Research on Constant Current Drive Source and Its Application in Solar LED Street Lights

I. Overview Although there are many ways to use solar energy, high-power solar power plants and home rooftop power generation are not suitable for China's national conditions. The only simplest and easiest to implement is solar street lights. The total number of street lamps in China exceeds 100 million baht. As long as 60 million gongs are converted into solar street lamps, the annual electricity saved exceeds the power generated by a Three Gorges hydropower station. If the 20 million new street lamps added in the future are all converted to solar street lamps, it will be a Three Gorges hydropower station in three years. The effect of energy conservation is very impressive, and there is no need to mobilize the power of the whole country to build, so that the power of local governments at all levels and the power of all relevant enterprises can be completed.

The composition of solar street lights is very simple .

The installation of solar street lights is also very simple, almost like planting a tree, dig a pit and bury it. Unlike street lights that use AC power, you need to lay cables, build transformer rooms, and dig maintenance wells... Although the cost of a single solar street light is higher than the cost of a conventional high-pressure sodium lamp, if the overall cost of laying a cable or the like is considered, the two are similar. Coupled with the considerable annual electricity savings that can be saved in the future, it can be said that the electricity cost saved by solar street lamps within one or two years can make up for the difference, and the electricity savings in the future will be pure income.

As can be seen from the block diagram shown in Figure 1, the most central components are the PWM dimming controller and the constant current module. In the past, many investments have been made in solar cells and LEDs . Few people care about these two parts, so that most of the controllers on the market today do not have the ability to PWM dimming, and most of the current constant current modules use chips imported from abroad. The state does not attach importance to this input. This is a big problem, and of course there are great opportunities.

Second, why do solar LED street lights require constant current drive?

In addition to luminous efficiency, there are a number of issues that need to be addressed to make LEDs a practical luminaire. The most important of these is its constant current drive. This is determined by several reasons.

(1) The output voltage of the battery used in solar LED street lamps is not constant

In solar street lamps, lead batteries are usually used as energy storage units, and the output voltage of lead batteries is from full charge to full discharge, and the voltage change is close to 20% (Fig. 2). Therefore, the LED current change caused by it may exceed 4 times.

Assume that the initial voltage is 3.25V and the forward current is 350mA. If the supply voltage drops to 2.6V (20%), then the current is less than 40mA, which is nearly 8.75 times lower.

The brightness of the LED is directly related to its forward current. The relationship between the relative luminous intensity and the forward current of the same LED of the same manufacturer is shown 

As can be seen from the figure, if the forward current is reduced by 8.75 times to 40 mA from 350 mA, the relative luminous intensity will be reduced from 100 to 20. Reduced by nearly 5 times. Obviously this is totally unacceptable. So be sure to keep the current constant.

(2) The temperature of the LED light is unstable

LED street lights usually work in the open air and their ambient temperature changes are large.
The forward current of the LED is also related to the junction temperature. Figure 5 shows the volt-ampere characteristics of the LED at different junction temperatures.

The temperature coefficient of the LED is usually negative, that is, when the temperature rises (T1->T2), the volt-ampere characteristic shifts to the left. Its value is about -2mV / °C, then when its junction temperature increases by 50 degrees, its forward voltage will be reduced by 0.1V, if it is powered by a constant voltage power supply, its forward current will increase. For example, when the ambient temperature is 25 °C, the optimal operating current of the LED is 20 mA. When the ambient temperature rises to 85 °C, the PN junction voltage VF decreases, and the operating current increases sharply to 35 mA to 37 mA. However, the increase of current does not produce brightness. Increased, called brightness saturation. Similarly, when the ambient temperature drops to -40 ° C, the junction voltage VF rises, the optimal operating current will decrease from 20 mA to 8 mA to 10 mA, and the illuminance will decrease with the decrease of current, which will not reach the illuminance required for the application. 

Moreover, when the temperature changes, the luminescence spectrum of the LED also changes. Generally, the maximum value of the spectrum when the temperature is increased is drifting in the direction of the long wavelength. It is about 1 nm drift for every 10oC increase, and a 50nm rise will produce a 5nm change (see Figure 6).

In fact, the spectrum of the LED also changes as its forward current changes. This is also undesirable, so be sure to keep its forward current constant. After a constant current source is used, the current change caused by this temperature change is converted into a change in its forward voltage, so that it does not cause changes in brightness and spectrum.

The above content is provided by WOSEN. WOSEN is a professional manufacturer and supplier of Led Flood Light, Led Street Light, Led Solar Light, etc. For more information, please visit https://www.wosenled.com/ or contact admin@wosenled.com or WhatsApp +86-13425434349