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.