A Single-Stage LED Tube Lamp Driver with Power-Factor Corrections and Soft Switching for Energy-Saving Indoor Lighting Applications

Abstract

This paper presents a single-stage alternating current (AC)/direct current (DC) light-emitting diode (LED) tube lamp driver for energy-saving indoor lighting applications; this driver features power-factor corrections and soft switching, and also integrates a dual buck-boost converter with coupled inductors and a half-bridge series resonant converter cascaded with a bridge rectifier into a single-stage power-conversion topology. The features of the presented driver are high efficiency (>91%), satisfying power factor (PF > 0.96), low input-current total-harmonic distortion (THD < 10%), low output voltage ripple factor (<7.5%), low output current ripple factor (<8%), and zero-voltage switching (ZVS) obtained on both power switches. Operational principles are described in detail, and experimental results obtained from an 18 W-rated LED tube lamp for T8/T10 fluorescent lamp replacements with input utility-line voltages ranging from 100 V to 120 V have demonstrated the functionality of the presented driver suitable for indoor lighting applications.

1. Introduction

Latest developments and applications of solid-state lighting have begun gaining greater attention due to the requirements for efficient energy usage nowadays [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]. With features of energy-savings, lower maintenance costs and long lifetime, light-emitting diode (LED) tube lamps have begun to be favorable alternatives to replace traditional T8/T10-type fluorescent lamps in building-lighting applications [17,18,19]. According to the specifications shown in [20,21] with almost the same color temperature and color-rendering index, the LED tube lamp has better lighting efficiency, less power consumption, longer lamp lifetime, and contains no mercury inside the lamp tube, as compared to its T8-type counterparts. Therefore, T8-type LED tube lamps have become increasingly popular lighting sources for indoor lighting applications such as public architecture, offices, classrooms, and so on [17,18,19,20,21]. Figure 1 shows the commercial two-stage driver for supplying a T8-type LED tube lamp; it is composed of an AC-DC (alternating current-direct current) converter (typically a boost converter) with power-factor corrections (PFCs) as the first stage, and a DC-DC converter (typically a buck converter) as the second stage for regulating the voltage/current of the LED tube lamp. Separate controllers for each stage are required, and the circuit efficiency is restricted due to the two-stage power conversion. Some single-stage AC-DC drivers for providing a T8-type LED tube lamp to replace T8/T10-type fluorescent lamps have been presented: reference [18] utilizes a flyback PFC converter, and a buck converter with input-current shaping is presented in reference [19]. These single-stage versions are cost-effective in comparison to their two-stage counterpart; however, their power switches do not include the soft-switching function, which results in limited efficiency.

In response to these concerns, this paper presents a single-stage LED tube lamp driver suitable for indoor lighting applications with features of power-factor corrections and soft-switching characteristics. Theoretical analyses of operating modes, design equations for key circuit parameters, and experimental results from a prototype driver circuit for supplying an 18 W-rated T8-type LED tube lamp with input voltages ranging from 100 V to 120 V are demonstrated. This paper is organized as follows. Section 2 introduces and analyzes the presented single-stage LED tube lamp driver. Section 3 shows design equations of key components in the presented driver. Section 4 demonstrates experimental results of a prototype circuit for supplying an LED tube lamp. Finally, some conclusions are provided in Section 5.

2. The Description and Analysis of the Presented LED Tube Lamp Driver

Figure 2 shows the presented driver, which integrates the dual buck-boost converter with coupled inductors (one buck-boost converter includes a diode D_b1, a coupled inductor L_PFC1, a switch S₁, a body diode of the switch S₂, and a DC-linked capacitor C_DC1; another buck-boost converter includes a diode D_b2, a coupled inductor L_PFC2, a switch S₂, a body diode of the switch S₁, and the capacitor C_DC2) with the half-bridge series resonant converter cascaded with a bridge rectifier (including two DC-linked capacitors C_DC1 and C_DC2, two switches S₁ and S₂, a resonant inductor L_r, a resonant capacitor C_r, a full-bridge rectifier (including diodes D₁, D₂, D₃ and D₄), and an output capacitor C_o) into a single-stage topology for supplying an LED tube lamp [17]. In addition, an LC filter (an inductor L_f and a capacitor C_f) is connected with the input utility-line voltage.

In order to analyze the operations of the presented LED tube lamp driver, the following assumptions are made.

(a)

Since the switching frequencies of the two switches (S₁ and S₂) are much higher than that of the utility-line voltage v_AC, the sinusoidal utility-line voltage can be considered as a constant value for each high-frequency switching period.

(b)

Power switches are complementarily operated, and their intrinsic diodes and inherent drain-source capacitors are considered.

(c)

The conducting voltage drops of all diodes (including D_b1, D_b2, D_b3, D_b4, D₁, D₂, D₃ and D₄) are neglected.

(d)

Two coupled inductors (L_PFC1 and L_PFC2) are designed to be operated in discontinuous-conduction mode (DCM) for naturally achieving power-factor correction (PFC).

The operating modes and key waveforms of the presented LED tube lamp driver during the positive half-cycle of input utility-line voltage are shown in Figure 3 and Figure 4, respectively, and the analysis of operations for the presented driver are described in detail as follows.

Mode 1 (t₀ ≤ t < t₁; in Figure 3a): The drain-source voltage v_DS1 of S₁ is decreased to zero at time t₀; thus, the body diode of switch S₁ is forward-biased. The resonant capacitor C_r provides energy to resonant inductor L_r, capacitors C_DC1, C_DC2, C_o and the LED tube lamp through S₁’s body diode, D₂, and D₃. When power switch S₁ turns on with zero-voltage switching (ZVS) at t₁, this mode ends.

Mode 2 (t₁ ≤ t < t₂; in Figure 3b): This mode begins when switch S₁ achieves ZVS turn-on at t₁. The input voltage v_AC provides energy to coupled inductor L_PFC1 through diode D_b1 and switch S₁. The inductor current i_LPFC1 linearly increases from zero, and can be expressed as

$$i_{LPFC1}(t)=\frac{\sqrt{2}{v}_{AC-rms}\sin (2\mathsf{\pi }{f}_{AC}t)}{L_{PFC1}}\left(t-t_1\right).$$

(1)

where v_AC-rms is the root-mean-square (rms) value of the input utility-line voltage, and f_AC is the utility-line frequency.

The resonant capacitor C_r still provides energy to inductor L_r, capacitors C_DC1, C_DC2, C_o and the LED tube lamp through S₁, D₂, and D₃. This mode finishes when the resonant inductor current i_Lr is zero at t₂.

Mode 3 (t₂ ≤ t < t₃; in Figure 3c): The input voltage v_AC still provides energy to the coupled inductor L_PFC1 through diode D_b1 and switch S₁. The DC-bus capacitors C_DC1 and C_DC2 supply energy to resonant inductor L_r, capacitors C_r and C_o, and the LED tube lamp through switch S₁, D₁, and D₄. This mode ends when inductor current i_LPFC1 reaches its peak value at t₃, which is denoted as i_LPFC1-pk(t), and can be expressed as

$$i_{LPFC1-pk}(t)=\frac{\sqrt{2}{v}_{AC-rms}\sin (2\mathsf{\pi }{f}_{AC}t)}{L_{PFC1}}D{T}_S.$$

(2)

where D and T_S are the duty cycle and period, respectively, of the power switch.

Mode 4 (t₃ ≤ t < t₄; in Figure 3d): This mode begins when S₁ turns off at t₃. The utility-line voltage v_AC and the coupled inductor L_PFC1 supply energy to the drain-source capacitor of S₁ through diode D_b1.

The DC-bus capacitors C_DC1 and C_DC2, the drain-source capacitor of S₂ and resonant inductor L_r provide energy to capacitors C_r and C_o and the LED tube lamp through diodes D₁ and D₄. When the drain-source voltage v_DS2 of S₂ is decreased to zero at t₄, this mode ends.

Mode 5 (t₄ ≤ t < t₅; in Figure 3e): This mode begins when the body diode of switch S₂ is forward-biased at t₄. The coupled inductor L_PFC1 provides energy to C_DC1 through diode D_b3. The inductor current i_LPFC1 linearly decreases, and can be given by

$$i_{LPFC1}(t)=\frac{\sqrt{2}{v}_{AC-rms}\sin (2\mathsf{\pi }{f}_{AC}t)-V_{DC}/2}{L_{PFC1}}\left(t-t_4\right).$$

(3)

where V_DC/2 is the voltage of the DC-linked capacitor C_DC1.

Inductor L_r provides energy to capacitors C_r and C_o and the LED tube lamp through S₂’s body diodes, D₁ and D₄. When power switch S₂ is turned on with ZVS at t₅, this mode ends.

Mode 6 (t₅ ≤ t < t₆; in Figure 3f): This mode begins when switch S₂ achieves ZVS turn-on at t₅. Inductor L_r continues providing energy to capacitors C_r and C_o and the LED tube lamp through S₂, D₁ and D₄. The mode ends when the resonant inductor current i_Lr decreases to zero at t₆.

Mode 7 (t₆ ≤ t < t₇; in Figure 3g): This mode begins when current i_Lr decreases to zero at t₆. Capacitor C_r provides energy to inductor L_r, capacitor C_o and the LED tube lamp through S₂ and diodes D₂ and D₃. The mode ends when switch S₂ turns off at t₇.

Mode 8 (t₇ ≤ t < t₈; in Figure 3h): In this mode, capacitor C_r and the drain-source capacitor of switch S₁ provide energy to inductor L_r, the drain-source capacitor of S₂, capacitors C_DC1, C_DC2, C_o, and the LED tube lamp through diodes D₂ and D₃. When the drain-source voltage v_DS1 of S₁ is decreased to zero at t₇, this mode ends. Then, Mode 1 begins for the next high-frequency switching period.

Figure 5 shows the circuit diagram for controlling the single-stage LED tube lamp driver. Referring to Figure 5 and utilizing a constant voltage/current controller (IC1 SEA05) for regulating the LED tube lamp’s output voltage and current, the output voltage V_o can be sensed through resistors R_vs1, VR₁ and R_vs2, and the output current can be sensed through resistor R₃. The sensed output signal from pin 5 of the IC1 feeds into the high-voltage resonant controller (IC3 ST L6599) through a photo-coupler (IC2 PC817). Two gate-driving signals v_GS1 and v_GS2 are generated from pin 15 and pin 11, respectively, of the IC3, to carry out regulating the LED tube lamp’s output voltage and current. In addition, an AC-DC switching power module with dual output voltages is utilized for providing the 15 V power supply voltages of IC1 and IC3 in the control circuit.

3. Design Equations of Key Components in the Presented LED Tube Lamp Driver

3.1. Design of Coupled Inductors L_PFC1 and L_PFC2

The design equation of the coupled inductor L_PFC1 (L_PFC2) is expressed as [22]

$$L_{PFC1}=\frac{\mathsf{\eta }{v_{AC-rms}}^2{D}^2}{2P_{lamp}{f}_S}=L_{PFC2}$$

(4)

where η is the estimated circuit efficiency, P_lamp is the rated power of the LED tube lamp, and f_S is the switching frequency.

With a η of 0.9, a v_AC-rms of 110 V and a P_lamp of 18 W, an f_S of 55 kHz and a D of 0.5, the coupled inductors L_PFC1 and L_PFC2 are given by:

$$L_{PFC1}=L_{PFC2}=\frac{0.9\times {110}^2\times {0.5}^2}{2\times 18\times \mathrm{55,000}}=1.34\mathrm{mH}.$$

3.2. Design of Series Resonant Tank L_r and C_r

Figure 6 depicts the equivalent circuit during the design of the series resonant tank; R_o is the equivalent resistance of the T8-type LED tube lamp, and can be represented by R_o = V_o/I_o. By using the fundamental approximation theory, the fundamental components of the switch voltage v_DS2 can be expressed by [23,24]

$$v_{DS2-fund}=\frac{2}{\mathsf{\pi }}{V}_{DC}\sin 2\mathsf{\pi }{f}_{S}t.$$

(5)

Referring to Figure 6, the equivalent load resistance R_eq can be expressed as [24]

$$R_{eq}=\frac{8V_o}{{\mathsf{\pi }}^2{I}_o}=\frac{8}{{\mathsf{\pi }}^2}{R}_o.$$

(6)

With a V_o of 60 V and an I_o of 0.3 A, the equivalent load resistance R_eq is given by

$$R_{eq}=\frac{8}{{\mathsf{\pi }}^2}\left(\frac{60}{0.3}\right)=162.1\mathsf{\Omega }.$$

Referring to Figure 6, the series resonant tank is composed of a resonant inductor L_r in series connection with a resonant capacitor C_r; and the resonant frequency f_o can be expressed by

$$f_o=\frac{1}{2\mathsf{\pi }\sqrt{L_r{C}_r}}.$$

(7)

The loaded quality factor Q_L is expressed by

$$Q_L=\frac{\sqrt{\frac{L_r}{C_r}}}{R_{eq}}.$$

(8)

In order to obtain ZVS for the two active switches, the switching frequency f_S is designed to be larger than the resonant frequency f_o so that the resonant tank resembles an inductive network [24].

Therefore, the relationship between switching frequency f_S and resonant frequency f_o is selected as

$$f_S=4f_o.$$

(9)

Combining (7) and (8) with (9), the resonant capacitor C_r is given by

$$C_r=\frac{2}{\mathsf{\pi }{f}_S{R}_{eq}{Q}_L}.$$

(10)

By selecting an f_S of 55 kHz and a Q_L of 0.9, the resonant capacitor C_r is computed as

$$C_r=\frac{2}{\mathsf{\pi }\times 55k\times 162.1\times 0.9}=79.3\mathrm{nF}.$$

In addition, C_r is selected to be 82 nF.

Combining (7) with (9), the resonant inductor L_r is given by

$$L_r=\frac{4}{{\mathsf{\pi }}^2{f_S}^2{C}_r}.$$

(11)

With an f_S of 55 kHz and a C_r of 82 nF, the resonant inductor L_r is computed as

$$L_r=\frac{4}{{\mathsf{\pi }}^2\times {(55k)}^2\times 82n}=1.63\mathrm{mH}.$$

4. Experimental Results

A prototype driver has been built and tested for supplying an 18 W-rated T8-type LED tube lamp (EVERLIGHT FBW/T8/857/U/4ft, ELECTRONICS Co., Ltd., New Taipei City, Taiwan), whose rated voltage and current are 60 V and 0.3 A, respectively. The components utilized in the LED tube lamp driver are shown in

Table 1. Key components used in the presented led tube lamp driver.

Component	Value
Filter Inductor (Lf)	2 mH
Filter Capacitor (Cf)	0.68 μF/250 V
Diodes (Db1, Db2)	MUR460
Power Switches (S1, S2)	IRF840
Coupled Inductors (LPFC1, LPFC2)	1.34 mH
DC (direct current)-Linked Capacitors (CDC1, CDC2)	100 μF/450 V
Resonant Inductor (Lr)	1.68 mH
Resonant Capacitor (Cr)	82 nF
Diodes (D1, D2, D3, D4)	MUR460
Output Capacitor (Co)	200 μF/63 V

.

Figure 7 presents the measured inductor current i_LPFC1. The measured switch voltage v_DS2 and inductor current i_Lr are shown in Figure 8, and the series resonant tank resembles an inductive load. The measured DC-bus voltage is shown in Figure 9, and the average value of V_DC is approximately 318 V. Figure 10a,b presents the measured voltages v_DS1 and v_DS2 and currents i_DS1 and i_DS2 of the power switches S₁ and S₂, respectively. ZVS is obviously achieved on the power switches; thus, the circuit efficiency is increased.

Figure 11 shows the measured output voltage and current waveforms, and the average values of V_o and I_o are 60 V and 0.3 A, respectively.

Table 2. Measured output voltage and current of the presented led tube lamp driver under different input voltages.

Input Voltage	100 V	105 V	110 V	115 V	120 V
Parameters
Output Voltage (mean)	60.07 V	60.1 V	60.08 V	60.11 V	60.16 V
Output Voltage (pk–pk)	3.76 V	3.84 V	3.93 V	4.11 V	4.33 V
Output Current (mean)	0.321 A	0.311 A	0.308 A	0.304 A	0.301 A
Output Current (pk–pk)	23.9 mA	22.1 mA	21.3 mA	20.6 mA	20.3 mA

presents the measured output voltage and current of the presented LED tube lamp driver under different input voltages. Figure 12 shows the calculated voltage and current ripple factors of the presented LED tube lamp driver under different input utility-line voltages. In addition, the output voltage (current) ripple factor is obtained by the peak-to-peak (pk–pk) level divided by the average value of output voltage (current). According to Figure 12, the highest and lowest measured output-voltage ripple factors are 7.2% and 6.26%; these occurred at a utility-line rms voltage of 120 V and 100 V, respectively. Moreover, the highest and lowest measured output-current ripple factors are 7.45% and 6.74%; these occurred at a utility-line rms voltage of 100 V and 120 V, respectively.

The measured input utility-line voltage and current are shown in Figure 13, and the input current is in phase with the input voltage. Figure 14 presents the measured input-current harmonics compared with the International Electrotechnical Commission (IEC) 61000-3-2 Class C standards under input utility-line voltages ranging from 100 V to 120 V; all measured current harmonics meet the requirements. Figure 15 shows the measured power factor (PF) and current THD at input utility-line voltages ranging from 100 V to 120 V. At a utility-line rms voltage of 110 V, the measured PF and current THD are 0.96 and 9.3%, respectively. The maximum PF is 0.97 occurred at a utility-line rms voltage of 120 V, and the minimum current THD is 6.9% occurred at a utility-line rms voltage of 100 V.

Figure 16 shows the measured efficiency of the presented LED tube lamp driver under input utility-line voltages from 100 V to 120 V. The highest and lowest measured efficiency levels are 94.16% and 91.24%; these occurred at utility-line rms voltages of 100 V and 120 V, respectively. In addition, the efficiency which drops with increased utility-line voltages is related to the voltage gain of the LC series-resonant tank. For providing rated output power (voltage/current), the voltage gain of the LC series-resonant tank will decrease when the utility-line voltages increase, resulting in increasing the switching frequency of the power switches. Thus, the switching losses of power switches and conduction losses of power diodes will increase resulting in lowering the circuit efficiency.

The comparisons between the existing T8-Type LED tube lamp drivers and the proposed one are shown in

Table 3. Comparisons between the existing t8-type led tube lamp drivers and the proposed one.

Item	Existing LED (Direct Current) Tube Lamp Driver in [18]	Existing LED Tube Lamp Driver in [19]	Proposed LED Tube Lamp Driver
Circuit Topology	Flyback-type AC-DC (alternating current-direct current) Converter	Buck-type AC-DC Converter	Integration of a Dual Buck-Boost Converter with coupled inductors and a Half-Bridge Series Resonant Converter cascaded with a bridge rectifier
Input Utility-Line Voltages	90~264 V	90~264 V	100~120 V
Rated Output Power (Rated Output Voltage/Current)	19 W (42 V/0.45 A)	18.3 W (39 V/0.47 A)	18 W (60 V/0.3 A)
Required Power Switch	1 (Without Soft-Switching)	1 (Without Soft-Switching)	2 (With Soft-Switching)
Measured Maximum Power Factor	0.99 @ 110 V	0.96 @ 110 V	0.97 @ 120 V
Measured Minimum Input Current THD (total-harmonic distortion)	9% @ 180 V	21.54% @ 110 V	6.86% @ 100 V
Measured Maximum Efficiency	87.8% @ 180 V	88.56% @ 180 V	94.16% @ 100 V

. A flyback-type AC-DC converter as an LED tube lamp driver is presented in [18], and a buck-type AC-DC converter as an LED tube lamp driver is presented in [19]. In addition, these existing LED tube lamp drivers ([18,19]) operate with universal input voltages while the proposed version operates with American utility-line voltages. Although the circuit components counts in the proposed driver are higher than these existing ones, this table illustrates that the proposed single-stage LED tube lamp driver has smaller input current THD and achieves soft-switching on the power switches to increase circuit efficiency over the two existing versions.

5. Conclusions

This paper has presented and implemented a single-stage driver suitable for energy-saving indoor lighting applications, which integrates a dual buck-boost converter with coupled inductors and a half-bridge series resonant converter cascaded with a bridge rectifier, for supplying an LED tube lamp for T8/T10 fluorescent lamp replacements with power-factor corrections and soft switching. A prototype circuit has been successfully built and tested for supplying an 18 W-rated LED tube lamp with utility-line voltages ranging from 100 V to 120 V. The experimental results have demonstrated high efficiency (>91%), satisfying power-factor (>0.96), low current THD (<10%), and soft-switching of power switches in the presented LED tube lamp driver.