A CLCL Resonant DC/DC Converter for Two-Stage LED Driver System

A CLCL Resonant DC/DC Converter for Two-Stage LED Driver System 1 K. NAGARAJU, 2 K. JITHENDRA GOWD 1 PG Scholar, Dept. of Electrical Power System (EPS), Jawaharlal Nehru Technological University, Anantapuramu, AP. 2 Assistant Professor, Dept. of Electrical and Electronics Engineering (EEE), Jawaharlal Nehru Technological Abstract:- A CLCL resonant DC/DC converter has been proposed for two-stage LED drivers. The circuit performs zero voltage switching turn-on and quasi-zero current switching turn-off. Then, a twostage system has been designed using a power factor correction circuit before the proposed converter. Optimum input impedance angle, dead time and components parameters have been achieved after thoughtful design, thus obtaining good soft-switching performance and reduced voltage stress. A 100 W prototype has been realized and tested demonstrating its high feasibility and efficiency at full load and during dimming operations. I. INTRODUCTION In virtue of their excellent light intensity per Watt, very long lifetime, eco-friendliness and good color rendering, Light-Emitting Diodes (LED), are quickly replacing previous artificial light sources in indoor and street lighting, display backlighting, lighting in plant factory and automotive applications. LEDs require constant current which can be supplied by an electronic circuit commonly known as driver which usually consists of either a two-stage circuit (the most common) or a single-stage circuit. Though single-stage drivers use less number of switches, they have some disadvantages, such as high voltage stress and very wide variation of bus voltage. In the twostage drivers, for the PFC stage and the DC/DC stage working separately, the bus voltage can be regulated to a relatively constant value which is independent of the load and the input voltage in two-stage structure, resulting high power factor and low total harmonic distortion. Due to these concerns, two-stage structure is adopted in the proposed LED driving system. For DC/DC circuits in two-stage drivers, many researchers choose fly back topology, which is simple because only one switch and one transformer are needed. With this topology, cost and complexity can be further minimized through primary side current sensing technology. However, its efficiency is not very high because of high switching losses. Reference proposed quasi-resonant fly back LED drivers, in which the switching losses decrease; however, the switch still did not work in a real zero-voltage-switching (ZVS) state, and the system efficiency results not very high. In order to decrease switching losses, many researchers focused their attention on resonant University, Anantapuramu, AP. topologies with soft switching capability. The authors of [23] proposed a LED driver with Class D resonant converter, where primary side switches turn-on in ZVS mode, but the secondary diodes turnoff in hardswitching mode, thus resulting high turn-off losses and high voltage stress. Those of [24] proposed a LED driver with CLL resonant converter. The primary side switches can turn-on in ZVS mode, thus efficiency was improved. In [25], LCL type and CLC type resonant tanks were analyzed and an excellent current balancing system was proposed based on them. LLC resonant circuit has been a widely used circuit in LED drivers because of its soft switching feature, resulting high efficiency [26]-[28]. However, LLC resonant circuit can only allow that the primary side switches turn-on in ZVS mode, while the turn-off current of the switches is large which means that switches cannot turn off in ZCS mode. In order to improve efficiency, operating frequency at full load has to be chosen no more than the resonant frequency, thus, primary side switches turn-on in ZVS mode and secondary side diodes turnoff in ZCS mode [26]. Under this condition, circulating current reduces, too. However, when the input voltage increases or load reduces, operating frequency becomes higher than resonant frequency, thus ZCS operation of secondary side diodes is lost and turn-off losses increase. Two-stage LED driver is proposed in this paper, consisting of a boost circuit operating as PFC front-end and a novel CLCL resonant converter. At full load operating condition, MOSFETs at primary side turn-on in ZVS mode and turn-off at extremely low current condition, which is named quasi-zcs mode. Also the ZVS turn-on characteristics of the switches and the ZCS turn-off characteristics of diodes are ensured both at full load and during dimming, thus resulting reduced power losses and increased overall efficiency. II PROPOSED CLCL CONVERTER Fig. 1. Proposed CLCL resonant converter Page No:330

Fig. 1 shows the proposed half-bridge CLCL converter. Q1 and Q2 are the two switches, Cr and Lr are in series; the series branch, Cp and Ls form a π - type network. Lm is the magnetizing inductor, and Rled is the equivalent impedance of the LED load. Fig. 2 shows the main voltage and current waveforms obtained through proper operations, which consists of ten distinct modes as shown in Fig. 3 and described in this section. ils and ilm reverses, so on the secondary side, the diode D1 turns on, then Lm voltage begins to be clamped to nvo by the output voltage. The current ilm starts to increase positively, Cp keeps charging. At t3, the current ils resonates to zero, then this mode ends. Mode 4: (t3 t4): Inductor Lr discharges and Ls charges, the current ils begins to increase, the current ilm keeps increasing, the resonant current icp decreases. At t4, the current icp resonates to zero and the voltage Ucp reaches the peak value in the positive direction. Mode 5: (t4 t5): Cp begins to discharge, the current icp resonates to the negative direction, capacitor Cr keeps charging, the resonant current of Ls and secondary diode D1 both increase, the resonant current icr is decreasing. Until t5, the driving signal applied to Q1 disappears and the current icr resonates to almost zero, so the turn-off current of Q1 is extremely low and the switch turns off in quasi-zcs mode. The next half period working modes are not detail introduced here. Fig: 2 Voltage and current waveforms of the proposed CLCL DC/DC converter Mode 1: (t0 t1): At t0, Q2 turns off, the resonant current icr decreases negatively, capacitor Cds2 is charging and capacitor Cds1 is discharging. On the secondary side, D2 keeps turned on, the voltage of Lm is clamped by the output voltage. The current icr resonates decreasing and the drain-source voltage of Q1decreases to be zero. Then, the current flows through the body diode contained in Q1. So, it can turn on in ZVS mode and this mode ends at t1. Mode 2: (t1 t2): At instant t1, Q1 turns on in ZVS mode and at the same time, Ucr reaches its minimum value. Then, the current icr increases forward, Cr and Lr begin to be charged. On the secondary side, D2 keeps turning on, the voltage across Lm terminals is still clamped to the voltage - nvo, so that ilm decreases lineally, Cp is charging, the current ils resonates to zero in the negative region, when ilm equals to ils, the current transferring to the secondary side is zero, the diode D2 turns off in ZCS mode, at this time, ilm reaches the maximum value in the negative direction and this mode ends. Mode 3: (t2 t3): A t2, the current flowing across Ls keeps increasing towards zero, the difference between Fig: 3 Working modes of the proposed CLCL DC/DC converter A Analysis And Design of The CLCL Converter The analysis of ZVS and Quasi-ZCS features Fig.4 shows the equivalent circuit of the CLCL resonant converter based on the fundamental wave analysis method. Transformer leakage inductors of the primary side and secondary side are neglected. The equivalent fundamental input voltage is: The output voltage transferred to the primary side can be calculated as follows For LED loads, using the equivalent linear model, the primary equivalent resistance can be calculated as follows Page No:331

same rate. According to the above analysis, follows that Where Vth is the threshold voltage, Rd is the slope of the I-V curve and n is the transformer turns ratio. From equation (3) it can be found that Re is no longer a constant resistor, but a function of Io, i.e. the LED forward current. As shown in equation system's input impedance Zeq can be calculated according to Fig. 4. The angle of system input impedance can be calculated as shown in equation (5) where Ideally, when fr = fs and the impedance angle φ=0, input current is the same phase with the input voltage and switches can work in ZVS turn-on and ZCS turn-off mode ignoring dead time. Also, working frequency needs to be close to system's resonant frequency. However, in such a situation switches cannot turn-on or turn-off instantaneously, hence, a certain dead time has to be ensured. In practical situations, the impedance angle φ is set a little bit more than zero, so switches can turn-on in ZVS mode and turn-off at very low current condition, here named quasi-zcs mode. According to Fig. 3 and working mode analysis, when system impedance is Zeq and the impedance angle is φ, the resonant current can be calculated as follows Fig: 5 Equivalent circuit during the dead time The analysis of impedance angle and system gain At this frequency, the input impedance can be seen as resistive. Fig. 6a shows the curves of the input impedance angle with Cp and Ls changing when Cr=150 nf. Fig. 6b shows similar curves when Cr =50 nf. It can be seen from these two figures that when Cp is larger, the input impedance angle is almost uncorrelated with Ls and the angle keeps at high value, which cannot satisfy system demand, hence, Cp should not be very large. However, when Cp is small, with Ls variations, the angle will enter into the negative value region, which makes the system working in a capacitive zone and cannot realize ZVS feature. So, in order to avoid a change of φ; due to Ls deviation, the value of the capacitor Cp cannot be too small. Comparing Fig. 6a and Fig. 6b, it can be observed that when taking both the same values of Cp and Ls in the two figures, the smaller the value of Cr is, the smaller the input impedance angle is. Fig: 4 Equivalent circuit of the CLCL resonant converter Fig: 6 Loci of angle φ with Cp and Ls Generally, for LED driving systems, output current is often selected as the controlled object. From equation the curves of current gain can be obtained as shown in Fig. 7, where Mi is set: Mi =1/Mv when the efficiency is set equal to 100% in design procedure. Where ton1 = 0.5Ts t dead. To guarantee turn-on in ZVS mode during the whole dimming process, a suitable dead time should be identified. The equivalent circuit in dead time can be obtained as shown in Fig. 5 capacitor Cds1 is discharging and Cds2 is charging, while assuming Cds1 = Cds2, the voltage of the two capacitors changes linearly at a Fig. 7. The curves of system current gain with load changing Page No:332

Assuming constant current gain, the relationship between load changes and corresponding frequency variations can be seen: the proposed system own a small frequency variation which is expected in the converter design. The working range of the converter is within the left part, thus with frequency lower than fr. There is also another condition for the LED driver, which is the output current kept constant while the output voltage changes. This makes the driver compatible with LEDs from different vendors. The analysis shows that, when the output voltage changes to 60 % and the output current is kept constant, the variation of operating frequency can be reduced up to around 5%, which is a very narrow range and suitable for LED driver. The relationship between the current gain, Cp and Ls when Cr =150 nf is shown in Fig. 8a. Curves in Fig. 8b show what does occur when Cr goes to 50 nf. As shown in Fig. 8a, current gain changes rapidly in those regions where Cp and Ls both take small values. It means that little deviations cause significant changes of system operating point, which was not expected during system design. From Fig. 8b it can be seen, that current gain Mi forms a linear relationship between Ls and Cp. However, the slope of current gain curve is higher than the value found inside the linear region shown in Fig. 4.8a. So, when Cr takes small values, a limited change of Cp or Ls greatly affects operating point. function of control to output current in the two-stage LED driver. Fig.10 shows current and drain-source voltage waveforms for Q2. The switches turn-on in ZVS mode and turn off at very low current (quasi-zcs mode) which is in agreement with analysis. Fig: 10 Waveforms of the current and drain-source voltage of Q2 Fig. 11 shows current and voltage waveforms of the secondary diode D1 at the full load condition. The secondary side diode works in the ZCS state; working frequency is 100 khz, which is in agreement with analysis. Fig: 4.8 The main circuit and control circuit of the proposed two stage LED driving system III SIMULATION RESULTS Fig: 9 Matlab Simulink Diagram Since the CLL resonant converter is the load of the first stage buck converter, the transfer function of control to bus voltage in the first stage can be derived with Z s in, which is the input impedance of CLL resonant converter. Combine with the transfer function of bus voltage to output current, the transfer Fig: 11 Waveforms of the voltage and current of the secondary diode D1 IV CONCLUSION A CLCL DC/DC resonant converter for a 100 W two-stage LED driving system has been proposed in this paper. The novel LED driver offers satisfactory soft switching characters and limited losses in switches and diodes, thus resulting high efficiency. After design analysis, a prototype has been realized and successfully tested in laboratory. CLL resonant converter is proposed for the multiple LED strings application. Its voltage gain characteristic and the operation principle are similar to the LLC s. However, its unique characteristics are investigated for driving LEDs. Larger magnetizing inductance Lm is good for current balance among LED strings, especially when the load is unbalanced. ZVS of primary side witches could be achieved by designing Lr1 properly. Voltage gain of CLL at resonant frequency point is higher than one, which is useful for the voltage step-up application as well. After investigate the characteristic of MC3 CLL resonant converter, a prototype of two-stage LED Page No:333

driver with CLL resonant converter as the second stage is carefully designed. The complete design procedure of MC3 CLL resonant converter is presented. The key point of the design procedure is how to design the resonant tank. First of all, the turns ratio of the transformer is determined according to the specification of the LED driver. Then the primary side switches, dead time td and Lp are set in order to achieve the lowest conduction loss. Finally, Lr1 and Le2 are ascertained on account of the ZVS requirement. Cr is determined based on the resonant frequency. In the design process, the junction capacitors of the secondary-side rectifiers have significant influence on the ZVS achievement of the primary-side switches. Since the rectifiers junction capacitors resonate with Le2 and output capacitors of the primary-side switches during dead time, it must be taken into account with the voltage step-up transformer. Future Scope Beside voltage mode control, other advanced control strategies could be applied to this two-stage LED driver. For buck converter, current mode control is a good candidate to achieve better control performance. For CLL, the switching frequency could be fine-tuned around the resonant frequency according to the load condition to find the best efficiency point. REFERENCES [1] N. Narendran and Y. Gu, Life of LED-ased white light sources, Journal of Display Technology, vol. 1, no. 1, pp. 167-171, Sep. 2005. [2] J. Y. Tsao, Solid-state lighting: Lamps, chips and materials for tomorrow, IEEE Circuits Devices Mag., vol. 20, no. 3, pp. 28 37, May 2004. [3] X. Qu, W. S. Chung, and C. K. Tse, Ballast for independent control of multiple LED lamps, in Proc. IEEE ECCE, 2009, pp. 2821-2826. [4] 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. [5] C.-C. Chen, C.-Y. Wu, Y.-M. Chen, and T.-F. Wu, Sequential color LED backlight driving system for LCD panels, IEEE Trans. Power Electron., vol.22, no. 3, pp. 919 925, May 2007. [8] B. White, Y. Liu, H. Wang, X. Liu, An Average Current Modulation Method for Single Stage LED Drivers with High Power Factor and Zero Low Frequency Current Ripple, IEEE Jour. Emerg. Sel. Topics Power Electron. online doi: 10.1109/JESTPE.2015.2424680 [9] J.R. de Britto, A.E. Demian, L.C. de Freitas, V.J. Farias, E.A.A. Coelho, J.B. Vieira, A proposal of Led Lamp Driver for universal input using Cuk converter, in Proc. IEEE PESC, pp.2640-2644, Jun. 2008 [10] T. Gao, Q. Wang, Y. Yang, Z. Yan, High power factor LED power supply based on SEPIC converter, Electronics Letters, vol.50, no.24, pp.1866-1868, Nov. 2014 [11] D. Gacio, J. M. Alonso, J. Garcia, L. Campa, M. J. Crespo, and M. Rico-Secades, PWM series dimming for slow-dynamics HPF LED drivers: the high-frequency approach, IEEE Trans. Ind. Electron., vol. 59, no. 4, pp. 1717-1727, Apr. 2012. [12] S. Y. Chen, Z. R. Li, and C. L. Chen, Analysis and design of single-stage AC/DC LLC resonant converter, IEEE Trans. Ind. Electron., vol. 59, no. 3, pp. 1538 1544, Mar. 2012. [13] Y. Wang, Y. Guan, X. Zhang, and D. Xu, Single-stage LED driver with low bus voltage, Electronics Letters, vol. 49, no. 7, pp. 455 456, Mar. 2013. [14] Y. C. Li, and C. L. Chen, A Novel Primary-Side Regulation Scheme for Single-Stage High-Power- Factor AC DC LED Driving Circuit, IEEE Trans. Ind. Electron., vol. 60, no. 11, pp. 4978-4986, Nov. 2013. [15] C.-H. Chang, C.-A. Cheng, H.-L. Cheng, and C.- F. Lin, Analysis and design of a novel interleaved single-stage LLC resonant AC-DC converter, in Proc. Conf. Power Electron. and Drive Systems, 2013, pp.602-607. [6] A. Leon, H. Valderrama-Blavi, J. Bosque- Moncusi, L. Martinez- Salamero, A High Voltage SiC-based Boost PFC for LED Applications, IEEE Trans. Power Electron., online doi: 10.1109/TPEL.2015.2418212 [7] X. Xie, J. Wang, C. Zhao, Q. Lu, S. Liu, A Novel Output Current Estimation and Regulation Circuit for Primary Side Controlled High Power Factor Single- Stage Flyback LED Driver, IEEE Trans. Power Electron., vol.27, no.11, pp.4602-4612, Nov. 2012 Page No:334