Multi-string AC-powered LED driver with current ...

LETTER

IEICE Electronics Express, Vol.11, No.19, 1–8

Multi-string AC-powered LED driver with current regulation reduction based on simple circuitry Kilsoo Seo1, Van Ha Nguyen2, Jinwoo Jung3, Jusung Park4a), and Hanjung Song2b) 1

Power Semiconductor Research Center, KERI, Changwon 642–120, Korea

2

Department of Nanoscience and Engineering, Inje University,

Gimhae 621–749, Korea 3

MagnaChip Semiconductor, Ltd., Kangnam-Gu, Seoul, 135–738, Korea

4

Department of Electronic Engineering, Busan University, Busan 609–735, Korea

a) [email protected] b) [email protected]

Abstract: In this letter, a new simple circuitry LED driver with current reduction is proposed, realized and experimentally validated. The proposed circuit comprises with very few components intended for making a costeffective and compact design while still shows a high performance. The circuit reduces non-conducting time of LEDs to enhance power factor (PF) and total harmonic distortion (THD). The flicker can also be relatively reduced. By using current limiter cells, lighting variation is eliminated with respect of the variation of AC input voltage. A worst case of a 4 W with only two LED-strings driver is implemented and tested by using a 1 um 650 VBCDMOS high voltage process to verify advantageous characteristics of the suggested scheme. Experimental results demonstrate a PF of 0.97 with a THD of 24.62% and an efficiency of 87.1% at a 220 V AC supply. Keywords: LED, AC driver, shunt regulator, flicker, multi-string LED Classification: Integrated circuits References

© IEICE 2014 DOI: 10.1587/elex.11.20140810 Received August 21, 2014 Accepted August 22, 2014 Publicized September 10, 2014 Copyedited October 11, 2014

[1] N. Ning, W. B. Chen, D. J. Yu, C. Y. Feng and C. B. Wang: Electron. Lett. 49 (2013) 1170. DOI:10.1049/el.2013.2093 [2] A. Wilkins, J. Veitch and B. Lehman: IEEE Energy Conversion Congress and Exposition (2010) 171. DOI:10.1109/ECCE.2010.5618050 [3] Y. Hu, L. Huber and M. M. Jovanović: IEEE Trans. Power Electron. 27 (2012) 1579. DOI:10.1109/TPEL.2010.2082564 [4] K. I. Hwu and W. C. Tu: Proc. IEEE Applications Power Electronics Conf. (2011) 713. DOI:10.1109/APEC.2011.5744674 [5] R. Dayal, K. Modepalli and L. Parsa: Proc. IEEE Energy Conversion Congress Expo. (2012) 4230. DOI:10.1109/ECCE.2012.6342248 [6] J. Kim, J. Lee and S. Park: Proc. Int. Solid State Circuits Conf. (2013) 376. DOI:10. 1109/ISSCC.2013.6487777

1

IEICE Electronics Express, Vol.11, No.19, 1–8

[7] M. E. Poplawski and N. M. Miller: Proc. of CIE Centenary Conference Towards a New Century of Light (2013) 188.

1

Introduction

LED lighting has been concerned as an effective lighting generation for the last few years due to its advantages: compact, long lifetime, environmental friendliness, as compared to other lighting systems [1]. In the literature, a lot of AC-DC drivers have been proposed with high efficiency [3, 4]. Most of them use a bulky transformer and electrolytic capacitors which shorts the lifetime of LED module and cannot be integrated inside the chip. Then, some researchers have been trying to design AC directly-controlled LED drivers which eliminate totally transformer and electrolytic capacitors [5, 6]. In the design presented in [5], the design gains a high PF and improves THD; however, it may results in a current glitch due to hard switching operation that degrades the THD due to severe current distortion which then degrade PF [1]. With the design in [6], although the glitch phenomenon is eliminated due to the use of a soft-switching mechanism, it owns another shortcoming of the change of reference voltage level if the input voltage changes. In other hand, at the workplaces where the ac power supply is unstable (without using stabilizers) the fluctuation of the input voltage may cause the variation of LED brightness, shorten the LED lifetime or even damage LED bubs in case of the overvoltage rating conditions. Furthermore, by using these designs, the chip cost keeps relative high because of the complexity of the circuit. Recently, flicker and its human health-related effects have been drawn much more concerns [2]. Flicker from LED luminaires can be much more significant than flicker from conventional sources, and becomes critical in any AC LED-based lighting systems because this topology cannot totally eliminate flickers as in converter-based drivers. However, the reduction of flicker in AC LED-based lighting systems is possible. In this letter, we propose a new AC LED driver topology which owns some dominant advantages of circuit simplicity, high efficiency and low cost. The proposed circuit does not only eliminate the aforementioned effects caused by the fluctuation of the input voltage but also reduce flicker via a simple mechanism. 2

Proposed multiple-string AC LED driver

Fig. 1 illustrates the proposed AC LED driver. The proposed AC driver is composed of four simple sub-blocks: a bridge, an LED module, a bias circuit (comprised of a resistor RB , three zener diodes ZB1 -ZB3 , and a high voltage power MOSFET MB ), a group of current limiter cells and a switching system (which is not fully shown for simplicity of analysis). The core of the proposed LED driver is the current limiter cells which is compounded of two resistors, one power MOSFET and a shunt regulator (SR). These current limiters are configured in a constant current sink topology with the sink current expressed as: © IEICE 2014 DOI: 10.1587/elex.11.20140810 Received August 21, 2014 Accepted August 22, 2014 Publicized September 10, 2014 Copyedited October 11, 2014

Isink ¼ VREFi =Ri :

ð1Þ

where VREFi is the voltage on the reference terminal of the ith shunt regulator. 2

IEICE Electronics Express, Vol.11, No.19, 1–8

Fig. 1.

Proposed multi-string AC-powered LED driver.

Assume that the forward voltage drops on the LED string is indicated by VLED . The basic operating principle of the proposed LED driver is described as follows: At a low voltage, VIN < VLED1 , no current flows through the circuit since the forward voltage of LED1 cannot overcome by the input voltage. When VLED1 < VIN < VLED1 þ VLED2 , the LED1 turns on and the current limiter CLC1 operates in the constant current status. Note that the CLCs cause an additional power loss due to the voltage drops across the transistors and resistors; hence, in order to obtain a high efficiency, only one CLC1 turns on while the others turn off by using the switching system. The switching signals are generated by detecting the value on current-sensing resistor RS1 (is the voltage from the reference voltage terminal VREF1 of the shunt regulators SR1 ). The switching system detects when VREF1 of SR1 is close to 2.5 V, and it generates control signals to turn off the other power MOSFET Msw2 ; Msw3 . . . Mswn . These switching MOSFETs operate in active regions with low resistances that means the drain-source voltages which equal to the gate voltages of the power MOSFETs drop down to a low level then turns off accompanying CLCs. As VIN continues increasing, all the LEDs will be turned on. This process repeats in the next cycles. Fig. 2 shows the current generated from CLCs and the input current (approximately to the LED current without taking into account the bias current) according to the input voltage of the proposed multi-string AC LED driver. The current flowing through the LED is the sum of the currents generated from CLCs. Fig. 3 shows the input which equals to LED current for three cases of one, two and fourstring topologies, respectively. As shown in this figure, by increasing the number of LED strings, the step-like current approximates to the sinusoidal input voltage. The current flows from the section in which the input voltage is low and the input current wave shape widened, which leads to a lower THD and a higher PF. To evaluate the flicker of the proposed AC LED driver, the flicker index is adopted because this metric takes into account the shape of the LED brightness waveform while the metric in percent of flicker does not [7]. The flicker index is calculated as: © IEICE 2014 DOI: 10.1587/elex.11.20140810 Received August 21, 2014 Accepted August 22, 2014 Publicized September 10, 2014 Copyedited October 11, 2014

Flicker Index ¼ area1=area2:

ð2Þ

3

IEICE Electronics Express, Vol.11, No.19, 1–8

Fig. 2. Operation modes of LED current.

Fig. 3. Operation modes of LED current.

© IEICE 2014 DOI: 10.1587/elex.11.20140810 Received August 21, 2014 Accepted August 22, 2014 Publicized September 10, 2014 Copyedited October 11, 2014

where area1 and are2 are the areas above and under the mean line value of the LED output brightness. In fact, the LED forward current is relatively proportional to its luminous flux (which is essentially the brightness) in a linear relationship, especially within a low LED current range [2]. This means that the LED output flux wave shape is approximately to the LED current wave shape so that the flicker can be indirectly calculated through the LED current wave shape. Because of this, the ratio (flicker index) between the area1 and are2 in Eq. 2 now can be calculated via the ratio of the areas above and below the mean line value of the LED current. Fig. 3 shows the approximated calculation of flicker indexes for the proposed scheme. As seen in Fig. 3, the area2 becomes dominant with more string driver which results in a lower flicker index. The area1 and are2 are calculated using Matlab program. With one-string circuit, the flicker index is approximately 0.5 while decreasing to 0.23 when 4-string driver is used. Although the LEDs may not be driven that entire time of duration, the off-time of LED current is shrunk when more string LED is adopted. This demonstrates an improvement of flicker of the proposed topology of the proposed driver. In AC-powered LED driver based lighting systems, beside the effect of the flicker, the brightness variation of LED caused by the LED current variation which derived from the variation of the input voltage potentially raises the fatigue of the eye. In this design, since the peak current is limited by utilizing the current limiter cells, the size of the input current according to the input voltage is kept as a constant. As a consequence, the proposed driver removes the brightness variation of LED and keeps the LED lifetime longer.

4

IEICE Electronics Express, Vol.11, No.19, 1–8

3

Simulation and experimental results

To demonstrate the high performance of the proposed scheme, only two-string driver was conducted. The reason for this is that with more string LED, a higher PF and a lower THD can be gained [1]. However, for a low cost-effective target, we need to balance the performance and the chip cost. Also, with only two-string driver, a control circuit can be easily realized by using only a resistor and a MOS transistor. The eight LEDs were used and divided into two strings, one string consist of 2 sub-strings (Str1 and Str2) with 2 LEDs for each and connected in parallel. The LED has a 20 mArms operating current and a 50 Vrms forward voltage. Note that two-string driver owns a relatively high flicker index as demonstrated in Fig. 3. The circuit simplified schematic without the bridge is presented in Fig. 4(a).

Fig. 4.

© IEICE 2014 DOI: 10.1587/elex.11.20140810 Received August 21, 2014 Accepted August 22, 2014 Publicized September 10, 2014 Copyedited October 11, 2014

(a) Two-string AC LED driver. (b) Operation of two-string AC LED driver.

First, the circuit was simulated via SPICE using 1 um technology to verify the feasibility of the proposed AC LED driver operation at rated conditions. The value of the sensing resistors, RS1 and RS2 are set of 250 Ohms and 62.5 Ohms for constant currents of CLCs of 10 mA and 40 mA, respectively. Fig. 4(b) illustrates the operation of the testing circuit. As expected, the first LED string Str1 starts to conduct when the input voltage is higher than V1 ¼ VLED1 þ VLED2 ¼ 100 V, and the both of LED strings Str1 and Str2 conduct when the input voltage comes over V2 ¼ VLED1 þ VLED2 þ VLED5 þ VLED6 ¼ 200 V. The first current limiter which controls LED Str1 is turned off when VIN > V1 to lower the power loss. Fig. 5 shows the simulation waveform of the input current versus the input voltage at the 220 V and 265 V, respectively. As shown in this figure, for different input voltages the input currents is always in phase with the input voltages. In addition, the current profile is expanded to a lower input voltage as compared with conventional LED driver. As a consequence, the PF and THD of the proposed LED driver are efficiently improved. Furthermore, there are no current glitches due to a soft turn on of the switch MOS because of a slow change of the reference voltage of the SR2, then the disadvantages (such as decrease the quality of lighting due to blink, shortening of the LED lifetime, and degrading of the THD due to severe current distortion, which in turn degrade the PF) caused by the hard switching operation as 5

IEICE Electronics Express, Vol.11, No.19, 1–8

encountered in [5] are no longer exist. It can also be seen that the input current is relatively insensitive with a large fluctuation of the input voltage by using two CLCs. The input currents shown in Fig. 5 are kept at a relatively constant at different input voltages with the current regulation is reduced to 54.2 uA/V. This demonstrates that the aforementioned effects of the input voltage variation are nearly removed.

Fig. 5. Simulation waveform of input current versus input voltage.

The two-string LED driver was designed using a 1-um 650 V-BCDMOS process technology that provides lateral double-diffused MOS transistors with a 350 V of breakdown voltage. All of the components are integrated inside the chip without any external components. The microphotograph of fabricated monolithic IC is shown in Fig. 6(a) with the zener-zap diodes for trimming to ensure the proper value of RS1 and RS2 . The fabricated chip has 1.72 mm2 of area. The chip was packaged directly on board with the mounted LEDs of Seoul semiconductors AN4240 for a cost-effective solution and cooling method for LED bubs (Fig. 6(b)). The prototype board was tested with 220 Vac input voltage and exhibited a consistent light as shown in Fig. 6(c).

Fig. 6.

© IEICE 2014 DOI: 10.1587/elex.11.20140810 Received August 21, 2014 Accepted August 22, 2014 Publicized September 10, 2014 Copyedited October 11, 2014

From left to right: (a) Microphotograph of the proposed 2-string LED driver IC. (b) Prototype board. (c) Practical testing light

Fig. 7 shows the measurement waveform of the input voltage and input current of the prototype board. The measurement waveform in Fig. 7 is same as in the case of simulations results shown in Fig. 5, hence, proving the proposed AC LED driver works as expected. Table I shows the measurement summary of the prototype driver. The measurements of the prototype are THD of 24.6% with a PF of 0.97,

6

IEICE Electronics Express, Vol.11, No.19, 1–8

Fig. 7. The measured waveforms of input voltage and input current.

Table I.

Measured performance summary of the prototype LED driver

Subject Input Voltage Input Current (RMS)

Result 220/60 Vrms/Hz

Unit -

21.3

mA

Power of LED Lighting

4

W

Number of LED Strings

2

-

Power Factor

0.97

-

Total Harmonic Distortion

24.62

%

Efficiency

87.1

%

LED Module PCB Area

23.04

cm2

IC Process

1-um 650 V BCDMOS

-

IC Active Area

1.72

mm2

IC package

COB

-

and efficiency of 87.1% of a 4 W. The PF and efficiency totally satisfy the international standard IEC-6200-3-2 for lighting system while the THD does not. This can be easily understood because the tested circuit only used 2-string LED. If we use more LED strings with a lower forward voltage of LED device (VLED < 50 V as used in this test case), the THD will be further lowered to be satisfied for this requirement. 4

© IEICE 2014 DOI: 10.1587/elex.11.20140810 Received August 21, 2014 Accepted August 22, 2014 Publicized September 10, 2014 Copyedited October 11, 2014

Conclusion

In this paper, we have proposed a new, simple, low cost and high efficiency AC LED driver. For the evaluation, only 2-string LED driver was fabricated using the 1-um 650 V-BCDMOS process technology. Measurement results demonstrated a high performance that the driver can gain. The LED driver proposed can be flexibly customized to get a higher performance according to the requirement of the lighting systems by choosing the number of LED string. The simplicity and straight forward design produce a cost-effective, compact solution for LED lighting applications.

7

IEICE Electronics Express, Vol.11, No.19, 1–8

Acknowledgments This work was supported by the project “AC direct driving LED IC with dimming control” of KERI (Korea Electrotechnology Research Institute), Korea and MSIP (Ministry of Science, ICT and Future Planning), Korea, under the IT/SW Creative research program supervised by the NIPA (National IT Industry Promotion Agency)” (NIPA-2013-H0502-13-1111).

© IEICE 2014 DOI: 10.1587/elex.11.20140810 Received August 21, 2014 Accepted August 22, 2014 Publicized September 10, 2014 Copyedited October 11, 2014

8