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Design of a Boost Converter

Submitted by

Abdul Fathah (109EE0612)

Department of Electrical Engineering National Institute of Technology Rourkela

Design of a Boost Converter

A Thesis submitted in partial accomplishment for the degree of

Bachelor of Technology in “Electrical Engineering”

By

Abdul Fathah (109EE0612)

Under guidance of

Prof. B.D. Subudhi

Department of Electrical Engineering

National Institute of Technology

Rourkela-769008 (ODISHA)

June-

a

ACKNOWLEDGEMENTS

On the submission of my thesis entitled “ Design of a Boost Converter ”, as a final year project, I would like to extend my appreciation & my sincere thanks to my project supervisor, a very generous guide in fact, Prof. B.D. Subudhi , Department of Electrical Engineering for his ceaseless encouragement and support during the course of my work. I verily appreciate and value his prestigious guidance and motivation from the beginning to the end of this work. His knowledge and support at the time of crisis will be remembered lifelong. He has been great source of inspiration to me and I thank him from the bottom of our hearts. I would also like to thank the staff of Electrical engineering department for constant support and providing place to work during project period. Last but not the least I would also like thank my friends, specially Soumya Ranjan Panda and Subhransu Satpathy who was with me during thick and thin.

Abdul Fathah B. Tech (Electrical Engineering)

b

Dedicated to

My beloved parents

ii

CONTENTS

ACKNOWLEDGEMENTS a ABSTRACT i TABLE OF CONTENTS ii

Chapter 1

INTRODUCTION

1.1 RATIONALE

In many technical applications, it is required to convert a set voltage DC source into a variable-voltage DC output. A DC-DC switching converter converts voltage directly from DC to DC and is simply known as a DC Converter. A DC converter is equivalent to an AC transformer with a continuously variable turns ratio. It can be used to step down or step up a DC voltage source, as a transformer. DC converters are widely used for traction motor control in electric automobiles, trolley cars, marine hoists, forklifts trucks, and mine haulers. They provide high efficiency, good acceleration control and fast dynamic response. They can be used in regenerative braking of DC motors to return energy back into the supply. This attribute results in energy savings for transportation systems with frequent steps. DC converters are used in DC voltage regulators; and also are used, with an inductor in conjunction, to generate a DC current source, specifically for the current source inverter. [1]

1.2 PURPOSE OF THE PROJECT Efficiency, size, and cost are the primary advantages of switching power converters when compared to linear converters. The switching power converter efficiencies can run between 70- 80%, whereas linear converters are usually 30% efficient. The DC-DC Switching Boost Converter is designed to provide an efficient method of taking a given DC voltage supply and boosting it to a desired value.

Chapter 2

BACKGROUND AND LITERATURE OVERVIEW

2.1 OVERVIEW

Power for the boost converter can come from any suitable DC sources, such as DC generators, batteries, solar panels and rectifiers. The method that changes one DC voltage to a different DC voltage is called DC to DC conversion. Generally, a boost converter is a DC to DC converter with an output voltage greater than the source voltage. It is sometimes called a step-up converter since it “steps up” the source voltage. [2]

2.2 HISTORY

For high efficiency, the SMPS switch must turn on and off quickly and have very less losses. The coming of a commercial semiconductor switch such as the boost converter in the 1950s represented a major milestone that made SMPSs possible. The main DC to DC converters were developed in the early 1960s when semiconductor switches were available.

Switched systems such as SMPS are a challenge to design since its model depends on whether a switch is opened or closed. R. D. Middlebrook from Caltech in 1977 published the models for DC to DC converters in market today. He averaged the circuit configurations for each switch state in a technique called state-space average modelling. This simplification resulted in reduction of two systems into one. This model led to insightful design equations which helped SMPS growth. [2]

2.3 APPLICATIONS

Battery powered systems often stack cells in series to obtain higher voltage. However, sufficient heaping of cells is not possible in many high voltage applications due to insufficient space. Boost converters can increase the voltage and reduce the cell numbers. Two battery-powered applications that use boost converters are hybrid electric vehicles (HEV) and lighting systems.

The NHW20 model Toyota Prius HEV utilizes 500 V. If there is no boost converter, the HEV would need nearly 417 cells to power its motor. In reality, a Prius actually uses only 168 cells and boosts the battery voltage from 202 V to 500 V. On a smaller scale application, boost converters also power devices such as portable lighting systems and emergency lights. A white LED typically

Chapter 3 METHODOLOGY

3.1 GENERAL BOOST CONVERTER CONFIGURATION

In a boost converter, the output voltage is greater than the input voltage – hence the name “boost”. A boost converter using a power MOSFET is shown below.

Fig. 3.1.1 Circuit diagram of Boost Converter. [1]

The function of boost converter can be divided into two modes, Mode 1 and Mode 2. Mode 1 begins when transistor M 1 is switched on at time t=0. The input current rises and flows through inductor L and transistor M 1. Mode 2 begins when transistor M 1 is switched off at time t=t 1. The input current now flows through L, C, load, and diode Dm. The inductor current falls until the next cycle. The energy stored in inductor L flows through the load.

The circuits for the two modes of operation are shown below:

(a)

(b) Fig. 3.1.2 Circuit operation (a) Mode 1 and (b) Mode 2. [1]

The voltage-current relation for the inductor L is:

Or

For a constant rectangular pulse:

When the transistor M 1 is switched:

Or

And when the transistor is switched off the current is:

Or

Here VD is the voltage drop across the diode Dm, and VTrans is the voltage drop across the transistor M 1.

By equating through delta i, we can solve for Vout:

Neglecting the voltage drops across the diode and the transistor:

So it is clear that the output voltage is related directly to the duty cycle. The main challenge when designing a converter is the sort of inductor to be used. From above equations, it can be seen that the inductance is inversely proportional to the ripple current. So, to reduce the ripple, a larger inductor should be used. [5]

3.2 BLOCK DIAGRM

Fig. 3.2.1 shows the basic blocks of building a boost converter circuit:

Fig. 3.2.1 Block diagram. [6] The voltage source provides the input DC voltage to the switch control, and also to the magnetic field storage element. The block which contains switch control directs the action of the switching element, whereas the output rectifier and filter deliver an acceptable DC voltage to the output. [6]

Switching Element

Switch Control

Magnetic Field Storage Element Voltage Source

Output Rectifier and Filter

3.5.3 CAPACITOR

∆V = ESR = 2.622×10-3V

C =

=

New Ripple Voltage= = 2.424×10-3V

Capacitor Specifications - Low ESR Tantanum Capacitor No. - TRSE 22006K050R Capacitance - 2.2mF Maximum O/P Voltage - 50V Tolerance - ±10%

3.5.4 INDUCTOR

L =

Ripple current ∆Io = 10% of Io

Inductor Specifications –

Family – 54z Minimum Quality Factor – 27 at 8MHz Test Frequency – 8MHz Tolerance – 10%

C = 2.034×10-^3 F

L = 870μH

Lead Style – Axial Type – Lower Choke Product Length – 19mm Diameter – 0.8mm Technology- Wire Wound Inductance – 1mH

3.5.5 DIODE

Diode Specifications – No. - IN Maximum voltage – 30V Maximum Current – 5A Schottky Diode Forward voltage drop at peak current = 0.4V

3.5.6 MOSFET

MOSFET Specifications – Output Voltage – 30V Load Current – 5A RDS = 0.002Ω F = 50 KHz N Channel MOSFET. [5]

3.6 LOSS CALCULATION

3.6.1 DIODE LOSS Diode loss = Vd×Io(1-D)

= 0.07 W