# How to choose a constant current module for designing led street lights (2) A: Solar Led Street Lights

The biggest feature of solar LED street lights is that they are usually powered by batteries, and batteries have some features that need to be considered:

1. There are only a few specified voltages for batteries, 12V is the most commonly used because it is the standard voltage for car batteries. 24V requires two in series. 36V is even rarer. For this reason, a boost-type constant current drive module is often required in solar LED street lamps. This affects the efficiency of the constant current module.
2. Another issue that needs to be considered in solar street lights is the change of input voltage.

For the output power and efficiency of the constant current source, in order to obtain the highest efficiency, it is necessary to keep the ratio of the input and output voltage as close to 1 as possible. But if the input voltage is unstable. Then it is very difficult to maintain the optimum state of the highest efficiency. Solar LED street light systems usually use batteries as energy storage units. The voltage of the battery will change greatly when it is just fully charged and when it is about to be discharged. Usually this change is more than 30%. For example, for a 12V battery, the output voltage can vary from 14, 8V to 10, 8V. Of course, for a constant current module, such a change is completely acceptable. That is, the output current can be kept constant within such a large variation range. But it also comes at a price, which is not working optimally. The so-called optimal state refers to the most efficient state. or the state with the highest output power.

For the constant current module, whether it is a boost or a buck, the efficiency is highest only when the output voltage is closest to the input voltage. Usually the output voltage is determined by the load and rarely changes. So when the input voltage varies within a range, its efficiency also varies. In order to ensure that it can work in the worst case, it can’t work in the best state!

Now take a specific example to illustrate: if the solar LED street light uses a 12V battery and the load is 8 strings of 3 parallel 1W LEDs, then the voltage of the load at the output end is roughly 26, 4V, the current is 1, 05A, and the output power is If the 12V battery is used, the highest voltage is 14, 8V, and the boost ratio is 1, 78. At that time, the case temperature is about 60 degrees when the QFN package is used without a radiator. It is also acceptable, but when the battery voltage is reduced to 10, 8V, the boost ratio becomes 2, 44 times, and the case temperature will rise to more than 70 degrees, and if the TSSOP package is used, then the case temperature It will rise to about 85 degrees, which is not allowed!

In addition to the case temperature, consider the maximum input current. Because the maximum input current of SLM2842 is 3A. When calculating the maximum input current, the efficiency of the constant current module itself should be considered. Now assume that it is 90%, and assume that the output power is 32W, divided by 0, 9 is 35, 5W, and when the input voltage is 14, 8V, the input current is 2, 4A, not exceeding the allowable 3A; but when the battery voltage When it is reduced to 10, 8V, the input current reaches 3, 28A, which exceeds 3A, the chip will automatically stop working.

Therefore, it is a very dangerous design if the voltage change of the battery is not considered!

So is it possible to use 2 batteries in series? This is also not possible here, because in this case, the input voltage is exactly the same as the output voltage, both around 24V. The characteristic of the buck-boost constant current module is that no matter how the input voltage changes, the output current remains unchanged when it is lower or higher than the output voltage. That is, it can automatically change from boost to buck according to the relationship between the input voltage and the output voltage.

Can three batteries be connected in series and a step-down constant current module be used? It is theoretically possible, and higher efficiency can be obtained at this time, but because three batteries are connected in series, it may use too much capacity (ampere-hour), and increase the cost and increase the probability of replacing batteries during maintenance. Overall it’s not worth it.

Therefore, the design of solar LED street lights can be summarized as follows:

As mentioned above, it is not recommended to use 3 batteries in series, so if the total power is greater than 60W, the size of the solar panel will be too large and the wind resistance will be reduced, so it is recommended to use a dimmable controller to minimize the brightness Lamp time and brightness. This saves about 40-50% of the capacity. In this way, the scheme of connecting two 12V batteries in series can still be used.

Electronic Led Street Light

In the case of commercial power, of course, it is best to use the AC-DC direct constant current output solution.

Therefore, in the case of commercial power, it is best to use a constant voltage power supply to stabilize the voltage first, and then use multiple constant current modules for constant current. The biggest advantage of this is that the highest efficiency can be obtained under a variety of different LED connection architectures. Because the output voltage of the constant voltage source can be arbitrarily selected to achieve the best matching. And high flexibility, it is easy to change its combination.

The constant voltage source of commercial power is a very mature product, which usually has the following advantages:

1. The input voltage range is wide and can be applied to different voltage specifications in various countries.
2. High efficiency, usually more than 90%.
1. The input voltage range is wide and can be applied to different voltage specifications in various countries.
2. High efficiency, usually more than 90%.
1. The input voltage range is wide and can be applied to different voltage specifications in various countries.
2. High efficiency, usually more than 90%.
3. High power factor, usually above 0.99
4. With anti-surge measures, it can resist lightning strikes above 4kv, and can protect the circuit behind
5. With perfect over-current, over-voltage, short-circuit, over-power protection functions
6. Low cost
7. The biggest advantage is that it is easy to choose its output voltage to be close to the load voltage for the highest efficiency.

Here’s an example:

This is a street light with LED power of 112W. The LED modules are divided into 4 groups, each group is 7 series and 4 parallel, 28W. Because the load is 7 series and 4 parallel. The voltage is about 23-24V, so a switching power supply with an output of 28V is selected, so that a step-down constant current source can be used. And because the output voltage is very close to the input voltage, high efficiency can be achieved. The measured result is that the total efficiency from input to output is 90.9%, and the power factor is 0.996.

And the biggest advantage of this separation of constant voltage and constant current is that a program dimming system can be inserted. This can further save power by more than 40%, which cannot be achieved by any direct constant current output scheme.

Moreover, this solution can be matched with various LED connection structures. For example, assume that the LEDs are 10 in series and 3 in parallel. Then as long as the output of the voltage regulator in the above picture is changed to 36V, everything else remains the same. At this time, the efficiency of the constant current source itself can be as high as 98%.

All in all, LED street light designers must have a full understanding of the characteristics of various constant current power supplies in order to design a high-performance and highly reliable street light system!