Application example of constant current drive source in solar LED street light

I. Overview

There are many ways to use solar energy, but ultra-high-power solar power stations and household rooftop power generation are not suitable for China's national conditions. The only simplest and easiest to achieve is solar street lights. There are more than 100 million street lights in China. If only 60 million of them are converted to solar street lights, the annual electricity saved will exceed the power generation of a Three Gorges Hydropower Station. If all the 20 million street lights added each year are converted to solar street lights, it will be another Three Gorges Hydropower Station in three years. The energy-saving effect is very considerable, and there is no need to mobilize the country's strength to build it. It can be completed by mobilizing the strength of local governments at all levels and the strength of various related enterprises.

The structure of solar street lights is very simple (Figure 1).

Figure 1. The structure of a solar street light

The installation of solar street lights is also very simple, almost like planting a tree, just dig a hole and bury it. Unlike street lights that use AC power, it is necessary to lay cables, build transformer rooms, and dig maintenance wells... Although the cost of a single solar street light is more expensive than that of an ordinary high-pressure sodium lamp, if the overall cost of laying cables is considered, the difference between the two is not much. In addition to the considerable electricity bills saved every year in the future, it can be said that the electricity bills saved by solar street lights within one or two years can make up for the price difference, and the electricity bills saved in the future are pure income.

As can be seen from the block diagram shown in Figure 1, the most core components are PWM dimming controllers and constant current modules. In the past, many investments were made in solar cells and LEDs. Few people pay attention to these two parts, so that most of the controllers on the market currently do not have the ability to dim PWM, and most of the current constant current modules use chips imported from abroad. And the country does not attach importance to investment in this area. This is a big problem, and of course there are great opportunities.

2. Why do solar LED street lights need constant current drive?

In addition to luminous efficiency, there are a series of issues that need to be solved to make LED a practical lamp. The most important one is its constant current drive. This is determined by the following reasons.

(I) The output voltage of the battery used in solar LED street lights is not constant.
Lead-acid batteries are usually used as energy storage units in solar street lights. The output voltage of lead-acid batteries changes by nearly 20% from full charge to full discharge (Figure 2). Therefore, the change in LED current caused by it may exceed 4 times.

Figure 2. Discharge curve of lead-acid battery

LEDs have very steep volt-ampere characteristics (Figure 3).

Figure 3. The volt-ampere characteristics of a company's LED

Assuming the initial voltage is 3.25V, the forward current is 350mA. If the supply voltage is reduced to 2.6V (20%), 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 curve between the relative luminous intensity and the forward current of the same LED from the same manufacturer is shown in Figure 4.

Figure 4. Relationship between relative light intensity and forward current

As can be seen from the figure, if the forward current is reduced by 8.75 times from 350mA to 40mA, its relative luminous intensity will be reduced from 100 to 20. It is reduced by nearly 5 times. Obviously, this is completely unacceptable. Therefore, the current must be kept constant.

(II) The temperature of LED light emission is unstable.

LED street lights usually work outdoors, and the ambient temperature changes greatly. The forward current of LED is also related to the junction temperature. Figure 5 shows the volt-ampere characteristics of LED at different junction temperatures.

Figure 5. LED volt-ampere characteristics at different ambient temperatures

The temperature coefficient of LED is usually negative, that is, when the temperature rises (T1->T2), the volt-ampere characteristic moves to the left. Its value is about -2mV/℃, so when its junction temperature increases by 50 degrees, its forward voltage will decrease by 0.1V. If it is powered by a constant voltage power supply, its forward current will increase. For example, the optimal operating current of LED is 20mA at room temperature of 25℃. When the ambient temperature rises to 85℃, the PN junction voltage VF decreases, and the operating current increases sharply to 35mA~37mA, but the increase in current does not increase the brightness at this time, which is called brightness saturation. Similarly, when the ambient temperature drops to -40℃, the junction voltage VF rises, the optimal operating current will decrease from 20mA to 8mA~10mA, and the luminous brightness will also decrease with the decrease in current, which cannot reach the illumination required by the application site.

Moreover, when the temperature changes, the luminous spectrum of LED will also change. Usually, when the temperature increases, the maximum value of the spectrum drifts in the direction of longer wavelength. The drift is approximately 1nm for every 10°C increase, and a 50°C increase will produce a 5nm change (see Figure 6).

Figure 6. The peak wavelength of LED light varies with ambient temperature.

In fact, the spectrum of LED also changes with the change of its forward current. This is also undesirable, so its forward current must be kept constant. After using a constant current source for power supply, the current change caused by this temperature change will be converted into a change in its forward voltage, which will not cause changes in brightness and spectrum.

(III) PWM dimming of LED

In solar LED street lights, it is often necessary to adjust the brightness of street lights according to the working hours to reduce the area of ​​the solar panel.

In order to change the brightness of the LED, the simplest way is to change its forward current. However, the change of forward current will cause a change in the spectrum. For white light LEDs, it will cause a change in its apparent color temperature, which is obviously undesirable (Figure 7).

Figure 7. Changes in the emission spectrum caused by changes in forward current

The best way is to use pulse width modulation (PWM) to dim the light. This actually takes advantage of the residual vision of the human eye, so that although the LED still works at full current, it works intermittently in a switching manner. By changing the ratio of on and off, its apparent brightness can be changed. In order not to cause the feeling of flicker, the switching cycle must be less than the residual time of the human eye's vision, that is, the frequency of PWM must be higher than the flicker frequency that the human eye can perceive. It is about 200Hz or more. However, as the power of LEDs is getting bigger and bigger, it is very troublesome to generate a high-power PWM signal directly to the LED. Fortunately, most of the current constant current sources are a switching DC converter, which can accept a very small power PWM signal and output a high-power switching signal to the LED, while maintaining the constant current function, that is, its peak value still maintains the original set current value.

Therefore, in order to achieve PWM dimming, a constant current drive source is also needed. The dimming function is very important in solar LED street lights. For example, it can be changed to half-power operation after midnight, or even changed to 1/3 power operation later, which can greatly reduce the area of ​​the solar panel, thereby reducing the cost of the entire lamp.

(IV) LED inconsistency

Even for the same model of LED, its volt-ampere characteristics are different between individual devices, not to mention that they are even more different between different manufacturers (Figure 8).

Figure 8. The discreteness of the volt-ampere characteristics of LEDs from the same manufacturer (solid line) and the discreteness of the volt-ampere characteristics of LEDs from different manufacturers (dashed line)

As can be seen from the figure, if a constant voltage power supply is used, there will be a big difference in the forward current between them. Excessive forward current will also lead to accelerated light decay, so a constant current source must be used for power supply.

3. Selection of various constant current sources

The constant current source used in solar LED street lights can be divided into three types: boost type, buck type, and buck-boost type: the so-called boost type means that its output voltage is higher than the input voltage. Buck type means that the output voltage is lower than the input voltage. The buck-boost type can automatically adjust its working mode to boost or buck according to the situation that the input voltage is lower or higher than the output voltage.

In solar LED street lights, lead-acid batteries are usually used as energy storage devices, and their voltage is usually 12V or 24V. The required output voltage is determined by the architecture of the connected LED. In order to make the forward current of all LEDs consistent, the LEDs are usually connected in series. At this time, the required output voltage is the sum of the forward voltages of all LEDs in series. For example, if 10 LEDs are connected in series (Figure 9a), the sum of their forward voltages is approximately 10x3.3V=33V. In fact, since the LEDs produced by different manufacturers are different, and each LED is also different. Therefore, the sum of the forward voltages of the 10 LEDs is also different. In fact, in a constant current source, the current is constant rather than the voltage. Therefore, it is not necessary to know the correct value of the sum of the forward voltages, but only to know whether it is higher or lower than the input voltage. Here, whether a 12V or 24V battery is used, it requires a boost type constant current source.

(a) (b)
Figure 9. 10-series and 3-parallel LED structure

If the LED used is a 10V, 1A 10-watt LED. Then whether it is a 12Vd1 battery or a 24V battery, a step-down constant current source must be used.

If the voltage of the LED is close to the power supply voltage, for example, the load is 4 1-watt LEDs in series, then its voltage is about 13V, and the battery will reach more than 14V when it is fully charged. At this time, a step-down constant current source must be used, but if the battery is about to be discharged, its voltage is only about 10.4V. At this time, a step-up constant current source is needed. Therefore, in this case, a step-up and step-down constant current source must be used.

Multiple LEDs can also be connected in series and parallel, which we usually call several series and several parallels. For example, 10 series and 3 parallels are the structures shown in Figure 9b.

At this time, a constant current source can certainly be used for power supply, but the constant current source at this time can only keep the total current of the 3 series constant. The distribution of this total current in each series is based on their volt-ampere characteristics. Since the voltage applied to the three strings is the same, and the current of each LED in each string is the same, it is necessary to balance the working point that meets these two conditions. Moreover, if one LED in one string is broken, the total current of the three strings will be distributed to two strings, which increases the current in each string. In order to reduce the current imbalance between the strings, all LEDs in each string can be connected in parallel to form a grid structure. At this time, if one LED in a string is broken, it will not affect the other LEDs. However, if the broken LED is short-circuited, the LEDs in the other two strings will also be short-circuited. However, when LEDs are damaged, they are mostly open-circuited, and short-circuited are relatively rare. Of course, the best way is to use a protection diode (usually a Zener diode) in parallel with each LED, but this increases the cost. Of course,

multiple LEDs can also be connected in parallel, but because the volt-ampere characteristics of each LED are different, if a constant voltage source is used to supply power at this time, it will cause great problems (Figure 10).

Figure 10. When multiple LEDs are connected in parallel and powered by a constant voltage source, the current in each LED is different.

At this time, even if a large current constant current source is used for power supply, it cannot guarantee that the current in each LED is the same. Usually, a resistor needs to be connected in series with each LED to achieve balance, but that will reduce efficiency. Therefore, it is not recommended to connect too many LEDs in parallel.

4. Basic working principle and characteristics of constant current source

(I) The basic structure of the boost constant current source is shown in Figure 11.

Figure 11. Basic schematic diagram of boost constant current source

The figure shows a basic inductor boost circuit, in which the controller (Control) gives a PWM switch signal to control the high-power switch tube, which charges the inductor when it is turned on, and the energy in the inductor charges the capacitor when it is turned off. After several switches, the input voltage can be pumped to a higher voltage, thus completing the task of boosting. Changing the duty ratio of the PWM signal can change its output voltage. In order to keep the output current constant, it is required to measure the output current value, which is measured by a small resistor in series with the LED. The voltage on this resistor is compared with a reference voltage, and the error signal obtained by the comparison is sent to the controller to change the duty ratio of the PWM signal, thus realizing a closed-loop automatic control to control its output current to be constant.

When selecting this type of boost constant current source, there are several issues to pay attention to.

1. The constant current characteristic should be good

Figure 12 shows a constant current characteristic curve of SLM2842S. It is better to put the detection resistor at the high end than at the low end.

Figure 12. Constant current characteristics of SLM2842S

2. The boost ratio should be as small as possible, or the input voltage should be as high as possible. Generally speaking, the smaller the boost ratio, the higher the efficiency. Figure 13 below shows the relationship between the efficiency and input voltage of the SLM2842S when the output current is 0.7A and the output power is 30W.

Figure 13. Efficiency of the SLM2842S vs. input voltage when the output current is 0.7A and the output power is 30W.

As can be seen from the figure, if a 12V battery is used, its efficiency is only about 90%, while if a 24V battery is used instead, its efficiency can reach 94%. This can reduce the power consumption of the chip, reduce the temperature of the chip, and improve the reliability of the chip.

3. The detection resistor should be as small as possible to avoid consuming power on it. In fact, this also means that the internal reference voltage should be as low as possible. This is a certain difficulty for chip designers. If this reference voltage is around 0.1 volts, it is a very good chip. At this time, the external detection resistor can use a very small value, which means that the entire system has a very high efficiency.

(II) Buck constant current source

The basic working principle of the buck constant current source is similar to that of the boost type. Its basic schematic diagram is shown in Figure 14.

Figure 14. Basic schematic diagram of a step-down constant current source

Usually, the energy storage inductor is connected in series with the LED. When the switch is turned on, the inductor stores energy; when the switch is turned off, the inductor continues to have current flowing through the diode. Since the output voltage is the input voltage minus the inductor voltage, it is a step-down type. The current in the LED is measured by the detection resistor and fed back to the controller. To control the PWM duty ratio to achieve constant current control.

The disadvantage of the step-down constant current source is that the input voltage is required to be higher than the output voltage, and the battery used in the solar LED street lamp is often 12V, at most 24V. The LEDs in the street lamp are usually 10 in series, and the total output voltage required is often around 33V-36V, so it is difficult to use a step-down constant current source. The advantage of the step-down constant current source is high efficiency. Since the input voltage is higher than the output voltage, the input current is less than the output current. Small current helps to reduce resistive losses. Figure 15 shows the relationship between the efficiency and output voltage of a step-down constant current source when the input voltage is fixed at 35V and the output current is fixed at 2A.

Figure 15. Efficiency and output voltage curve of step-down constant current source

As can be seen from the figure, when the output voltage is 30V, its efficiency is as high as 98%. The constant current characteristic of this constant current source is also very good (Figure 16).

Figure 16. Constant current characteristics of SLM2862J

In fact, this step-down constant current source can also be used in AC LED street lights. As long as a constant voltage power supply with an output of 36V is added in front, it can drive a 60-watt (1-watt LED 10 series and 6 parallel) LED, and the efficiency is as high as 98%.

(III) Constant current source with PWM dimming

For the human eye, it is difficult to detect the change of a few nanometers in the wavelength of red, green or blue LEDs, especially when the light intensity is also changing. However, the color temperature change of white light is very easy to detect. And the method of changing the forward current to dim the light can easily make the human eye feel the change of color temperature, which is not desirable. Therefore, in high-power LED street lights, we often use PWM dimming to avoid changes in apparent color temperature. On the other hand, analog dimming that changes the forward current will also reduce the setting accuracy of the output current, which is also undesirable.

In PWM dimming, a very important indicator is the dimming frequency. The dimming frequency should be high enough to avoid the human eye from feeling flickering, so it should be at least above 200Hz. On the other hand, the dimming frequency cannot be too high. Since there is a process for the output current to increase from 0 to the specified value, that is, it takes a certain amount of time, we hope that the proportion of this time to the entire cycle is as small as possible, so the cycle cannot be too small. Otherwise, the contrast that can be achieved will be reduced.

In addition, there is a buck-boost constant current source. However, this constant current source is more used in occasions such as automobiles where the input voltage fluctuates within a large range, and is rarely used in solar LED street lights. I will not introduce it here.

Conclusion

Solar LED street lights have excellent market prospects and are also the best way to develop renewable energy. However, from the current situation, most people focus on solar panels and LED lamps. Few people pay attention to the performance and development of constant current sources and controllers. Choosing a high-performance LED constant current source can not only improve the reliability of LED street lights, but also greatly reduce the area of ​​the required solar panels by matching a constant current source with PWM dimming performance and a controller with PWM dimming output. This is extremely important for reducing the cost of the entire lamp and accelerating the popularization of solar LED street lights. I hope that all the workers engaged in solar LED street lights will pay attention to this work and improve our country's solar LED street lights to the most advanced international level!