LETTER IEICE Electronics Express, Vol.10, No.13, 1 7 LED lamp driving technology using variable series-parallel charge pump Jeongduk Ryeom a) Department of Electrical Engineering, Soongsil University, 369 ngdo-ro, Dongjak-ku, Seoul 156 743, Korea a) cosmos01@ssu.ac.kr Abstract: A novel high power light-emitting diode lamp driving technology that uses a reverse charge pump circuit and digital dimming method instead of a conventional DC-DC converter is proposed to obtain a high circuit efficiency at all dimming levels. In this method, the voltages of all capacitors are kept constant by the period in which the capacitors are connected in parallel. From the experimental results, the circuit efficiency was kept constant irrespective of the dimming level and lighting condition. In addition, an average circuit efficiency of 91.8% was obtained when all of the light-emitting diode lamps were turned on at the same dimming level. Keywords: circuit efficiency, driving scheme, dimming, LED lighting Classification: Electron devices, circuits, and systems References [1] C. Wang, W. S. Lee, F. Zhang and N. Y. Kim: Electron. Lett. 46 [17] (2010) 1220. [2] C. H. Liu, C. Y. Hsieh, Y. C. Hsieh, T. J. Tai and K. H. Chen: IEEE Trans. Circuits Syst. I, Reg. Papers 25 [2] (2010) 1384. [3] X. O. Qu, S. C. Wong and C. K. Tse: IEEE Trans. Power Electron. 25 [2] (2010) 331. [4] H. Broeck, G. Sauerlander and M. Wendt: IEEE Applied Power Electronics Conference 2007 (2007) 1319. [5] X. Long, R. Liao and J. Zhou: IET Optoelectronics 3 [1] (2009) 40. [6] J. A. Starzyk, Y. W. Jan and F. Qiu: IEEE Trans. Circuits Syst. I, Fundam. Theory Appl. 48 [3] (2001) 350. 1 Introduction Recently, the conventional incandescent, halogen, and fluorescent lamps have been replaced with light-emitting diode (LED) lamps [1]. As a result, various studies have been conducted on the high efficiency of the driving circuit for LED lamps [2, 3]. In general, the operating voltage of an LED ranges from 2 to 4 V. Therefore, when a commercial power source is used, a DC-DC converter (which appropriately reduces the DC in accordance with the LED operating voltage) is required. The buck converter has been used 1
for current LED lamp converters as well [4]. However, a change in the dimming level for LED lamps entails a change in the load factor. The efficiency of the buck converter varies from approximately 80% to 90%, depending on the load factor [5]. In this paper, a novel driving technology for high power LED lamps using the lightings is proposed. The proposed driving circuit applies a charge pump reversely instead of a conventional DC-DC converter to improve the circuit efficiency degradation triggered by the dimming level change. In addition, a digital dimming control method of achieving a higher efficiency is also proposed in lieu of a conventional pulse width modulation (PWM) dimming method. 2 Principle of variable series-parallel charge pump for LED lamp driving A charge pump circuit is mainly used for obtaining a voltage higher than the source voltage by charging and discharging voltage with multiple capacitors connected in series [6]. Conversely, the charge pump circuit is used for reducing the input voltage in this study. The proposed LED lamp driving circuit has a charge pump structure with three capacitors C 1, C 2, and C 3 connected in series, as well as three LED lamps LED 1, LED 2 and LED 3 that are turned on by using the electric energy charged in the respective capacitors as shown in Fig. 1. Fig. 1(a) shows the charging period obtained by connecting the capacitors C 1 C 3 in series. In this period, the current flows along the dotted line1. The voltage obtained by subtracting Fig. 1. Schematic diagrams of proposed LED lamp driving circuit (a) Charging period and discharging period with current flow (b) Parallel connection period with current flow 2
the sum of the forward voltage drops across the diodes D 1 D 3 from the input voltage V S is charged to the three capacitors C 1 C 3 (about 1/3 voltage each). Fig. 1(a) also shows the discharging period in which each capacitor C 1 C 3 discharges electricity. In this period, the flow of current is shown by the dotted lines labelled2. In the Fig. 1(a), finally, owing to the charging period, each capacitor becomes an independent power source, and electrical power is supplied to each LED lamp from this capacitor during the discharge period. At this point, each LED lamp is independently controlled for dimming light by field-effect transistor (FET) switches M 3 M 5. If the timing of the on state for LED lamps do not coincide, some of the capacitors will be discharged and the others will not be discharged during the same period. When only one of three LED lamps is on state in Fig. 1(a), after the k th charging period, V ON and V OFF, the voltages of the capacitors for the LED lamp turned on and with the LED lamp turned off, are expressed as in Eqs. (1) and (2), respectively. Here, the LED lamp is equivalent to the resister R and LED turn-on voltage V LED and the voltage drops by the diode D 1 D 3 and FET switches M 3 M 5 are ignored. V S represents the source voltage, C represents capacitance of capacitor, and T ON represents the turn-on time of LED lamp. Also, it is assumed that (V S sum of capacitor voltage)/3 is uniformly charged to each capacitor during the charging period. If this charging and discharging processes are infinitely repeated, finally the Eqs. (1) and (2) have become Eqs. (3) and (4) respectively. Therefore, the voltage of the capacitor that sustains the LED lamp in the on state decreases down to the turn-on voltage of LED and the voltages of the other capacitors that sustain the LED lamp in the off state exceed V S /3. V k ON ¼ V S 3 V LED 1 þ 2 e T ON RC 3! k 1 þ V LED; k 2 (1) VOFF k ¼ V S 3 þ 1 V S 3 3 V LED 1 e T ON RC! Xk 1 þ 2 e T ON i 2 RC ; k 2 3 i¼2 (2) lim k!1 V k ON ¼ V LED (3) lim V OFF k ¼ V S k!1 3 þ 1 2 V S 3 V LED (4) Eventually, if the charging and discharging periods were continuously repeated in this state, the voltage of the capacitors would become highly non-uniform, and thus, this makes the dimming control of LED lamp difficult. To prevent this problem, the capacitors C 1 C 3 have been designed to be all connected in parallel by using FET switches M 6 M 9, right after the discharging period when the LED lamps are turned on or off. Fig. 1(b) shows a parallel connection period. During the parallel connection period, the current flows from the high-voltage capacitor to the low-voltage 3
capacitor as shown on dotted line3, and thus, the voltage of each capacitor becomes about the same. The timing chart of the FET switches used in the proposed driving circuit is shown in Fig. 2. The main period T is set to 8.96 ms (=115 Hz) to prevent flicker in the light emitted from the LED lamps. The main period comprises multiple sub-periods included the capacitor charging, discharging and parallel connecting periods. In the timing chart, t 1 denotes the period in which each capacitor is connected in parallel, and the pulse width of t 1 is 8 s. In the chart, t 2 denotes the period for charging the capacitors connected in series, and the pulse width thereof is 20 s. Here, t 3 denotes the capacitor discharging period in which the LED lamps are turned on, and the pulse width thereof is 28 s. If the dimming level is D 0 (=1/255) or D 1 (=2/255), the charged voltage in the capacitor during t 2 is supplied to the LED lamp during t 3 or 2 t 3, respectively. When the dimming level is D 2 (=4/255), the capacitors are charged during t 2 and discharged during 4 t 3 to turn on the LED lamps. This sub-period of D 2 is repeated to turn on the LED lamps for all dimming levels other than D 0 and D 1. For instance, as shown in the figure, sub-period is repeated twice for dimming level D 3 (=8/ 255), and for the final period when the dimming level is D 7 (=128/255), this is repeated 32 times. Fig. 2. Timing chart for FET switches used in proposed circuit (a) FET M 6 M 9 (b) FET M 1 (c) FET M 2 (d) FET M 3 M 5 Each LED lamp is independently controlled by the FET switches M 3 M 5 according to the 8-bit digital control signals. Each bit from the lowest to the highest of the 8-bit control signal corresponds to a lighting time D 0, D 1,..., D 7 of the LED lamp. The dimming of LED lamps is controlled in 256 levels by combining these bits. For example, when the digital input is 0000 0001 2, the LED lamps are turned on for a time period corresponding to the dimming level of D 0 (=1/255). If the input is 0000 0010 2, the lamps are turned on for the time period of D 1 (=2/255). In addition, if the digital input is 0000 0011 2, the lamps are on for D 0 +D 1 (=3/255) times. In addition, in the case of the highest value, which is 1111 1111 2, the lamps are on for D 0 +D 1 +...+D 7 (=255/255). 4
3 Experimental results and discussions Table I shows the circuit constants using the experiments. The value of each capacitor C 1 C 3 is 1000 F and resistor R 1 R 3 inserted in parallel with the capacitor is 10 k respectively. The switch timing pulses applied to the FET gate are generated by using FLEX 10K (FPGA by ALTERA Corp.). Fig. 3(a) shows the voltage and current waveforms of the capacitors when the LED lamp is turned off. The current escaping to the other capacitors when they are connected in parallel is observed from the figure. Fig. 3(b) shows the voltage and current waveforms of the capacitor when the lamp is turned on. The current entering from the other capacitors at the parallel connection is seen in the figure. Accordingly, as shown in the voltage waveforms of the two capacitors in Figs. 3(a) and (b), there is no substantial difference in the voltages between the capacitors when the lamp is turned on and then off after the lighting period is completed. Therefore, even though each LED lamp is individually operated in the proposed circuit, the capacitor voltages are kept uniform without significant differences. Table I. Circuit constants using experiments Fig. 3. Measured voltage and current waveforms of capacitor when (a) LED lamp is turned off and (b) LED lamp is turned on Table II shows the measurement results of luminous flux and circuit efficiency when 50% (=128/255) of dimming level is realized by driving three LED lamps in five different cases of dimming levels. According to the table, the luminous flux is constant at about 230 lm with five different cases of bit codes. From the results, 92% of circuit efficiency was achieved almost consistently even when three LED lamps were turned on in different timings. Here, the efficiency of the driving circuit is calculated by measuring the input wattage and the power consumption of the LED lamps. Because power consumption of the FET switching control circuit varies depending on its structure and is very small compared to that of LED lamps, the power consumption of the control circuit is excluded in this experiments. Fig. 4 shows the measurement results for the luminous flux and circuit 5
Table II. Measurement results (50% dimming level) efficiency at each dimming level when the three LED lamps are turned on with the same dimming level. From the results, the proposed driving circuit has a constant circuit efficiency, irrespective of the dimming level for the LED lamps. On average, a circuit efficiency of 91.8% was obtained. The proposed digital dimming control method divides a main period into 65 sub-periods. Under these conditions, the discharge time of capacitor is reduced, which enables the capacitor to become nearly the constant-voltage power source. Therefore, a low rush current is generated during the capacitor charging period as shown in Fig. 3(b) and high circuit efficiency is obtained. One disadvantage of the proposed LED lamp driving circuit is that it cannot turn on the LED lamp in either the capacitor-charging period or the period when the capacitors are connected in parallel. Thus, further studies are required to develop a circuit that does not have such time loss. Fig. 4. Measured luminous flux and circuit efficiency at each dimming level 4 Conclusion A novel driving technology is proposed for dimming control of high power LED lamps using a charge pump reversely. The proposed driving technology makes it possible to reduce the input voltage for the operating voltage of LED lamps without using a conventional DC-DC converter, and to individually control the dimming of each LED lamp by digital method. Moreover, even though each LED lamp is individually operated, the voltages of all capacitors are kept constant by utilizing a period in which the capacitors are connected in parallel. From the experimental results, the circuit efficiency was kept constant irrespective of the dimming level and 6
the lighting condition of LED lamps. In addition, an average circuit efficiency of 91.8% was obtained when all of the LED lamps were turned on at the same dimming level. Acknowledgments This research was supported by the MSIP, Korea, under the CITRC support program (NIPA-2013-H0401-13-1002) supervised by the NIPA. 7