A Feedback Resonant LED Driver with Capacitive Power Transfer for Lighting Applications

Transcription

1 A Feedback Resonant LED Driver with Capacitive Power Transfer for Lighting Applications Shreedhar Mullur 1, B.P. Harish 2 1 PG Scholar, 2 Associate Professor, Department of Electrical Engineering, University Visvesvaraya College of Engineering, Bangalore University, Bengaluru shreedharmullur@gmail.com, dr.bp.harish@ieee.org Abstract: The proposed work presents a single stage resonant topology for the Light Emitting Diode (LED) driver with the feedback control to regulate the constant set point value of either the input inductor current or the output capacitor voltage of the ac-dc converter. In order to provide for galvanic isolation between the input and the load, this topology uses capacitive power transfer in place of bulky transformers. The circuit operates in zero current switching and zero voltage switching mode on a LED string load of 15 W, 30 V. The feedback controller is provided to regulate the output voltage constant against fluctuations in input voltage. A simulated maximum power efficiency is found to be 94.4 %for open loop operation and 94.2 % for closed loop operation. A prototype of single stage resonant LED driver with closed loop control is implemented in hardware and tested to drive a 3-segment LEDs string. The experimental results justify the simulated results. Keywords: LED driver, Capacitive Power Transfer, ZVS-ZCS, Feedback Control. 1 I. INTRODUCTION LEDs are gaining importance compared to the other light sources like, incandescent light bulbs and compact fluorescent lamps (CFLs). LEDs have several advantages like small size, low power per unit of light generated, long life span of around 50,000 hours and help reduce greenhouse gas emissions from power plants. Hence, LED lighting has the potential to replace all other forms for general lighting applications. LEDs work on the principle of electro-luminescence that directly converts the electricity into light. There has been significant research carried out to optimize the efficiency and component count of power electronic driver circuits used to power LEDs. The driver should provide sufficient current for required brightness and limit excess current to protect the string of LEDs. Various power driver topologies and control schemes have been proposed for LED drivers. Chen-yang et al., have analyzed two wireless power transfer systems, capacitive power transfer system and inductive power transfer system and their selection based on maximum power transfer capacity [1]. Boser and Sanders have presented a capacitor-isolated LED driver that relies on a pair of high voltage isolation capacitors of a series resonant tank and integrated with a balanced ladder stepdown switched capacitor front-end. While the series resonant conversion stage function with any line voltage, the resonant stage preserves the efficient voltage regulation capability [2]. Jiejian Dai et al. have reported that CPT is normally applied for low power applications as it requires a small coupling capacitance, while power electronics for CPT at high power level and low cost is a requirement [3]. Hence, single switch and single diode converter topologies like Cuk, SEPIC, buck-boost and Zeta are studied to decrease the cost and losses. Liu et al. have presented theoretical analysis and design of capacitive power flow control to reduce the voltage/current overshoots, improve system reliability, and to achieve a wide range of output power control [4]. Aguilar et al. have proposed a low cost buck-boost topology operating in discontinuous conduction mode (DCM) as an off line LED driver with unity power factor [5]. But this driver suffers from high component stress levels due to DCM operation. Almeida et al. have proposed to drive a multistring LED driver based on a DCM buck-boost power factor corrector stage and four linear current equalizers to feed LED strings [6]. The driver is shown to have good efficiency at full power, low input current distortion and high power factor, but continue to suffer from high component stress levels. Broeck et al. have proposed a series resonant galvanic isolated LED driver that generates a pulsating current to power LEDs and investigated the implications of pulsating current waves, EMI generation and switching losses [7]. Cheng et al. have presented a single stage AC- DC resonant converter with interleaving power-factorcorrection (PFC) for street-lighting applications [8]. The driver exhibits low input current ripple, reduced switching losses, good power factor and reduced component count, but increases the cost due to the isolation transformer. Shmilovitz et al., have proposed resonant LED driver with Capacitive Power Transfer (CPT) which features a power factor correction and dimming [9]. This topology uses the Capacitive Power Transfer (CPT) method for its advantages over inductive power transfer [1][10]. With inductive power transfer, system gets magnetically coupled with the magnetic material present outside the system resulting in core losses and electromagnetic interference. CPT eliminates these losses as it employs two pairs of loosely coupled capacitor plates as shown in Fig. 1.

2 Figure 1: Block Diagram of Capacitive Power Transfer. Here, the power is transmitted by electric fields between the electrodes such as metal plates. Capacitive interface makes it simple, low cost and provides the galvanic isolation. Further, it facilitates low switches count leading to better system reliability. The main objective of LED driver for its wide adoption in lighting applications is to decrease its size and cost with improved efficiency. The normal approach of transformer coupling for power transfer increases the cost and size of the circuit. But the use of electrolytic capacitors in converter topologies is avoided as they decrease the reliability and increase the volume. In order to achieve stable flicker-free light output for lighting applications, closed loop operation with current mode control is proposed. II. THE PROPOSED TOPOLOGY The circuit schematic of resonant LED driver for open loop operation is shown in the Fig. 2 [9]. The supply and load side are isolated using capacitors and power is transferred through capacitive coupling. The proposed circuit looks similar to the Cuk converter, but it varies in its components property and their design values. To obtain a high power transfer efficiency, the circuit should operate with a higher switching frequency and zero voltage and zero current (ZVS-ZCS) switching operation. The ceramic capacitors C s of few nano farads are used in place of electrolytic capacitors. Figure 2: Open Loop LED Driver [9] The AC supply is stepped down and converted to DC using a diode rectifier bridge and a capacitor filter, then switched to the LED load by switching the MOSFET switch at high switching frequency of 200 KHz and capacitive coupling. The circuit operates in four intervals, during which the MOSFET is turned ON and OFF [9]. It is made sure that the LEDs are supplied during all the intervals. Figure 3: Closed Loop LED Driver 2

3 It is observed that the open loop operation as illustrated in [9] suffers from regulation issues i.e., output power and hence the light output varies with fluctuations in supply voltage. This variation in the output may cause flickering of LEDs, affecting human vision. To address this issue of flickering feedback controller is proposed. Current mode control is employed to maintain output power constant. The closed loop operation of frequency. The feedback operation is simulated in MATLAB and implemented in hardware using Arduino controller. III. SIMULATION RESULTS The resonant LED driver in open loop and closed loop configuration are simulated using MATLAB. The design values for all components in the driver circuit are listed in Table I. The simulated waveforms of gate pulse, I Lin, I Lout, MOSFET drain voltage (V drain ) and diode voltage (V d ) in closed loop operation are shown in Fig. 5. The output voltage of LED driver in open loop and closed loop configurations is tabulated for variations in AC supply voltage in Table II and the corresponding graphical representation is shown in Fig. 6. It can be seen that the closed loop operation of LED driver provides excellent output voltage regulation leading to constant lumen output. The maximum power efficiency is found to be 94.4 % for open loop operation and 94.2 % for closed loop operation. Further, in closed loop operation, the output is found to be insensitive to variations in switching frequency as shown in Fig. 7. Table I: Parameter Values Parameters Value V in V out 100 V 30 V L in 120 µh L out 300 µh Cs 2 nf f s 200 KHz Fig. 4: Flowchart of The Feedback Control LED driver is as shown in Fig. 3. The process of feedback control is described by the flowchart in Fig. 4. The output voltage V o is compared with the reference voltage V ref to obtain an error signal e 1 (t) which is processed using the PI controller to generate c 1 (t). The input inductor current I Lin is compared with the control signal c 1 (t) and an error signal e 2 (t) is obtained that is processed using an another PI controller to generate c 2 (t). This control signal c 2 (t) is compared against the repeating sequence i.e., a ramp carrier signal and the generated output control signal V g is used as gate drive for MOSFET switching. The output voltage is maintained constant in the presence of supply voltage fluctuations by adjusting the duty cycle of the MOSFET gate pulse drive at constant switching Figure 5: Waveforms of Gate Pulse, I Lin, I Lout, MOSFET Drain Voltage (V drain ) and Diode Voltage (V d ) of LED Driver in Closed Loop Operation 3

4 Table II: Simulated Variation of Output Voltage in Closed Loop and Open Loop LED Driver with Supply Voltage Variations AC Supply Voltage Vo (Open Loop) Vo (Closed Loop) in output voltage with variations in supply voltage is listed in Table III. The experimental reading of power efficiency is found to be 92%. These experimental results justify the simulation results about the efficacy of closed loop control in providing constant light output. Table III: Experimental Variation of Output Voltage in Closed Loop LED Driver with Supply Voltage Variations Supply (Vin) Output Voltage (Vo) V. CONCLUSION A resonant LED driver with closed loop control is implemented to turn on the LED string using capacitive coupling for isolation to achieve low cost and small size. Further, ceramic capacitors replace electrolytic capacitors to increase the reliability of the driver. The proposed feedback operation ensures excellent regulation against variations in supply voltage leading to flicker free lighting. Figure 6: Variation of Output Voltage Without and With Feedback Figure 7: Sensitivity Analysis of Output Voltage with Respect to Switching Frequency in Feedback Operation of LED Driver IV. 4 EXPERIMENTAL RESULTS As proof of concept, a prototype of single stage resonant LED driver with closed loop is implemented in hardware and tested to drive a 3-segment LEDs string of 3 W, 10 V rating. An autotransformer is used to step down the power supply from 230 V to a range of 17 V to 27 V to generate an output voltage of 10 V. The corresponding variations VI. REFERENCES [1] XIA Chen-yang, LI Chao-wei and ZHANG Juan, Analysis of Power Transfer Characteristic of Capacitive Power Transfer System and Inductively Coupled Power Transfer System, International Conference on Mechatronic Science, Electric Engineering and Computer, August 2011,pp no [2] M. Kline, I. Izyumin, B. Boser and S. Sanders, "A transformerless galvanically isolated switched capacitor LED driver," in Applied Power Electronics Conference and Exposition (APEC), 2012 Twenty- Seventh Annual IEEE, 2012, pp [3] Jiejian Dai and Daniel C. Ludois, Single Active Switch Power Electronics for Kilowatt Scale Capacitive Power Transfer, IEEE Journal of Emerging and Selected Topics in Power Electronics, 2013, pp [4] C. Liu and A. P. Hu, "Power flow control of a capacitively coupled contactless power transfer system," in Industrial Electronics, 2009.IECON '09. 35th Annual Conference of IEEE, 2009, pp

5 [5] D. Aguilar and C. P. Henze, "LED driver circuit with inherent PFC," in 25 th IEEE Applied Power Electronics Conference and Exposition (APEC), 2010, pp [6] P. S. Almeida, J. M. Jorge, D. Botelho, D. P. Pinto and H. A. C. Braga, "Proposal of a low-cost LED driver for a multistring street lighting luminaire," in IECON th Annual Conference on IEEE Industrial Electronics Society, 2012, pp [7] H. Van der Broeck, G. Sauerlander and M. Wendt, "Power driver topologies and control schemes for LEDs," in 22 nd IEEE Applied Power Electronics Conference, APEC, 2007, pp [8] Chun-An Cheng, Hung-Liang Cheng, Chien-Hsuan Chang, Fu-Li Yang and Tsung-Yuan Chung, "A single-stage LED driver for street-lighting applications with interleaving PFC feature," in IEEE International Symposium on Next-Generation Electronics (ISNE), 2013, pp [9] Doron Shmilovitz and Shaul Ozeri, A LED Driver With Capacitive Power Transfer, 29 th IEEE Applied Power Electronics Conference, APEC, pp [10] Mitchell Kline, Capacitive power transfer, Electrical Engineering and Computer Sciences University of California at Berkeley, Technical Report No. UCB/EECS EECS html Dec