St. Thomas’ College of Engineering and Technology

Project Report on ‘Design of Half bridge converter and it’s application’ by

Name Roll Number Avirup Kundu 46 Soumyabrata Patra 56 Sounak Biswas 06 Tanushree Dutta 28 Sudip Chakraborty 09 Sujatro Laskar 47 Sovan Dutta 26

Guided ByProf. Sukanya Dasgupta

PROJECT REPORTSubmitted in partial fulfillment of the requirement for B.Tech degreeDepartment Of Electrical EngineeringSt Thomas College Of Engineering and TechnologyWest Bengal University of TechnologyYear 2014-15

Certificate of Approval

To whom it may concern,This is to certify that the project entitled ‘Design of Half bridge converter and it’s application’ is upto the standard of WBUT 8th semester syllabus. The project work has been done with precision and is quite satisfactory.

Prof (Dr.) B.B. Sen

Project Coordinator

Prof. S. Dasgupta

Mentor, Electrical Engineering dept.

Prof (Dr.) A. Ganguly

HOD Electrical Engineering dept. External Examiner

Acknowledgement

We would like to articulate our deep gratitude to our project guide Prof. S. Dasgupta who have always been a source of motivation and firm support for carrying out the project. We would like to convey our sincerest gratitude to all other faculty members and staff of Department Of Electrical Engineering, St Thomas College Of Engineering and Technology, who bestowed their great effort and guidance at appropriate times without which the completion of our project would have been impossible.Finally, Prof G. Banerjea (Director, Admin & Finance), and Prof. A. Ganguly (H.O.D of EE Dept.) owe special mention as without their disciplined guidance and care, the completion of the project within the given deadline would have been a distant dream.

Avirup Kundu

Soumyabrata Patra

Sounak Biswas

Tanushree Dutta

Sudip Chakraborty

Sujatro Laskar

Sovan Dutta

Contents

1.Introduction

2.Objective

3.Theory

4.Implementation Details

4.1. Generation of Gate Pulse

4.2. Working of Implemented Circuit

4.3. Equations Used

4.4. Values of Parameters

4.5. Output Observed

5.Matlab Simulation

6.Observation and Discussion

Objective

The main objective of our project is to understand the various components required for power converter circuits and then implement them to develop a Half bridge converter which can gain output 240v from 12v DC input.

Introduction

Dc-Dc converters are widely used in regulated switched mode dc power supplies and in dc motor drive applications. Often the input to the converters is an unregulated dc voltage which is obtained by rectifying the line voltage and therefore it will fluctuate due to changes in the line voltage magnitude.Switched mode dc-dc converters are used to convert the unregulated dc input into a controlled dc output at the desired voltage level. Converters are very often used with an electrical isolation transformer in the switched mode dc power supply and almost always without an isolation transformer in case of dc motor drives.The aim of this project is to design and analyse the Half bridge converter. This converter may be used in conjunction with a high frequency transformer to boost the output voltage with the advantage of having a smaller transformer and providing isolation between input and output.

TheoryBasic Design of a Half bridge Converter

A half bridge converter is a type of DC-to-DC converter that uses a transformer to change the voltage of a DC power supply. The DC bus is divided into two halves and capacitors of same value are connected in parallel to them. This is to hold the charge and give supply during the ON time of each switch.

The transformer primary is supplied with current from the input line by a pair of controlled switches in a symmetrical Half bridge circuit. The switches are alternately switched on and off, periodically reversing the

current in the transformer. Therefore current is drawn from the line during both halves of the switching cycle.

Power MOSFETs are often chosen as the switching device due to their high current switching capability and their inherently low ON state resistance.

The cycle starts with no voltage and no current. Then one MOSFET turns on, a constant voltage is applied to the primary, current increases linearly, and a constant voltage is induced in the secondary. After some time T the MOSFET is turned off and the other MOSFET turns on. However there should be a gap between switching off the first MOSFET and switching on the second MOSFET to ensure short circuit conditions do not occur. The cycle is then repeated.

Half-Bridge Converter belongs to the primary switched converter family since there is isolation between input and output. It is suitable for output powers up to 1kW.

PID Controller

The governing equation of the PID controller is

y (t )=K p e (t )+K i∫0

t

e (τ ) dτ+Kd

de ( t )dt

Where, y(t) is the output and e(t) is the error and u(t) is the reference.

The PID controller is used in the closed loop feedback system to maintain the output voltage constant. The PID controller works by first calculating an error as the difference between measured process output and a reference level and then tries to minimize the error by use of a manipulated variable.

Implementation DetailsGeneration of Gate Pulse

Pulse width modulation is used to control the MOSFETs in the implemented MATLAB simulation. The scheme to generate the pulses in MATLAB as described below.

A Triangular wave block is compared with a constant to get the required gat pulse for 1st MOSFET. The complement of this waveform is taken as the gate pulse for 2nd MOSFET.

This results in a pulsed output with the desired Ton and Toff.

Waveforms

Working of implemented circuit

Two MOSFETs are used as the switching devices to generate the continuous pulsed input to the transformer. A Gate Pulse generation circuit is designed which generates two pulses each of which has a duty cycle of 50%.

When the first MOSFET is turned on by the one of the gate pulses, the other MOSFET is kept off. the dc bus is splitted in two equal halves and same valude capacitors are connected across each of them.this enables us to give half of the supply to the transformer for each MOSFET. One half of the center tapped transformer is energized at the time. Then the first MOSFET is turned off and the second MOSFET is also kept off to avoid the short circuit condition.

After a suitable time, the second gate pulse fires the second MOSFET. The other half of the transformer is then energized. After the cycle is complete both MOSFETs are kept off for a while so that junctions can get enough time to recombine and we can avoid short circuit and then the cycle is repeated.

The transformer winding has a turns ratio of 40 to boost the input voltage of 6V to 240V to account for losses in rectification.

An uncontrolled rectifier is used to convert the pulsating output from the transformer to a constant DC voltage.

The low pass RLC filter is used to reduce the ripple component, and the output is taken across the load resistor.

Equations usedTransformer

1.V o

V ¿=0.5×

N s

N p

Where,

Vo = Output Voltage of transformer Vin = Input Voltage of transformer

Full Bridge Rectifier

1. V d=2π

√2V s

2. V r=0.217×√2V s

Where,

Vd = Average value of DC output voltage from full bridge rectifier Vs = RMS value of input sine wave Vr = RMS value of AC component in output

Low Pass RLC filter

1.V o (s )V ¿ ( s)

=

1LC

s2+s1

CR+1LC

Where,

Vo(s) = Output of filter in s domain Vin(s) = Input of filter in s domain L = Inductance

C = Capacitance R = Resistance

Values of Parameters

V input=12V

T on=5×10−4 s

Duty Ratio=0.50

Control Circuit parametersf Triangular=1kHz

PID

P=20 , I=45 ,D=0

Gain

Gain=1/240

MOSFET parametersFET Resistance=0.1Ω

Internal diodeinductance=0H

Internal diode resistance=0.01Ω

Internal diode forward voltage=0V

Snubber parametersResistance=1e5Ω

Capacitance=1e-12F

RLC Filter parametersC=1×10−3 F

L=1×10−6H

Winding parametersR1=0.000288Ω

L1=1.1833×10−5H

R2=1.152Ω

L2=3.667×10−4 H

Diode parameters

Vd=0.8V

Snubber resistance=500Ω

Snubber capacitance=250×10−9F

Output Observed

Vo=236.82V

Io=.148 A

Pout=35W

Oscillation in output

V r=0.5V

I r=0.0004 A

MATLAB SimulationHalf Bridge Converter circuit in MATLAB

Gate Pulse generation subsystem

Gate Pulses

Input Voltage

Output Voltage

Output Current

Observations and Discussion

1. This type of topology is mainly used for high voltage operation and power limit is in the range of 1 kw.

2. The splitting of DC bus and the capacitors are extra arrangements that have to be made for this topology.

3. Transformer is used for two reasons, first for stepping up the voltage, and second, for giving electrical isolation to secondary side.

4. There was a voltage spike being generated at the drain of the MOSFET after turning it off. we used a snubber to keep it down.

5. Unsteady was being observed in the open circuit output. Therefore a closed circuit system with a PID controller was implemented.

The objective of designing a Half Bridge converter with a low ripple output was successful. The input of 12V is successfully transformed to 235V at the output.

A ripple was observed in the output which is within tolerance limits and which we will try to reduce further during the hardware implementation.