ENERGY saving through efficient equipment is an essential

IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 61, NO. 9, SEPTEMBER 2014 4649 Isolated Switch-Mode Current Regulator With Integrated Two Boost LED Drivers Jae-Kuk Kim, Student Member, IEEE, Jae-Bum Lee, Student Member, IEEE, and Gun-Woo Moon, Member, IEEE Abstract A new isolated switch-mode current regulator is proposed for an LED driving system. The two boost LED drivers are integrated with the dc/dc converter, which results in a simple structure and low component count. The primary side provides an ac voltage source to the secondary side in which one boost inductor, two switches, and diodes comprise two boost drivers. Each secondary switch controls each LED current to be balanced. The voltage stresses of the primary switches are clamped to the input voltage, and those of secondary switches and diodes are clamped to the output voltages. Furthermore, all switches can easily achieve zero-voltage switching by using the transformer magnetizing current without additional auxiliary circuits. The validity of this proposed circuit is confirmed by the experimental results from a 400-V-input and 150-W-output prototype with two 75-W LED strings. Index Terms Boost LED driver, isolated, switch-mode current regulator, zero-voltage switching (ZVS). I. INTRODUCTION ENERGY saving through efficient equipment is an essential component of international efforts to slow down global warming. Recently, LEDs have received much attention due to the advantages of long lifetime, low voltage driving, and mercury-free device in lighting applications such as liquidcrystal-display backlight or display panel, streetlights, signage, and general-purpose lighting [1], [2]. Presently, the power ratings of individual LED devices are a few watts, limited by the packaging technology and heat dissipation. To obtain sufficient luminance, many LEDs are connected and arranged in parallel LED strings. The parallel structure inevitably leads to current imbalance problem due to the LED parameter variations, aging, and temperature changes, which will in turn affect the luminous intensity and even color in each string [3]. Most importantly, if the current imbalance causes one or more LED strings to exceed their rated current values, the lifetime of the LED strings (and, hence, the LED system) will be drastically reduced. There are several methods of driving multiple LED strings connected in parallel. Among them, active current regulation with a linear or switching circuit in each LED string can achieve precise current control for a multichannel LED driver. The approach employing Manuscript received April 22, 2013; revised September 2, 2013 and October 25, 2013; accepted November 17, 2013. Date of publication December 5, 2013; date of current version March 21, 2014. J.-K. Kim is with Samsung Electro-Mechanics Company, Ltd., Suwon 443-743, Korea (e-mail: jaekuk99@naver.com). J.-B. Lee and G.-W. Moon are with the Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIE.2013.2293700 Fig. 1. Typical block diagram of an LED driving system with boost drivers. linear regulators offers a simple control and low cost but suffers from poor operating efficiency because the voltage drop across the linear regulator, i.e., the voltage difference between the input and output voltages of the linear regulator cannot be minimized under all operating conditions. The efficiency of linear regulator LED drivers can be improved by sensing and regulating the minimum voltage of the linear regulators [4] [6]. The drawback of the linear current regulator LED driver can be basically overcome by employing a more efficient switchmode current regulator. Fig. 1 shows a typical block diagram of an LED driving system with boost LED drivers as switch-mode current regulators. This system is composed of the input stage, including a power factor correction (PFC) converter, which reduces the line current harmonics and makes the regulated link voltage V S about 400 V, by which the dc/dc stage is followed and provides the galvanic isolation and the bus voltage V B for boost LED drivers. Each driver controls the LED current of each string. The conventional boost converter is a good candidate for the LED driver because of simple structure and driving circuit, as well as high-efficiency capability. However, it has hard switching operation that makes electromagnetic interference (EMI) problem worse and low power density due to the limitation of the switching frequency. To overcome these problems, soft-switched boost converters are proposed using additional passive and active snubbers [7], [8]. In addition, to improve the overall performance or make a simple structure of overall circuit, many current regulators are researched. Among them, the isolated dc/dc converter, including LED driving functions, is proposed in [9] [11]. For street lighting applications, soft-switching asymmetrical half-bridge converters for each string are proposed [9]. In [10], an isolated and unregulated dc/dc converter provides two bus voltages from which twoinput buck converters regulate the currents of each array. These approaches have high overall efficiency and high accuracy to control the LED currents. However, as the number of LED string increases, the overall system becomes more complex and expensive. In [11], post regulators using a magnetic amplifier are applied to control each LED current in a single-ended forward converter. The secondary-side post regulator is extended according to the number of LED strings. Thus, it has lower 0278-0046 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

4650 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 61, NO. 9, SEPTEMBER 2014 Fig. 2. Circuit diagram of the proposed regulator. component count than the prior circuits. However, it still has the disadvantage that every single string has an output filter inductor, including a magnetic amplifier inductor with reset circuits, which make complex structure. In this paper, a new isolated switch-mode current regulator is proposed. The conventional two boost LED drivers are integrated with the dc/dc converter, which makes more simple and cost-effective structure. In addition, the proposed circuit has many advantages, e.g., the voltage stresses of all semiconductors are clamped to the input or output voltage. Furthermore, all switches can easily achieve zero-voltage switching (ZVS) without additional auxiliary circuits, which increases the overall efficiency and relieves the EMI problem. II. CIRCUIT OPERATIONS AND FEATURES Fig. 2 shows the circuit diagram of the proposed regulator. The proposed circuit consists of the half-bridge inverter in the primary side and the two boost converters using one inductor in the secondary side. Two LED currents are sensed, i.e., i sense1 and i sense2, and compared with the same current reference I REF. The errors are compensated by adjusting the duty ratios of secondary switches. Figs. 3 and 4 show the operational key waveforms and equivalent circuit of the proposed regulator, respectively. It is assumed that all parasitic components except for those specified in Fig. 2 are neglected and the blocking capacitor voltage V C in the half-bridge inverter is constant with half of the input voltage V S /2 and transformer turns ratio n = N P /N S. Primary switches Q P 1 and Q P 2 have the same duty ratio of 0.5, neglecting the dead time. Secondary switches Q S1 and Q S2 are used to regulate each LED current and are turned on simultaneously with Q P 2 and Q P 1, respectively. In addition, the minimum duty ratio of Q S1 and Q S2 is the duty ratio of Q P 2 and Q P 1, respectively. The overlapping on-time of Q S1 and Q S2 is the build-up interval of the boost inductor L B. The operational mode can be divided into eight modes. In mode 1, Q P 2 and Q S1 are turned on with ZVS with the help of the magnetizing current i Lm, and Q S2 is at on state. V S /2 is transferred to the secondary side and is applied to the boost inductor L B, which is build-up mode. In mode 2, Q S2 is turned off and the boost diode D S2 is turned on. The voltage of Q S2 is clamped to V O1. Powering occurs through Q S1 and D S2. Mode 3 starts when the boost inductor current decreases to 0 A. D S2 is turned off with zero-current switching (ZCS), and i Lm is only reflected to the primary side. In mode 4, Q P 2 is turned off and each output capacitance of Q P 2 and Q P 1 is charged and discharged by the reflected transformer magnetizing current, respectively. Accordingly, the transformer secondary voltage reflected from v pri is all applied to the drain-to-source voltage on Q S2. When v pri reaches 0 V, the drain-to-source voltage on Q S2 also reduces to 0 V, and the body diode of Q S2 is turned on. Therefore, if there exists enough dead time between Q P 1 and Q P 2,theZVSofQ P 1 and Q S2 is guaranteed. When v pri increases over 0 V, the output capacitance values in the primary switches are resonated with L B until v pri reaches V S /2, which means that the voltage stress of Q P 2 is clamped to V S.The modes from 5 to 8 are symmetric with those from 1 to 4. The dc conversion ratio of the proposed circuit is similar with conventional boost converter operating with discontinuous conduction mode (DCM) and can be expressed as follows. It is assumed that the duty ratio D B1 = D B2 = D B, the output voltage V O1 = V O2 = V O, and the load current I O1 = I O2 = I O V O = 1 1+ V S 4n 1+ 2R OD 2 B T S L B (1) where T S is the switching cycle, and R O is the equivalent load resistance (= V O /I O ). To normally operate the proposed circuit, the integrated boost converter should be operated with DCM in half of the switching cycle T S /2. Therefore, duty ratio D B and boost inductance L B should be designed small enough. The maximum value of D B,

KIM et al.: ISOLATED SWITCH-MODE CURRENT REGULATOR WITH INTEGRATED TWO BOOST LED DRIVERS 4651 Fig. 3. Operational key waveforms of the proposed regulator. Fig. 5. Experimental waveforms. (a) Transformer primary voltage and current. (b) Gate signal, drain-to-source voltage, and current of Q P 1. (c) Gate signal, drain-to-source voltage, and current of Q P 2. Fig. 4. Equivalent circuit of the proposed regulator. (a) During the mode from 1 to 4. (b) During the mode from 5 to 8. i.e., D B_MAX, and that of L B, i.e., L B_MAX, are obtained as follows: D B_MAX 1 ( 1 V ) S (2) 2 2nV O L B_MAX 2R ODB_MAX 2 T S ( ) 2. (3) 4nVO V S 1 1 For example, if V S =400, V O =250 V, R O =833.3 Ω, n =1, and T S =11.1 μs, then D B_MAX =0.1 and L B_MAX =148 μh. To extend the multistring over two LED strings, the same secondary circuits are simply added using multiple secondary windings according to the number of LED ones. However, the proposed converter can be only extended to even number of LED strings with the same structure. III. EXPERIMENTAL RESULTS To verify the operation and analysis of the proposed circuit, the prototype is implemented with specification of 400-V input from the output of the PFC converter and 150-W output prototype with two 75-W/0.3-A LED strings. Transformer = PQ3220, L m =1mH, L B = 100 μh, N P = N S =40, Q P 1, Q P 2, Q S1, Q S2 = IPP60R385, D S1, D S2 =10ETF06 C O1,

4652 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 61, NO. 9, SEPTEMBER 2014 and Q P 2, respectively. Both switches are turned on with ZVS by using the transformer magnetizing current. The dead times between the primary switches are about 420 ns. In addition, the voltages of the primary switches are clamped to 400 V. Fig. 6 shows the experimental waveforms in the secondary side. Fig 6(a) shows v sec and i sec. The voltage stresses of the secondary semiconductors are each output voltage about 250 V. The boost inductor L B is operated in DCM, and diodes D S1 and D S2 are turned off with ZCS. Fig. 6(b) and (c) shows the ZVS waveforms of secondary switches Q S1 and Q S2, respectively. Both switches are turned on with ZVS. Finally, each LED current is balanced, as shown in Fig. 6(d), and the efficiency of the proposed circuit is measured about 96.2% at 150-W output. IV. CONCLUSION A new isolated switch-mode current regulator has been proposed for an LED driving system. The operational principle and feature are illustrated in this paper. The conventional two boost LED drivers are integrated, which results in a simple structure and low component count. Furthermore, all semiconductors have the voltage stress of the input voltage or output voltage, and all switches can easily achieve ZVS without additional auxiliary circuits. The validity of the basic operational principle is confirmed by the experiment with a 150-W prototype with two 75-W LED strings. From the experimental results, the proposed regulator has high efficiency. Fig. 6. Experimental waveforms (a) v sec and i sec. (b) Gate signal, drain-tosource voltage, and current of Q S1. (c) Gate signal, drain-to-source voltage, and current of Q S2. (d) LED currents I O1 and I O2. C O2 = 220 μf, and f S =90kHz. Fig. 5 shows the experimental waveforms in the primary side. Fig 5(a) shows the transformer primary voltage and current. The transformer primary voltage is symmetric with 200 V due to the same duty ratio of the primary switches. The duty ratio D B is about 0.08. Fig. 5(b) and (c) shows the ZVS waveforms of the primary switches Q P 1 REFERENCES [1] H. J. Chiu and S. J. Cheng, LED backlight driving system for large-scale LCD panels, IEEE Trans. Ind. Electron., vol. 54, no. 5, pp. 2751 2760, Oct. 2007. [2] Y.-K. Lo, K.-H. Wu, K.-J. Pai, and H.-J. Chiu, Design and implementation of RGB LED drivers for LCD backlight modules, IEEE Trans. Ind. Electron., vol. 56, no. 12, pp. 4862 4871, Dec. 2009. [3] X. Qu, S.-C. Wong, and C. K. Tse, Temperature measurement technique for stabilizing the light output of RGB LED lamps, IEEE Trans. Instrum. Meas., vol. 59, no. 3, pp. 661 670, Mar. 2010. [4] L. Burgyan and F. Prinz, High efficiency LED driver, U.S. Patent 6 690 146, Feb. 10, 2004. [5] M. Doshi and R. Zane, Digital architecture for driving large LED arrays with dynamic bus voltage regulation and phase shifted PWM, in Proc. IEEE APEC, 2007, pp. 287 293. [6] Y. Hu and M. M. Jovanovic, LED driver with self-adaptive drive voltage, IEEE Trans. Power Electron., vol. 23, no. 6, pp. 3116 3125, Nov. 2008. [7] J.-J. Yun, H.-J. Choe, Y.-H. Hwang, Y.-K. Park, and B.-K. Kang, Improvement of power-conversion efficiency of a DC DC boost converter using a passive snubber circuit, IEEE Trans. Ind. Electron.,vol.59,no.4, pp. 1808 1814, Apr. 2012. [8] S.-H. Park, G.-R. Cha, Y.-C. Jung, and C.-Y. Won, Design and application for PV generation system using a soft-switching boost converter with SARC, IEEE Trans. Ind. Electron., vol. 57, no. 2, pp. 515 522, Feb. 2010. [9] M. Arias, D. G. Lamar, F. F. Linera, D. Balocco, A. A. Diallo, and J. Sebastian, Design of a soft-switching asymmetrical half-bridge converter as second stage of an LED driver for street lighting application, IEEE Trans. Power Electron., vol. 27, no. 3, pp. 1608 1621, Mar. 2012. [10] M. Arias, D. G. Lamar, J. Sebastian, D. Balocco, and A. A. Diallo, Highefficiency LED driver without electrolytic capacitor for street lighting, IEEE Trans. Ind. Appl., vol. 49, no. 1, pp. 127 137, Jan./Feb. 2013. [11] W. Chen and S. Y. R. Hui, A dimmable light-emitting diode (LED) driver with mag-amp postregulators for multistring applications, IEEE Trans. Power Electron., vol. 26, no. 6, pp. 1714 1722, Jun. 2011.

KIM et al.: ISOLATED SWITCH-MODE CURRENT REGULATOR WITH INTEGRATED TWO BOOST LED DRIVERS 4653 power supplies. Jae-Kuk Kim (S 08) received the B.S. degree from Inha University, Incheon, Korea, in 2004 and the M.S. and Ph.D. degrees from the Korea Advanced Institute of Science and Technology, Daejeon, Korea, in 2007 and 2011, respectively, all in electrical engineering. Since 2011, he has been a Senior Engineer with Samsung Electro-Mechanics Company, Ltd., Suwon, Korea. His research interests are in power electronics, including analysis, modeling, control methods, power factor correction, LEDs, adapters, and server Jae-Bum Lee (S 12) was born in Korea in 1983. He received the B.S. degree in electrical engineering in 2010 from Korea University, Seoul, Korea, and the M.S. degree in electrical engineering and computer science in 2012 from the Korea Advanced Institute of Science and Technology, Daejeon, Korea, where he is currently working toward the Ph.D. degree. His main research interests are dc/dc converters, ac/dc converters, soft-switching techniques, server power supplies, and digital control methods. Gun-Woo Moon (S 92 M 00) received the M.S. and Ph.D. degrees in electrical engineering from the Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, in 1992 and 1996, respectively. He is currently a Professor with the Department of Electrical Engineering, KAIST. His research interests include modeling, design, and control of power converters, soft-switching power converters, resonant inverters, distributed power systems, power factor correction, electric drive systems, driver circuits of plasma display panels, and flexible ac transmission systems. Dr. Moon is a member of the Korean Institute of Power Electronics, Korean Institute of Electrical Engineers, Korean Institute of Telematics and Electronics, Korea Institute of Illumination Electronics and Industrial Equipment, and Society for Information Display.