vary during the
switch transition interval from the ON state to the OFF state and
vice versa. If the converter inductor current is represented as a
current , the basic hard-switched converter can be generally be
represented by Fig. 2 as having three major nodes (node 1, 2 and
3). is the converter main unidirectional active switch, is
the main converter diode or freewheeling diode, while is the
dc voltage between nodes 1 and 2.
Fig. 2 The generalized equivalent circuit of the basic dc to dc
converter.
The following nodes are noted;
Node 1: Located at the anode (input) of the active switch, .
Node 2: Located at the anode of the freewheeling diode ,
Node 3: Point of outflow of the inductor current from the
common point conectiong the cathodes of and [5].

98 | P a g e
At time t = 0, is closed. In the ON state of , is
reverse biased by and therefore is OFF, thus forcing the
current to flow through . For an interval (1-D) seconds
in a switching cycle, the switch is open thus causing the
current to flow through as long as . Based on the
generalized basic converter of fig. 2, the Buck dc to dc hard-
switched converter topology is presented and analyzed in this
work.
III. MODE OF CIRCUIT OPERATION
The step down or Buck dc to dc converter shown in fig. 3
converters the unregulated dc input voltage to a regulated dc
output voltage which can be varied from zero to maximum dc
voltage equal to the input dc voltage . It is assumed that the
input and the output filter capacitors and are large enough
to make the input voltage and current ( , ) ripple content
negligible. The principal nodes (1, 2, 3) in fig. 3 identify the
generalized form of the basic converter as depicted in fig. 2.
Fig. 3 The basic buck dc to dc converter circuit configuration
Suppose the active switch is turned ON as shown in fig.
4, for a time interval DT seconds (0 < D < 1), the freewheeling
diode, becomes reverse biased and the input provides energy
to the load as well as to the inductor.
Fig. 4 Buck Converter Operation when is ON
Furthermore, if the active switch is turned OFF as
shown in fig. 5, for the remaining interval of (1-D)*T seconds in
a cycle of period T seconds. During this interval, the inductor
current flows through the freewheeling diode, transferring
some of its stored energy to the load.
Fig. 5 Buck Converter Operation when is OFF
Under this operation condition the converter current
conduction can be continuous or discontinuous. Continuous
current conduction is the case if the freewheeling diode is on
conducting the inductor current for the entire interval (1-
D)*T seconds in a switching cycle operation. Discontinuous
current conduction is the case if the freewheeling diode is on
conducting the inductor current for an interval (
seconds where is less than T, implying that the diode current
( ) decayed to zero thus turning OFF (T-
seconds in a switching cycle.
A. Continuous Current conduction operation
Under continuous current conduction mode, the inductor
current is greater than zero at all times in a cycle except at
the instant of turn ON of the active switch when can be
greater than or equal to zero. A more generalized definition of
continuous current operation is that the freewheeling diode
remains ON conducting current in the interval . Fig.
6 shows the converter steady state circuit current and voltage
waveforms for converter operation at continuous current
conduction mode. For the interval , the switch is
ON and is reverse biased by = and therefore OFF. In
this interval therefore the inductor voltage and current are given
by
= = (1)
From Equation (1) inductor current can be determined as
= )*t + K (2)
The integration constant K can be determined by evaluating
equation (2) at the value of t = 0 and the corresponding value of
the inductor current ie , which implies that the value
of K is . Thus equation (2) can be modified as
= )*t + (3)
From equation (3) when t = DT, becomes , hence the
change in inductor current can be determined as
= (4)
For the interval DT < t < T, is turned off and conducts
the inductor current as show in fig. 5 giving and as
= = (5)

99 | P a g e
Fig. 6 Circuits waveforms of the basic buck converter under
continuous current conduction operation.
From Equation (5) inductor current can be determined as
= *(t-DT) + M (6)
The integration constant M can be determined by evaluating
equation (6) at the value of t=DT and the corresponding value of
the inductor current i.e. , which implies that the value
of M is . Thus equation (6) can be modified as
= (t-DT) + (7)
At t = T, becomes again and the cycle is repeated.
Hence the change in inductor current is expressed by
= (8)
From the equations (4) and (5), the ratio of the converter output
to input voltage is obtained as
(9)
The input and output capacitor currents and and active
switch current in the interval 0 are given (see fig.
4) as
(10)
(11)
(12)
Where and are the average converter input and output
currents respectively. From the converter waveforms of fig. 6,
and are seen to be the average values of the main switch
current and the inductor current ,
= = (13)
= = (14)
From equations (13) and (14), the ratio of the converter output
current to the input current can be shown to be inverse of
the ratio of the output voltage to the input voltage This is
the common characteristic of all dc to dc converters assuming
lossless circuit components.
(15)
Alternatively, the voltage ratio, , can be determined by
equating the average voltage across the filter inductor to zero
since, in a practical and stable switching converter circuit, an
inductor has no dc or average voltage across it. This is same as
equating the area under the inductor voltage waveform over a
cycle to zero. From fig. 6, the inductor average voltage equated
to zero is
(16)
On simplifying equation (16) yields the same result as given in
equation (9).
Similarly, can alternatively be determined by equating the
average current through each of the filter capacitors ( )
to zero. This is equivalent the average current through each
capacitor current ( ) to zero. From the waveforms of
( ) in fig. 6 give the same result as obtained in
equation (15).
The filter inductor peak to peak ripple current is given in
equations (4) and (8) as
= = (17)
From equation (17) the maximum value of occurs at D=
and is given by
= (18)
The ripple factor of the inductor current is the ratio of the
inductor peak ripple current to the inductor average current,
= = (19)
The minimum and the maximum instantaneous inductor current
are obtained by the simultaneous solution of
equations (14) and (17)
(20)
(21)
The condition for maximum current conduction mode is that the
minimum inductor current must be greater than or equal to zero.
Therefore the condition for continuous current conduction is
(22)
This implies (from equations (17), (20) and (22)) that
1 (23)
Where the output load resistance,
From equation (23), the minimum filter inductance that
just makes the inductor current continuous for a given duty
cycle, specified load and operating frequency is
(24)
The peak to peak input filter capacitor voltage ripple is the
change in the capacitor voltage during the interval that the
capacitor current is either positive or negative. Assuming an
operating condition in which , is positive during
the interval (1-D)T seconds when is off thus giving as,

100 | P a g e
= (25)
It can be easily deduced from equation (25), that the maximum
value of occurs at D = and that this maximum value is
expressed as
(26)
The ripple factor of the converter input voltage is
defined as the ratio of the capacitor peak ripple voltage to the
input voltage.
= (27)
Note that has its maximum value at D = .
Similarly the peak to peak ripple voltage of the output filter
capacitor is the change in the capacitor voltage in the interval,
in a cycle of operation, during which is either charge or
discharge. From fig (6), is charge by in the interval
Therefore, is the area under waveform
in this interval divided by the capacitance .
. (28)
It is seen from equation (28) that the maximum value of
occurs at D = and this maximum value
(29)
The ripple factor of the converter output voltage is defined
as the ratio of its ripple voltage to its average voltage.
= (30)
Note again that has its maximum value at D =
Considering the boundary condition between continuous and
discontinuous conduction, the boundary average inductor current
is given by
= = (30a)
In fig. 7, is plotted against the duty cycle, D. From the plot it
is observed that the maximum boundary inductor current occurs
at D = . Therefore during operating condition, if the
average output current becomes less than , then will
become discontinuous [6].
Fig. 7 Current at boundary of continuous-discontinuous
conduction
B. Discontinuous Current Conduction Operation
In discontinuous current conduction operation, the steady
state inductor current decreases from its maximum value to zero
at t = where DT < < T thus causing Dm to be OFF for the
rest of the cycle. In other words, the minimum inductor current
at instant of turn on of the active switch at t = 0 is zero
and the inductor current becomes zero again at t = . The
general condition for discontinuous current condition operation is
fulfilled if equation (24) is untrue. Therefore for discontinuous
current conduction operation, the following equation applies
1 (31)
Fig. 8 Circuits waveforms of the basic buck converter under
discontinuous current conduction operation.
In fig. 8 is shown the circuit waveforms of the buck converter
under discontinuous current conduction operation.
For the interval 0
= )*t + K (32)

101 | P a g e
The integration constant K can be determined by evaluating
equation (32) at the value of t = 0 and the corresponding value of
the inductor current i.e. , which implies that the
value of K is 0. Thus equation (32) can be modified as
= )*t (33)
At t = DT, =
= )*DT (34)
Similarly, for the interval DT (see equation (8), where
)
(35)
Comparing equations (34) and (35) yields the voltage gain
under discontinuous current conduction mode which is given as
(36)
Since is less than unity, the voltage gain under
discontinuous current conduction operation is higher than the
voltage gain at continuous current conduction operation for the
same duty cycle D.
From the waveforms of the inductor current and the main
switch current , the average output current and the input
current can be determined as
= (37)
Average input current can be computed as
= (38)
The current gain is calculated from equations (37) and (38)
which is given by
(39)
Putting equations (4), (with ) and (36) into equation
(37) yields
- = 0. (40)
Solving equation (40) using quadratic formula gives the
converter output voltage as
(41)
Equation (41) can be modified as
(42)
Where, = .
Fig. 9 Buck Converter Characteristics keeping constant
Fig. 9 shows the step-down converter characteristic in
both modes of operation for a constant source voltage, . The
voltage ratio ( ) is plotted as a function of for various
values of duty cycle using equations (9) and (42). The boundary
between the continuous and the discontinuous mode, shown by
the curved line is established by equations (9) and (30a).
C. Control of Buck DC to DC Converter
In dc – dc converter, the average dc output voltage must be
controlled to equal a desired level, though the input voltage and
the output load may fluctuate. Switch-mode buck dc – dc
converter utilizes one or more switches to transform dc from one
level to another. In a dc – dc converter with a given input
voltage, the average output voltage is controlled by controlling
the switch on and off duration. ( ).
Fig. 10 Pulse-width modulator with comparator signals.
In pulse-width modulation (PWM) switching at a
constant frequency, the switch control signal, which controls the
state (on and off) of the switch, is generated by comparing a
signal-level control voltage with a repetitive waveform
as shown in fig. 10. The frequency of the repetitive waveform
with a constant peak, which is shown to be triangular waveform,
, establishes the switching frequency. This frequency is
kept constant in a PWM control and is chosen to be in few
kilohertz to a few hundred kilohertz range [6]. A comparator
device can be used to compare the two signals to generate firing
pulses, for the power switch which is characterized by on and off
behavior.

102 | P a g e
IV. CALCULATION AND SIMULATION RESULTS
In the calculation and the simulation of Buck dc – dc
converter the following parameters will be assumed.
Table 1 Parameters of buck dc-dc converter model
Parameters Circuit Values
Input Voltage, 12.6V
Switching frequency, f 20kHz
Duty Cycle, D 0.397
Output Current, 200mA
Load Resistance, 25Ω
Inductance, 1mH
Input Capacitor, 470uF
Output Capacitor, 470uF
Table 2 Parameters for Buck dc – dc converter model
Parameters Continuous
Values
Calculated
Discontinuo
us values
Calculated
Continuo
us Values
Simulated
Discontinuo
us values
Simulated
Output
Voltage,
5.0V 5.44V 4.8V 5.40V
Minimum
inductor
current,
0.1245A 0A 0.110A 0A
Maximum
inductor
current,
0.2755A 0.457A 0.268A 0.470A
1. Calculation Results for continuous and discontinuous
current conduction modes
Fig. 11 Continuous current conduction mode
Fig. 12 Discontinuous Current Mode Waveforms
2. Simulation Results for continuous and discontinuous
current conduction modes
Fig. 13 Voltage Waveforms for Buck DC to DC Power
Regulator in continuous current mode of Operation.

103 | P a g e
Fig. 14 Current Waveforms for Buck DC to DC power
Regulator in Continuous Current Conduction mode.
Fig. 15 Voltage Waveforms for Buck DC to DC Power
Regulator in Discontinuous Current conduction Mode.
Fig. 16 Current Waveforms for Buck DC to DC Power
Regulator in Discontinuous Current Conduction mode.
V. CONCLUSION
In this work an attempt has been made to analyse
mathematically and simulate a buck dc-dc converter under a
resistive load application. The circuit is analysed under two
different modes of operation namely: Continuous and
Discontinuous current conduction modes. The circuit is
simulated using Ansoft Simplorer software. Now from all this,
following conclusions can be drawn that the calculated results
are approximately equal to the simulated results under both
modes of operations. The waveforms obtained are closely
related. This work successfully generated waveforms as desired
and can be further verified by carrying out the prototype of the
project.
REFERENCES
[1] Erickson R.W., “DC to DC Power Converters” Article in
wiley Encyclopedia of Electrical and Electronics Engineering.
[2] Muhammad H. Rashid, ‘Power Electronics Circuits, Devices
and Applications’ Prentice Hall of India Publishers Ltd, 2004.
[3] M. Ahmed, M. Kuisuma, P. Silventoinem, ‘Implementing
Simple Procedure for Controlling Switch mode Power Supply
Using Sliding Mode control as a control Technique’, XIII-th
International Symposium on Electrical Apparatus and
technologies (Siela). May 2003, pp 9-14, vol. 1
[4] Hongmei Li and Xiao Ye ‘Sliding-Mode PID Control of DC-
DC Converter’, 5th IEEE Conference on Industrial Electronics
and Applications.
[5] Ogbuka, C. U and Diyoke, G. C., “Analysis of switch Mode
DC to DC Power Regulators” International Journal for IEEE,
A publication of the IEEE, Nigeria Section. April 2011,
Volume 1, No1. IJIR (ISSN: 0978-303807), Pg. 53-56.
[6] Ned Mohan, Tore M. Undeland and William P. Robbins,
‘Power Electronics’ John Wiley & sons, inc 2003.

ANALYSIS AND SIMULATION OF BUCK SWITCH MODE DC TO DC POWER REGULATOR

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ANALYSIS AND SIMULATION OF BUCK SWITCH MODE DC TO DC POWER REGULATOR