Design and Implementation of High Frequency AC-LED Driver with Digital Dimming Chao-Lung Kuo Tsorng-Juu Liang Kai-Hui Chen Jiann-Fuh Chen

Advanced Power Electronics Center, Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan

Email: tjliang@mail.ncku.edu.tw

Abstract – In this paper a two-stage high frequency AC-LED driver is designed and studied. The front-stage is a boost power-factor correction (PFC) circuit and the rear-stage is a half-bridge series resonant inverter. The lighting characteristics of high frequency AC voltage driving LEDs, and the effect of LED to half-bridge resonant circuit are discussed. The proposed design uses the digital dimming control to reduce LED chromaticity shift, and connect the LEDs as a full-bridge rectifier to simplify the current sensing circuit. An 80-W high frequency AC-LED driver with universal voltage input (90~264 Vrms) is implemented to verify the design. Experimental results show that the variation of chromaticity coordinates (u’, v’) from brightness 100% to 20% is within 0.0023.

Keywords：Digital dimming, High frequency AC-LED.

I. INTRODUCTION Light emitting diodes (LEDs) nowadays draws high

attraction for general illumination. Because of energy saving, environmental friendly, long life, vibration endurance, high color saturation, and fast response, LEDs are widely used in traffic light, vehicle light, flash light, and so on. LEDs are treated as next generation lighting source, but the challenges of LEDs are the efficacy, thermal mechanism, and the cost. [1~3]

LED is driven by DC voltage conventionally. Thus, it needs to transfer AC utility source to DC voltage for LED. Recently, AC utility source directly driving LED is proposed as Fig.1(a).[4] The disadvantage of AC-LED is the conduction voltage of LEDs. The conduction angle causes current harmonic distortion and lowers the power factor, as shown in Fig. 1(b).

In this paper, a two stage high frequency AC-LED driving system is proposed and designed. The system do not need the output rectify diode to achieve higher efficiency. Also, the conduction angle issue of utility AC-LED is improved. Finally, an 80-W high frequency AC-LED driver with universal input voltage (90~264 Vrms) is implemented to verify the feasibility.

(a) (b)

Fig. 1 Equivalent circuit of AC LED and correlated waveforms

Ⅱ. HIGH FREQUENCY AC-LED DRIVER SYSTEM

A high frequency AC-LED driving system is proposed, as shown in Fig. 2. The front-stage, a boost PFC circuit, achieves high power factor and low input current harmonic distortion. The rear-stage is a half-bridge series resonant inverter to drive LEDs.

LEDs are full-bridge connected to form the high frequency AC-LED module. vo is the output voltage of the half-bridge series resonant inverter. The current sensing circuit of LEDs is simplified, because of the rectification characteristics of AC-LED module. io is a high frequency alternating current signal. By the full-bridge connection all LEDs current can be controlled by sensing id, rectified io.

Because the V-I curve of LED is shown as an exponential function, the non-linearity characteristics is difficult to be analyzed. To simplify the analysis, an approximated linear relationship is taken in Fig. 3(a). The linear relationship only takes the DC characteristics of LED into consideration, and the capacitance effect caused by LED junction is neglected. The capacitance effect of LED must be considered for high frequency AC voltage driving LED. The modified approximate linear relationship includes two parts: LED on and LED off. When LED is on, the equivalent circuit is shown in Fig. 3(b). When LED is off, the equivalent circuit is shown in Fig. 3(c).[5].

L1

S1vgs1

DPFC

Cbus

S2

S3

vgs2

vgs3

Lr Cr

LED module

vac

vDS3

Half-bridge resonant inverter

BCM boost PFC

D2 C2

D3 C3

vDS2

vLr vCrVbus

D1C1

vDS1

iL1 id,PFC

iS1

ioDP1 DN1

DP2DN2

idvoCin

iac

Fig. 2 Circuit configuration of the proposed converter

(a) (b) (c)

Fig. 3 V-I curve and equivalent circuit of LED (a) V-I curve (b) turn-on equivalent circuit (c) turn-on equivalent circuit

978-1-4244-5309-2/10/$26.00 ©2010 IEEE 3713

Ⅲ. OPERATING PRINCIPLES AND CIRCUIT ANALYSIS

1. Operating principles

The simplified equivalent circuit of LED is shown in Fig.4. When LED is on, the impedance of Con is much greater than Ron, consequently, the simplified circuit of LED-on is shown in Fig. 4(a). When LED is off, the impedance of Roff is much greater than Coff. The simplified LED-off circuit is shown in Fig. 4(b). Some conditions are assumed as follows to simplify the analysis.

The characteristics of S2 and S3 are the identical.

RDS,on of S2, S3 are neglected.

Switching frequency fs is higher than resonant frequency fr.

(a) (b)

Fig. 4 Simplified equivalent circuit of the AC-LEDs.

The key waveforms of the inverter are shown in Fig. 5. The two switches of the half-bridge inverter are driven by complementary signals with dead time, and the duty cycle of the two gate-driving signals is 50 %. There are eight operating modes in one full switching cycle. Since mode Ⅴ~

Ⅷ are symmetrical to the first four modes, only the first four operating modes are discussed.

Mode I： (t0 ~ t1)

At t = t0, switch S2 is on. The operation of this mode is shown in Fig. 6(a). In this mode, LEDs are not conducted. Lr, Cr and Coff form the resonant tank. Because of Coff << Cr, Coff is a high impedance, vo rises quickly to the conduction voltage VD,on. When vo rises to VD,on, DP conducts, next mode starts. The time duration of this mode is shown in equation (1).

])(

1[cos)( ,1

01busr

onDoffr

offr

offrr

VCVCC

CCCCL

ttt⋅

⋅+−⋅

+⋅⋅

=−=Δ − (1)

Mode II： (t1 ~ t2)

In this mode, switch S2 and DP are on. The operation of this mode is shown in Fig. 6(b). During this mode, Lr and Cr are resonant with Ron. The mode ends when S2 is turned off.

Mode III：(t2 ~ t3)

At t = t2, S2 and S3 are off. The operation of this mode is shown in Fig. 6(c). During this mode, io charges C2 and

discharges C3 to zero and the anti-parallel diode is conducted, ZVS of S3 achieved. When vgs3 is high, next mode starts.

Mode IV： (t3 ~ t4)

In this mode, switch S3 is on and S2 is off. The operation of this mode is shown in Fig. 6(d).When vo drops to VD,on, DP off and enters the next symmetrical mode.

Fig. 5 The key waveforms of the proposed converter

(a) Mode I (b) Mode II

(c) Mode III (d) Mode IV

Fig. 6 The operating modes of the proposed converter

2. Resonant characteristics

The output voltage vo can be separated into two parts. The first part is the conduction voltage of LEDs, vD,on, and the second part is the voltage vr, which is resonant by Lr, Cr, and Ron. The resonant tank is shown in Fig. 7(a), 7(b), and the relationship of vo is shown as equation (2). The resonant tank only formed by Lr, Cr, and Ron for constant conduction voltage of LEDs. va(t) is a square wave voltage, and its Fourier series is expressed as equation (3). The higher order

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harmonics effect is lowered and the resonant current is approximately to a sinusoidal wave by setting the switching frequency close to the resonant frequency. The voltage transfer function G(jω) can be obtained as equation (4). So, the output voltage of the half-bridge resonant inverter is shown as equation (5), and the curve of the voltage gain is shown in Fig. 7(c).

ronDo vVv += , (2)

1

,

2 1 ( 1)( ) sin( )2 2

2 ( )2

nI I

an

busI D on

V Vv t n tn

Vv V

ωπ

∞

=

− −= + ⋅ ⋅

= ⋅ −

∑ (3)

( ) 1( )( ) 1 ( )

1: ,2

o

s ra

r s

r

rr

s r r

v jG j f fv j jQf f

LC

where Q fR L C

ωωω

π

= =+ +

= =

(4)

,2 | ( ) |

2I

o D onvv V G jω

π= + ⋅ ⋅ (5)

(a) LED on (b) LED off

(c)

Fig. 7 Resonant network and voltage gain

3. Digital dimming

There are two methods for LEDs dimming, digital or analog dimming. Analog dimming can adjust the voltage gain by modulating the switching frequency to achieve. The disadvantages of analog dimming are the narrow dimming range and the chromaticity shift problem. This paper adopts digital dimming to adjust the light output of LEDs. Digital

dimming is using low-frequency pulse width modulation to adjust the light output of LEDs[6~7]. The dimming signal frequency is higher than human eyes can aware of the light fliker. The strategy of dimming is shown in Fig. 8(a), and the dimming circuit is shown in Fig 8(b).

(a)

VCO Control logic

High side driver

Low side driver

S3vgs3

D3 C3

S2vgs2 D2 C2

Q1 Q2

Dimming signal

FB

ref

Rc1 Rc2

Rb2Rb1

Soft start

Cf

L6574

(b)

Fig. 8 Digital dimming

Ⅳ. EXPERIMENTAL RESULTS

In order to verify the performance of the proposed converter, a laboratory prototype driving system is implemented. The system specifications and parameters are, vac: 90~264 Vrms, Vbus: 400 V, LED module: 1W LED 80 pics, Po,rated: 80 W, Dimming frequency:200 Hz, L1: 0.625 mH, Lr: 0.555 mH, Cr: 8.2 nF.

Fig. 9 shows the switching signals of half bridge stage, which can achieves ZVS. Fig. 10 shows the waveforms of output voltage vo and output current io, which are 145 Vrms and 570 mA and almost no conduction angle between vo and io. Fig. 11 shows the rectified current id, which is the current control signal. Fig.12 shows the waveforms of digital dimming. Fig. 13 shows one LED of the module light output versus dimming duty, and Fig. 14 shows the system efficiency under dimming process. Table 1 shows the light characteristics of LEDs, including CIE1931(x, y), CIE1976 (u’,v’), chromaticity shift Δu’v’ and dominant wave length DW. The variation of CIE1976 (u’,v’) is within 0.0023 which can comply with the standard of Energy star.

Fig. 9 The experimental waveforms of vgs2, vDS2, vgs3, and vDS3

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Fig. 10 The experimental waveforms of vo and io＝

Fig. 11 Waveform of id

(a) (b)

Fig. 12 Waveforms of vgs3, dimming signal and io at 20 % (a) and 50 % dimming(b)

0

10

20

30

40

50

60

20% 30% 40% 50% 60% 70% 80% 90% 100%

Duty cycle of dimming signal

lm

Fig. 13 Light output of one LED of the module at 220 Vrms input under

various duty cycle of dimming signal

80.00%

82.00%

84.00%

86.00%

88.00%

90.00%

92.00%

20% 30% 40% 50% 60% 70% 80% 90% 100%

Duty cycle of dimming signal

Effic

ienc

y

Fig. 14 System efficiency at 220 Vrms input under various duty cycle of

dimming signal

Table 1 The light characteristics of LEDs under various dimming condition

Ⅴ. CONCLUSIONS

In this paper, a two-stage high frequency AC-LED driver with digital dimming is implemented to verify the feasibility. The front-stage is a boost PFC circuit to improve power factor and reduce the input current harmonic distortion of the system. The rear-stage is a half-bridge series resonant inverter which provides the high frequency sinusoidal signal to drive LEDs. Finally, a laboratory prototype circuit with (90~264 Vrms) input and 80 1W-LEDs output is implemented to verify the design of the proposed system. The experimental results show that the half-bridge stage can achieve ZVS operation and nearly no conduction angle between the forward voltage and current of LEDs. The efficiency is 90.7% at 220 Vrms input in full-load condition. With digital dimming, the variation of chromaticity coordinate (u’, v’) from 100% brightness to 20% brightness is within 0.0023.

ACKOWLEDGEMENT The authors gratefully acknowledge financial support

from LED Lighting And Research Center of National Cheng Kung University and Advanced Optoelectronic Technology Center of National Cheng Kung University

Ⅵ. REFERENCE

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[2] M. S. Shur and A. Zukauskas, “Solid state lighting: toward superior illumination,” IEEE Proc. vol. 93, pp. 1691-1703, Oct. 2005.

[3] D. A. Steigerwald, J. C. Bhat, D. Collins, R. M. Fletcher, M. O. Holcomb, M. J. Ludowise, P. S. Martin, and S. L. Rudaz, “Illumination with solid state lighting technology,” IEEE Journal on selected topics in quantum electronics, vol 8, no 2, Mar./Apr. 2002.

[4]M. Miskin, J.N. Anderson, “AC light emitting diode and AC LED drive methods and apparatus,” U.S. PAT. No. 20070273299.

[5] C. Kittel, Introduction to solid state physics, John Wiley & Sons, Inc., 2004.

[6] C. C. Chen, C. Y. Wu, and T. F. Wu, “Fast transition current-type burst-mode dimming control for the LED back-light driving system of LCD TV,” IEEE proc. PESC’ 06, pp. 1-7, June 2007.

[7] X. Xu, X. Wu, “High dimming ratio LED driver with fast transient boost converter,” IEEE proc. PESC’ 08, pp. 4192-4195, June 2008.

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