A Novel Light-Emitting Diode Streetlight Driver Circuit Applied to a Direct Current-Input Voltage Source

Abstract

With the global advocacy of green lighting and the urgent need for energy saving and carbon reduction, more and more street lighting applications have entered the era of being replaced by light-emitting diode (LED) lighting sources. This paper presents a new LED streetlight driving circuit applied to a direct current (DC)-input voltage source, which consists of a buck converter combined with a flyback converter to reduce the number of circuit components required and to recover the leakage energy of the transformer to improve energy conversion efficiency. In addition, this study also completed the analysis of the operational principle of the new LED streetlight driving circuit, and developed a prototype LED streetlight driver with DC-input voltage of 48V and output power of 72 W (36 V/2 A). Finally, the measurement results of the prototype circuit show that the output voltage ripple rate was less than 15%, the output current ripple rate was less than 6%, and the circuit efficiency was as high as 91%.

1. Introduction

Road safety is a top priority when designing roads, not only for motorists, but also for pedestrians. Street lighting is a key contributor to road safety. Proper street lighting can improve visibility, make navigation easier, keep road users and pedestrians safe, and reduce crime. Street lighting systems facilitate the use of roadways for drivers and pedestrians. In addition to public safety, they also promote the effectiveness of roads as a means of transportation [1,2].

A high-pressure mercury lamp is a type of gas discharge lamp, which is a light source containing mercury vapor inside; it produces bright light in the form of gas discharge. High-pressure mercury lamps have the advantage of high luminous efficiency and long service life. As the lamp of a high-pressure mercury street lamp contains mercury, it has associated environmental pollution problems; high-pressure mercury street lamps also have high energy consumption, light decay, and are not environmentally-friendly lamps. Therefore, their use has been greatly reduced. A high-pressure sodium lamp is also a type of gas discharge lamp, which is not only used as a light source for road lighting, but also for lighting scenes and other occasions. Both high-pressure mercury lamps and high-pressure sodium lamps are high energy-consuming street lighting sources, commonly used for outdoor lighting on roads, plazas, streets, stadiums, ball fields, and parks [3,4].

Streetlights are vital to modern life and are an important infrastructure for social security and road safety. However, streetlights are high energy-consuming facilities for long-term lighting, which are a burden to environmental protection, electricity consumption, and government finance. In line with the global trend of clean energy, and taking into account energy saving and carbon reduction, as well as reducing the financial burden of the government, streetlights, as an important component of urban lighting, can meet the needs of environmental protection and energy saving using energy-saving and high-efficiency light sources. LED streetlights, compared with traditional high-pressure mercury and high-pressure sodium lamps, have a longer life span, lower energy consumption, high lighting efficiency, and also provide a clearer view of the road at night and reduce maintenance costs. As well, the costs of installing and maintaining LED streetlights have virtually bottomed out. Consequently, LED streetlights have replaced traditional sources of street lighting and play an important role in energy-efficient outdoor lighting [5,6,7,8,9,10]. Replacing old energy-consuming traditional streetlight sources with energy-efficient LED lights not only reduces the environmental pollution caused by high-pressure mercury lamps and carbon dioxide emissions, but also significantly reduces the power consumption of streetlights and lowers the power generation load and costs for power companies. In addition, complete replacement using energy-saving streetlights can provide a safer and more comfortable living environment and quality of life for citizens, and enable a city to move towards becoming a green city with environmental protection, energy savings, and low-carbon emissions [11,12,13,14,15].

Solar energy is an inexhaustible sustainable source of energy. Solar photovoltaic panels make it possible to convert solar energy into electric energy for streetlights. During the day, energy from sunlight is captured by solar photovoltaic panels and converted into electrical energy stored in the battery, and the energy of the battery can be used to power streetlights at night [16,17,18,19,20,21,22]. The literature describes some LED driver circuits that are applied to a DC-input voltage source, such as a solar photovoltaic panel or a battery, suitable for powering LED street lighting applications [23,24,25,26,27,28,29]. Reference [27] proposed a Zeta/flyback integrated DC-to-DC converter applied to photovoltaic power generation arrays. The integrated converter combined a Zeta converter with a flyback converter, and the photovoltaic power generation array was used as the input voltage source for LED street lighting systems or digital signage. A battery charger and discharger are required when solar photovoltaic panels are installed in an LED illumination system. In the presented solar photovoltaic panel-powered LED lighting system, a Zeta converter was used as a battery charger and a flyback converter was used as a battery discharger due to its simple circuit topology.

Reference [28] presented a full-bridge resonant DC-to-DC converter as an LED driver circuit. In this circuit, an LED light was powered by two voltage sources connected in series. One of the voltage sources supplied power directly to the main lamp, and the other delivered low power across the full bridge for regulation. The presented driver utilized the fifth harmonic component in the bridge output voltage, which reduced the size of the reactive components and enabled lower switching losses in the full bridge to achieve high efficiency. Reference [29] presented a two-stage DC-to-DC driver circuit for LED lighting applied to automotive headlights. The front stage was a step-up DC-to-DC converter, and the rear stage was a step-down converter. The entire driver circuit consisted of a boost converter and two buck converters, and was used to drive two sets of automotive headlight LED arrays.

A review of driver circuits for LED lighting applications classified according to whether the topology is isolated or non-isolated is presented in [30]. For basic isolated topology applied to a DC-input voltage source for LED streetlight applications, the main circuit is typically a flyback converter with good electrical isolation characteristics. The disadvantage of the flyback converter is its low transformer utilization due to its unidirectional operation; a snubber circuit is recommended to discharge the energy stored in the leakage inductor of the transformer when the power switch is turned off. For non-isolated converters that are applied to DC-input voltage sources and supply power for LED streetlight applications that are lower than the input voltage level, the main circuits are generally buck converters and buck-boost converters. In addition, buck converters have the attractive features of non-inverting output and continuous output current compared to buck-boost converters. Therefore, this study proposed and developed a novel driver circuit applied to a DC-input voltage source for LED streetlight applications, which combines a buck converter with a flyback converter into a single-stage single-switch non-isolated buck-flyback power converter. In addition, the proposed driver circuit is suitable for applications where the rated voltage of the LED is lower than the DC-input voltage level, and can recover the energy stored in the leakage inductance of the transformer without using a snubber circuit in order to improve the circuit efficiency.

This paper is organized as follows. Section 2 describes and analyzes operational modes of the proposed LED streetlight driver circuit applied to a DC-input voltage source. Section 3 presents design considerations regarding the proposed LED streetlight driver circuit. In Section 4, experimental results for the prototype LED streetlight driver circuit applied to a DC-input voltage source are demonstrated. Finally, conclusions and future work are presented in Section 5.

2. Descriptions and Operational Modes Analysis of the Proposed LED Streetlight Driver Circuit Applied to a DC-Input Voltage Source

Figure 1 shows the proposed driver circuit applied to a DC-input voltage source to supply an LED streetlight module, which integrates a DC–DC buck converter with a DC–DC flyback converter into single-stage power conversion topology and includes a power switch, S_B; two diodes, D_B and D_F; a transformer, T_R,with a magnetizing inductor, L_M, and a leakage inductor, L_lk; two output capacitors, C_O1 and C_O2; and the LED streetlight module. In addition, the magnetizing inductor, L_M, was designed to operate in continuous conduction mode (CCM), and the proposed driver circuit recycles energy stored in the leakage inductor, L_lk, of the transformer, T_R, in order to improve circuit efficiency.

Figure 2 shows the equivalent circuit of the proposed LED streetlight driver applied to a DC-input voltage source, obtained while analyzing the operational modes. In order to analyze the circuit operation of the proposed LED streetlight driver, the following assumptions were made:

(a)

The magnetizing inductor, L_M, of the transformer, T_R, is designed to operate in continuous conduction mode, and L_lk1 and L_lk2 are the primary-side leakage inductance and the secondary-side leakage inductance of the transformer, T_R, respectively.

(b)

Assuming that the capacity of energy storage capacitors C_O1 and C_O2 is large enough, the output voltage can be regarded as a constant value.

(c)

The rest of the circuit elements are considered ideal.

Figure 3, Figure 4, Figure 5, Figure 6 and Figure 7 show the operating modes and key waveforms of the LED streetlight driver applied to a DC-input voltage source; the operational analysis is described in detail below.

Operation Mode 1 (t₀ ≤ t < t₁): Figure 3 is an equivalent circuit diagram of operation mode 1 of the proposed LED streetlight driver circuit powered by DC voltage. At time t₀, the power switch S_B is turned on, the diode D_B is reverse biased, and the input voltage source V_IN provides energy to the magnetizing inductor L_M, the primary-side leakage inductance L_lk1, and the energy storage capacitor C_O1 through the switch S_B. The diode D_F continues the previous mode and presents a forward bias conduction, and the energy is released from the secondary-side leakage inductance L_lk2 to the LED streetlight module through the diode D_F. When the secondary-side leakage inductance current I_Llk2 is equal to zero at t₁, operation mode 1 ends.

Operational Mode 2 (t₁ ≤ t < t₂): Figure 4 is an equivalent circuit diagram of operation mode 2 of the proposed LED streetlight driver circuit powered by DC voltage. At t₁, the power switch S_B is continuously turned on and the input voltage source V_IN continues to provide energy to the magnetizing inductor L_M, the primary-side leakage inductance L_lk1, and the capacitor C_O1 through the power switch S_B. At the same time, the energy storage capacitors C_O1 and C_O2 provide energy to the LED streetlight module. When the power switch S_B is turned off at t₂, operation mode 2 ends.

Operational Mode 3 (t₂ ≤ t < t₃): Figure 5 is an equivalent circuit diagram of operation mode 3 of the proposed LED streetlight driver circuit powered by DC voltage. At t₂, after the power switch S_B is turned off, the diode D_B is in a state of forward bias, and the magnetizing inductor L_M and the primary-side leakage inductance L_lk1 provide energy to the energy storage capacitors C_O1 and C_O2, the secondary-side leakage inductance L_lk2, and the LED streetlight module through diodes D_B and D_F. When the primary-side leakage inductance current i_Llk1 is equal to zero at t₃, operational mode 3 ends.

Operational Mode 4 (t₃ ≤ t < t₄): Figure 6 is an equivalent circuit diagram of operation mode 4 of the proposed LED streetlight driver circuit powered by DC voltage. At t₃, the power switch S_B is still off, and the magnetizing inductor L_M and the secondary-side leakage inductance L_lk2 provide energy to the capacitors C_O1 and C_O2, as well as the LED streetlight module, via the diode D_F. Energy storage capacitors C_O1 and C_O2 continuously provide energy to the LED streetlight module. When the power switch S_B is turned on again at t₄, operation mode 4 ends, and the circuit returns to operation mode 1.

3. Design Considerations Regarding Magnetizing Inductor L_M and Output Capacitors C_O1 and C_O2 in the Proposed LED Streetlight Driver Circuit

According to the volt–second balance theorem, the voltage occurred on the magnetizing inductor L_M multiplied by the turn-on time of the switch is equal to the voltage occurred on the magnetizing inductor L_M multiplied by the turn-off time of the switch, and can be expressed using the following formula:

$$V_{IN}\times D\times T_S=\frac{N_P}{N_S}\times \frac{V_{OUT}}{2}\times \left(1-D\right)\times T_S$$

(1)

where D is the duty cycle of the power switches and T_S is the switching period.

The relationship between the output voltage V_OUT and the input voltage V_IN can be expressed as

$$\frac{V_{OUT}}{V_{IN}}=\frac{N_S}{N_P}\times \frac{2D}{1-D}$$

(2)

The peak-to-peak value of the magnetizing inductor ΔI_LM can be expressed as

$$\Delta I_{LM}=\frac{V_{IN}\times D\times T_S}{L_M}=\frac{V_{OUT}\times N_P\times \left(1-D\right)\times T_S}{2\times L_M\times N_S}$$

(3)

In the boundary case between continuous conduction mode and discontinuous conduction mode, the average value of the magnetizing inductor current I_LMB can be expressed as

$$I_{LMB}=I_{OB}=\frac{\Delta I_{LM}}{2}$$

(4)

Therefore, in order to operate in continuous conduction mode so that the magnetizing inductor current does not drop to zero, the design consideration of the output current I_O is required to be larger than I_LMB, as shown below:

$$I_O>I_{LMB}=I_{OB}=\frac{V_{OUT}\times N_P\times \left(1-D\right)\times T_S}{2\times 2\times L_{MB}\times N_S}$$

(5)

Therefore, the magnetizing inductor L_M is required to be greater than the magnetizing inductor in the boundary conduction mode L_MB, and can be expressed as the following formula:

$$\begin{array}{l}{L}_M>L_{MB}=\frac{V_{OUT}\times N_P\times \left(1-D\right)\times T_S}{2\times 2\times I_{OB}\times N_S}\\ =\frac{V_{OUT}\times N_P\times \left(1-D\right)}{2\times 2\times I_{OB}\times N_S\times f_S}\end{array}$$

(6)

According to Equation (6), Figure 8 shows the relationship between the magnetizing inductor in the boundary conduction mode L_MB and the duty cycle D at different switching frequencies.

Assume that the average output current I_OB in the boundary condition is 0.8 times the average output current I_O, and the value of the magnetizing inductor in the boundary conduction mode L_MB can be calculated as follows, using Equation (6) with a V_OUT of 36 V, an N_P of 10, an N_S of 9, an I_OB of 1.6 A, and an f_S of 50 kHz.

$$L_{LMB}=\frac{V_{OUT}\times N_P\times \left(1-D\right)}{2\times 2\times I_{OB}\times N_S\times f_S}=\frac{36\times 10\times \left(1-0.3\right)}{2\times 2\times 1.6\times 9\times \mathrm{50,000}}=87.5 \mathsf{\mu }\mathrm{H}$$

In order to allow the magnetizing inductor current to operate in continuous conduction mode when implementing the circuit, the magnetizing inductor L_M was selected as 100 μH.

Regarding the design of the output capacitors C_O1 and C_O2, the peak-to-peak value of the output voltage ripple ΔV_OUT of the output capacitor under continuous conduction mode can be written as

$$\Delta V_{OUT}=\frac{\Delta Q}{\frac{C_{O1}}{2}}=\frac{2}{C_{O1}}\times \frac{1}{2}\times \frac{\Delta I_{LM}}{2}\times \frac{T_S}{2}=\frac{\Delta I_{LM}\times T_S}{4\times C_{O1}}$$

(7)

Substituting Equation (3) into Equation (7), the expression of the output voltage ripple ΔV_OUT can be obtained using

$$\Delta V_{OUT}=\frac{V_{OUT}\times N_P\times (1-D)\times T_S{}^2}{16\times L_M\times C_{O1}\times N_S}$$

(8)

The percentage of the output voltage ripple ΔV_OUT/V_OUT can be expressed as

$$\frac{\Delta V_{OUT}}{V_{OUT}}=\frac{N_P\times (1-D)\times T_S{}^2}{16\times L_M\times C_{O1}\times N_S}\times 100\%$$

(9)

After calculating Equation (9), the design expressions of output capacitors C_O1 and C_O2 can be obtained using

$$C_{O1}=C_{O2}=\frac{N_P\times (1-D)\times V_{OUT}}{16\times L_M\times N_S\times f_S{}^2\times \Delta V_{OUT}}$$

(10)

Substituting the circuit parameters into Equation (10), with a V_OUT of 36 V, an N_P of 10, an N_S of 9, an f_S of 50 kHz, an L_M of 100 μH, and a ΔV_OUT of 0.5 V, the values of capacitors C_O1 and C_O2 can be obtained as follows:

$$C_{O1}=C_{O2}=\frac{N_P\times (1-D)\times V_{OUT}}{16\times L_M\times N_S\times f_S{}^2\times \Delta V_{OUT}}=\frac{10\times \left(1-0.3\right)\times 36}{16\times 100\times {10}^{-6}\times 9\times {\mathrm{50,000}}^2\times 0.5}=22.85 \mathsf{\mu }\mathrm{F}$$

In order to reduce the ripple of the output voltage when implementing the circuit, the output capacitors C_O1 and C_O2 were selected as 220 μF.

4. Experimental Results of Prototype LED Streetlight Driver Circuit Applied to a DC-Input Voltage Source

Figure 9 presents a photograph of the LED streetlight module used for the experiment. The specifications of the LED streetlight module used in the experiment are as follows: the rated power was 72 W, the rated input voltage was 36 V, the rated input current was 2 A, the luminous flux was 6000 lm, the luminous efficiency was 63.7 lm/W, the color temperature ranged between 5500 K~6500 K, the weight was 8.6 kg, and the service life of the LED streetlight module was longer than 50,000 h.

A prototype driver circuit from a DC-input voltage of 48 V was successfully implemented and tested for powering a 72 W-rated LED streetlight module with an output rated voltage of 36 V and an output rated current of 2 A.

Table 1. Specifications of the proposed LED streetlight driver circuit applied to a DC-input voltage source.

Parameter	Value
DC-Input Voltage Source VIN	48 V
Rated Output Power PO	72 W
Rated Output Voltage VO	36 V
Rated Output Current IO	2 A

and

Table 2. Key components used in the proposed LED streetlight driver circuit applied to a DC-input voltage source.

Component	Value
Diodes D1, D2	SB1060FCT
Power Switches SB	STF13NM60N
Transformer TR
Magnetized Inductor LM	100 μH
Leakage Inductance in the Primary-Side Llk1	1.86 μH
Leakage Inductance in the Secondary-Side Llk2	1.32 μH
Turns Ratio NP:NS	10:9
Output Capacitors CO1, CO2	220 μF/100 V

show the specifications and key components, respectively, used in the proposed LED streetlight driver circuit applied to a DC-input voltage source.

Figure 10 presents the measured input voltage V_IN and input current I_IN; their measured mean values were 47.61 V and 2.101 A, respectively. The measured switch voltage V_DS and switch current I_DS are shown in Figure 11. Figure 12 presents the measured output voltage V_OUT and output current I_OUT; their measured mean values were approximately 36 V and 2 A, respectively. Figure 13 shows measured ripple waveforms of output voltage V_OUT-ripple and output current I_OUT-ripple.

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

Parameters	Values
Mean value of the output voltage	36.014 V
Peak-to-peak value of the output voltage	5.137 V
Ripple factor of the output voltage	14.266%
Mean value of the output current	2.024 A
Peak-to-peak value of the output current	112.24 mA
Ripple factor of the output current	5.545%

shows the measured output voltage ripple and output current ripple of the proposed LED streetlight driver circuit applied to a DC-input voltage of 48 V. The mean value and peak-to-peak value of the output voltage were 36.014 V and 5.137 V, respectively. In addition, the mean value and peak-to-peak value of the output current were 2.024 A and 112.24 mA, respectively. Moreover, the ripple factor of the output voltage (current) was obtained by dividing the peak-to-peak value by the mean value of the output voltage (current). According to this table, the measured output voltage ripples and current ripples were 14.266% and 5.545%, respectively. Figure 14 presents a photograph of the proposed driver circuit supplying the experimental LED streetlight module with a DC-input voltage source of 48 V.

Table 4. Comparisons between the existing DC–DC LED driver in [26] and the proposed driver.

Item	Existing DC–DC LED Driver in Reference [26]	Proposed DC–DC LED Driver
Circuit Topology	Buck converter with coupled inductors	Integration of a buck converter and a flyback converter
Input DC Voltage	12 V	48 V
Output Power	8 W (8 V/1 A)	72 W (36 V/2 A)
Number of Required Switches	1	1
Number of Required Capacitors	2	2
Number of Required Magnetic Elements	1	1
Number of Required Diodes	3	2
Measured Circuit Efficiency	91%	91.8%

shows a comparison between the DC–DC LED driver in [26], which supplied an 8 W-rated power with an input DC voltage of 12 V, and the proposed driver, which supplied a 72 W-rated power with an input DC voltage of 48 V. As can be seen from Table 4, both LED drivers used a single power switch, two capacitors, and a magnetic element; only two diodes were required in the proposed driver compared to the three diodes required in [26]. In addition, the circuit efficiency of the proposed LED driver was slightly better than that of the driver in [26].

5. Conclusions

This study proposed and implemented an LED streetlight driver applied for a DC input voltage source, integrating a buck converter with a flyback converter into a single-stage power conversion topology with the function of recovering the leakage inductance energy from the converter. In addition, the proposed circuit architecture reduced the number of power switches and components used, reduced the cost of the driver circuit, and improved the energy conversion efficiency. A prototype driver was developed and tested to supply a 72 W LED streetlight module with a rated output voltage of 36 V and a rated output current of 2 A with a DC-input voltage of 48 V. The experimental results for the presented LED streetlight driver circuit demonstrated high circuit efficiency (>91%), and the ripple factors of output voltage and output current were smaller than 15% and 6%, respectively. In the future, by redesigning and adjusting the circuit parameters, the DC–DC LED streetlight driver proposed in this paper can be applied to LED streetlights of different wattages. In addition, the proposed driver can be applied to a DC-input voltage source, such as a solar photovoltaic panel or a battery, and is suitable for LED street lighting applications.