Selection of constant current source module in LED street lamp design

We know that all LEDs must be powered by a constant current source, but many street lamp manufacturers currently look for a suitable constant current module after the LED module has already been designed. Little do they know that this design method will encounter problems, at least making the design not optimal. It is possible that the LED module will need to be redesigned.

1. LED connection architecture

When designing and selecting LED modules, you need to consider some requirements related to constant current modules and others.

1. The number of LEDs in series should be less than 10, because there is a safety requirement for LED lamps. According to the EU IEC 61347-2-13 (5/2006) standard, the voltage in LED lamps using DC or AC power supply must not exceed the maximum safety extra low voltage (SELV), that is, its operating output voltage ≤ 25 Vrms (35.3 Vdc), so the total number of LEDs in series must not exceed 10.

2. As street lamps, LED modules are usually used for easy repair and replacement. Each module is usually designed in a rectangular shape, and the constant current module is also designed in it. It is recommended that the power of each module is less than 30W. In this way, a street lamp of about 100W uses 3-4 LED modules, while a 150W LED street lamp uses 5-6 LED modules. For higher power, 7-8 modules can be used.

3. In each LED module, 1W LED is usually used, and the connection method is series-parallel. If the LED has a protection diode, then it can be connected in series first and then in parallel. If there is no protection diode, then it should be connected in parallel first and then in series, so as to avoid one damage and the whole string not lighting up.
4. To determine how many series and parallels to use, you need to further understand the performance and characteristics of the constant current module.

2. Performance and characteristics of constant current module

Ordinary voltage-stabilized power supplies keep the output voltage constant when the load changes. Constant current modules, on the other hand, are power supplies that keep the output current constant when the input voltage changes. Before using them, you must understand their various characteristics.

1. Current setting: Generally, the output current of a constant current module can be adjusted within a wide range according to user requirements by simply changing the output current setting resistor.

2. Type: There are three types of constant current modules: boost type, buck type and buck-boost type. The choice is entirely determined by the relationship between the required input voltage and output voltage. However, if you want to achieve maximum efficiency, you should choose the buck type. The buck type means that the input voltage is relatively high. Therefore, the input current is relatively small, so the loss caused by copper loss is relatively small, and the buck type constant current module usually has a higher efficiency. Therefore, the number of LEDs in series in the load must be minimized so that a buck type constant current drive module can be used.

3. Relationship between input voltage and output voltage

Whether it is a step-up or step-down constant current module, the closer the step-up ratio or step-down ratio is to 1, the higher the efficiency. Of course, it cannot be equal to 1, and 2-3V must be reserved for the constant current module to consume. This is entirely determined by the overall lamp design of the street lamp. Therefore, when designing the street lamp as a whole, this factor must be considered to select the power supply voltage and the number of LEDs connected in series with the load. Instead of designing it well and then requiring the constant current module to provide a high efficiency.

4. Dimming capability: Constant current modules usually have dimming capability, and this dimming capability is not simply to adjust its output current, but to use a dimming method called pulse width modulation (PWM), which uses the fast switching capability of LEDs and the residual vision of the human eye to change the apparent brightness. This avoids the change in the color spectrum caused by adjusting the current. This dimming capability is very important in street lamp design, because this performance can make the brightness of street lamps change according to traffic flow, thereby further achieving the purpose of energy saving.
Let's discuss the two cases of solar LED street lamps and AC LED street lamps.

3. Solar Street Lights

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

3.1 There are only a few specified battery voltages, 12V is the most commonly used because it is the standard voltage of 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 lights. This affects the efficiency of the constant current module.

3.2 Another issue that needs to be considered in solar street lights is the change in input voltage. For the output power and efficiency of the constant current source, to obtain the highest efficiency, the ratio of input to output voltage should be kept as close to 1 as possible. However, if the input voltage is unstable, it is difficult to maintain the best state of 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 almost discharged. Usually this change exceeds 30%. For example, for a 12V battery, its output voltage can change from 14.8V to 10.8V. Of course, for the constant current module, such a change is completely tolerable. That is, it is completely possible to keep its output current constant within such a large range of changes. But this also has a price, that is, it cannot work in the best state. The so-called best state refers to the state with the highest efficiency. Or the state with the highest output power.
For the constant current module, whether it is a boost type or a buck type, 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 changes within a range, its efficiency also changes. In order to ensure that it can work in the worst case, it cannot work in the best condition!

For example, for the SLM2842S boost constant current module, if the output power is 27 watts, the closer the boost ratio is to 1, the higher the efficiency, the lower the temperature rise of the chip, and the higher the life and reliability (see Figure 1).

Figure 1. Relationship between chip surface temperature and boost ratio when the SLM2842S output power is 27.5W.

As can be seen from the figure, chip packaging is also very important. Chip packaging using QFN is obviously better than chip packaging using TSSOP because its thermal resistance is much lower. Therefore, when purchasing a module, you should also understand the chip packaging used (use QFN packaging as much as possible, and try not to use TSSOP packaging).
Now let's take a specific example to illustrate: if the solar LED street light uses a 12V battery and the load is 8 series and 3 parallel 1W LEDs, then the voltage of the load at the output end is approximately 26.4V, the current is 1.05A, and the output power is 27.7W. If a 12V battery is used, the highest voltage is 14.8V, and the step-up ratio is 1.78. At that time, when the heat sink is not used and the QFN package is used, the shell temperature is about 60 degrees. It is acceptable, but when the battery voltage is reduced to 10.8V, the step-up ratio becomes 2.44 times, and the shell temperature will rise to more than 70 degrees. If the TSSOP package is used, the shell temperature will rise to about 85 degrees, which is not allowed!

In addition to considering the shell temperature, the maximum input current must also be considered. Because the maximum input current of SLM2842 is 3A. When calculating the maximum input current, the efficiency of the constant current module itself must 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, which does not exceed the allowed 3A; but when the battery voltage drops to 10.8V, the input current reaches 3.28A, which exceeds 3A, and the chip will automatically stop working. So if you don't consider the voltage change of the battery, it is a very dangerous design! So can we use two batteries in series? It's not possible here either, because in this case, the input voltage is just the same as the output voltage, both of which are around 24V. At this time, the buck-boost type SLM2842SJ must be used. The characteristic of the buck-boost type constant current module is that no matter how the input voltage changes, when it is lower or higher than the output voltage, its output current does not change. That is to say, it can automatically change from boost to buck according to the relationship between input voltage and output voltage. For example, the buck-boost constant current module SLM2842SJ can maintain a constant output current of 1A when the output current is 1A and the output voltage is 13V, and the input voltage rises from 7.5V to 25V. So should the buck-boost type be used as much as possible in all cases? This is not the case, because the output power of the buck-boost type is relatively small, and its efficiency is low and the price is expensive, so it should be used as little as possible unless it is necessary.

Can we use three batteries in series and use a step-down constant current module? Theoretically, yes, and higher efficiency can be achieved. However, because three batteries in series may use too much capacity (ampere-hours), and increase costs and the chance of replacing batteries during maintenance. Overall, it is not cost-effective.
Therefore, the design of solar LED street lights can be summarized as follows:

As mentioned above, it is not recommended to connect 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. Therefore, it is recommended to use a dimmable controller to minimize the lighting time and brightness. This can save about 40-50% of the capacity. In this way, you can still use the solution of connecting 2 12V batteries in series.

4. Mains LED Street Lights

In the case of mains power, it is of course best to use an AC-DC direct constant current output solution. However, most of these solutions can only provide low power output (<40W). Currently, only the TOP250YN of Power Integration in the United States can provide a 75W power output solution. However, its circuit is very complex, expensive, and inflexible. Although the domestic company Moso also provides power supplies with direct constant current output, they usually only have single-channel output. If you want multiple channels, you must customize (number of channels and current per channel). On the one hand, it increases the cost, and any changes in the future will be very troublesome.

Therefore, in the case of AC 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 you can get the highest efficiency under various LED connection architectures. Because you can arbitrarily choose the output voltage of the constant voltage source to achieve the best match. And it is highly flexible and easy to change its combination. The constant voltage source of AC 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 different countries.

2. High efficiency, usually more than 90%.

3. High power factor, usually above 0.99

4. It has surge protection measures and can withstand lightning strikes above 4kV, which can protect the subsequent circuits.

5. With complete over-current, over-voltage, short-circuit and over-power protection functions

6. Low cost

7. The biggest advantage is that it is easy to select its output voltage so that it is close to the load voltage to obtain the highest efficiency

Here is an example:

This is a street light with an LED power of 112W. The LED modules are divided into 4 groups, each group is 7 series and 4 parallel, 28W. We use the 150W, 28V AC regulated power supply VP28SA150U from Hong Kong Weida Electronics Company. The overall block diagram is shown in Figure 2:

Figure 2. General structure of a mains powered LED street light

Because the load is 7 series and 4 parallel. Its 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 its output voltage is very close to the input voltage, it can achieve high efficiency. The measured result is that the total efficiency from input to output is 90.9% and the power factor is 0.996.

The biggest advantage of this solution of separating constant voltage and constant current is that it can be inserted into a program dimming system, which can further save more than 40% of electricity, which is impossible to achieve with any direct constant current output solution.

Moreover, this solution can be used with various LED connection architectures. For example, assuming that the LEDs are 10 in series and 3 in parallel, then just change the output of the voltage regulator in the above figure to 36V, and keep everything else the same. At this time, the efficiency of the constant current source itself can be as high as 98%. In short, LED street light designers must have a full understanding of the characteristics of various constant current power sources in order to design high-performance and high-reliability street light systems!