Table of contents

Declaration …............................................................................................................................................i
ACKNOWLEDGEMENT.........................................................................................................................ii
ABSTRACT ..............................................................................................................................................iii
1.0 INTRODUCTION ...............................................................................................................................1
1.1 Objective...........................................................................................................................................1
1.2 The need for isolated buck...….........................................................................................................1
2.0 DESIGN OF ISOLATED BUCK........................................................................................................2
2.1 Rectifier………...............................................................................................................................2
2.2 Inverter………................................................................................................................................2
2.3 High frequency transformer............................................................................................................2
2.4 High frequency rectifier..................................................................................................................3
2.5 Buck converter……………………………………………………………………………………3
3.0 COMPLETE CIRCUIT DIAGRAM……………………………..…………………………..………4
4.0 REFRENCES………………………………………………………………………………………...4
 Declaration
We declare that the work mentioned in the report is done by us and has not been
submitted to gain any other degree or project. Moreover, no part of this work has been copied or
plagiarized from any other source.
 ACKNOWLEDGEMENT
First and foremost, we would like to thank Almighty Allah for bringing us this far. Most
gratitude goes to our project supervisor, Dr. Rabia Nazir for his unrelenting advice and guidance
in the design and implementation of the project.
Lastly, we would like to take this opportunity to give special thanks to all our classmates
who helped us in achieving our task.
 ABSTRACT
Buck converters are step-down DC-DC converters that are widely being used in different
electronic devices like laptops PDA’s, cell phones and also electric vehicles to obtain different
level of voltages. These converters are nothing but, high frequency switching devices operating
on PWM principle.
An isolated buck converter uses a synchronous buck converter with coupled inductor
windings to create isolated outputs. Isolated converters use a smaller transformer for an
equivalent power transfer as the transformer primary and secondary turns ratios are better
matched. There is no need for an optocoupler or auxiliary winding as the secondary output
closely tracks the primary output voltage, resulting in smaller solution size and cost. The high
frequency transformer incorporated in proposed converter provides the necessary isolation and
safety with high power density, as it eliminates the need for external bulky line frequency
transformer.

Our aim is to design and implement an isolated buck converter with an output power
rating upto 400W with switching frequency 40kHz.
1.0 INTRODUCTION
1.1 Objective:
The project aimed to design a buck converter with the following specifications and
providing source isolation using a specially designed high frequency transformer:

Input Voltage 220V AC

Input Frequency 50Hz
Switching Frequency 40 KHz
Output Power 400W

1.2 The need for isolated buck converter:

Although optocouplers can also be used for isolation but they are low power devices and
cannot be use in HVDC applications, hence transformer is used for isolation purpose.
Moreover we know that for buck converter

Vo = DVs
Where Vo is output voltage of buck converter, D is duty cycle and Vs is source voltage.
If we want to step down high input DC value up to low value DC, it implies that duty
cycle will be very small that is why we first step down voltage through high frequency
transformer up-to a reasonable value then and after that we further step down the rectified
output through buck converter.

1.3 Transformer Isolation

In a large number of applications, it is desired to incorporate a transformer into a
switching converter, to obtain dc isolation between the converter input and output. For
example, in off-line applications (where the converter input is connected to the ac utility
system), isolation is usually required by regulatory agencies. Isolation could be obtained
in these cases by simply connecting a 50 Hz or 60 Hz transformer at the converter ac input
input. However, since transformer size and weight vary inversely with frequency,
significant improvements can be made by incorporating the transformer into the converter,
so that the transformer operates at the converter switching frequency of tens or hundreds of
kilohertz.
When a large step-up or step-down conversion ratio is required, the use of a transformer
can allow better converter optimization. By proper choice of the transformer turn ratio, the
voltage or current stresses imposed on the transistors and diodes can be minimized, leading
to improved efficiency and lower cost.
2.0 DESIGN OF ISOLATED BUCK
In Isolated buck converter we first rectified the voltage coming from main supply and then
inverted the output of rectifier into high frequency square wave. Output of inverter is fed to
high frequency transformer and step it down. After that we again rectified the transformer
output and fed it into the buck converter.

2.1 Rectifier
A rectifier is an electrical device that converts alternating current (AC), which
periodically reverses direction, to direct current (DC), which flows in only one direction.
The process is known as rectification, since it "straightens" the direction of current. In
our design we have used rectifier to convert 12V AC output of transformer in DC, and its
peak value is approximately 17V.

2.2 Inverter
In our project we used single phase half bridge to invert the output of rectifier into square
wave. We used IR2103 as a gate driver to drive the gates of MOSFETS. We used IRF450
MOSFETS for switching purpose. Moreover we have used capacitor divider in parallel
with switches leg which provides virtual ground.

2.3 High Frequency Transformer

For high frequency transformer we are using ferrite core and transformer ratio of 155/40.
For the design of transformer we have done the calculations, the results are given below.

Primary side Secondary side

Voltage 155 V 40 V
Current 2.5 A 10 A
Turns 44 11

The conductor used for both primary and secondory winding is AWG 25

Boben length=Height of window=27mm

 Width of window= 8mm
Total radial build=6.3mm
In our circuit we have used half bridge topology for transformer design.
220x1.414 = 311V ~ 310V

According to half bridge topology

310x0.5 = 155V
Design frequency =40kHz

2.4 High Frequency Rectifier

From the high frequecy transformer we get AC output which has high frequncy so, to
rectify at this high frequency we have to use high frequency diodes in bridge rectifier.
That’s why we used IRF1560 diodes which can work at high frequncy.

2.5 Buck Converter

Buck converter is a dc to dc converter, it step downs the output voltage.The circuit
diagram for the buck converter is given below.

3.0 CIRCUIT DIAGRAM

4.0 Refrencens
Book : Fundamental of Power Electronics by Erikson

Website : www.IEEE.org