 Open Access
 Total Downloads : 1182
 Authors : M. Gupta , S. P. Phulambrikar
 Paper ID : IJERTV3IS031844
 Volume & Issue : Volume 03, Issue 03 (March 2014)
 Published (First Online): 05042014
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Design and Analysis of Buck Converter
M. Gupta and S. P. Phulambrikar
Department of Electrical Engineering, Samrat Ashok Technological Institute, Vidisha, MP, INDIA
AbstractDesigns of the power electronics circuitry are now adays reducing the size, space and weight of converter / inverter circuits. This is possible because of the availability of new high switching frequency devices. This paper presents a generalized model of buck converters. The converter used for stepping down the voltage is called buck converter. Buck converter is designed, analyzed, simulated & developed. The proposed model of Buck converter consists of two parts: (i) Main converter circuits with the components like switch, inductor, diode, capacitor, and a load, (ii) A control circuit for controlling the operation of the switch using microprocessor through optocoupler. This model can accurately predict the steadystate behaviors for average load current and voltage across the load of a single switch dcdc converter. To verify the proposed model, the circuit is prepared & their experimental results were compared with the results obtained by simulation of the circuit in pspice.
KeywordsDuty cycle(Dr), Average output voltage(Vo), Input voltage (VIN), ONtime of switch, OFFtime of switch, ptop ripple in capacitor voltage(VC) & inductor current(IL), frequency (f).

INTRODUCTION
The dcdc converter is a dc power supply that is small, lightweight & highly efficient, and uses a semiconductor switching element. It can response quickly and suitable to changes in input voltage within the scope of normal operating conditions to return to the normal operating state. It is comprised of (i) switching power supply unit which, can turn ON/OFF switching elements that can be turned ON/OFF at high frequency to convert a dc input voltage VIN into a dc output voltage VOUT, and (ii) a control unit, which is used to control the ON/OFF operation of the switching element of said switching power supply unit [1, 2]. The dc input voltage to the converters is assumed to have zero internal impedance &in the output stage of the converter, a small filter is treated as an integral part of the dctodc converter. The output is assumed to supply a load that can be represented by an equivalent resistance. Computerbased analysis/simulation tools have been used for powerelectronics circuits for many years. Challenges in the area of analysis/simulation tools for powerelectronics are described in literature [3]. The simulation package (pspice) is used to calculate the circuits wave forms, dynamic and steady state performance of the developed system.
The buck converter is introduced using the evolutionary approach. Let us consider the circuit in Fig.1, containing a single pole doublethrow switch. For the circuit in Fig.1, the output voltage equals the input voltage when the switch is in position A and it is zero when the switch is in position B. By varying the duration for which the switch is in position A and B, it can be seen that the average output voltage can be varied, but the output voltage is not pure dc. The output voltage
contains an average voltage with a squarevoltage superimposed on it as shown in Fig.2. Usually the desired outcome is a dc voltage without any notice able ripple content and the circuit in Fig.1 is to be modified accordingly. This is performed by addition of components in steps and observing their waveforms.
The circuit in Fig.1 can be modified as shown in Fig.3 by adding an inductor in series with the load resistor. An inductor reduces ripple in current passing through it and the output voltage would contain less ripple content. Since the current through the load resistor is the same as that of the inductor.
Fig. 1. A resistor with a singlepole doublethrow switch
Fig. 2. Output waveform decomposed
Fig. 3. Effect of adding inductor
Fig. 4. Effect of adding capacitor
When the switch is in position A, the current through the inductor increases and the energy stored in the inductor increases. When the switch is in position B, the inductor acts as a source and maintains the current through the load resistor. During this period, the energy stored in the inductor decreases and its current falls. It is important to note that there is continuous conduction through the load for this circuit.
If the time constant due to the inductor and load resistor is relatively large compared with the period for which the switch is in position A or B, then the rise and fall of current through inductor is more or less linear, as shown in Fig.3.
The next step in evolutionary development of the buck converter is to add a capacitor across the load resistor and this circuit is shown in Fig.4. A capacitor reduces the ripple content in voltage across it, whereas an inductor smoothes the current passing through it. The combined action of LC filter reduces the ripple in output to a very low level [4].

EQUATIONS
The equations that govern the operation of the circuit is as follows:
In the first state (when the switch is on)
Third state (Both diode and switch are off and inductor is at rest with no energy stored in it)
dVO / dt = ( VO / R) / C (5)

SELECTION OF CONVERTER
Table (1) shown below is the converter decision matrix that illustrates, why a Buck converter topology has been selected for this paper. Each factor was weighted equally with a true or false value, with 1 (one) being true and 0 (zero) being false. The assigned points were summed and the converter design with the greatest total was chosen. The linearity criterion refers to the linear relationship between the voltage transfer ratio and the duty cycle [5].
The Buck converter is perfectly linear and has a 1:1 ratio between the voltage transfer ratio and the duty cycle. To meet project specifications a positive voltage is required on the output rather than a negative. Also, the voltage will need to be reduced as the specifications state that the input voltage is to be a nominal 12V and the output voltage a constant 7V. The Buck converter meets both of the aforementioned criteria, as it will reduce the input voltage without inversion.
Fewer number of components leads to a simpler design, which again, the Buck converter has. Therefore, the Buck converter is chosen as the design because it meets all of the specified criteria. Then, the basic circuit topology of the Buck converter, which includes five standard components: (1) a pulsewidth modulating controller, (ii) a switch, (iii) an inductor, (iv) a capacitor, and (v) a diode.
Table 1. Converter decision matrix
Linearity
Positive Output Polarity
Fewest Number of Components
Reduces Output Voltage
Total
Buck
1
1
1
1
4
Boost
0
1
1
0
2
Buck Boost
0
0
1
1
2
Cuk
0
0
0
0
0
SEPIC
0
1
0
0
1
diL/ dt = VS
– VO
/ L (1)

OPEN LOOP CONTROLLER
The control circuit acts upon the sensed input and output
dVO/ dt = ( IL – V / R) / C (2)
Second state (When the switch is off and diode is on)
diL/ dt = – VO / L (3)
dVO/ dt = ( IL – VO / R) / C (4)
characteristics of the dcdc converter and ultimately adjusts the duty cycle to allow the converter to respond to system component and load variations. The key component of the control circuit, which would dictate how the control circuit would do its processing, was chosen based on the results of the decision matrix as shown in Table 2.
Table 2. Control circuit decision matrix
Ease of Use
Cost
2
1
0
Additional Components
Total
Microprocessor
2
1
5
FPAA
1
2
4
DSP
0
0
0
The microprocessor and DSP are well known control components. The FPAA (Field Programmable Analog Array) is a new integrated circuit, which can be configured to implement analog functions using a set of configurable analog blocks and a programmable interconnection network, and is programmed with the use of onchip memories.
The criteria in the matrix were rated as 0, 1, or 2. For ease of use, 2 represent the easiest component to implement, while 0 would be the most difficult. The most costly component will receive a 0 in the cost criterion while the least expensive will receive a 2. In the additional components column a rating of 2 means this implementation will have the least amount of extra components and a 0 rating will need the most additional components. The option with the highest cumulative score is selected in this case, the control circuit decision matrix indicate that, the microprocessor is the best choice for buck converter as control circuit.


ANALYSIS OF BUCK CONVERTER

BUCK CONVERTER CIRCUIT & ITS PARAMETERS
VC = [VIN X Dr ( 1 Dr)] / 8LC f2

Inductor current (IL ):
IL = [VIN X Dr (1 – D r )] / f L

Maximum and minimum current of inductor:
IMAX = IO+ VOTOFF / 2 L IMIN=IOVOTOFF / 2L

Determination of filter inductance:
L = 3.33 VO (TS T ON) / IO

Determination of filter capacitance:

VC = VIN Dr ( 1 Dr) / 8LC f2


DESIGN CONSIDERATION OF BUCK CONVERTER
In designing a buck switching converter, certain specifications are needed as: (i) Nominal input dc voltage = VIN volt, (ii) Variation of input voltage = 25% of VIN (say),
(iii) Output voltage = VOUT volt, (iv) Maximum load current = IMAX (max) A, (v) Minimum load current = IMIN (min) A, and

Output maximum ripple voltage = VO volt (ptop) [6, 7].
Buck converter circuit is shown in Fig.5.
Specifications Parameters


HARDWARE REQUIREMENT

SPECIFICATION OF THE CIRCUIT
The specifications of the components used in the circuit
VDC = 12V
FREQUENCY = 82.24 KHZ MOSFET (IRFZ44)
DIODE (1N4500) DUTY CYCLE = 0.63
L=151.5uH C=4.7uF
R=10ohm
(Fig. 6) are mentioned below:

Main circuit: Input supply of 12 volt, Switch MOSFET (IRFZ44), Diode (1N4007), Inductor (Ferrite core)
= 151.5 H, Capacitor (electrolytic)=4.7F, Resistive load (R)=10ohm/5 watts.

Auxiliary circuit: – Input supply of 12volt, Opto coupler (MCT2E), Transistor (2C547), Three Resistors (100 ohm, 1 kilo ohm, 1 kilo ohm).


EQUATIONS
Fig. 5. Buck converter circuit

PRACTICAL VALUES

Generation of square wave:
In Intel 8085 microprocessor, Time for 1T state is 320nsec. Therefore,
Delay Time = 320 109 24
= 7.68 Âµsec
OFF Time = 320 109 14

Output voltage:
Vo= Dr X VIN

PtoP ripple in capacitor voltage:
= 4.48 Âµsec
Total time period (TS) is given by
= TON + TOFF
= (7.68+4.48) Âµsec
TS = 12.16 Âµsec F = 82.24 K HZ


Duty cycle :
Dr = TON / T
= 7.68/12.16
Dr = 0.63

Determination of L: We have, L = 3.33 VO (TS T ON) / IO
VI. SIMULATION RESULTS Buck converter with pspice:

Make the circuit for Buck converter, using the following parts:
VDC (Voltage source) V Pulse
IRFZ44 (MOSFET switch) R (Resistance)
L (Inductance)
Output Power = ( I
)2 R
C (Capacitance)
O 2 L IN4007 (Diode)
5 = ( IO ) 10 IO = 0.70 amp
Therefore, L = 3.33 VO (TS T ON ) / IO
L = 3.33 7.56 (12.16 7.86 ) 106 L = 154.64 ÂµH


Determination of C

VC = VO (VIN VO ) / 8LC f2 VIN
/ 0.70
GNDsignal

Connect the differential voltage marker and current marker at different position [7].

Get plots for the following: –
Load voltage & current, Inductor voltage & current, Capacitor current. Results of the simulation of these
As ripple is independent of the output load power, so long as the converter operates in continuous conduction mode. It should be less than or equal to one. i.e VC 1.Let the ripple in output voltage be 0.07.
From above the calculation, value of C is given as 4.7ÂµF
i.e. C =4.7ÂµF.
V. OBSERVATION
It is observed that output voltage across load is come out to be approximately 7.2 volt, which is about 40% of the input (12 volt).
Fig. 6. Hardware circuit
waveforms are shown in Fig.7, Fig.8, Fig.9, Fig.10, and Fig.11 respectively.

CONCLUSION
In this work, Buck converter is analyzed, designed for low output voltage ripple (0.07) with the output voltage of 7.2 volts & maximum current of 1A. The circuit has been simulated using pspice software and then hardware has been developed & controlled by microprocessor. The simulation results and experimental results are almost in agreement with each other.

SUGGESTION FOR FUTURE WORK
The developed buck converter can be modified to operate in a closed loop. If microcontroller is used for controlling the switching operation, the circuit will become more compact and reliable.
Fig. 7. Load – Voltage waveform
Fig. 8. Load – Current of Buck converter
Fig.9: Inductor – Voltage of Buck converter
Fig. 11. Capacitor – Current of Buck converter
REFERENCES

N. Mohan, T. M. Undeland and W. R. Robbins, Power Electronics: Converters, Applications and Design, 2nd Edition, John Wiley & Sons, New York, 1995.

P. T. Krein, Elements of Power Electronics Oxford University Press, New York, 1998.

V. J. Thottuvelil, Challenges in ComputerBased Analysis / Simulation and Design of Power Electronic Circuits, IEEE workshop on computers in power Electronics, pp. 17, 1998.

Z. Salam, Power Electronics & Drives UTMJB (version 2), 2002.

Jaycar Electronics Reference Data Sheet: Â© Jaycar Electronics, 2001

KwangHwa Liu, and F.C.Y. Lee, Zerovoltage switching technique in DC/DC converters, IEEE Transaction on power Electronics, Vol. 5, No. 3, 1990.

M. H. Rashid, Power Electronics: Circuit, Devices and Application, Prentice Hall (4 edition), 2013.
Fig. 10. Inductor – Current of Buck converter