Design and implementation of push pull converter forrgb led lighting system-2

1. INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING &
Proceedings of the 2nd International Conference on Current Trends in Engineering and Management ICCTEM -2014
TECHNOLOGY (IJEET)
17 – 19, July 2014, Mysore, Karnataka, India
ISSN 0976 – 6545(Print)
ISSN 0976 – 6553(Online)
Volume 5, Issue 8, August (2014), pp. 100-106
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IJEET
© I A E M E
DESIGN AND IMPLEMENTATION OF PUSH-PULL CONVERTER
FORRGB LED LIGHTING SYSTEM
Avinash A, Karna Yaswanth, K.V.Devadas, K.Suryasen
E & EE, K.V.G.College of Engineering, Sullia, D.K. Karnataka, India,
100
ABSTRACT
Recently LED lighting raised interest among consumers because of its certain advantages
over conventional lighting systems. It is very compact, has high luminous efficiency and low heat
dissipation. Single LED (called RGB LED) giving different coloured light are available in market. In
this paper a DC-DC push-pull converter is designed and rigged up to drive RGB LED’s to produce
different coloured light. In this system accurate control of colour of light can be achieved and it can
be used in all modern lighting applications.
PWM signals are generated using microcontroller to drive the DC to DC push-pull converter.
By varying the duty cycle of the push-pull converter, voltage applied to RGB LED’s is controlled to
get different colour lights. The technique has been automated and can effectively change the colour
of LED’s without the need of using expensive feedback systems that involve light sensors. RGB
LED based lighting system of 30watts is developed. This project identifies three modes of operation
i.e., one LED ON, two LED’s ON and three LED’s ON.
Keywords: Arduino, Push-pull Converter, RGB LEDs, High Frequency Transformer, Multicolour
Lighting
1. INTRODUCTION
CURRENTLY, fluorescent lamps represent the most popular lighting solution due to their
high luminous efficacy and low running cost. However, because of their limited programmability
fluorescent lamps cannot meet the requirements of many modern applications. Unlike fluorescent
lamps, red–green–blue (RGB) light-emitting diodes (LEDs), with their capability to generate
instantly different colors and intensities, are expected to find many applications in areas such as
display boards, traffic lights, detector systems and general decorative illuminations. Thus, LEDs are
expected to become a major kind of light sources in the coming decades [1].LED lighting clearly
offers an eco-friendly alternative to energy-wasting conventional lighting. RGB LED lighting
fixtures offer endless colour changing possibilities to enhance the environment. Intensity of light
produced by an LED is proportional to its forward current as shown in Fig1. The forward

2. Proceedings of the 2nd International Conference on Current Trends in Engineering and Management ICCTEM -2014
17 – 19, July 2014, Mysore, Karnataka, India
characteristics of Red, Green and Blue LED’s, indicate the region of control to produce acceptable
level of light intensities as shown in Fig 2. Suitable region has to be identified to explore the energy
saving options [1, 3]. This paper demonstrates the application based on voltage mode control scheme
to meet the market needs [2]. RGB LED’s forward current varies with voltage in the region as
marked in the figures shown below. For RED LED when voltage is varied from 2.2V to 2.4V current
varies from 100mA to 350mA. This region is exploited in this paper to control the light intensity of
RED Led. Similarly for Green LED linier range is 3.2V to 3.4V and for Blue LED linier range is
3.0V to 3.4V.
Fig. 1: Characteristics of RGB LED’S
Fig. 2: Luminous Flux versus Forward Current
Two LED’s are connected in series, and 5 such sets are connected in parallel. 3W RGB
LED’s are considered, so current rating is 1.75A. The requirement for Red LED is considered as
4.8V, for Green and Blue LED as 7.0V. Total power output of LED array is 30W. Input supply to
converter is considered as 24V. This is derived from 230V, 50Hz single phase supply. Hence
230V/24V, 250VA transformer is considered. Diode rectifier bride along with filter capacitor is used
to get DC voltage.
101

3. Proceedings of the 2nd International Conference on Current Trends in Engineering and Management ICCTEM -2014
17 – 19, July 2014, Mysore, Karnataka, India
102
2. HARDWARE SELECTION
2.1. Converter Selection
It looks DC – Dc buck converter is natural choice. To get the required output voltages (4.8V
for Red LED, 7.0V for Green and Blue LED) buck converter has to operate at smaller duty ratio,
which puts stress on filter capacitance to maintain the output voltage during the switch off period.
Also buck converter is non isolated converter. To overcome these draw backs push pull converter is
used [4]. Two switching devices are required but it provided isolation for control circuit. Three push-pull
converters are required to control Red, Green and Blue LED’s separately.
2.2 Control of Converter
Three push-pull converters required 6PWM pulses. Microcontroller (arduino) is considered
since 6PWM signals can be derived from this microcontroller. To get desired gate voltages for
MOSFET switches opto-isolator can be used [5]. One can write program for this microcontroller in
computer in embedded C and program can be directly loaded to microcontroller.
2.3 Push-Pull Converter
The Push-Pull converter is dc-dc converter as shown in Fig3 which produces an output
voltage lesser than the source. Pulses of opposite polarity are produced on the primary and secondary
windings of the transformer by switching Sw1 & Sw2. The Push-Pull converter utilizes a center-tapped
transformer for both the primary and secondary windings. The primary winding is controlled
by two transistors, which allow one of them to conduct during each half-cycle, so the output is
receiving voltage directly through one of them at all times [4].
Fig. 3: Push Pull Converter
The transformer is assumed to be ideal for this analysis. Switches Sw1, Sw2 turn on and off
with the switching sequence. When Sw1 is closed,
(1)
The voltage across the lower primary winding P1 is transformed to the secondary windings.
D1 diode is forward biased and D2 is reverse biased. Assuming a constant output voltage Vo, the
voltage across L is a constant, resulting in a linearly increasing current in L. Closing Sw2 established
the voltage across upper primary winding P2 at,

4. Proceedings of the 2nd International Conference on Current Trends in Engineering and Management ICCTEM -2014
17 – 19, July 2014, Mysore, Karnataka, India
Parameters RED BLUE GREEN
Input voltage Vs 20V-24V-28V 20V-24V-28V 20V-24V-28V
Out put voltage Vo 4.8V 7V 7V
Switching frequency fs 25khz 25khz 25khz
Out put current Io(max) 350mA 350mA 350mA
Out put current Io(min) 105mA 110mA 70mA
Output current ripple Io 10% 10% 10%
Output voltage ripple Vo 1% 1% 1%
103
(2)
Diode D2 is forward biased D1 is reverse biased. The current in L increases linearly while
Sw2 is closed. When both the switches are open, the current in each of the primary winding is zero.
The current in the filter inductor L, must maintain continuity, resulting in both D1 D2 becoming
forward biased. Inductor current divides evenly between the transformer secondary windings. The
relevant waveforms pertaining to the operation of the Push-Pull converter is shown in Fig 4.
Fig. 4: Waveforms of Push Pull Converter
3. HARDWARE IMPLEMENTATION
The component selection and design of magnetic materials for all three push-pull converter is
done [3,4]. The control logic is also developed using Aurdino microcontroller and integrated along
with the converters. Push-pull converter is designed based on the following specifications [6].
Table 1: specifications for push pull converter
Design of Push Pull Converter is done based on the above specifications [7]. The following table 2
shows the list of components employed

5. Proceedings of the 2nd International Conference on Current Trends in Engineering and Management ICCTEM -2014
17 – 19, July 2014, Mysore, Karnataka, India
Table 2: List of components
Sl.no Name of Component Specifications Quantity
104
1 Single phase Step down
Transformer
230V/24V,10A 1
2 Bridge rectifier Diode BR 101 4
3 Capacitor 50V,300μF 3
4 Ferrite Core EE -25ₓ13ₓ7
EE-30ₓ15ₓ7
6
5 Copper wire (SWG-
20,21,22,25)
10 meters each
6 Schottky diode 45V, 20A 6
7 Capacitor 10uf 3
8 RGB LEDs(Edison make),6
terminal
3W 10
9 MOSFET IRF Z44f 60V, 30A with suitable
heat sinks
6
10 General purpose PCB 3
11 Microcontroller Arduino Uno assembly
board
1
12 Pot 10K 6
4. RESULTS AND DISCUSSIONS
Three push-pull converters are implemented using the above components. Arduino
Microcontroller is used to generate required PWM signals for the push-pull converter.
It is tested for the following modes:
Mode 1:
Pulses will be given for red LED only
Pulses will be given for blue LED only
Pulses will be given for green LED only
MODE 2:
Red LED is ON, Green LED if OFF and duty ratio of Blue LED is varied
Red LED is ON, Blue LED is if OFF and duty ratio of Green LED is varied
Blue LED is ON, Red LED is OFF and duty ratio of Green LED is varied
Blue LED is ON, Green LED is OFF and duty ratio of Red LED is varied
Green LED is ON, Red LED is OFF and duty ratio of Blue LED is varied
MODE 3:
Two LEDs are ON and varying duty ratio of other LED
This chapter demonstrates the results obtained in hardware and substantiates the distinctive
acceptable colour intensities with that of the modes of operations.

6. Proceedings of the 2nd International Conference on Current Trends in Engineering and Management ICCTEM -2014
17 – 19, July 2014, Mysore, Karnataka, India
105
4.1 TEST RESULTS OF RGB LED ARRAY
The multi-colours produced using RGB LED’s powered by the designed push-pull converters
are displayed in this section.
4.1.1 Individual LED ON – Mode I
Fig. 5: Red LED ON Fig. 6: Green LED ON
Fig. 7: Blue LED ON
The above results are shown for the operation of RGB LED in Mode- I where individual
LEDₓs are switched ON. The operation is shown for the variation in intensities at different levels so
as to obtain acceptable distinctive colours.
4.1.2 Two LED’s ON – Mode II
Fig. 8: Red and Blue LED’s ON Fig. 9: Red and Green LED’s ON

7. Proceedings of the 2nd International Conference on Current Trends in Engineering and Management ICCTEM -2014
17 – 19, July 2014, Mysore, Karnataka, India
The above results are shown for the operation of RGB LED in Mode- II where two LED’s
are switched ON. The operation is shown for the variation in intensities at different levels so as to
obtain acceptable distinctive colours. Similarly the results obtained other modes of operation
experimentally are matches with the expected results.
106
5. CONCLUSION
The projected hypothesis of obtaining distinctive multi-colors is accomplished using RGB
LED’s. The linear region in the operation of these LED’s is explored to attain variety of distinctive
colors by recognizing the different modes. The hardware results are also conclusive towards the
projected objectives of the work. These results signify the prominence of the proposed concept being
valid and hence saleable in commercial applications.
6. REFERENCES
1. Stephen Johnson “LEDS an overview of the state of the Art in Technology Applications,
IEEE transaction on Industry Applications Vol: 1a-19 No.5 September/October 1983
2. Development of Computer Controlled colour mixing illumination Network using RGB
LED’s by Mr.S.S.Umare, Prof. A.M.Jain, Dr.B.E. Kushale KKWIEER, Nasik in IOSR
Journal of Engineering (IOSRJEN) ISSN: 2250-3021 ISBN: 2878-8719 PP44-47
3. Sameer Ram Pujari “Application of Pushpull Converter for RGB LED based commercial
lighting systems” PESC, MIT, M.Tech Thesis -2011
4. Daniel W. Hart, “Introduction to Power Electronics”, Prentice Hall International Editions,
ISBN 0-13-180415-4
5. Laszlo Balogh, “Design and Application Guide for High Speed MOSFET Gate Drive
Circuits”, Texas Instruments
6. Abraham I. Pressman, “Switching Power Supply Design”, McGraw Hill, Second Edition,
ISBN 0-07-116707-2
7. L. Umanand, S. R. Bhat, “Design of Magnetic Components for Switched Mode Power
Converters”New Age International (P) Limited, ISBN 81-224-0339-5