A Single-Stage LED Streetlight Driver with Soft-Switching and Interleaved PFC Features

Abstract

This paper presents a single-stage driver with soft-switching and interleaved power-factor correction (PFC) features suitable for light-emitting diode (LED) energy-saving streetlight applications. The proposed LED streetlight driver integrates an interleaved buck-boost PFC converter with coupled inductors and a half-bridge LLC resonant converter into a single-stage power-conversion circuit with reduced voltage stress on the DC-linked capacitor and power switches, and it is suitable for operating at high utility-line voltages. Furthermore, coupled inductors in the interleaved buck-boost PFC converter are operated in discontinuous-conduction mode (DCM) for accomplishing PFC, and the half-bridge LLC resonant converter features zero-voltage switching (ZVS) to reduce switching losses of power switches, and zero-current switching (ZCS) to decrease conduction losses of power diodes. Operational modes and design considerations for the proposed LED streetlight driver are introduced. Finally, a 144 W (36V/4A)-rated LED prototype driver is successfully developed and implemented for supplying a streetlight module and operating with a utility-line input voltage of 220 V. High power factor, low output-voltage ripple factor, low output-current ripple factor, and high efficiency are achieved in the proposed LED streetlight driver.

1. Introduction

With recent developments in green lighting and energy saving around the world, light-emitting diodes (LEDs) are characterized by their small size, long life, high brightness and environmental friendliness [1,2,3]. As a result, LEDs have begun to play important roles as new solid-state light sources for indoor and outdoor energy-saving applications in our daily lives [4,5,6,7,8,9,10,11].

Streetlights that illuminate the road are designed to provide a safe night-time environment for cars, motorcycles, cyclists and pedestrians. The traditional source of illumination for streetlight applications is high-pressure mercury lamps, because of their low installation cost. However, high-pressure mercury lamps consume more energy and save less electricity. In addition, the discharge tubes of these lamps contain mercury vapor, which is harmful and can cause pollution to our environment when the lamp is exhausted. Therefore, LED streetlights with energy-saving features have begun replacing traditional high-pressure mercury street lamps [12,13]. Traditional two-stage drivers for LED streetlight applications include AC–DC converters with power-factor correction (PFC) and DC–DC converters that provide rated voltage and current to the LED streetlight [14,15]. However, the circuit is not efficient, and requires more power switches and components in a conventional two-stage streetlight driver. The literature presents some single-stage streetlight drivers that integrate an AC–DC converter with a DC–DC converter [16,17,18,19,20]. Figure 1 shows an existing single-stage LED streetlight driver, which combines an interleaved boost converter with a half-bridge-type LLC resonant converter into a single-stage power converter for supplying the LED street-lighting module at a utility-line voltage of 110V [16]. The LED streetlight driver comprises a low-pass filter, a bridge rectifier (D_r1, D_r2, D_r3 and D_r4), two capacitors (C_in1 and C_in2), two diodes (D_B1 and D_B2), two inductors (L₁ and L₂), two power switches (S₁ and S₂), a DC-linked capacitor C_DC, a resonant capacitor C_r and an inductor L_r, a center-tapped transformer T with two output windings, two diodes (D₁ and D₂), a capacitor C_o and the LED streetlight module. This kind of single-stage streetlight driver based on interleaved boost conversion is suitable for operating at utility-line voltages from 100~120 V in American and Asian countries, but will tolerate high voltage levels on the DC-linked capacitor C_DC when it operates at higher utility-line voltages due to boost-type power conversion, such as the 220~240 V in European countries. In addition, the voltage stresses of the power switches in this version will increase. Another existing single-stage LED streetlight driver, which integrates an interleaved boost PFC converter with a half-bridge-type series-resonant converter cascaded with a bridge rectifier for supplying the LED street-lighting module at a utility-line voltage of 220 V was proposed in [20], and high voltage stresses of power switches occurred in this version due to the boost-type power conversion; therefore, the level of DC-bus voltage is increased, and two DC-linked capacitors are required.

To meet these challenges, this paper proposes and implements a single-stage LED streetlight driver based on interleaved buck-boost conversion with PFC and soft-switching functions, which is suitable for operating at high utility-line voltages along with reduced voltage levels on the DC-linked capacitor and decreased voltage stresses on the power switches due to a buck-boost-type power conversion. This paper introduces the description and analysis of the operating modes, the design considerations of the key circuit components in the proposed LED streetlight driver, and experimental results obtained from the 144 W (36V/4A)-rated prototype circuit are included.

2. Circuit Derivation and Analysis of the Proposed Single-Stage LED Streetlight Driver

Figure 2a shows the original two-stage LED streetlight driver, which consists of buck-boost PFC converter #1 and buck-boost PFC converter #2 with interleaved operation in series connection with a half-bridge LLC resonant converter. In addition, the two coupled inductors are employed instead of single-winding inductors in order to accomplish buck-boost conversion. Figure 2b shows the presented LED streetlight driver with soft-switching and interleaved PFC feature, which integrates an interleaved buck-boost PFC converter with a half-bridge LLC resonant converter into single-stage power conversion and includes a low-pass filter (L_f and C_f), a bridge rectifier (D_r1, D_r2, D_r3 and D_r4), two capacitors (C_in1 and C_in2), two coupled inductors (L_B1 and L_B2; L_B3 and L_B4), four diodes (D_B1, D_B2, D_B3, and D_B4), two power switches (S₁ and S₂), a DC-bus capacitor (C_DC), a resonant capacitor (C_r), a resonant inductor (L_r), a center-tapped transformer T with a magnetizing inductor L_m and two output windings, two output diodes D₁ and D₂, an output capacitor (C_o) and the LED streetlight module. In addition, the diodes D_B2 and D_B3 are used to prevent current from entering the inductors L_B2 and L_B3 from the AC mains voltage sources. Furthermore, diodes D_B1 and D_B4 are capable of preventing the inductor currents from returning to the input capacitors C_in1 and C_in2. Since the voltage on the capacitor C_in1 or C_in2 is half of the utility-line voltage, the DC-bus voltage and the peak current of each coupled inductor will also be half. Due to the reduced DC-bus voltage, power switches with decreased voltage-stress can be utilized in the proposed LED streetlight driver, which is advantageous for high utility-line voltage applications.

Figure 3 shows a simplified circuit of the proposed single-stage LED streetlight driver when analyzing its operating modes. To describe the operation of the proposed LED streetlight driver, the following assumptions are made.

(a)

Since the switching frequency of the power switches is much higher than the utility-line frequency, the sinusoidal utility-line voltage can be considered to be a constant value in each high-frequency switching period.

(b)

The voltage sources V_REC1 and V_REC2 of capacitors C_in1 and C_in2, respectively, represent the rectified input utility-line voltages.

(c)

The power switches S₁ and S₂ operate complementarily, and their intrinsic body diode and drain-source capacitance are taken into consideration.

(d)

The turn-on voltage drops of diodes (D_B1, D_B2, D_B3, D_B4, D₁ and D₂) are omitted.

(e)

To naturally obtain PFC, the coupled inductors (L_B1 and L_B2; L_B3 and L_B4) are designed to operate in discontinuous-conduction mode (DCM).

The operational modes and key waveforms of the LED streetlight driver proposed in this paper are shown in Figure 4 and Figure 5, respectively, and the analysis of the operation is described in detail below.

Mode 1 (t₀ ≤ t < t₁; in Figure 4a): When the switch voltage v_DS1 decreases to zero and the body diode of switch S₁ is forward-biased at time interval t₀, this mode begins and the power switch S₁ turns on with zero-voltage switching (ZVS). The voltage source V_REC1 charges the coupled inductor L_B1 through diode D_B1 and switch S₁. The inductor current i_LB1 increases linearly from zero and can be given by:

$$i_{LB1}(t)=\frac{\left|\sqrt{2}{v}_{AC-rms}\sin (2\pi f_{AC}t)\right|}{2L_{B1}}(t-t_0)$$

(1)

where v_AC-rms represents the rms value of input utility-line voltage, and f_AC represents the utility-line frequency.

The resonant inductor L_r and magnetizing inductor L_m provide energy to the resonant capacitor C_r and to DC-linked capacitor C_DC through the body diode of switch S₁, and to the output capacitor C_o and the LED streetlight module through transformer T and output diode D₁. The diode D_B3 is forward-biased, and the coupled inductors L_B3 and L_B4 provide energy to the drain-source capacitor of switch S₂ through diode D_B3. This mode ends when the resonant inductor current i_Lr is zero at time t₁.

Mode 2 (t₁ ≤ t < t₂; in Figure 4b): This mode is activated when the resonant inductor current i_Lr reaches zero at t₁. The voltage source V_REC1 continues charging the coupled inductor L_B1 through diode D_B1 and switch S₁.

The capacitors C_DC, the magnetizing inductor L_m and the coupled inductors L_B3 and L_B4 provide energy to the drain-source capacitor of switch S₂, the resonant inductor L_r and the resonant capacitor C_r through D_B3, and to the output capacitor C_o and the LED streetlight module through transformer T and output diode D₁. When the magnetizing inductor current i_Lm and inductor current i_LB4 become zero at t₂, this mode finishes.

Mode 3 (t₂ ≤ t < t₃; in Figure 4c): At t₂, the voltage source V_REC1 continues charging the coupled inductor L_B1 through D_B1 and S₁. The capacitors C_DC provides energy to the resonant inductor L_r, the resonant capacitor C_r, and the magnetizing inductor L_m through S₁, and to the output capacitor C_o and the LED streetlight module through transformer T and output diode D₁. This mode ends when the diode current i_D1 becomes zero at t₃.

Mode 4 (t₃ ≤ t < t₄; in Figure 4d): This mode activates when i_D1 is zero at t₃. The voltage source V_REC1 continues charging the coupled inductor L_B1 through D_B1 and S₁. The capacitor C_DC continues providing energy to the inductors L_r and L_m and to C_r through S₁. The output capacitor C_o provides energy to the LED streetlight module. The coupled-inductor current i_LB1 reaches its peak value at t₄, which is denoted as i_LB1-pk(t), and is given by:

$$i_{LB1-pk}(t)=\frac{\left|\sqrt{2}{v}_{AC-rms}\sin (2\pi f_{AC}t)\right|}{2L_{B1}}D{T}_S$$

(2)

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

When switch S₁ turns off at t₄, this mode finishes.

Mode 5 (t₄ ≤ t < t₅; in Figure 4e): This mode begins when S₁ is off and i_LB1 is at its maximum level at t₄. The diode D_B2 is forward-biased and coupled inductors L_B1 and L_B2 provide energy to the drain-source capacitor of S₁ through D_B2. The coupled-inductor current i_LB1 linearly decreases, and it can be given by:

$$i_{LB1}(t)=\frac{V_{DC}}{4L_{B1}}(t-t_4)$$

(3)

where V_DC represents the voltage of the DC-bus capacitor.

The capacitor C_DC and the drain-source capacitor of S₂, provide energy to inductors L_r and L_m and to C_r. The output capacitor C_o continues providing energy to the LED streetlight module. At time interval t₅, the voltage v_DS2 of power switch S₂ is decreased to zero; then this mode ends.

Mode 6 (t₅ ≤ t < t₆; in Figure 4f): When the switch voltage v_DS2 is decreased to zero and the body diode of switch S₂ is forward-biased at t₅, this mode activates and the power switch S₂ turns on with ZVS feature. The voltage source V_REC2 provides energy to coupled inductor L_B4 through diode D_B4 and switch S₂, and the inductor current i_LB4 increases linearly from zero. The coupled inductors L_B1 and L_B2 continue providing energy to the drain-source capacitor of S₁ through D_B2, and the inductor current i_LB1 continues linearly decreasing. The capacitor C_DC continues providing energy to inductors L_r and L_m and to the resonant capacitor C_r through the body diode of S₂. The output capacitor C_o continues providing energy to the LED streetlight module. This mode finishes when the magnetizing inductor current i_Lm reaches its peak value at t₆.

Mode 7 (t₆ ≤ t < t₇; in Figure 4g): This mode begins when the magnetizing inductor current i_Lm is at its maximum level at t₆. The voltage source V_REC2 continues providing energy to coupled inductor L_B4 through diode D_B4 and switch S₂, and i_LB4 continues linearly increasing. The coupled inductors L_B1 and L_B2 continue providing energy to the drain-source capacitor of S₁ through D_B2, and i_LB1 continues linearly decreasing. The DC-linked capacitor C_DC provides energy to the drain-source capacitor of S₁ through S₂. The resonant inductor L_r provides energy to resonant capacitor C_r through switch S₂. The magnetizing inductor L_m provides energy to the output capacitor C_o and the LED streetlight module through transformer T and diode D₂. This mode ends when the inductor current i_LB1 is decreased to zero at t₇.

Mode 8 (t₇ ≤ t < t₈; in Figure 4h): This mode activates when the current i_LB1 is zero at t₇. The voltage source V_REC2 continues providing energy to coupled inductor L_B4 through diode D_B4 and switch S₂. The coupled inductors L_B1 and L_B2 continue providing energy to the drain-source capacitor of S₁ through D_B2. The DC-linked capacitor C_DC continues providing energy to the drain-source capacitor of S₁ through S₂. The resonant inductor L_r and the magnetizing inductor L_m provide energy to resonant capacitor C_r through switch S₂ and to the output capacitor C_o along with the LED streetlight module through transformer T and diode D₂. This mode finishes when the magnetizing inductor current i_Lm is decreased to zero at t₈.

Mode 9 (t₈ ≤ t < t₉; in Figure 4i): This mode begins when the magnetizing inductor current i_Lm is zero at t₈. The voltage source V_REC2 continues providing energy to coupled inductor L_B4 through diode D_B4 and switch S₂, and i_LB4 continues linearly increasing. The DC-linked capacitor C_DC continues providing energy to the drain-source capacitor of S₁ through S₂. The inductors L_r and L_m continue providing energy to the capacitor C_r through switch S₂, and to the output capacitor C_o along with the LED streetlight module through transformer T and diode D₂. When the diode current i_D2 decreases to zero, this mode ends.

Mode 10 (t₉ ≤ t < t₁₀; in Figure 4j): This mode activates when the current i_D2 is zero at t₉. The voltage source V_REC2 continues providing energy to the coupled inductor L_B4 through D_B4 and S₂, and i_LB4 continues increasing linearly. The DC-linked capacitor C_DC continues providing energy to the drain-source capacitor of S₁ through S₂. The inductors L_r and L_m continue providing energy to the capacitor C_r through switch S₂. The output capacitor C_o supplies energy to the LED streetlight module. When S₂ turns off and i_LB4 reaches its peak value at t₁₀, this mode finishes.

Mode 11 (t₁₀ ≤ t < t₁₁; in Figure 4k): This mode begins when switch S₂ is turned off and i_LB4 is at its maximum level at t₁₀. The coupled inductors L_B3 and L_B4 provide energy to the drain-source capacitor of S₂ through D_B3, and the inductor current i_LB4 linearly decreases. The drain-source capacitor of S₁ and the inductors L_r and L_m supply energy to capacitors C_r and C_DC. The output capacitor C_o still provides energy to the LED streetlight module. When the switch voltage v_DS1 decreases to zero at t₁₁, this mode ends, and Mode 1 begins again for the next switching period.

3. Design Considerations in the Presented LED Streetlight Driver

3.1. Design of Coupled Inductors L_B1, L_B2, L_B3 and L_B4

Referring to Figure 3, the rectified voltages V_REC1 and V_REC2 are theoretically equal due to the same capacitance of capacitors C_in1 and C_in2, and they can be expressed by

$$V_{REC1}(t)=V_{REC2}(t)=\frac{\sqrt{2}{v}_{AC-rms}\left|\sin (2\pi f_{AC}t)\right|}{2}$$

(4)

The switching frequency f_s is much larger than the line frequency f_AC; thus, rectified voltages V_REC1 and V_REC2 could be regarded as a constant value during one switching period. Referring to Figure 2b, the peak level of diode currents i_DB1 and i_DB4 can be represented by

$$i_{DB1,pk}(t)=i_{DB4,pk}(t)=\frac{\sqrt{2}{v}_{AC-rms}\left|\sin (2\pi f_{AC}t)\right|Duty}{2L_B{f}_s}$$

(5)

where L_B represents the inductance of coupled inductors L_B1, L_B2, L_B3 and L_B4, and Duty is the duty cycle of the switches S₁ and S₂.

The peak level of the rectified input current i_rec, denoted as i_rec,pk, can be represented by

$$i_{rec,pk}(t)=i_{DB1,pk}(t)+i_{DB4,pk}(t)=\frac{\sqrt{2}{v}_{AC-rms}\left|\sin (2\pi f_{AC}t)\right|Duty}{L_B{f}_s}$$

(6)

By filtering the high-frequency components of i_rec,pk(t), the input current i_AC is equal to the average level of i_rec,pk(t) during one switching period and can be expressed as

$$i_{AC}(t)=\frac{1}{T_{AC}}{\displaystyle {\int }_0^{T_{AC}}{i}_{rec,pk}(t)\cdot dt}=\frac{\sqrt{2}{v}_{AC-rms}Dut{y}^2\left(\sin (2\pi f_{AC}t)\right)}{2L_B{f}_s}$$

(7)

where T_AC is the utility-line period.

The average value of input utility-line power P_in is obtained by:

$$P_{in}=\frac{1}{T_{AC}}{\displaystyle {\int }_0^{T_{AC}}{v}_{AC}(t)i_{AC}(t)dt}=\frac{v_{AC-rms}{}^{2}Dut{y}^2}{4L_B{f}_s}$$

(8)

The rated output power P_o of the LED street-lighting module is related with input power P_in and is given by

$$P_o=\eta P_{in}$$

(9)

where η is the estimated efficiency of the LED driver.

From (8) and (9), the design equation of the inductance L_B of coupled inductors is given by

$$L_B=\frac{\eta v_{AC-rms}{}^{2}Dut{y}^2}{4P_o{f}_S}$$

(10)

With a η of 0.85, a Duty of 0.5, a P_o of 144 W, a switching frequency f_S of 100 kHz, and a v_AC-rms of 220 V, the inductance L_B of coupled inductors is given by

$$L_B=\frac{0.85\cdot {220}^2\cdot {0.5}^2}{4\cdot 144\cdot 100k}=178.6\mu H$$

3.2. Determining the Transformer Turns-Ratio n

The turns-ratio n of transformer T is given as

$$n=\frac{n_p}{n_s}\ge \frac{D\sqrt{2}{v}_{AC-rms}}{V_o+V_F}$$

(11)

where V_F is the forward voltage drop of the output-rectifier diodes D₁ and D₂; and V_o is the output voltage.

With a V_o of 36 V and a V_F of 0.7 V, the turns-ratio n is given by

$$n=\frac{n_p}{n_s}\ge \frac{0.5\cdot \sqrt{2}\cdot 220}{36+0.7}=4.3$$

The turns-ratio n is selected as 5.

3.3. Determining the LLC Resonant Network

The quality factor Q_r is defined as

$$Q_r=\frac{\sqrt{L_r}}{R_a\sqrt{C_r}}$$

(12)

where R_eq is the equivalent output resistor referring to the primary side of transformer T, and which can be expressed by the following equation:

$$R_{eq}=\frac{8n^2{V}_o}{{\pi }^2{I}_o}$$

(13)

The main resonant frequency ω_r1 and secondary resonant frequency ω_r2 of the LLC resonant network are respectively defined as

$${\omega }_{r1}=2\pi f_{r1}=\frac{1}{\sqrt{L_r{C}_r}}$$

(14)

$${\omega }_{r2}=2\pi f_{r2}=\frac{1}{\sqrt{\left(L_m+L_r\right)C_r}}$$

(15)

The inductance ratio A is defined as

$$A=\frac{L_m}{L_r}$$

(16)

In addition, substituting (16) into (14) and (15), the secondary resonant frequency f_r2 is given by

$$f_{r2}=\sqrt{\frac{f_{r1}{}^2}{A+1}}$$

(17)

With an f_r1 of 120 kHz and an A of 5, the secondary resonant frequency f_r2 is computed by

$$f_{r2}=\sqrt{\frac{{\left(120k\right)}^2}{5+1}}\cong 49kHz$$

Dividing (12) by (14), the resonant inductor L_r can be expressed by

$$L_r=\frac{Q_r{R}_{eq}}{2\pi f_{r1}}$$

(18)

The resonant capacitor C_r can be obtained by

$$C_r=\frac{1}{{\left(2\pi f_{r1}\right)}^2{L}_r}$$

(19)

With an f_r1 of 120 kHz, an R_eq of 182.4Ω, an A of 5 and a Q_r of 0.4, the resonant inductor L_r is given by

$$L_r=\frac{Q_r{R}_{eq}}{2\pi f_{r1}}=\frac{0.4\cdot 182.4}{2\pi \cdot 120k}=96.8\mu H$$

In addition, the inductor L_r is selected as 90 μH, and the magnetic inductor L_m is selected as 450μH according to (16).

The resonant capacitor is given by

$$C_r=\frac{1}{{\left(2\pi f_{r1}\right)}^2{L}_r}=\frac{1}{{\left(2\pi \cdot 120k\right)}^2\cdot 90\mu }=19.5nF$$

Additionally, the resonant capacitor C_r is selected as 22 nF.

3.4. Design Guidelines of Achieving Soft-Switching in the Proposed LED Streetlight Driver

By using the fundamental approximation method, the voltage gain |M_V| of the LLC resonant network is given by [18]:

$$\begin{array}{l}\left|M_V\left(2\pi f_S\right)\right|=\frac{n\frac{4V_O}{\pi }\sin 2\pi f_{S}t}{\frac{2}{\pi }{V}_{DC}\sin 2\pi f_{S}t}=\frac{2n\cdot V_o}{V_{DC}}\cong \frac{2n\cdot V_o}{\sqrt{2}\cdot v_{AC-rms}}\\ =\left|\frac{A{\left(\frac{f_S}{f_{r1}}\right)}^2}{\left[\left(A+1\right){\left(\frac{f_S}{f_{r1}}\right)}^2-1\right]+j{Q}_{r}A\left(\frac{f_S}{f_{r1}}\right)\left[{\left(\frac{f_S}{f_{r1}}\right)}^2-1\right]}\right|\end{array}$$

(20)

In this design procedure of the proposed LED streetlight driver, the main resonant frequency f_r1, the inductance ratio A, and quality factor Q_r are selected to be 120 kHz, 5, and 0.4, respectively. According to (20), Figure 6 shows the relationship between voltage gain |M_V| and switching frequency f_S under different quality factor Q_r. In addition, the right-hand side and left-hand side of the pink line (which is the constraint line for achieving soft-switching) are inductive region and capacitive region, respectively. To achieve soft-switching for reducing power losses in the proposed LED streetlight driver, the LLC resonant network inside the driver is recommended to be operated at inductive region. Moreover, with a turns-ratio n of 5, an output voltage V_O of 36V and a rated input utility-line voltage v_AC-rms of 220V, the rated voltage gain M_V-rated is obtained by

$$M_{V-rated}=\frac{2n{V}_O}{\sqrt{2}\cdot v_{AC-rms}}=\frac{2\cdot 5\cdot 36}{\sqrt{2}\cdot 220}=1.16$$

In addition, the switching frequency f_S is designed at 100 kHz under a rated voltage gain M_V-rated of 1.16. If the rated input utility-line voltage has some variations (for example, 10V), the required maximum voltage gain M_V-max occurred at minimum input voltage and the required minimum voltage gain M_V-min occurred at maximum input voltage are respectively calculated by

$$M_{V-\max }=\frac{2n{V}_O}{\sqrt{2}\cdot v_{AC-\min }}=\frac{2\cdot 5\cdot 36}{\sqrt{2}\cdot (220-10)}=1.21$$

$$M_{V-\min }=\frac{2n{V}_O}{\sqrt{2}\cdot v_{AC-\max }}=\frac{2\cdot 5\cdot 36}{\sqrt{2}\cdot (220+10)}=1.11$$

Please see Figure 6, by using variable frequency control scheme, the switching frequencies under required maximum and minimum voltage gains (M_V-max and M_V-min) are adjusted to be 90 kHz and 110 kHz due to variations of input utility-line voltage, respectively. Furthermore, these switching frequencies are located at the right-hand side of the pink line. As a result, soft-switching features are also achieved when the rated input utility-line voltage has variations of 10 V.

4. Experimental Results of the Prototype LED Streetlight Driver

A prototype driver has been successfully developed and implemented for supplying a 144 W-rated (36V/4A) LED streetlight module with an input utility-line voltage of 220 V.

Table 1. Specifications of the presented singles-stage LED streetlight driver.

Parameter	Value
Input Utility-Line Voltage vAC	220 V (rms)
Output Rated Power PO	144 W
Output Rated Voltage VO	36 V
Output Rated Current IO	4 A

and

Table 2. Key components utilized in the presented LED streetlight driver.

Component	Value
Capacitors Cin1, Cin2	330 nF
Inductors LB1, LB2, LB3, LB4	179 μH
Diodes DB1, DB2, DB3, DB4	MUR460
Power Switches S1, S2	STP20NM60
DC-Linked Capacitor CDC	220 μF/450 V
Magnetizing Inductor Lm	450 μH
Resonant Inductor Lr	90 μH
Resonant Capacitor Cr	22 nF
Diodes D1, D2	MBR30H100CT
Output Capacitor Co	2200 μF/63 V
Filter Inductor Lf	2.5 mH
Filter Capacitor Cf	1 μF

, respectively, show the specifications and key components utilized in the presented single-stage LED streetlight driver. Additionally, Figure 7 shows the proposed LED streetlight driver with control block diagram. A constant-voltage and constant-current (CV-CC) controller is adopted to sense the output voltage through resistors R_VS1 and R_VS2, while simultaneously sensing the output current through the resistor R_CS for supplying the rated voltage and current to the experimental LED street-lighting module. The output signal of the CV-CC controller feeds into the high-voltage resonant controller through a photo-coupler. Two gate-driving signals v_gs1 and v_gs2 generating from the resonant controller regulate the output voltage and current of the LED street-lighting module by utilizing variable-frequency control scheme. Moreover, the coupled-inductors (L_B1, L_B2, L_B3 and L_B4) are designed to be operated at discontinuous conduction mode (DCM) for naturally achieving input-current shaping without utilizing a power-factor-correction controller with a feed-forward controlling path.

The measured waveforms of coupled-inductor currents i_LB1 and i_LB4 are shown in Figure 8; both have interleaved features and operate in DCM. Figure 9 shows the measured switch voltage v_DS2 and switch current i_DS2; thus, ZVS has occurred on the power switch for lowering switching losses. Figure 10 presents the measured switch voltage v_DS2 and resonant inductor current i_Lr. Figure 11 presents the measured switch voltage v_DS2 and current i_D2 of the output rectified diode D₂; thus, ZCS has occurred on the power diode for decreasing the conduction losses. Figure 12 depicts the measured output voltage V_O and current I_O; their average values are approximately 36 V and 4 A, respectively.

The measured waveforms of input utility-line voltage v_AC and current i_AC are shown in Figure 13, and the input current is in phase with utility-line voltage, which results in high power factor. In addition, the measured power factor and the circuit efficiency are 0.9684 and 89.69%, respectively, as measured by a power analyzer (Tektronix PA 4000). Figure 14 shows the measured input utility-line current harmonics at an input utility-line voltage of 220 V in comparison with the International Electrotechnical Commission (IEC) 61000-3-2 Class C standards; all utility-line current harmonics meet the requirements.

Table 3. Measured output voltage ripple and current ripple of the presented LED streetlight driver at a utility line voltage of 220V.

Parameter	Value
Mean Value of Output Voltage VO	35.85 V
Peak-to-Peak Value of Output Voltage VO	1.53 V
Voltage Ripple Factor	4.28 %
Mean Value of Output Current IO	3.95 A
Peak-to-Peak Value of Output Current IO	48.9 mA
Current Ripple Factor	1.24 %

shows the measured output voltage ripple and current ripple of the presented LED streetlight driver at a utility-line voltage of 220 V; additionally, the output voltage (current) ripple factor is obtained by the peak-to-peak level divided by the mean value of output voltage (current). It can be seen that the measured voltage and current ripple factors are smaller than 5% and 2%, respectively. Figure 15 presents a photo of supplying the LED streetlight module with the proposed streetlight driver at an input utility-line voltage of 220V. In addition,

Table 4. Comparisons between the existing single-stage LED streetlight driver in [16,17,18,19], and the version proposed in this paper.

Item	Presented Driver in Reference [16]	Presented Driver in Reference [17]	Presented Driver in Reference [18]	Presented Driver in Reference [19]	Proposed Driver
Circuit Topology	Integration of interleaved boost PFC converter and LLC resonant converter	Integration of dual buck-boost converter with coupled inductors and LLC resonant converter	Integration of modified bridgeless boost PFC converter and LLC resonant converter	Integration of dual boost converter with coupled inductors and LLC resonant converter	Integration of interleaved buck-boost converter with coupled inductors and LLC resonant converter
Number of Required Power Switches	2	2	2	2	2
Number of Required Diodes	8	6	4	4	10
Number of Required Capacitors	6 (Including one DC-bus capacitor)	5 (Including two DC-bus capacitors)	4 (Including one DC-bus capacitor)	4 (Including one DC-bus capacitor)	6 (Including one DC-bus capacitor)
Number of Required Magnetic Components	5	4	4	4	5
Input Utility-Line Voltage	110V	110V	110V	110V	220V
Output Power	144W (36V/4A)	144W (36V/4A)	144W (36V/4A)	144W (36V/4A)	144W (36V/4A)
Voltage Stress of Power Switches	$\frac{1}{1-D}\sqrt{2}{v}_{AC-rms}$	$\frac{2D}{1-D}\sqrt{2}{v}_{AC-rms}$	$\frac{1}{1-D}\sqrt{2}{v}_{AC-rms}$	$\frac{1}{1-D}\sqrt{2}{v}_{AC-rms}$	$\frac{D}{1-D}\sqrt{2}{v}_{AC-rms}$
Voltage Ripple Factor	< 6%	< 7%	< 8%	< 4%	< 5%
Current Ripple Factor	< 10%	< 5%	< 13%	< 4%	< 2%
Measured Power Factor	> 0.99	> 0.99	> 0.99	> 0.98	> 0.97
Measured Circuit Efficiency	> 88 %	> 90%	> 92%	> 92%	≒ 90%

shows comparisons between the existing single-stage LED streetlight driver in references [16,17,18,19] and the one proposed in this paper. According to this table, the proposed single-stage LED streetlight driver has a beneficial feature of reduced voltage stress of power switches, which is favorable for operating with high utility-line voltages, in comparison to the existing single-stage versions in the references [16,17,18,19]. In addition, the proposed circuit has the lowest current ripple factor among these LED streetlight drivers.

5. Conclusions

This paper has presented and implemented a single-stage LED streetlight driver with soft-switching and PFC features; the proposed circuit integrates an interleaved buck-boost converter with coupled inductors and a half-bridge LLC resonant converter into a single power-conversion stage, and is suitable for operating at high utility-line voltages with reduced voltage stress on the DC-linked capacitor. A 144 W prototype LED driver has been developed and tested with an input utility-line voltage of 220 V. The experimental results of the presented LED streetlight driver display low output-voltage ripple factor (< 5%), low output-current ripple factor (< 2%), high power factor (> 0.97), ZVS on power switches, ZCS on output rectified diodes, and high circuit efficiency (approximately 90%); thus the functionality of the presented LED streetlight driver is validated.