INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING & TECHNOLOGY (IJEET) Proceedings of the 2 nd International Conference on Current Trends in Engineering and Management ICCTEM -2014 ISSN 0976 6545(Print) ISSN 0976 6553(Online) Volume 5, Issue 8, August (2014), pp. 100-106 IAEME: www.iaeme.com/ijeet.asp Journal Impact Factor (2014): 6.8310 (Calculated by GISI) www.jifactor.com 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, 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 100
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
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 pushpull 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 centertapped 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, 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, 102 (1)
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. (2) 3. HARDWARE IMPLEMENTATION Fig. 4: Waveforms of Push Pull Converter 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 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% Design of Push Pull Converter is done based on the above specifications [7]. The following table 2 shows the list of components employed 103
Table 2: List of components Sl.no Name of Component Specifications Quantity 1 Single phase Step down 230V/24V,10A 1 Transformer 2 Bridge rectifier Diode BR 101 4 3 Capacitor 50V,300µF 3 4 Ferrite Core EE -25ₓ13ₓ7 & 6 EE-30ₓ15ₓ7 5 Copper wire (SWG- 10 meters each 20,21,22,25) 6 Schottky diode 45V, 20A 6 7 Capacitor 10uf 3 8 RGB LEDs(Edison make),6 3W 10 terminal 9 MOSFET IRF Z44f 60V, 30A with suitable 6 heat sinks 10 General purpose PCB 3 11 Microcontroller Arduino Uno assembly 1 board 12 Pot 10KΩ 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. 104
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 105
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. 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 81-224-0339-5 106