TheoryBasic Design of a Push Pull Converter

A pushpull converter is a type of DC-to-DC converter that uses a transformer to change the voltage of a DC power supply. The transformer primary is supplied with current from the input line by a pair of controlled switches in a symmetrical push-pull 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.Pushpull converters have steadier input current, create less noise on the input line, and are more efficient in higher power applications.

PID Controller

The governing equation of the PID controller is

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 Sine Wave block is used to generate a repeating sinusoidal signal of desired frequency. This signal is compared with a constant value that is set according to the required Ton given by the relation

Where y=constant value, T=time period of sine wave, t=Ton This results in a pulsed output with the desired Ton and Toff. The constant value is complemented and used to generate the pulsed output for the other MOSFET.

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 is set to generate two pulses each of which has a duty cycle of 0.41.When the first MOSFET is turned on by the one of the gate pulses, the other MOSFET is kept off. 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 to avoid short circuit and then the cycle is repeated.The transformer winding has a turns ratio of 20 to boost the input voltage of 12V to 240V to account for losses in rectification.A full bridge 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 resistor.

Equations usedTransformer1. Where, Vo = Output Voltage of transformer Vin = Input Voltage of transformer

Full Bridge Rectifier1. 2. 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 filter1. 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

Control Circuit parameters

PID

Gain

MOSFET parameters

Snubber parameters

RLC Filter parameters

Winding parameters

Diode parameters

Output Observed

Oscillation in output

MATLAB SimulationPush Pull Converter circuit in MATLAB

Gate Pulse generation subsystem

Gate Pulses

Input Voltage

Output Voltage

Output Current

Ripple in Output Voltage

Observations and DiscussionProblems faced1. After increasing the frequency of sine wave in the Gate Pulse generation subsystem there was a lot of disturbance in the output. The cause was determined to be an error in MATLAB which causes the sine wave to behave unexpectedly at higher frequencies. It was solved by settling the sampling rate of the constant block to a value comparable to the time period of the sin wave.2. There was a voltage spike being generated at the drain of the MOSFET after turning it off. The cause was determined to be the back EMF generated in the transformer windings due to sudden chopping of current. A snubber circuit was used to limit the spike.3. 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 Push-Pull converter with a low ripple output was successful. The input of 12V is successfully transformed to 234V 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.

St. Thomas College of Engineering and Technology

Project Report on Design of Push Pull converter and its application by

NameRoll Number

Avirup Kundu46

Soumyabrata Patra56

Sounak Biswas06

Tanushree Dutta28

Sudip Chakraborty09

Sujatro Laskar47

Sovan Dutta26

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 Push Pull converter and its application is upto the standard of WBUT 7th semester syllabus. The project work has been done with precision and is quite satisfactory.

Prof (Dr.) B.B. SenProject Coordinator

Prof. S. DasguptaMentor, Electrical Engineering dept.

Prof (Dr.) A. GangulyHOD 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. Introduction2. Objective3. Theory4. Implementation Details4.1. Generation of Gate Pulse4.2. Working of Implemented Circuit4.3. Equations Used4.4. Values of Parameters4.5. Output Observed5. Matlab Simulation6. 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 push- pull converter which can gain output 240v from 12v DC input. After having achieved successfully the design in MATLAB our next objective is to implement it for practical purposes.

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 push-pull 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.