A Single-Stage High-Power-Factor Light-Emitting Diode ( LED ) Driver with Coupled Inductors for Streetlight Applications

This paper presents and implements a single-stage high-power-factor light-emitting diode (LED) driver with coupled inductors, suitable for streetlight applications. The presented LED driver integrates an interleaved buck-boost power factor correction (PFC) converter with coupled inductors and a half-bridge-type series-resonant converter cascaded with a full-bridge rectifier into a single-stage power conversion circuit. Coupled inductors inside the interleaved buck-boost PFC converter sub-circuit are designed to operate in discontinuous conduction mode (DCM) for achieving input-current shaping, and the half-bridge-type series resonant converter cascaded with a full-bridge rectifier is designed for obtaining zero-voltage switching (ZVS) on two power switches to reduce their switching losses. Analysis of operational modes and design equations for the presented LED driver are described and included. In addition, the presented driver features a high power factor, low total harmonic distortion (THD) of input current, and soft switching. Finally, a prototype driver is developed and implemented to supply a 165-W-rated LED streetlight module with utility-line input voltages ranging from 210 to 230 V. Experimental results demonstrate that high power factor (>0.99), low utility-line current THD (<7%), low-output voltage ripples (<1%), low-output current ripples (<10%), and high circuit efficiency (>90%) are obtained in the presented single-stage driver for LED streetlight applications.

Streetlights, which illuminate a road, aim to provide a safe environment during the night-time for motorcycle/bicycle drivers and pedestrians [3,17,18].Traditional lighting sources for streetlight applications have been high-pressure mercury lamps because of their low-cost.However, high-pressure mercury lamps are not energy efficient.In addition, the discharge tube containing mercury vapors is harmful in terms of polluting our environment when the lamp runs out.Therefore, LEDs have begun to replace the conventional high-pressure mercury streetlight.The conventional two-stage LED driver supplying a rated lamp power of greater than 70 W for streetlight applications, shown in Figure 1, consists of an input low-pass filter (L f and C f ) connected with a full-bridge rectifier (D 1 , D 2 , D 3 and D 4 ), an interleaved boost power factor correction (PFC) converter (including two capacitors C in1 and C in2 , two diodes D B1 and D B2 , two inductors L 1 and L 2 , two power switches S 1 and S 2 , and a DC-linked capacitor C B ), and a half-bridge-type LLC resonant converter (including a Direct-Current (DC)-linked capacitor C B , two power switches S 3 and S 4 , a resonant capacitor C r , a resonant inductor L r , a center-tapped transformer T 1 with two output windings, two output diodes D 5 and D 6 and an output capacitor C o ), along with a LED [19].Due to two-stage power conversions, the circuit efficiency is limited and more power switches and components are required in the conventional driver.
In response to these challenges, this paper presents and implements a single-stage LED driver with coupled inductors and high power factor for streetlight applications.Descriptions and analysis of operational modes, and design equations of key components in the presented LED driver, and experimental results obtained from a prototype circuit are demonstrated.
Appl.Sci.2017, 7, 167 2 of 18 (D1, D2, D3 and D4), an interleaved boost power factor correction (PFC) converter (including two capacitors Cin1 and Cin2, two diodes DB1 and DB2, two inductors L1 and L2, two power switches S1 and S2, and a DC-linked capacitor CB), and a half-bridge-type LLC resonant converter (including a Direct-Current (DC)-linked capacitor CB, two power switches S3 and S4, a resonant capacitor Cr, a resonant inductor Lr, a center-tapped transformer T1 with two output windings, two output diodes D5 and D6 and an output capacitor Co), along with a LED [19].Due to two-stage power conversions, the circuit efficiency is limited and more power switches and components are required in the conventional driver.
In response to these challenges, this paper presents and implements a single-stage LED driver with coupled inductors and high power factor for streetlight applications.Descriptions and analysis of operational modes, and design equations of key components in the presented LED driver, and experimental results obtained from a prototype circuit are demonstrated.

Descriptions and Operational Modes Analysis of the Presented Single-Stage LED Driver
Figure 2a shows the original two-stage driver circuit suitable for supplying an LED streetlight module, which consists of two buck-boost PFC converters with interleaved operation and a half-bridge-type series resonant converter cascaded with a full-bridge rectifier.The two coupled inductors are employed instead of single-winding inductors in order to accomplish buck-boost conversion.Figure 2b shows the presented LED driver with coupled inductors and interleaved PFC feature by utilizing the synchronous switch technique to simplify power switches and integrate the two-stage configuration into single-stage one.Figure 2b shows the presented LED driver for streetlight applications, which combines an interleaved buck-boost PFC converter with a half-bridge-type series resonant converter cascaded with a full-bridge rectifier into a single-stage power conversion.The interleaved buck-boost PFC converter sub-circuit consists of two capacitors (Cin1 and Cin2), two coupled inductors (LB11 and LB12; LB21 and LB22), four diodes (DB11, DB12, DB21, and DB22), two power switches (S1 and S2), and a DC bus capacitor (CDC).The half-bridge-type series resonant converter cascaded with a full-bridge rectifier sub-circuit includes a DC bus capacitor (CDC), two switches (S1 and S2), a resonant capacitor (Cr), a resonant inductor (Lr), four diodes (Do1, Do2, Do3 and Do4), and a capacitor (Co) along with the LED streetlight module.In addition, coupled inductors (LB11 and LB12; LB21 and LB22) are designed to be operated in discontinuous conduction mode (DCM) in order to naturally achieve input-current shaping.In addition, the diodes DB12 and DB21 are used to block the current going from the utility-line voltage source into the inductors LB12 and LB21.Besides, the diodes DB11 and DB22 are capable of preventing the inductor currents going back to the input capacitors Cin1 and Cin2.Since the input voltage of each buck-boost PFC converter (the voltage on the capacitor Cin1 or Cin2) is half of the utility-line voltage, the peak current of each coupled inductor and the DC bus voltage will also be half.Because the DC bus voltage is reduced, the presented LED driver is suitable for the applications with high utility-line voltage.Additionally, the input-current harmonics can

Descriptions and Operational Modes Analysis of the Presented Single-Stage LED Driver
Figure 2a shows the original two-stage driver circuit suitable for supplying an LED streetlight module, which consists of two buck-boost PFC converters with interleaved operation and a half-bridge-type series resonant converter cascaded with a full-bridge rectifier.The two coupled inductors are employed instead of single-winding inductors in order to accomplish buck-boost conversion.Figure 2b shows the presented LED driver with coupled inductors and interleaved PFC feature by utilizing the synchronous switch technique to simplify power switches and integrate the two-stage configuration into single-stage one.Figure 2b shows the presented LED driver for streetlight applications, which combines an interleaved buck-boost PFC converter with a half-bridge-type series resonant converter cascaded with a full-bridge rectifier into a single-stage power conversion.The interleaved buck-boost PFC converter sub-circuit consists of two capacitors (C in1 and C in2 ), two coupled inductors (L B11 and L B12 ; L B21 and L B22 ), four diodes (D B11 , D B12 , D B21 , and D B22 ), two power switches (S 1 and S 2 ), and a DC bus capacitor (C DC ).The half-bridge-type series resonant converter cascaded with a full-bridge rectifier sub-circuit includes a DC bus capacitor (C DC ), two switches (S 1 and S 2 ), a resonant capacitor (C r ), a resonant inductor (L r ), four diodes (D o1 , D o2 , D o3 and D o4 ), and a capacitor (C o ) along with the LED streetlight module.In addition, coupled inductors (L B11 and L B12 ; L B21 and L B22 ) are designed to be operated in discontinuous conduction mode (DCM) in order to naturally achieve input-current shaping.In addition, the diodes D B12 and D B21 are used to block the current going from the utility-line voltage source into the inductors L B12 and L B21 .Besides, the diodes D B11 and D B22 are capable of preventing the inductor currents going back to the input capacitors C in1 and C in2 .Since the input voltage of each buck-boost PFC converter (the voltage on the capacitor C in1 or C in2 ) is half of the utility-line voltage, the peak current of each coupled inductor and the DC bus voltage will also be half.Because the DC bus voltage is reduced, the presented LED driver is suitable for the applications with high utility-line voltage.Additionally, the input-current harmonics can be reduced by the interleaved operation, so that the size of the input low-pass filter can be miniaturized [20].
Appl.Sci.2017, 7, 167 3 of 18 be reduced by the interleaved operation, so that the size of the input low-pass filter can be miniaturized [20].The operating modes and the key waveform of the presented LED driver for streetlight applications are shown in Figures 5 and 6, respectively, and the analyses of operations are described in detail in the following.
Mode 1 (t0 ≤ t < t1; in Figure 5a): The body diode of switch S1 is forward-biased at time t0, and this mode begins.The resonant capacitor Cr provides energy to the inductor Lr, capacitors CDS2 and Co and to the LED through diodes Do2 and Do3.The diode DB21 is forward-biased and coupled inductors LB21 and LB22 provide energy to capacitor CDS2 through diode DB21.At time t1, the drain-source voltage vDS1 of power switch S1 is zero and S1 turns on with zero-voltage switching (ZVS); then this mode ends.The operating modes and the key waveform of the presented LED driver for streetlight applications are shown in Figures 5 and 6, respectively, and the analyses of operations are described in detail in the following.The operating modes and the key waveform of the presented LED driver for streetlight applications are shown in Figures 5 and 6, respectively, and the analyses of operations are described in detail in the following.
Mode 1 (t 0 ≤ t < t 1 ; in Figure 5a): The body diode of switch S 1 is forward-biased at time t 0 , and this mode begins.The resonant capacitor C r provides energy to the inductor L r , capacitors C DS2 and C o and to the LED through diodes D o2 and D o3 .The diode D B21 is forward-biased and coupled inductors L B21 and L B22 provide energy to capacitor C DS2 through diode D B21 .At time t 1 , the drain-source voltage v DS1 of power switch S 1 is zero and S 1 turns on with zero-voltage switching (ZVS); then this mode ends.
Mode 2 (t 1 ≤ t < t 2 ; in Figure 5b): When switch S 1 achieves ZVS turn-on at t 1 , this mode starts.The rectified input voltage source V REC1 provides energy to coupled inductor L B11 through diode D B11 and switch S 1 , and diode D B12 is reverse-biased during this mode.The inductor current i LB11 linearly increases from zero, and can be expressed as: where v AC-rms is the root-mean-square (rms) value of input utility-line voltage, and f AC is the utility-line frequency.
The DC bus capacitor C DC and resonant inductor L r provide energy to capacitors C DS2 , C r and C o and to the LED through diodes D o1 and D o4 .Coupled inductors L B21 and L B22 continue providing energy to capacitor C DS2 through diode D B21 .This mode ends when current i LB22 decreases to zero at t 2 .
Mode 3 (t 2 ≤ t < t 3 ; in Figure 5c): Voltage source V REC1 continues providing energy to coupled inductor L B11 through diode D B11 and switch S 1 .Capacitors C DC and C DS2 , along with resonant inductor L r provide energy to capacitors C r and C o and to the LED through diodes D o1 and D o4 .At t 3 , the coupled-inductor current reaches its peak value, defined as i LB11-pk (t), and is given by: where D and T S are the duty cycle and period of the power switch, respectively.This mode ends when diode D B12 becomes forward-biased at t 3 .Mode 4 (t 3 ≤ t < t 4 ; in Figure 5d): This mode begins when power switch S 1 turns off at t 3 .The diode D B12 is forward-biased and coupled inductors L B11 and L B12 provide energy to capacitor C DS1 .The coupled-inductor current i LB11 linearly decreases from its peak level, and can be given by: where V DC is the voltage of the DC bus capacitor C DC .Capacitors C DC and C DS2 and resonant inductor L r continue providing energy to capacitors C DS1 , C r and C o and to the LED through diodes D o1 and D o4 .When the drain-source voltage v DS2 of S 2 decreases to zero at t 4 , this mode ends.
Mode 5 (t 4 ≤ t < t 5 ; in Figure 5e): The body diode of switch S 2 is forward-biased at time t 4 , and this mode begins.The resonant inductor L r provides energy to capacitors C DS2 , C r and C o and to the LED through the body diode of power switch S 2 and diodes D o1 and D o4 .The diode D B12 is forward-biased and coupled inductors L B11 and L B12 provide energy to capacitor C DS1 through diode D B12 .At time t 5 , the drain-source voltage v DS2 of power switch S 2 is zero and S 2 turns on with ZVS; then this mode ends.
Mode 6 (t 5 ≤ t < t 6 ; in Figure 5f): When switch S 2 achieves ZVS turn-on at t 5 , this mode starts.The rectified input voltage source V REC2 provides energy to coupled inductor L B22 through diode D B22 and switch S 2 , and diode D B21 is reverse-biased during this mode.The DC bus capacitor C DC and resonant capacitor C r provide energy to inductor L r , capacitors C DS1 and C o and to the LED through diodes D o2 and D o3 .Coupled inductors L B11 and L B12 continue providing energy to capacitor C DS1 through diode D B12 .This mode ends when current i LB11 decreases to zero at t 6 .
Mode 7 (t 6 ≤ t < t 7 ; in Figure 5g): Voltage source V REC2 continues providing energy to coupled inductor L B22 through diode D B22 and switch S 2 .The resonant capacitor C r provides energy to resonant inductor L r , output capacitor C o and the LED through diodes D o2 and D o3 .This mode ends when diode D B21 is forward-biased at t 7 .
Mode 8 (t 7 ≤ t < t 8 ; in Figure 5h): This mode begins when power switch S 2 turns off at t 7 .The diode D B21 is forward-biased and coupled inductors L B21 and L B22 provide energy to capacitor C DS2 .The resonant capacitor C r continues providing energy to resonant inductor L r , capacitors C DS2 and C o and to the LED through diodes D o2 and D o3 .When the drain-source voltage v DS1 of S 1 decreases to zero at t 8 , this mode ends.Then Mode 1 begins for the next high-frequency switching period.

Design of Coupled Inductors LB11, LB12, LB21 and LB22
The coupled inductors (LB11 and LB12; LB21 and LB22) are designed to be operated in DCM for naturally achieving PFC, and the design equation of them can be expressed as follows [12,20]: where Vac-pk is the peak value of utility-line voltage; η is the estimated efficiency; D is the duty cycle of the power switches; fs is the switching frequency; and Po is the output rated power.
In reference to Equation (4) with a η of 0.85, a Vac-pk of 220 2 V, a D of 0.45, a Po of 165 W and an fs of 50 kHz, the coupled inductors LB11, LB12, LB21 and LB22 are given by:

Design of Resonant Inductor Lr and Resonant Capacitor Cr
In reference to Figure 2b, the resonant frequency fr is given by:  The coupled inductors (L B11 and L B12 ; L B21 and L B22 ) are designed to be operated in DCM for naturally achieving PFC, and the design equation of them can be expressed as follows [12,20]: where V ac-pk is the peak value of utility-line voltage; η is the estimated efficiency; D is the duty cycle of the power switches; f s is the switching frequency; and P o is the output rated power.In reference to Equation (4) with a η of 0.85, a V ac-pk of 220 √ 2V, a D of 0.45, a P o of 165 W and an f s of 50 kHz, the coupled inductors L B11 , L B12 , L B21 and L B22 are given by:

Design of Resonant Inductor L r and Resonant Capacitor C r
In reference to Figure 2b, the resonant frequency f r is given by: The switching frequency f s is designed to be larger than the resonant frequency f r so that the resonant tank resembles an inductive network in order to obtain ZVS on for the two power switches [21].The relationship between the switching frequency f s and the resonant frequency f r is selected as: The quality factor Q r is defined as: where R a is the equivalent output resistor referring to the left side of the full-bridge rectifier, and could be expressed by the following equation: Combining Equation (2) with Equations ( 3)-( 5), the design equations of resonant capacitor C r and inductor L r are given by: and L r = 4 According to Equation ( 8), with a V o of 235 V and an I o of 700 mA, the equivalent resistor R a is given by: R a = 8 • 235 In reference to Equation (9), with an R a of 272.1Ω, the relationship between the resonant capacitor C r and the switching frequency under different levels of quality factor Q r is shown in Figure 7.With a Q r of 0.15 and a switching frequency f s of 50 kHz, the capacitor C r is selected to be 1.22 µF according to Figure 7.
In reference to Equation (10), with a C r of 1.22 µF, the resonant inductor L r is given by:

Design of Input Low-Pass Filter
The input low-pass filter is composed of an inductor Lf and a capacitor Cf, and the cut-off frequency of the input low-pass filter is given by: The design consideration of the cut-off frequency in the input low-pass filter is determined to be one-tenth of the switching frequency (which is 5 kHz) in order to filter the high-frequency switching noises.The design equation of the inductor Lf is represented by: On choosing a capacitor Cf of 2 μF (two capacitors of 1 μF in parallel connection), the inductor Lf is given by: ( )

Experimental Results
A prototype driver has been successfully implemented and tested for powering a 165 W-rated LED streetlight module (LMD003 from AcBel Polytech Inc., New Taipei City, Taiwan) with input utility-line voltages of 220 V ± 5% (from 210 to 230 V).Tables 1 and 2 show the specifications and key components utilized in the presented single-stage LED driver for streetlight applications, respectively.

Design of Input Low-Pass Filter
The input low-pass filter is composed of an inductor L f and a capacitor C f , and the cut-off frequency of the input low-pass filter is given by: The design consideration of the cut-off frequency in the input low-pass filter is determined to be one-tenth of the switching frequency (which is 5 kHz) in order to filter the high-frequency switching noises.The design equation of the inductor L f is represented by: On choosing a capacitor C f of 2 µF (two capacitors of 1 µF in parallel connection), the inductor L f is given by:

Experimental Results
A prototype driver has been successfully implemented and tested for powering a 165 W-rated LED streetlight module (LMD003 from AcBel Polytech Inc., New Taipei City, Taiwan) with input utility-line voltages of 220 V ± 5% (from 210 to 230 V).Tables 1 and 2 show the specifications and key components utilized in the presented single-stage LED driver for streetlight applications, respectively.The measured waveforms of coupled inductor currents i LB11 and i LB22 are shown in Figure 8; both of which have interleaved features and operate in DCM. Figure 9 shows the measured switch voltage v DS1 and current i DS1 ; Figure 10 presents the measured switch voltage v DS2 and current i DS2 ; thus, ZVS occurred on both switches for lowering the switching losses.
Figure 11 presents the measured switch voltage v DS2 and resonant inductor current i Lr .The current i Lr lags with respect to voltage v DS2 so that the series resonant tank resembles an inductive load.Figure 12 depicts the measured output voltage V O and current I O ; their average values are approximately 235 V and 0.7 A, respectively.Figure 13 shows measured voltages on the diodes D B11 and D B22 .The voltage spikes on D B11 and D B22 are approximately 360 V.In addition, the voltage rating of the diode (C3D10060) is 600 V. Therefore, the utilized diodes are capable of sustaining these voltage spikes.The measured waveforms of input utility-line voltage v AC and current i AC are shown in Figure 14, and the input current is in phase with utility-line voltage, which results in high power factor.In addition, experimental waveforms from Figure 8 to Figure 14 are measured at a utility-line voltage of 220 V.The measured waveforms of coupled inductor currents iLB11 and iLB22 are shown in Figure 8; both of which have interleaved features and operate in DCM. Figure 9 shows the measured switch voltage vDS1 and current iDS1; Figure 10 presents the measured switch voltage vDS2 and current iDS2; thus, ZVS occurred on both switches for lowering the switching losses.
Figure 11 presents the measured switch voltage vDS2 and resonant inductor current iLr.The current iLr lags with respect to voltage vDS2 so that the series resonant tank resembles an inductive load.Figure 12 depicts the measured output voltage VO and current IO; their average values are approximately 235 V and 0.7 A, respectively.Figure 13 shows measured voltages on the diodes DB11 and DB22.The voltage spikes on DB11 and DB22 are approximately 360 V.In addition, the voltage rating of the diode (C3D10060) is 600 V. Therefore, the utilized diodes are capable of sustaining these voltage spikes.The measured waveforms of input utility-line voltage vAC and current iAC are shown in Figure 14, and the input current is in phase with utility-line voltage, which results in high power factor.In addition, experimental waveforms from Figure 8 to Figure 14 3 shows the measured output voltage ripple and current ripple of the presented LED streetlight driver among   Figure 15 shows the measured input utility-line current harmonics comparing with the International Electrotechnical Commission (IEC) 61000-3-2 Class C standards at input utility-line voltages ranging from 210 to 230 V, and all current harmonics meet the requirements.Table 3 shows the measured output voltage ripple and current ripple of the presented LED streetlight driver among   3 shows the measured output voltage ripple and current ripple of the presented LED streetlight driver among input voltages ranging from 210 to 230 V; additionally, the output voltage (current) ripple is obtained by the peak-to-peak level divided by the average value of output voltage (current).According to this table, the measured output voltage ripples and current ripples are smaller than 1% and 10%, respectively, during the tested input voltages.
Appl.Sci.2017, 7, 167 14 of 18 input voltages ranging from 210 to 230 V; additionally, the output voltage (current) ripple is obtained by the peak-to-peak level divided by the average value of output voltage (current).According to this table, the measured output voltage ripples and current ripples are smaller than 1% and 10%, respectively, during the tested input voltages.Figure 16 presents the measured power factor and current total-harmonics distortion (THD) of the presented LED driver under utility-line voltages ranging from 210 to 230 V.At a utility-line rms voltage of 220 V, the measured power factor and current THD are 0.992 and 6.55%, respectively.In addition, the measured highest power factor and lowest current THD are 0.993 and 6.5%; these occurred at a utility-line rms voltage of 230 and 210 V, respectively.Figure 17 shows the measured circuit efficiency of the presented LED driver under utility-line voltages ranging from 210 to 230 V; additionally, the measured maximum circuit efficiency is 91.23%, at a utility-line rms voltage of 210 V.In addition, the efficiency which drops with the increase 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 an increase in the switching frequency of the power switches.Thus, the switching losses of power switches and conduction losses of power diodes will increase, resulting in lowered circuit efficiency.Figure 18 presents a photograph of supply of the experimental LED streetlight module using the presented driver at a utility-line voltage of 220 V.
Besides, Table 4 shows some measurements of the relationship between output voltage, efficiency and output load current under an input utility-line voltage of 220 V by altering the equivalent load resistor to represent the specific load current.In addition, the measured minimum  Figure 16 presents the measured power factor and current total-harmonics distortion (THD) of the presented LED driver under utility-line voltages ranging from 210 to 230 V.At a utility-line rms voltage of 220 V, the measured power factor and current THD are 0.992 and 6.55%, respectively.In addition, the measured highest power factor and lowest current THD are 0.993 and 6.5%; these occurred at a utility-line rms voltage of 230 and 210 V, respectively.Figure 17 shows the measured circuit efficiency of the presented LED driver under utility-line voltages ranging from 210 to 230 V; additionally, the measured maximum circuit efficiency is 91.23%, at a utility-line rms voltage of 210 V.In addition, the efficiency which drops with the increase 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 an increase in the switching frequency of the power switches.Thus, the switching losses of power switches and conduction losses of power diodes will increase, resulting in lowered circuit efficiency.Figure 18 presents a photograph of supply of the experimental LED streetlight module using the presented driver at a utility-line voltage of 220 V.
Besides, Table 4 shows some measurements of the relationship between output voltage, efficiency and output load current under an input utility-line voltage of 220 V by altering the equivalent load resistor to represent the specific load current.In addition, the measured minimum efficiency is 86.51%, in a minimum load current of 0.3 A. Moreover, Table 5 shows comparisons between the existing single-stage LED driver for streetlight applications in [18] and the proposed one.From Table 5, it can be seen that the proposed LED streetlight driver has better current THD and efficiency than the existing one.
Appl.Sci.2017, 7, 167 15 of 18 efficiency is 86.51%, in a minimum load current of 0.3 A. Moreover, Table 5 shows comparisons between the existing single-stage LED driver for streetlight applications in [18] and the proposed one.
From Table 5, it can be seen that the proposed LED streetlight driver has better current THD and efficiency than the existing one.5 shows comparisons between the existing single-stage LED driver for streetlight applications in [18] and the proposed one.
From Table 5, it can be seen that the proposed LED streetlight driver has better current THD and efficiency than the existing one.

Conclusions
This paper has presented and implemented a single-stage LED driver with a high power factor which is suitable for streetlight applications and integrates an interleaved buck-boost PFC converter with coupled inductors and a half-bridge-type series-resonant converter cascaded with a full-bridge rectifier into a single power conversion stage.A 165-W prototype LED driver has been developed

Conclusions
This paper has presented and implemented a single-stage LED driver with a high power factor which is suitable for streetlight applications and integrates an interleaved buck-boost PFC converter with coupled inductors and a half-bridge-type series-resonant converter cascaded with a full-bridge rectifier into a single power conversion stage.A 165-W prototype LED driver has been developed and tested with input utility-line voltages ranging from 210 to 230 V.The experimental results of the presented LED driver display low-output voltage ripple (<1%) and output current ripple (<10%), high power factor (>0.99), low total harmonic distortion of input utility-line current (<7%), zero-voltage switching on power switches, and high circuit efficiency (>90%); thus the functionality of the presented LED streetlight driver is demonstrated.

Figure 2 .
Figure 2. (a) Original two-stage LED driver circuit; (b) the presented single-stage LED driver with coupled inductors and interleaved PFC for streetlight applications.

Figure 3
Figure 3 shows the utilized control circuit diagram of the presented single-stage LED driver for streetlight applications.With using a constant-voltage/constant-current controller (IC1 SEA05) for regulating the LED streetlight module's output voltage and current, the output LED voltage Vo can be sensed through resistors Rvs1, VR1 and Rvs2, and the output LED 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 vGS1 and vGS2 are generated from pin 15 and pin 11, respectively, of the IC3, to carry out regulation of the LED streetlight module's output voltage and current.

Figure 2 .
Figure 2. (a) Original two-stage LED driver circuit; (b) the presented single-stage LED driver with coupled inductors and interleaved PFC for streetlight applications.

Figure 3
Figure 3 shows the utilized control circuit diagram of the presented single-stage LED driver for streetlight applications.With using a constant-voltage/constant-current controller (IC1 SEA05) for regulating the LED streetlight module's output voltage and current, the output LED voltage V o can be sensed through resistors R vs1 , VR 1 and R vs2 , and the output LED current can be sensed through resistor R 3 .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 regulation of the LED streetlight module's output voltage and current.

Figure 3 .
Figure 3.The utilized control circuit of the presented single-stage LED driver for streetlight applications.

Figure 4
Figure4presents the simplified circuit of the presented single-stage LED driver for streetlight applications, obtained while analyzing the operational modes.In order to analyze the operations of the presented LED driver, the following assumptions are made.
(a) Since the switching frequencies of the two switches S1 and S2 are much higher than those of the utility-line voltage vAC, the sinusoidal utility-line voltage can be considered as a constant value for each high-frequency switching period.(b) VREC1 and VREC2, respectively, represent the rectified input voltage sources for the capacitors Cin1 and Cin2.(c) Power switches are complementarily operated, and their inherent body diodes and drain-source capacitors (CDS1 and CDS2) are considered.(d) The conducting voltage drops of diodes DB11, DB12, DB21, DB22, Do1, Do2, Do3 and Do4 are neglected.(e) Coupled inductors (including LB11 and LB12; LB21 and LB22) are designed to be operated in DCM for naturally achieving PFC.

Figure 4 .
Figure 4. Simplified circuit of the presented single-stage LED driver for streetlight applications.

Figure 3 .
Figure 3.The utilized control circuit of the presented single-stage LED driver for streetlight applications.

Figure 4 18 Figure 3 .
Figure 4 presents the simplified circuit of the presented single-stage LED driver for streetlight applications, obtained while analyzing the operational modes.In order to analyze the operations of the presented LED driver, the following assumptions are made.(a) Since the switching frequencies of the two switches S 1 and S 2 are much higher than those 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) V REC1 and V REC2 , respectively, represent the rectified input voltage sources for the capacitors C in1 and C in2 .(c) Power switches are complementarily operated, and their inherent body diodes and drain-source capacitors (C DS1 and C DS2 ) are considered.(d) The conducting voltage drops of diodes D B11 , D B12 , D B21 , D B22 , D o1 , D o2 , D o3 and D o4 are neglected.(e) Coupled inductors (including L B11 and L B12 ; L B21 and L B22 ) are designed to be operated in DCM for naturally achieving PFC.

Figure 4
Figure4presents the simplified circuit of the presented single-stage LED driver for streetlight applications, obtained while analyzing the operational modes.In order to analyze the operations of the presented LED driver, the following assumptions are made.

( a )
Since the switching frequencies of the two switches S1 and S2 are much higher than those of the utility-line voltage vAC, the sinusoidal utility-line voltage can be considered as a constant value for each high-frequency switching period.(b) VREC1 and VREC2, respectively, represent the rectified input voltage sources for the capacitors Cin1 and Cin2.(c) Power switches are complementarily operated, and their inherent body diodes and drain-source capacitors (CDS1 and CDS2) are considered.(d) The conducting voltage drops of diodes DB11, DB12, DB21, DB22, Do1, Do2, Do3 and Do4 are neglected.(e) Coupled inductors (including LB11 and LB12; LB21 and LB22) are designed to be operated in DCM for naturally achieving PFC.

Figure 4 .
Figure 4. Simplified circuit of the presented single-stage LED driver for streetlight applications.
Mode 1 (t0 ≤ t < t1; in Figure 5a): The body diode of switch S1 is forward-biased at time t0, and this mode begins.The resonant capacitor Cr provides energy to the inductor Lr, capacitors CDS2 and Co and to the LED through diodes Do2 and Do3.The diode DB21 is forward-biased and coupled inductors LB21 and LB22 provide energy to capacitor CDS2 through diode DB21.At time t1, the drain-source voltage vDS1 of power switch S1 is zero and S1 turns on with zero-voltage switching (ZVS); then this mode ends.

Figure 4 .
Figure 4. Simplified circuit of the presented single-stage LED driver for streetlight applications.

Figure 6 .
Figure 6.Key waveforms of the presented LED driver for streetlight applications.

3 .
Design Equations of Key Circuit Components in the Presented LED Driver 3.1.Design of Coupled Inductors L B11 , L B12 , L B21 and L B22

Figure 7 .
Figure 7. Resonant capacitor Cr versus the switching frequency fs under different levels of quality factor Qr.

Figure 7 .
Figure 7. Resonant capacitor C r versus the switching frequency f s under different levels of quality factor Q r .
are measured at a utility-line voltage of 220 V.

Figure 15
Figure15shows the measured input utility-line current harmonics comparing with the International Electrotechnical Commission (IEC) 61000-3-2 Class C standards at input utility-line voltages ranging from 210 to 230 V, and all current harmonics meet the requirements.Table3shows the measured output voltage ripple and current ripple of the presented LED streetlight driver among

Figure 15
Figure15shows the measured input utility-line current harmonics comparing with the International Electrotechnical Commission (IEC) 61000-3-2 Class C standards at input utility-line voltages ranging from 210 to 230 V, and all current harmonics meet the requirements.Table3shows the measured output voltage ripple and current ripple of the presented LED streetlight driver among

Figure 15
Figure15shows the measured input utility-line current harmonics comparing with the International Electrotechnical Commission (IEC) 61000-3-2 Class C standards at input utility-line voltages ranging from 210 to 230 V, and all current harmonics meet the requirements.Table3shows the measured output voltage ripple and current ripple of the presented LED streetlight driver among input voltages ranging from 210 to 230 V; additionally, the output voltage (current) ripple is obtained by the peak-to-peak level divided by the average value of output voltage (current).According to this table, the measured output voltage ripples and current ripples are smaller than 1% and 10%, respectively, during the tested input voltages.

Figure 16 .Figure 17 .
Figure 16.Measured power factor and current total harmonic distortion (THD) under utility-line voltages ranging from 210 to 230 V.

Figure 16 .
Figure 16.Measured power factor and current total harmonic distortion (THD) under utility-line voltages ranging from 210 to 230 V.

Figure 16 .Figure 17 .
Figure 16.Measured power factor and current total harmonic distortion (THD) under utility-line voltages ranging from 210 to 230 V.

Figure 17 .
Figure 17.Measured circuit efficiency under utility-line voltages ranging from 210 to 230 V.Figure 17. Measured circuit efficiency under utility-line voltages ranging from 210 to 230 V.

Figure 18 .
Figure 18.Photograph of supply of the experimental LED streetlight module using the presented driver at a utility-line voltage of 220 V, AC: Alternating-Current.

Figure 18 .
Figure 18.Photograph of supply of the experimental LED streetlight module using the presented driver at a utility-line voltage of 220 V, AC: Alternating-Current.

Table 1 .
Specifications of the presented single-stage LED driver.

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

Table 1 .
Specifications of the presented single-stage LED driver.

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

Table 3 .
Measured output voltage ripple and current ripple in the presented LED streetlight driver.

Table 3 .
Measured output voltage ripple and current ripple in the presented LED streetlight driver.

Table 4 .
Measured output voltage and efficiency versus output load current under an input utilityline voltage of 220 V.

Table 5 .
[18]arisons between the existing single-stage LED driver for streetlight applications in[18]and the proposed one.

Table 4 .
Measured output voltage and efficiency versus output load current under an input utility-line voltage of 220 V.

Table 5 .
[18]arisons between the existing single-stage LED driver for streetlight applications in[18]and the proposed one.