by Agasthya Ayachit, Veda Prakash Galigekere, and Marian K. Kazimierczuk, Wright State University, Dayton, Ohio

Transcription

1 Power Electronic Circuitry In LED Modules: An Overview* ISSUE: December 2011 by Agasthya Ayachit, Veda Prakash Galigekere, and Marian K. Kazimierczuk, Wright State University, Dayton, Ohio One of the greatest inventions in solid-state technology is the light emitting diode (LED). The introduction of LEDs has created a benchmark for more advanced and efficient forms of lighting. In the present day scenario, LEDs are gradually replacing incandescent light bulbs and fluorescent lamps. This is mainly due to the basic advantages such as light weight, higher flexibility, long life, robustness, and ease in control. Their usage in industries is growing substantially. Sophisticated forms of LED lighting have been implemented in a myriad of applications namely, the automotive industry, household lighting, illumination in aircraft, handheld devices, traffic control lights, and so on. Presently, commercially available forms of LEDs have an output range of about 150 to 200 lumens/watt, and are about five times more efficient than incandescent sources of light. The main advantage of LEDs is their electro-luminescence phenomenon, i.e., direct conversion of electrical energy into light. Therefore, they do not flicker during turn-on, which eliminates the need for highly inductive chokes and starters. LEDs have turn-on time on the order of milliseconds and this feature is more helpful when they are used for optical signal transmission in communication systems. Since the operating temperatures of LEDs are relatively low, they possess very long lifespans on the order of about 30,000 to 50,000 operating hours. This article presents a detailed overview of the power electronic circuitry that constitutes an LED driver. To begin, electrical characteristics of LEDs are discussed briefly, and the demands and criteria for designing efficient drivers are mentioned. When these drivers are connected to the supply line, active power factor correctors become highly essential, especially when the operating power level is more than 25 W. Usually, they are accommodated in the ac-dc conversion stages of the power converters. These stages are described in brief and their merits and demerits are explained. To maintain high efficiency and optimum brightness of LED lamps, dimming control and bypass diodes become essential. These topics are briefly discussed as well. Physical Characteristics Of LEDs The structure of LEDs and their device characteristics are similar to that of p-n junction diodes. The variation of diode current ID with applied voltage VD is illustrated in Fig. 1. Fig. 1. Electrical characteristics of LED [15] How2Power. All rights reserved. Page 1 of 11

2 LEDs conduct and emit light once they are forward biased and when applied voltage is above their threshold value VT. Due to the flow of electrons, radiative electron-hole recombination takes place releasing energy in the form of photons. The intensity of these photons depends upon the band-gap energy of the semiconductive material under consideration. As the band-gap energy increases, the light emitted shifts to higher frequency regions of the electromagnetic spectrum. This results in the production of different colors. For example, red LEDs are composed of aluminium gallium arsenide (AlGaAs), LEDs emitting light in infrared regions are made of gallium arsenide (GaAs), and white LEDs have a combination of blue LEDs with phosphor material coated over them. Table 1 shows the typical forward voltages and currents for green, blue, red, and amber colored LEDs [1]. Table 1. V D and I D values for different LEDs. Color Green Blue Red Amber V D at 25 C 2.9 V 3.03 V 2.90 V 2.90 V (350 ma) V D at 25 C 3.25 V 3.30 V 3.60 V 3.60 V (700 ma) DC forward current rating 1000 ma 1000 ma 700 ma 700 ma Brightness or the luminous flux emitted by LED lamps is directly proportional to the forward current as shown in Fig. 2. In applications where color mixing is required, accurate current control methods become essential in order to provide the required brightness and the color [2]. Fig. 2. Luminous flux variation with forward current [15]. LED Driver Considerations Industrial grade standard dc-dc power converters, which are suitable for general applications cannot be directly used as LED drivers. They offer constant voltage at their output, but LED drivers require constant current throughout their operation [3]. Unlike linear loads, LEDs conduct only when the diode voltage exceeds the threshold value after which they are forward biased. So, there is the need for a minimum voltage to be supplied 2011 How2Power. All rights reserved. Page 2 of 11

3 to these diodes. When forward biased, they offer very low on-state resistance to the flow of current, usually on the order of hundreds of milliohms. This necessitates the use of current-limiting resistors. However, these current-limiting resistors dissipate power as heat and adversely affect the efficiency of the entire LED module. These resistors would not be able to directly regulate current for fluctuation in input voltage and current levels. Therefore, power converters are specifically designed for driving LED modules in order to obtain optimum performance. Control schemes are employed with these converters to improve the performance of these LED lamps. These drivers are designed such that they comply with EMI and measurement standards, namely, MIL, EN harmonic emission standards, etc. Digitally controlled interfaces such as, digital multiplex (DMX) and digital addressable lighting interface (DALI), are used to vary the illumination of LED lamps depending on the application. Therefore, these converters must provide dimming control with faster light to current response times. They should be capable of operating under universal input voltage levels, (i.e. 110 to 277 V rms ac) and provide multiple outputs with different current ratings. Certain applications will experience sudden inrush in current during start-up, for example, LEDs for automobile lighting, which can damage the lamps. Therefore, they should also provide inherent overvoltage and overcurrent protection. Owing to the several advantages and features of LED lamps, they are gradually replacing incandescent and fluorescent sources of lighting. Hence, these drivers need to be designed such that they are highly costeffective and have better performance with longer lifespans. The driver configuration can be classified into (a) single-stage and (b) two-stage categories. For low-power applications, a single-stage circuit that performs both power-factor correction and ac-dc rectification is employed. In such applications, the component count is a major consideration. LED drivers are cost effective and efficient due to reduced per-component losses. Several topologies, which provide both these functions, are discussed in [4] - [6]. A block diagram of a single-stage driver configuration is illustrated in Fig. 3. Fig. 3. Block diagram of single-stage LED driver. For higher-power applications, a two-stage conversion process is more feasible. The pre-regulator provides acto-dc conversion along with power-factor correction and the second stage drives the LED lamp with constant current and voltage. Usually, a half-bridge network with series-parallel resonant converter topology is employed at the second stage in order to (1) provide complete isolation for the LED array from the input side, (2) reduce voltage ripple at the input to the LED array, (3) provide wide load regulation by switching frequency variation, and (4) provide soft-switching operation. Fig. 4 shows the block diagram of a typical two-stage LED driver circuit How2Power. All rights reserved. Page 3 of 11

4 Fig. 4. Block diagram of two-stage LED driver. LED Driver Topologies When an LED driver module is directly connected to the ac mains, the presence of harmonics in the line causes severe voltage distortion, losses due to unwanted noise, and other undesired effects. In initial stages of the driver, EMI filters are used to eliminate these disturbances and maintain the operation of the driver according to the prescribed standards. Usually, the input filter consists of an inductor-capacitor network that passes only the required frequencies and also limits sudden current inrush. The presence of these filters helps in minimizing the interference caused to the neighboring devices, which may be powered from the same source. The ac-dc rectifier power-factor corrector (PFC) is employed in order to achieve unity power factor, irrespective of the power levels. PFC is used for input current shaping in offline power supplies in order to improve the effect of real power derived from the supply line. The PFC design is an important consideration in an LED driver. Several power factor correction schemes have been discussed in the literature [13], [14]. PWM Type Due to faster response times, these LEDs are usually fed with constant amplitude input current pulses with the help of pulse-width modulated LED drivers [7]. Literature suggests different topologies for the single-stage LED driving schemes. Buck, boost, buck-boost topologies are the most commonly used PWM type drivers [12]. The literature suggests a flyback converter to provide effective ac-to-dc conversion and effective power-factor correction [4], [5], [8], [12]. A flyback converter can be operated either as a stepup or a stepdown converter by adjusting the turns of the secondary winding. The circuit of a flyback ac-dc converter with PFC is shown in Fig. 5. Fig. 5. Flyback ac-dc converter connected to LED network. Optocouplers and transformers are used to provide complete dc isolation between the input and the output. Also, the transformer can have several secondary windings; hence, a flyback converter can accommodate multiple outputs How2Power. All rights reserved. Page 4 of 11

5 The flyback converter offers the following advantages: The MOSFET is driven with respect to ground, thus eliminating the need for complex gate-drive circuits. They offer galvanic isolation and the LED lamp is completely isolated from the input. The number of turns can be adjusted in order to accommodate more LED strings. The flyback converter is known for its lower parts count and also for eliminating the need for an output filter inductor. Single-stage driving schemes have the advantage of having a lower parts count since they employ only one MOSFET for switching and a single magnetic component for energy storage, resulting in a compact and portable system. On the other hand, they suffer from several disadvantages. Voltage ripples with frequency twice that of the line frequency appear at the input to the LED network. This demands the utilization of bulky capacitors at the output end of the power supply. Another limitation is that only one MOSFET processes all the power, hence the stress on that MOSFET is relatively high. If the flyback mode is used, the presence of higher leakage inductance causes higher voltage spikes resulting in higher stresses on the MOSFET, which requires clamping and snubber circuits. However, a topology such as that suggested in [10] proves to be an effective method for reducing these stresses on the MOSFET switches, and can be extended for use in LED drivers. Resonant Type Present research is concentrated on reducing the size of LED drivers so that they can be used in portable or offline applications. This can be achieved by reducing the size of the driver components to a certain extent. The sizes of passive components are reduced when operated under very high frequencies. Resonant converters are used where switching of transistors takes place at frequencies on the order of hundreds of kilohertz to several megahertz. Resonant dc-dc converters have the capability to produce constant current under wide operating conditions. This eliminates the need for complex external current control circuitry and smoothing capacitors [3]. The most widely used topology is the series-parallel resonant dc-dc converter. The basic circuit of a series-parallel resonant dc-dc converter is shown in Fig. 6. This type of converter offers galvanic isolation for the LED strings from the input fluctuations. LEDs are known for their high efficiencies when operated under normal junction temperature. Thus, it is essential to design drivers that offer fewer losses during energy conversion. Fig. 6. Circuit diagram of a series-parallel resonant ac-dc converter driving an LED load. Fig. 7 shows the voltage conversion characteristics of the series-parallel resonant converter. The presence of two resonant frequencies determines whether the converter operates in zero-current switching (ZCS) or zerovoltage switching (ZVS) mode. When the converter operates in ZVS, the switching losses will be reduced to a very large extent. Therefore, it is possible to achieve overall efficiencies of 95% to 97% by using resonant converters as LED drivers How2Power. All rights reserved. Page 5 of 11

6 Fig. 7. Variation of voltage gain with normalized switching frequency of a series-parallel resonant converter. A major drawback of this topology is that it cannot offer short-circuit and open-circuit protection at frequencies close to the resonant frequency, resulting in the flow of heavy currents that may damage the LEDs and the driver components. To avoid this effect, usually, resonant converters are designed as a two-stage set-up. As illustrated in Fig. 4, the first stage in the driver module is designated as a preregulator. It is usually a boost input-current shaper and power-factor corrector. A few schemes have been proposed that use a flyback converter as the preregulator. The second stage is a half-bridge series-parallel converter as discussed. LED Driver Auxiliary Circuit Dimming Methods White LEDs are amongst the most commonly used type of LEDs. Certain applications demand uniform luminosity of all the LEDs in the string. White LEDs are produced by mixing light from red, green and blue (RGB) colored LEDs in proper proportions. Therefore, adequate brightness of these colors is to be provided in order to generate white light, which needs accurate control circuitry. Also, flickering should be absent when the brightness changes from one level to another. In order to avoid this, the flicker frequency is increased to more than 120 Hz, i.e. the frequency above which the human eye cannot perceive flickering. Several dimming schemes have been suggested in the literature [9]. Two main types of dimming can be achieved (1) analog dimming and (2) pulse-width-modulated dimming. It was discussed earlier that the brightness of an LED depends upon the forward current. If the current through the LED can be controlled, then the required brightness can be achieved, i.e. if 50% of the total current is made to flow through the LED, then 50% brightness can be achieved. Analog control of LEDs can be achieved using linear current regulators. Fig. 8 shows an analog dimming technique using linear current regulators, which are connected in series with the LED strings. Variation in forward current causes color shift and for applications like LCD panels in televisions, laptop screens, etc, which require accurate brightness levels, color shift is not acceptable. Therefore, analog dimming techniques are not extensively used How2Power. All rights reserved. Page 6 of 11

7 Fig. 8. Circuit diagram of analog-type dimming control in LED network. In PWM dimming, the duration of the peak value of the LED forward current is varied by adjusting the conduction period of the switches. If the duty cycle is set at 50%, then the LEDs would glow at 50% of their maximum brightness. One of the main types of PWM dimming is burst-mode (BM) dimming. When the LED strings are connected in the form of a lattice structure consisting of series LEDs in parallel, current sharing becomes an important consideration. The electrical properties of all the LEDs connected in the matrix will not be the same, i.e., for the same operating temperature, the forward voltages will differ for different LEDs in conduction. Because of this irregularity, the forward currents through them will also differ by a certain factor. This causes differences in the illumination levels of the LEDs in the matrix. Therefore, uniform current sharing or current balancing methods need to be implemented. Fig. 9 shows the set-up of BM brightness dimming control. VO represents the output voltage from the dc-dc converter. The BM-mode transistors are connected in series with the LED string. They are made to operate as low-frequency transistors in order to reduce conduction losses. They are switched at 400 Hz in practical applications [9]. In this method, average currents are obtained by varying the duty-cycle or the pulse-width of the individual switches. Reference [11] suggests an improved methodology for burst-mode dimming How2Power. All rights reserved. Page 7 of 11

8 Fig. 9. Circuit diagram of burst-mode PWM-type dimming control in LED network. Bypass Diodes LEDs are connected in the form of a series and parallel connection of individual LEDs, in order to obtain the required brightness for large-scale and backlight LED panels. In such applications, there is a possibility that a single LED may cease to operate, disturbing the operation of several other LEDs in that string. Hence, bypass Zener diodes are connected in parallel with the individual LEDs as shown in Fig. 10(a). Under normal operating conditions, the Zener diode is turned off and the current flows through the LED. When an LED ceases to turn-on, the flow of current is deviated through the anti-parallel Zener diode, thus, maintaining the current flow. An alternate modified structure is shown in Fig. 10(b). In this case, during failure of an LED, the Zener diode conducts, allowing the current to flow through the resistor and capacitor. The capacitor is charged and triggers the silicon controlled rectifier (SCR). The current flow deviates thru the SCR without affecting the operation of the system [9] How2Power. All rights reserved. Page 8 of 11

9 (a) (b) Fig. 10. Zener bypass diode across individual LEDs (a). Modified bypass design (b) How2Power. All rights reserved. Page 9 of 11

10 Conclusion LED lamps offer an optical efficiency of up to 90% to 95%. By proper design of the LED drivers, an overall efficiency of up to 90% to 92% can be achieved. The need for LED drivers for different applications is discussed. Two types of LED drivers are suggested. Dimming schemes used for maintaining brightness levels are discussed. LED bypass design in order to improve the reliability of the drivers is also mentioned. References 1. Lumileds Luxeon Rebel datasheet. 2. O. Ronat, P. Green, and S. Ragona, Accurate current control to drive high power LED strings, Proc in IEEE APEC, pp , March Heinz ven der Broeck, G. Sauerlander, and M. Wendt, Power driver topologies and control schemes for LEDs, Proc in IEEE APEC, pp , May C. Qiao and K. M. Smedley, A topology survey of single-stage power factor corrector with a boost type input-current-shaper, IEEE Trans. Power Electronics, vol. 16, no. 3, pp , May M. T. Madigan, R. W. Erickson, and E. H. Ismail, Integrated high-quality rectifier regulators, IEEE Trans. Ind. Electron., vol. 46, no. 4, pp , August M. Daniele, P. K. Jain, and G. Joos, A single-stage power-factor-corrected ac/dc converter, IEEE Trans. Power Electronics, vol. 14, no. 6, pp , November B. Ackermann, V. Schulz, C. Martiny, A. Hilgers, and X. Zhu, Control of LEDs, Proc in IEEE IAS, vol. 5, pp , October O. Garcia, J. A. Cobos, R. Prieto, P. Alou, and J. Uceda, Single phase power factor correction: A survey, IEEE Trans. Power Electronics, vol. 18, no. 3, pp , May C. Huang-Jen and C. Shih-Jen, LED backlight driving system for large-scale LED panels, IEEE Trans. Ind. Electron., vol. 54, no. 5, pp , October D. Murthy-Bellur and M. K. Kazimierczuk, Two switch flyback-forward PWM DC-DC converter with reduced switch voltage stress, IEEE Trans. Circuits Syst., pp , May H. Yuequan and M. M. Jovanovic, LED driver with self-adaptive drive voltage, IEEE Trans. Power Electronics, vol. 23, pp , November M. K. Kazimierczuk, Pulse-width modulated DC-DC converters, Chichester, United Kingdom: Wiley & Sons, Ltd, K. Jirasereeamornkul, M. K. Kazmimierczuk, I. Boonyaroonate, and K. Chamnongthai, Single-stage electronic ballast with class-e rectifier as power factor corrector, IEEE Trans. Circuits Syst., vol. 53, no. 1, pp , January J. Yimin and F. C. Lee, Single-stage single-phase parallel power factor correction scheme, Proc in IEEE PESC, pp , June PHILIPS Lumileds DS64 datasheet. *This paper was originally presented at the 2010 Electrical Manufacturing & Coil Winding Expo, held October 18-20, 2010 in Dallas, Texas. For more information, see How2Power. All rights reserved. Page 10 of 11

11 About The Authors Agasthya Ayachit received his B.E degree in Electrical and Electronics Engineering from Visvesvaraya Technological University, India in 2009 and M.S degree in Electrical Engineering from Wright State University in Currently, he is working as a lecturer at Penn State Erie The Behrend College. His research interests are in resonant and PWM dc-dc power converters, modeling and control of dc-dc converters, and energy harvesting. Veda Prakash Galigekere received the B.E, M.S Degrees from Visveswaraiah Technological University, Belgaum, India and Wright State University, Dayton, Ohio in 2004 and 2007, respectively. He is currently pursuing his PhD at Wright State University. His areas of interests are PWM dc-dc and dc-ac inverters, power semiconductor devices, renewable energy systems and system-level simulations of the same. Marian K. Kazimireczuk received the M.S., and Ph.D., and D.Sci. degrees in electronics engineering from the Department of Electronics, Technical University of Warsaw, Warsaw, Poland, in 1971, and 1978, and 1984, respectively. He was a teaching and research assistant from 1972 to 1978 and assistant professor from 1978 to 1984 with the Department of Electronics, Institute of Radio Electronics, Technical University of Warsaw, Poland. In 1984, he was a project engineer for Design Automation, Inc., Lexington, MA. In , he was a visiting professor with the Department of Electrical Engineering, Virginia Polytechnic Institute and State University, VA. Since 1985, he has been with the Department of Electrical Engineering, Wright State University, Dayton, OH, where he is currently a professor. His research interests are in high-frequency high-efficiency switching-mode tuned power amplifiers, resonant and PWM dc-dc power converters, dc-ac inverters, high-frequency rectifiers, electronic ballasts, modeling and control of converters, high-frequency magnetics, and power semiconductor devices. For more on LED driver design, see the How2Power Design Guide, select the Advanced Search option, go to Search by Design Guide Category, and select Lamp ballasts and LED drivers in the Power Supply function category How2Power. All rights reserved. Page 11 of 11