The challenges of solar LED street lights and ON Semiconductor energy-efficient solutions
In recent years, the industry's growing concern about the use of clean renewable solar energy for street lighting. Typical solar street lighting system consists of solar panels, charge controllers, batteries, light sources, as well as the composition of the poles, etc., as shown in Figure 1. Light source in the lighting, the experience from incandescent to fluorescent and High Intensity Discharge (HID) such as the three important stages, so pre-and HID lamps have been used in solar-powered street lights.
Figure 1: A typical solar power system of street lighting.
In comparison, the light-emitting diode (LED) light source was regarded as an important stage of the fourth. LED with an energy-efficient, ultra-long working life, low voltage DC, issued a point to light, can provide a variety of colors and white light, compact, solid-state device has a robust, non-mercury and many other advantages, therefore, a growing industry and more will be used for solar LED street lighting. LED and the energy efficiency and light output performance has significantly improved the strongest publicly that white LED R & D capability has reached 132 to 136 lumens / watt (lm / W), this level of energy efficiency than traditional fluorescent and HID lamps metal. Especially in 2008, the white LED has been the realization of high-volume commercial production for more LED street lighting a large scale application of solar energy into opening the door.
Maximum peak power tracking technology to enhance energy efficiency of solar panels
For the purposes of solar street lights, solar panels to improve the photoelectric conversion efficiency (currently only about 30%) is very important. Solar panel voltage - current (VI) characteristic curve and a non-linear variability, drawn from the maximum amount of power is very difficult. This solar-powered LED street lights need to charge controllers and other related electronic circuits (in general the use of micro-controller to achieve) as far as possible, the use of effective control measures to improve energy efficiency in order to maximize the advantages.
Basic type of charge controller is designed to protect the batteries from over charge or charge less, and to prevent reverse current. Pulse width modulation (PWM)-type controller will control the electricity to charge the battery and trickle charge to achieve (trickle charge), so as to protect and extend the useful life of the battery. The latest support for the largest peak power tracking (MPPT) controller can function in the ever-changing solar V / I characteristic curve to provide compensation, and optimize the power output of solar cells, energy efficiency and to optimize battery power.
In particular, when we actually can not change the load, MPPT feature so that solar cells, "that" is changing load; this way, MPPT "deceit" by the solar panels to be hoped that the output voltage and current, allowing more power input to the battery.
Semiconductor solar panels for battery charging solution for control applications, the core of the use of enhanced CS51221 voltage mode PWM controller, support for the largest peak power tracking, the input voltage between 12 and 24 V, output current for 12 V @ 2 A, and to provide Adjustable-by-pulse current limiting, input undervoltage lockout and output over-voltage protection features such as locking. The controller provides auxiliary input for remote transmission and control; to adapt to the high power of 90 W to the application of solar panels.
Figure 2: ON Semiconductor CS51221 controller to control the application of solar panels charging diagram
In the application circuit, the CS51221 need to choose the right topology. Topology chosen to be able to in the case of a battery of solar panels will be the output voltage to 12 V, while in two cases or more batteries can be easily modified to support the step-up to 24 V. Itself can be configured to CS51221 forward, flyback or boost topology. ON Semiconductor charge control for the application of solar panels put in place by the reference design, the choice is the flyback topology.
In the application, the ISET pin through dynamically adjust current limit in order to achieve the maximum peak power tracking. Once the input voltage drop-by-pulse current limit will be reduced until the resumption of the input voltage. This approach eliminates the high cost of using the micro-controller (MCU) needs. This charge controller will achieve the peak power point and found that the dynamic adjustment in line with the changing characteristics of the power supply.
Maximum peak power through the use of tracking technology can be about 30% of the additional charge transfer from the solar panels to the batteries so that you can make solar street lighting system uses a smaller-size solar panels. Receive the same as in the case of electricity, MPPT feature can be used with 60 W of power using solar panels to replace the basic charge controller Power 90 W solar panels. Output-per-watt of power in accordance with the needs of about 4 U.S. dollars worth of solar panels to calculate the power reduction caused by 30 W solar panels can be achieved cost savings of 120 U.S. dollars, resulting in significant cost reduction proceeds.
To deal with performance LED driver circuit design strategies and solutions
As mentioned above, solar street lighting system in the source area, LED is to replace the traditional fluorescent and High Intensity Discharge (HID). Including metal halide HID lamps (referred to as "metal halide lamps"), high / low pressure sodium lamp and mercury vapor lamps, such as various types, of which, by virtue of a relatively high luminous efficiency, metal halide lamp applications more generally.
Now, with the rapid upgrade of LED performance, it shows a metal halide lamp to replace a greater potential to provide the same light output, the number of LED used will be less, thus providing the applicability of LED's economy. To 100 W metal halide lamp as an example, the average number of light output lumens 3,500 lumens (lm), this power level required for the number of LED used in 2007 was 30; is expected by 2012 the number will be reduced to 20! Therefore, LED will have a growing economy advantage.
In order to enhance the LED performance trends quickly and maintain the design in the application of a long period of time, to adopt the design of practical strategies, such as a modular alternative, life-cycle analysis and bill of materials (BOM) cost reduction and so on.
Figure 3: The alternative means of modular LED Lamp
First of all, in a modular way to replace metal halide light source, the LED light can be used for each fixed number of LED. With the LED light output in areas such as continuing to enhance performance, it is necessary to provide the same total light output, you can use the LED light be less, thereby reducing the need to use the cost of the LED, see Figure 3.
Secondly, during the design process, it is necessary to effective use of LED life-cycle analysis, in order to predict the possible consequences in advance. For example, in the current market the highest performing LED design for a prototype, although the associated costs are more expensive, but with the LED to enhance the performance and price decline, which can create a longer period of time in the next higher competitiveness and a longer product life. In addition, as the LED's to enhance performance and the resulting reduction in the amount of a single design in the design of LED drivers also need to be better related to the flexibility of planning so that the corresponding lower BOM cost.
To a typical solar LED street lights as an example-driven design, we can set this goal: the initial light output of 4,200 lm; energy efficiency applicable, the use of single-layer optical devices; +12 V battery used.
Accordingly, it is assumed that the LED specification used is as follows:
Output: a typical 100 lm @ 350 mA @ junction temperature (Tj) = 25 ℃
Drive current: 350 mA
Optoelectronic devices: single-layer, and a good coupling, optical loss was only 12%
Maximum ambient temperature: 40 ℃
Drive wear and tear: 10% (target 90% energy efficiency)
As a result, we first need to estimate the number and total power LED. Tj = 25 ℃ due to the time for the LED light output 100 lm, and when Tj increase LED light output will be reduced; Tj at 90 ℃ for, LED light output will drop 20 percent, that is, the output reduced to 80 lm. As the optical components of the optical loss of 12%, so the light output of each LED on for about 71 lm. Due to the total energy output of 4,200 lm, so calculated the number of required LED to about 60. Accordingly, the total output power: 3.6 V (LED operating voltage) × 0.350 A (output current) × 60 (LED number) = 76 W. The staff as a result of consumption drives about 15%, so the total lamp power of about 89 W.
In the topological structure, the structure needed to carry out constant-current drive. In addition, according to different LED need to be able to regulate the number of LED output current to meet higher energy efficiency requirements, the system is highly cost-effective way and easy to implement.
In response to these design requirements, can be used for ON Semiconductor NCP3066 voltage regulator to achieve the drive solution. NCP3066 is a high-brightness LED constant-current buck regulator with a special "opening" pin to achieve low standby power consumption, with an average current sensing function (current accuracy has nothing to do with the LED forward voltage) to provide 0.2 V voltage reference, suitable for small size / low-cost sensing resistor. The device hysteresis control, no loop compensation, easy to design. It should be noted that, NCP3066 can also be used as PWM controller, such as 100 V may be an external N-channel FET to boost. For the 4 to 30 W power of different applications may choose to provide different MOSFET.
Way in the design, we carry out modular design, that is, the use of LED light of Article 8, each optical section with a drive circuit and 8 LED. This LED is the total number of 64, close to the required number of 60 LED, you can provide the required power and light output, and has a very high energy efficiency, see Figure 4.
Figure 4: NCP3066 drives 8 CREE XRE LED when the output current and input voltage curves.
This paper discusses how to use ON Semiconductor CS51221 of charge controller, combined with the maximum peak power tracking (MPPT) function to enhance the maximum solar panel power supply for LED street lighting energy efficiency and reduce associated costs, as well as how to use ON Semiconductor flexible the NCP3066 controller to drive the battery-powered LED street lights and the related design strategy to help customers reduce the solar LED street lighting, the design process, accelerate products to market.
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