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Highly AC Voltage Fluctuation-Resistant LED Driver with Sinusoid-Like Reference
Highly AC Voltage Fluctuation-Resistant LED Driver with Sinusoid-Like Reference
Journal of Power Electronics. 2014. Mar, 14(2): 257-264
Copyright © 2014, The Korean Institute Of Power Electronics
  • Received : November 03, 2013
  • Accepted : January 03, 2014
  • Published : March 30, 2014
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About the Authors
Ning Ning
State Key Laboratory of Electronic Thin Film and Integrated Device, University of Electronic Science and Technology of China, Chengdu, China
ning_ning@uestc.edu.cn
Zhenxiao Tong
State Key Laboratory of Electronic Thin Film and Integrated Device, University of Electronic Science and Technology of China, Chengdu, China
Dejun Yu
State Key Laboratory of Electronic Thin Film and Integrated Device, University of Electronic Science and Technology of China, Chengdu, China
Shuangyi Wu
State Key Laboratory of Electronic Thin Film and Integrated Device, University of Electronic Science and Technology of China, Chengdu, China
Wenbin Chen
State Key Laboratory of Electronic Thin Film and Integrated Device, University of Electronic Science and Technology of China, Chengdu, China
Chunyi Feng
State Key Laboratory of Electronic Thin Film and Integrated Device, University of Electronic Science and Technology of China, Chengdu, China

Abstract
A novel converter-free AC LED driver that is highly resistant to the fluctuation of AC voltage is proposed in this study. By removing large passive components, such as the bulky capacitor and the large-value inductor, the integration of the driver circuit is enhanced while the driving current remains stable. The proposed circuit provides LED lamps with a driving current that can follow the sinusoid waveform to obtain a very high power factor (PF) and low total harmonic distortion (THD). The LED input current produced by this driving current is insensitive to fluctuations in the AC voltage. Users will thus not feel that LED lamps are flashing during the fluctuation. Experiment results indicate that the proposed system can obtain PF of 0.999 and THD as low as 3.3% for a five-string 6 W LED load under 220 V at 50 Hz.
Keywords
I. INTRODUCTION
LED is a photoelectric conversion element with high conversion efficiency. Given the absence of lamp filaments, this pure semiconductor luminous element has a long service life and functions as an environmentally friendly light source. With these advantages, LED has been widely used in various fields, including displays, decorations, backlights, general lighting, and urban landscape lighting [1] - [6] .
LED current determines the luminous flux output. Designing a highly reliable and efficient LED driving current is thus very important. AC-DC and DC-DC LED driving circuits [7] - [16] are widely utilized in LED lighting systems. They will also be inevitably applied to AC-DC or DC-DC converters; this condition means that large-value inductors and large electrolytic capacitors will be introduced to LED lighting systems. The lifetime of LED drivers is limited by large electrolytic capacitors. AC LED drivers can directly power LED lamps with an AC source. Given that an AC-DC or DC-DC converter is not required, the size of an LED driver is significantly decreased with high integration and its lifetime is increased. Two conventional AC LED connection structures exist. One is the inverse-parallel connection of two strings of LED directly connected to the AC source. In the other structure, the AC source directly controls the LED string after the bridge rectifier ( Fig. 1 ) [17] . Although no bulky capacitor is found in the rectifier output, both structures exhibit large current distortion and different high peak currents even under the same input voltage and average input current. An AC LED driving circuit controlled in segments was proposed in [18] , [19] . This circuit adopts a segmented driving structure to improve the circuit efficiency effectively, reduce the impact of AC source voltage fluctuation on the circuit, and consequently provide an LED with a stable driving current. However, given the stair-like input LED current, its degree of fitting with the input voltage limits the improvement of the power factor (PF) and total harmonic distribution (THD) of this structure. The driving current provided by a direct AC LED driver in [20] can greatly follow the AC source variation. However, the input current is very sensitive to the AC source fluctuation. Considering the traditional reference voltage generated from a resistor string that samples the power supply, the reference voltage encounters fluctuation along the AC source. The stability of the driving current is thus significantly affected. The fluctuation in the driving current upon that of the AC source leads to the instability of LED lamps and creates the feeling that the LED lamp is flashing.
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Bridge AC LED structure.
The traditional AC–DC and DC–DC has a low impact on LED brightness. However, LED in normal AC LED is easily influenced by voltage fluctuation. The AC LED proposed in this study can avoid the flash caused by LED current fluctuation, which is in turn generated by an AC source fluctuation. A novel reference voltage generator is applied in this driving structure. Compared with a conventional AC LED driver, this driving circuit provides a stable driving current for LED lamps without the effect of AC voltage fluctuation. At the same time, the high degree of fitting of the driving current waveform with the AC source obtains higher PF and THD in the LED system than the segment-controlled driving structure. The proposed driver, with amendments to the peripheral devices, can be applied to 110 and 220 V AC sources. Therefore, the proposed driver has excellent flexibility. This paper is organized as follows. A specific description of circuit topology is provided in Section 2, and simulation and experimental results are elaborated in Section 3.
II. ARCHITECTURE OF THE PROPOSED LED DRIVER
- A. Block Diagram of the Proposed AC LED Driver
The system block diagram of the proposed AC LED driving circuit is generated in this section. Fig. 2 shows the proposed LED driver. The LED string is connected in series to the output of the bridge rectifier, and the numbers of the input LED string increase along the ascendance of the AC source peak value. A five-string LED was adopted as the lighting unit of the LED system. Drivers 1, 2, 3, 4, and 5 of the driving current units provide the LED lamps with a constant current driver. Every driving current unit includes an operational amplifier (OPA), a current driving MOSFET ( Mn ), and a resistor soft-switching R SETn . The positive terminals of OPAs are connected to the output of the reference voltage generator, and the negative terminals are connected to the corresponding R SETn . Every driving current unit provides a segmented current for the corresponding LED strings. High-voltage transistors (i.e., HM 1, 2, 3, 4, and 5) serve as protectors of the driving circuit. The threshold voltage of HMn was set as V TH . When the AC source is extremely high, the output voltage of the driving current unit can only withstand V Limit ˗ V TH , and the extra high voltage is shared with the high-voltage transistor. The reference voltage generator provides the driving current units the referencevoltage, which can properly follow the waveform of the AC source and is insensitive to the fluctuation.
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Architecture of the proposed AC LED driver.
The first string of the LED is connected to the output voltage of the bridge rectifier, with the output voltage being V SIN . When V SIN increases from zero to V LED1 , the threshold voltage of the first string of the LED, Driver 1, is turned on. The current is then transmitted from resistor R STE1 to R SET5 by M1. At this point, the other four driving current units are closed, and the outputs of the five OPAs (i.e., V G1, 2, 3, 4, 5 ) remain at a high level. When V SIN continues to increase to V LED1 + V LED2 , the threshold voltage of the two strings of LED, Driver 2, begins to work and LED1 and LED2 are turned on. The current is transmitted through M 2 to R SET2 , R SET3 , R SET4 , and R SET5 . The current passing through R SET1 gradually decreases to zero, and Driver 1 is turned off. The output of OPA1 ( V G1 ) reaches a low level and produces a current that is approximate to zero. This driving structure does not require the use of a switch to control the working state of every driver. On the contrary, automatic switching is implemented. Therefore, no glitch occurs in the switch between every two driving current units. In the same manner, with the increase in V SIN , Drivers 3, 4, and 5 are turned on (in this order). More strings of LED are turned on accordingly. When Drivers 3 and 4 stop working, V G3 and V G4 drop to a low level, whereas V G5 remains high. As V SIN begins to decrease after its peak value, the operating status of the driving current units is symmetrical to the rising period of the voltage. The number of the lighted LED lamp strings decreases with the decrease in V SIN ; V G4 , V G3 , V G2 , and V G1 increases in order. The segment n current ( I LEDn ) of the LED lamps provided by every driving current unit is as follows:
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Considering that V REF is the reference voltage produced by the reference voltage generator that can closely follow V SIN and is not affected by the AC source, the LED driving current composed of every segmented current can properly fit V SIN ; thus, PF and THD are increased. Consequently, users would not feel that LED lamps are flashing when the AC source is unstable.
- B. Reference Voltage Generator
As shown in Fig. 3 , a novel reference voltage generator was employed in the AC LED driving circuit. The reference voltage ( V REF ) generated in this structure can well follow the half-sinusoidal waveform. Given that a separated current source exists for the charge and discharge of the capacitor, V REF is insensitive to the tremble of the AC source. The operating principle of this module is discussed in this study.
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Structure of the reference voltage generator.
The input terminals of the zero-crossing and peak comparators are simultaneously connected to the AC line voltage that has been divided by R 1 and R 2. The outputs of the zero-crossing and peak comparators are connected to two input terminals of the SR trigger. The output of the SR trigger is connected to selective switches SW 1 and SW 2 as the charge and discharge signal (CDS) of the current source. The zero-crossing comparator produces a transient high pulse when V SIN is zero. Before V SIN increases to the maximum value from zero, the peak comparator and SR trigger outputs remain low. SW 1 is then turned on and SW 2 is turned off. The source current provides the charging current for the capacitor of C Charge , and the capacitor voltage gradually increases from zero. The outputs of the five OPAs (i.e., V G1, 2, 3, 4, and 5 ) in the driving current units are connected to the comparator group. As mentioned previously, the increase in V SIN , V G1 , V G2 , V G3 , and V G4 turns from high to level in this order, whereas V G5 remains high. After going through the comparator group, these gate voltages generate a group of five bit-segmented control signal SCS i ( I = 1, 2, 3, 4, 5). When SCS i is high, current source i is connected to the circuit to provide a charging current for the capacitor. Otherwise, the current source is closed. During the process where V SIN increases from zero to the maximum value, SCS decreases from “11111” to “10000.” The total charging current changes along with the change in the V SIN gradient; thus, the output of C Charge closely follows V SIN . When V SIN reaches its maximum value, the peak comparator produces a transient high pulse. The SR trigger generates high voltage accordingly. SW 1 is then turned off and SW 2 is turned on. The current source provides a discharging current for the capacitor C Charge . With the decrease in V SIN , the value of SCS increases and the corresponding discharging current gradually increases. Reset transistor M 1 prevents the imbalance between charge and discharge of the current source from the voltage shift of the C Charge initial voltage. Therefore, the initial capacitor voltage is zero at the beginning of every cycle.
Fig. 4 is the waveform illustrative diagram of V SIN and V REF derived from AC source after the processing of the bridge rectifier and the reference voltage generator, respectively. We can divide the waveform of the figure into 5 regions. As mentioned before, from regions I to V, CDS is low; C Charge is charging; SCS turns from 11111 to 10000; the total charging current decreases; the corresponding charging voltage gradient also drops. The discharge stage is symmetrical to the charge one. The figure indicates a high fitting degree between the waveform of V REF and that of the V SIN . Considering that the current resource within the chip is directly used to charge the capacitor, V REF is also not sensitive to the AC source fluctuation.
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Illustrative diagram of VREF versus VSIN.
III. SIMULATED AND EXPERIMENTAL RESULTS
- A. Simulated Results
The simulation results of the proposed circuit are given in this subsection. The waveform of each part in the reference voltage generator is shown in Fig. 5 . Fig. 5 (a) demonstrates the waveform of CDS, which controls the charge and discharge of the current source. The change in SCS within a half cycle is shown in Fig. 5 (b). Fig. 5 (c) provides the total charging current, which is symmetrical to the discharging current. Finally, Fig. 5 (c) demonstrates V REF , which is the output voltage of the capacitor C Charge . Its voltage waveform greatly follows the half-sinusoid V SIN .
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Simulated results of the reference voltage generator.
The driving current is provided by Drivers 1, 2, 3, 4, and 5 consisting of a five-segment current. The waveform of I LED within the half cycle is shown in Fig. 6 . The switching node of every segment current coincides with the charge node of V REF . I LED and V SIN have a high fitting degree. In the simulation, the PF and THD for the five-string 6 W LED load under 220 V at 50 Hz are 0.999 and 3%, respectively. Fig. 7 shows the fluctuation of I LED along V SIN to demonstrate the impact of the AC source on I LED . The maximum value of V SIN is at ± 20%, whereas the fluctuation in I LED is only at ± 0.15%. This result indicates that the driving current is insensitive to the AC source fluctuation.
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Simulated ILED versus half-sinusoid VSIN under 220 V at 50 Hz.
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Simulated ILED versus half-sinusoid VSIN under 220 V at 50 Hz.
- B. Experiment Results
Fig. 8 shows the measurement result of V REF and V SIN after the rectifier bridge under 220 V at 50 Hz. The measurement result shows that V REF is very close to V SIN . Fig. 9 shows the waveform of driving current I LED and input voltage V SIN after the rectifier bridge. The input current fits input voltage V SIN well. The experiment results indicate that the fluctuation in input voltage V SIN is at ± 10% and the fluctuation in I LED is at ± 0.2%. In comparison with the previous structure, the reference of the driving current units was connected to a constant voltage instead of the reference voltage generator. The waveform of the driving current is shown in Fig. 10 . The current waveform is in the shape of stairs, and no obvious glitch occurs in the switch between each driving current unit. The measurement results of V SIN , sinusoid-like I LED , and stair-like I LED are obtained under 220 V at 50 Hz consuming 6 W. V SIN changes at ± 10%. Under the same power, the driving current of 110 V is twice that of 220 V. Fig. 11 shows the PF and THD of the sinusoid-like reference-equipped LED driver with different strings. The driver adopts the proposed sinusoid-like reference voltage generator. The achievable PFs are 0.997 and 0.999 for 110 V at 50 Hz and for 220 V at 50 Hz, respectively, with a five-string 6 W LED load. The THDs are 6.2% and 3.3%, respectively. The PFs and THDs for the sinusoid-like and stair-like drivers are provided in Fig. 12 . The proposed LED driver using a sinusoid-like reference voltage generator obtains high PF and low THD under the same load. Table I shows PF and THD in different conditions. The experiment results at 220 V/50 Hz and 6 W in this study compared with those in other references are summarized in Table II .
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Measurement result of the sinusoid-like reference voltage under 220 V at 50 Hz.
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Measured input voltage and driving current waveform
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Measured input voltage and stair-like current waveform.
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Power factor and THD versus the number of LED strings. Measurements were conducted at 50 Hz consuming 6 W under different AC supplies.
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PF and THD of sinusoid-like and stair-like drivers versus the number of LED strings. Measurements were conducted under 220 V at 50 Hz consuming 6 W.
EXPERIMENTS IN DIFFERENT CONDITIONS
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EXPERIMENTS IN DIFFERENT CONDITIONS
PERFORMANCE COMPARED WITH OTHER REFERENCES
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PERFORMANCE COMPARED WITH OTHER REFERENCES
Figs. 13 and 14 show the microphotograph of the fabricated IC and test circuit with an LED lamp matrix. The chip was fabricated through 0.25 μm BCD process, and the die area is 1190 μm × 1480 μm. Given the limitations of the process, the six high-voltage transistors are off-chip. Among the six high-voltage transistors, five were used to protect the driving circuit, and one was used to supply power for the chip. The resistors (i.e., R SET1 , R SET2 , R SET3 , R SET4 , and R SET5 ) were implemented outside the chip so that the driving circuit can be adjusted easily. The values of R SET1 , R SET2 , R SET3 , R SET4 , and R SET5 are 50, 25, 10, 10, and 30 ohms, respectively. Instead of being integrated, the resistors are out-of-chip; therefore, the heating problem of the resistors can be effectively controlled. The charge capacitor was also placed off-chip to obtain a more accurate V REF value.
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Micrograph of the fabricated AC LED driver.
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Photograph of the test circuit with an LED matrix.
IV. CONCLUSIONS
An AC LED driver with a sinusoid-like reference and highly resistant to AC voltage fluctuation was developed. The absence of passive components significantly improved the stability and integration of the LED system. The driving circuit of this structure allows the input current of LED to follow the waveform of the input voltage properly; thus, PF is increased and THD is decreased. The driving circuit is also insensitive to AC voltage fluctuation. As a result, users would not feel that LED lamps are flashing during input voltage fluctuation. The proposed driver, with amendments to the peripheral devices, can be applied to 110 and 220 V AC sources and thus has excellent flexibility. This suitability of this driver was demonstrated by a prototype implementation in a 0.25 μm BCD process. The PF and THD of the driving circuit under 6 W/110 V and 6 W/220 V AC voltage reached 0.997/0.999 and 6.2%/3.3%, respectively. The efficiencies are 90.3% and 91.6% for 110 V at 50 Hz and for 220 V at 50 Hz, respectively, with a power consumption of 6 W.
Acknowledgements
This work was supported by the Fundamental Research Funds for Central Universities under project no. ZYGX2012Z007.
BIO
Ning Ning received his B.S. and Ph.D. degrees from the University of Electronic Science and Technology of China, Chengdu, China, in 2002 and 2007, respectively. Since 2007, he has been with the State Key Lab of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, where he is now an associate professor. His current research interests include power electronics as well as analog and mixed-signal IC design.
Zhenxiao Tong received his B.S. degree from the University of Electronic Science and Technology of China, Chengdu, China, in 2011. He is currently pursuing his M.S. degree from the University of Electronic Science and Technology of China, Chengdu, China. His current research interests include power electronics and analog IC design.
Dejun Yu received his B.S. and M.S. degrees from the University of Electronic Science and Technology of China, Chengdu, China, in 2002 and 2006, respectively. He worked at Eutech Microelectronics Inc., Chengdu, China, from 2006 to 2012. He is currently pursuing his Ph.D. degree at the University of Electronic Science and Technology of China, Chengdu, China. His current research interests include power management and signal processing techniques.
Shuangyi Wu received his B.S. and M.S. degrees from the University of Electronic Science and Technology of China, Chengdu, China, in 2004 and 2007, respectively. He worked as a senior engineer at Sunplus Technology Co., Ltd., Chengdu, China, from 2007 to 2011. He is currently pursuing his Ph.D. degree at the University of Electronic Science and Technology of China, Chengdu, China. His current research interests include signal processing techniques as well as analog and mixed-signal IC design.
Wenbin Chen received his B.S. degree from Southwest Jiaotong University, Chengdu, China, in 2011. He is currently pursuing his M.S. degree at the University of Electronic Science and Technology of China, Chengdu, China. His current research interests include power electronics and analog IC design.
Chunyi Feng received his B.S. and M.S. degrees from the University of Electronic Science and Technology of China, Chengdu, China, in 2010 and 2013, respectively. He is currently working at JAVEE Microelectronics, Inc., Chengdu, China. His interests include power electronics and analog IC design.
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