Design and Implementation of a Novel Two‐Stage LED Driver

International Journal of Energy and Power, Volume 5 2016 www.ijep.org doi: 10.14355/ijep.2016.05.003

Design and Implementation of a Novel Two‐Stage LED Driver Yu‐En Wu Department of Electronic Engineering, National Kaohsiung First University of Science and Technology, 1 Uiversity Road, Yuanchau, Ksohsiung 824, Taiwan, R.O.C. yew@nkfust.edu.tw

Abstract This paper presents a novel two‐stage LED driver system which possesses two sets output source to keep the brightness of LED load. The first‐stage of the proposed system is the power factor correction (PFC) circuit which possesses fully voltage range (85~265V, 50/60Hz) and is implemented by boost converter. It operated in boundary conduction mode (BCM) to improve the power factor and the output voltage is 400V. The second‐stage of it is the forward converter which is used to convert input voltage 400V to 30V. The converter designed the third winding to transfer the energy of magnetizing inductance to the second winding for maintaining the output voltage stability. Finally, a two‐stage driver circuit is implemented to verify the functionality and feasibility of the proposed system.

Keywords Power Factor Correction (PFC), Forward Converter

Introduction According to the 2010 Energy Policy White Paper [1], LED lighting with the energy‐saving potential of improved efficiency and performance can reduce the cost. Application of LED includes traffic signals, exit lights, advertising lights, street lamps and house lighting etc., which can be estimated to save 6.4 billion KWH, and reduce 4.52 million carbon dioxide emissions till 2020. Therefore, it’s of great urgency to develop environmental friendly energy‐efficient lighting equipment. The traditional lightings, such as Metal Halide Lamp, Incandescent Lamp, Mercury Lamp, High Pressure Sodium Lamp, Halogen Lamp, and Fluorescent Lamp etc. have their individual shortcomings like short life, low efficiency, and poor color rendering. [2] In order to improving the above shortcomings of lighting and energy savings, many companies and academics invest their efforts to HB LED (High Brightness Light Emitting Diode). LED has many advantages, such as long life, good luminous efficiency, shock proof, small size, etc. In addition, it’s non‐toxic, which makes it more environmental friendly than traditional lightings; and owing to its dimming technique, fast switching, and low voltage drive, LED becomes the light of the new generation and replaces all traditional lighting equipments gradually. With the advancement and development of technology, LED’s luminous efficiency will gradually be improved so the prospect of LED‐developing is unlimited. Therefore, this paper presents a two‐stage LED driver technology as a choice to drive LED. System Architecture Fig.1 shows the proposed system architecture. The first stage is ac to dc converter to correct power factor, and then through the Forward dc‐dc converter to achieve a stable output voltage and drive the LED module. The advantage of the proposed two‐stage LED driver is independent control; the first stage was not influenced by the second stage circuit, so the output voltage can be relatively stable.

FIG. 1 SYSTEM ARCHITECTURE.

19

www.ijep.org International Journal of Energy and Power, Volume 5 2016

PFC Circuit Operational Principle Fig. 2 shows the control circuit of PFC stage, and its operation principle is: the input AC source (85V~265V) is rectified by bridge rectifier circuit to achieve half‐sine voltage, after dividing voltage into R5、R6; and it begins to amplify through the internal error amplifier and then multiply with the output signal as the reference signal Vm, as shown in Fig. 3. Comparing the reference signal Vm with VRs to produce the reset signal of RS Flip‐Flop, the reset signal and the output of PFC IC zero‐current detection are used to control the turn on/off time of switch, that is pulse‐width modulation (PWM), as shown in Fig. 4. Where, VL represents the voltage of inductor, VG is the internal reference voltage, Vm is the produced reference voltage of error amplifier, VRs represents the voltage of detective switching current, and Vg is the gate voltage of switch.

FIG. 2 CONTROL CIRCUIT OF PFC CIRCUIT.

FIG. 3 PFC IC INTERNAL ERROR AMPLIFIER.

FIG. 4 CONTROL SIGNAL OF PFC IC [3].

Design Of PFC Circuit The primary and secondary inductances of the transformer, the output capacitor and input capacitor in the proposed PFC circuit can be obtained according to [4]; and it can be listed as follows:

Ton  Toff  f 

L * I LPK (1) 2 * Virms

L * I LPK (2) Vo  2 * Virms

1 V 2 irms * (Vo  2 * Virms ) (3)  Ton  Toff Vo I LPK  2 * 2 *

20

Pi (4) Virms

International Journal of Energy and Power, Volume 5 2016 www.ijep.org

L

V 2irms *(Vo  2 *Virms ) (5) 2* f sw * Pi *Vo

Cin 

Po (6) 4 * f sw * r * Virms

Co 

Po (7) 4 * f * Vo * Vo

where, Ton and Toff represent the turn‐on and turn‐off time, respectively. ILPK is the maximum current of inductor, fsw is the switching frequency, f is the AC power frequency, L is the energy storage inductor, Cin and Co are the input and output capacitors, respectively. The maximum high frequency ripple voltage is usually restrained to 1%~10% of the minimum input voltage, it can be represented by r =0.01~0.1. Virms is the input rms voltage, Vo is the output voltage,  Vo is the ripple output voltage, and Pi is the input power. Table 1 shows the designed electrical specifications, and the inductor, input and output capacitors can be calculated by equations (5)‐(7); and the results can be obtained as L = 0.9mH, Cin = 3 uF and Co = 47uF. Operational Principle of Forward Circuit In this paper, the proposed forward converter used the third winding to connect with output for transferring the magnetizing inductance energy, as shown in Fig. 5. The operational modes are listed as follows: TABLE 1 DESIGN SPECIFICATIONS OF THE CIRCUIT.

Input Voltage

85~265Vrms

Input power frequency

50/60Hz

Output voltage

400V

Output power

40W

Power factor

>0.9

Efficiency

>90%

Minimum output ripple voltage

7V

Maximum over‐voltage protection

40V

Minimum switching frequency

40kHz

FIG. 5 PROPOSED FORWARD CIRCUIT.

FIG. 6(A) OPERATION MODE 1 OF FORWARD CIRCUIT.

21

www.ijep.org International Journal of Energy and Power, Volume 5 2016

FIG. 6(B) OPERATION MODE 2 OF FORWARD CIRCUIT.

FIG. 6(C) OPERATION MODE 3 OF FORWARD CIRCUIT.

Mode 1: During switching on, the input power Vs supplies energy from the primary inductance N1 of the transformer to the secondary inductance N2. Diode D1 is turned on while D2, D3 and D4 are turned off. The inductor L and capacitor C1 start to store energy, as shown in Fig. 6(a).

Mode 2: During switching off, inductor L and magnetizing inductance Lm of transformer begin to release energy from the N1 of transformer to the secondary inductance N3, and the diode D1 and D4 are turned off, as shown in Fig.6(b).

Mode 3 When the energy saved in magnetizing inductance Lm and inductor L is fully released, capacitor C2 begins to release energy. D4 is turned on while D1, D2 and D3 are turned off. When the energy of capacitor C2 is fully released, the switch is turned on again and back to the mode 1, as shown in Fig.6(c).

Design of Forward Circuit The designed electrical specifications of forward circuit are listed in Table 2. The designed procedure of the turn ratio of transformer can be calculated according to Table 2; and the procedures can be illustrated as follows: Step 1 : Determining the duty cycle and turns ratio Assuming the input voltage Vin(min) =380 V, output current Io=1A , setting the maximum duty cycle Dmax =0.3, then the turns ratio n is calculated by equation (8).

n 

V in (m in ) D m a x Vo



3 8 0  0 .3  3 .8 (8)

30

TABLE2. DESIGNED ELECTRICAL SPECIFICATIONS OF FORWARD CIRCUIT.

22

Input voltage

380~400V

Frequency

50kHz

Output voltage/current

30V/1A

Output power

30W

Output filter inductor

300uH

Output filter capacitor

470uF

Power switch

STF7N95K3

Diode

FR203

Transformer

EE42

International Journal of Energy and Power, Volume 5 2016 www.ijep.org

Let n = 4, and substituting into the equation (8), and there can be obtained Dmax≒0.3368.

Step 2: Select core size

A p  Aw Ae 

Pt  10 4 (9)

2  Bf s JK u

According to equation (9) to determine the core size of transformer where Pt is the apparent power of transformer, ∆B is flux density variation (T), fs is transformer frequency (Hz), J is current density (A), according to different cooling methods to determine J=300~600A/cm2, and Ku is winding factor, generally with the range of 0.2~0.4.

Selectting EE42‐type core, core area of the product Ap = 4.9484cm4, core effective cross‐sectional area Ae = 1.78cm2, core window‐type area Aw = 2.78cm2.

Step 3: Calculate the primary turns

Np 

V in (m in) D m ax T s  B Ac

(10)

From equation (10), the primary turns can be calculated; and there can be obtained Np=145.

Step 4: Double check the maximum flux density Bm

B 

V in (m in ) D m ax T s Ac N p

(11)

Substituting the specifications and equation (10) into equation (11); and there can be obtained ∆B=0.099173, therefore, Bm   B  Br  0.09917  0.15  0.249  Bsat .

Step 5: Calculating the secondary turns Ns1 and second power turns Ns2

N s1  N s 2 

N

p

n



145  3 8 (12) 4

Therefore, the winding of the transformer turn ratio is: Np: Ns1: Ns2= 145: 38: 38.

Experimental Results The experimental conditions are: input voltage: AC 110V, 60Hz, and the output load is 30W LED module. Fig. 7 shows the waveforms of the input voltage and current, both of which are sine wave and in phase, and the power factor is 0.98. Fig. 8 shows the output voltage waveform of the PFC circuit, it can be seen that the output voltage can be stabilized in 400V. Fig. 9 shows the waveform of the switch voltage stress, a RCD snubber circuit is designed in the Forward converter, so it can be seen there are no surge voltage in the waveform. Fig. 10 shows the output voltage waveform of the forward circuit, it can be seen that the output voltage can be stabilized in 30V. Fig.11 shows the relation of PF and input voltage, the larger input voltage, PF value is smaller. Fig.12 shows the relation of THD and input voltage: the larger input voltage is, the larger THD value will be.

(Vac: 50 V/div, Iac: 1A/div,time: 5ms/div) FIG. 7 WAVEFORMS OF INPUT VOLTAGE AND CURRENT.

23

www.ijep.org International Journal of Energy and Power, Volume 5 2016

(VO: 200 V/div,time: 20ms/div) FIG. 8 OUTPUT VOLTAGE WAVEFORM OF THE PFC CIRCUIT.

(VDS: 500 V/div,time: 20us/div) FIG. 9 WAVEFORMS OF SWITCH VOLTAGE STRESS OF THE FORWARD CIRCUIT.

Vo

(VO: 10 V/div,time: 20us/div) FIG. 10 OUTPUT VOLTAGE WAVEFORM OF THE FORWARD CIRCUIT.

FIG. 11 RELATION OF PF AND INPUT VOLTAGE.

FIG.12 RELATION OF THD AND INPUT VOLTAGE.

24

International Journal of Energy and Power, Volume 5 2016 www.ijep.org

Conclusions This paper proposes a two‐stage LED driver circuit for 85V~265V input voltage. The first stage is PFC circuit, which can achieve a stable 400V DC voltage, and the second stage is forward circuit which is used to drive the LED module. The proposed system possesses high stability, and the power factor is above 0.98 and THD is below 10%.

It can be verified the system to greatly improve the practicability and feasibility of self‐designed LED driver from experimental results.

REFERENCES

[1]

Bureau of Energy, Ministry of Economic Affairs，“2010 Energy Policy White Paper”.

[2]

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 J Sel. Topics Quantum Electron., vol. 8, no. 2, pp. 310‐320, 2002.

[3]

Song Ziheng, Lin Qingren, Academia Sinica Institute of Earth Sciences, “Power factor correction circuit of the theory and specification of common components”, the new electronic technology magazine pp. 217‐228, 2004.

[4]

ST Microelectronics, “AN966‐Handbook Application”, Datasheet, 2001.

[5]

Chun‐Chuan Chen,“Multi‐input Power Converter for Solar Energy and Wind Power Generation”, National Kaohsiung First University of Science and Technology, Department of Electronic Engineering, 2010.

[6]

J. W. Lim, and B. H. Kwon, “A power‐factor controller for single‐phase PWM rectifiers,” IEEE Trans. Industrial Electronics, vol. 46, pp. 398‐404, May 2002.

[7]

Y. K. Lo, H. J. Chiu, and S. Y. Ou, “Constant‐switching‐frequency control of switch‐mode recifiers without current sensors,” IEEE Trans. Industrial Electronics, vol. 47, no. 5, pp. 1172‐1174, Oct. 2000.

Yu‐En Wu was born in Chia‐Yi, Taiwan, R.O.C., in 1964. He received the B. S. degree in Electronics Engineering from Taiwan Institute of Technology, Taipei, Taiwan, R.O.C., in 1989, and the M. S. degree in Electrical Engineering from Sun Yat‐Sen University, Kaohsiung, Taiwan, R.O.C., and Ph.D. degree in Electrical Engineering from National Chung Cheng University, Chia‐Yi, Taiwan, R.O.C., in 1989, 1992 and 2005, respectively.

From 1992 to 2007, he was a Lecturer in the Department of Electronic Engineering, Wu Feng Institute of Technology, Chia‐Yi, Taiwan, R.O.C. Since 2007, he has been with the Electronic Engineering, National Kaohsiung First University of Science and Technology, Kaohsiung, Taiwan, R.O.C. His research interests include Power Electronic, Saving‐Energy Illumination, Photovoltaic Supply System, Design of Converters and Inverters, Multi‐Inverter Parallel System, and Smart Grid.

25