Maximize solar LED street light energy efficiency and reduce costs

In recent years, the industry has increasingly focused on street lighting using renewable, clean energy solar energy. A typical solar street lighting system consists of a solar panel, a charge controller, a battery, a light source, and a light pole, as shown in FIG. In terms of lighting sources, it has experienced three important stages, from incandescent lamps to fluorescent lamps and high-intensity discharge lamps (HIDs), so that both front fluorescent lamps and HIDs have been used in solar street lights.

Figure 1: Schematic diagram of a typical solar-powered street lighting system.

In comparison, light-emitting diodes (LEDs) are considered to be the fourth important stage of illumination sources. LEDs have many advantages such as high energy efficiency, long working life, low DC voltage operation, emitting direct light, providing a variety of colors and white light, compact, solid-state devices, and mercury-free. LEDs are used for solar street lighting. And the energy efficiency and light output performance of LEDs have been greatly improved. The publicly announced ability of the strongest white LED has reached 132 to 136 lumens per watt (lm/W), which is higher than traditional fluorescent lamps and HID metal lamps. Especially in 2008, white LEDs have achieved large-scale commercial production, opening the door for LEDs to enter solar streetlight applications on a larger scale.

Improve solar panel energy efficiency with maximum peak power tracking technology

For solar street lights, it is important to improve the photoelectric conversion energy efficiency of solar panels (currently only about 30%). The voltage-current (VI) characteristic curve of a solar panel exhibits nonlinearity and variability, and it is very difficult to extract the maximum amount of electrical energy therefrom. This requires the solar LED street light's charge controller and other related electronic circuits (usually implemented by a microcontroller) to maximize the benefits by using effective control methods to improve energy efficiency.

The basic charge controller is designed to protect the battery from overcharging or undercharging and to prevent reverse current. The Pulse Width Modulation (PWM) type controller controls the amount of charge on the battery and enables trickle charge to protect the battery and extend its life. The latest controllers that support Maximum Peak Power Tracking (MPPT) provide compensation for the changing V/I characteristics of solar cells, optimize solar cell power output, increase energy efficiency, and charge batteries to optimize power.

Specifically, when we can't actually change the load, the MPPT function causes the solar cell to "think" that the load is changing; in this way, the MPPT "spoofs" the solar panel to output the desired voltage and current, allowing more Power is input to the battery.

ON Semiconductor's solution for solar panel battery charge control applications uses a CS51221 enhanced voltage-mode PWM controller that supports maximum peak power tracking with an input voltage of 12 to 24 V and an output current of 12 V@2 A. Protection features such as pulse-by-pulse current limit, input undervoltage lockout, and output overvoltage lockout are adjustable. The controller provides an auxiliary input for remote transmission and monitoring; it can accommodate solar panel applications up to 90 W.

Figure 2: Schematic diagram of solar panel charging control application of ON Semiconductor CS51221 controller

In the application circuit, you need to choose the appropriate topology for the CS51221. The topology chosen should be able to reduce the solar panel output voltage to 12 V in the case of a battery, and in the case of two or more batteries, it can be easily modified to support boosting to 24 V. The CS51221 itself can be configured as a forward, flyback or boost topology. In the reference design introduced by ON Semiconductor for solar panel charging control applications, the flyback topology was chosen.

In applications, maximum peak power tracking is achieved by dynamically adjusting the current limit at the ISET pin. Once the input voltage drops pulse by pulse, the current limit is reduced until the input voltage is restored. This approach eliminates the need to use expensive microcontrollers (MCUs). The charge controller thus implemented will find the peak power point and dynamically adjust it to match the changing power supply characteristics.

By using maximum peak power tracking technology, approximately 30% of the extra charge can be transferred from the solar panel to the battery, which allows the solar street light system to use smaller solar panels. For example, in the case of the same electrical energy, a 60 W power solar panel with MPPT function can be used instead of a 90 W power solar panel with a basic charge controller. Calculated by outputting about $4 worth of solar panels per watt of electrical energy, the solar panel cost savings of 30 W can be as much as $120, resulting in significant cost reductions.

Drive circuit design strategy and solution for LED performance improvement

As mentioned earlier, LEDs are replacing traditional fluorescent lamps and high-intensity discharge lamps (HIDs) in terms of light sources for solar street lighting systems. HIDs include metal halide lamps ("metal halide lamps"), high/low pressure sodium lamps, and mercury vapor lamps. Among them, metal halide lamps are more commonly used due to their relatively high luminous efficiency.

Nowadays, with the rapid improvement of LED performance, it has shown greater potential in replacing metal halide lamps. To provide the same light output, the number of LEDs used will be less, thus providing the economical applicability of LEDs. Taking a 100 W metal halide lamp as an example, the average light output lumen is 3,500 lumens (lm). The number of LEDs required for this power level is 30 in 2007; it is expected that the number will be reduced to 20 by 2012! So LEDs will have an increasingly large economic advantage.

In order to respond to the rapid increase in LED performance and to maintain the applicability of the design over a longer period of time, practical design strategies such as modular replacement, life cycle analysis, and bill of materials (BOM) cost reduction must be used.

Figure 3: Replacement of metal halide lamps by modular LED approach

First, in the modular approach to replacing the metal halide lamp source, each LED strip can be used with a fixed number of LEDs. As LEDs continue to improve in terms of light output, etc., to provide the same total light output, fewer LED strips can be used, reducing the cost of the LEDs that need to be used, see Figure 3.

Secondly, in the design process, the LED life cycle analysis should be effectively utilized to predict the possible consequences in advance. For example, in terms of prototyping using the highest performing LEDs on the market today, although the associated costs are relatively high, as LED performance improves and prices drop, this approach can create higher levels in the future. Competitive and longer-life products. In addition, as LED performance increases and the resulting single design usage decreases, there is a need to better plan the associated flexibility in LED driver design, resulting in a corresponding BOM cost reduction.

Taking a typical solar street light LED driver design as an example, we can set the goal: the initial light output is 4,200 lm; the light energy efficiency is applicable, using a single layer of optics; working with a +12 V battery.

Correspondingly, the LED specifications used are assumed to be as follows:

Output: Typical 100 lm @ 350 mA @ Junction Temperature (Tj) = 25°C

Drive current: 350 mA

Optoelectronic device: single layer with good coupling and optical loss of only 12%

Maximum ambient temperature: 40 ° C

Drive loss: 10% (target energy efficiency 90%)

In this way, we first need to estimate the number of LEDs and the total power. Since the LED light output is 100 lm at Tj=25°C, the LED light output will decrease when Tj is increased; when Tj is 90°C, the LED light output will drop by 20%, that is, the output will drop to 80 lm. Since the optical loss of the optical device is 12%, the light output of each LED is about 71 lm. Since the total light energy output required is 4,200 lm, the calculated number of LEDs required is approximately 60. Correspondingly, the total output power is: 3.6 V (LED operating voltage) × 0.350 A (output current) × 60 (number of LEDs) = 76 W. Since the driver's power consumption is about 15%, the total power of the luminaire is about 89 W.

In terms of topology, a constant current architecture is required for driving. In addition, it is necessary to be able to adjust the LED output current according to the number of different LEDs, to meet higher energy efficiency requirements, to have a cost-effective system approach and to be easy to implement.

In response to the above design requirements, ON Semiconductor's regulator NCP3066 can be used to implement the drive solution. The NCP3066 is a high-brightness LED constant-current buck regulator with a dedicated “enable” pin for low standby power consumption, with average current sensing (current accuracy independent of LED forward voltage), 0.2 V available Voltage reference for small size / low cost sense resistors. The device features hysteresis control and eliminates the need for loop compensation for easy design. It should be noted that the NCP3066 can also be used as a PWM controller, such as a 100 V external N-channel FET for boosting. Different MOSFET options are available for different applications from 4 to 30 W.

In the design approach, we have a modular design that uses eight LED strips, each strip containing one driver circuit and eight LEDs. Thus, the total number of LEDs is 64, close to the required number of 60 LEDs, providing the required power and light output, and has extremely high energy efficiency, see Figure 4.

Figure 4: Output current vs. input voltage plot for the NCP3066 driving eight CREE XRE LEDs.

to sum up:

This article explores how to use ON Semiconductor's CS51221 charge controller combined with maximum peak power tracking (MPPT) to maximize the energy efficiency and cost associated with powering LED street lights, and how to use ON Semiconductor's flexibility. The NCP3066 controller drives battery-powered LED street lights and related design strategies to help customers shorten the design process for solar LED street lights and speed time to market.

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