Design LED street light how to choose constant current module

I often receive a phone call from the customer. How much power can your constant current drive module bring? This kind of problem is actually unanswerable. Because the power that a constant current module can carry is related to many factors. From the constant current module itself, it is mainly related to the heat dissipation requirements and heat dissipation conditions, and of course to the current drive capability of the driver chip. However, even if these have been determined, for example, the SLM2842 constant current module has been selected, then his driving ability seems to be completely certain! actually not! Although its ultimate drive capability is known, for example, its maximum switching current, its maximum input current, its maximum input voltage and maximum output voltage. However, the driving power of a constant current module is related to its heat dissipation capability, and its main efficiency is related to its working efficiency. And its work efficiency is related to many external factors, especially the user's use. Therefore, when designing or selecting LED modules, we must consider the requirements and characteristics of constant current sources.

First, LED connection architecture

There are some considerations related to constant current modules and other requirements when designing and selecting LED modules.

1. The number of LEDs connected in series should be less than 10, because there is a safety requirement as an LED luminaire. According to the European Union IEC61347-2-13 (5/2006) standard, for LED modules with DC or AC power supply, the voltage in the LED luminaire must not exceed the maximum safety extra low voltage (SELV), that is, its working output voltage ≤ 25Vrms ( 35, 3Vdc), so the total number of LEDs connected in series must not exceed 10.

2. As a street light, LED modules are usually used for repair and replacement. Each module is usually designed as a rectangular shape, and a constant current module is also designed. It is recommended that the power of each module be less than 30W. Thus, 3-4 LED modules are used for street lamps of about 100W, and 5-6 LED modules for 150W LED street lamps. For high power, you can use 7-8 modules.

3. In each LED module, 1W LED is usually used, and the connection mode is series-parallel. If the LED has a protection diode, then it can be stringed first, and if there is no protection diode, then it must be followed by a string, so as not to damage one string.

4, the specific use of several strings and several, we must further understand the performance and characteristics of the constant current module.

Second, the performance and characteristics of the constant current module

The ordinary regulated power supply is that the output voltage does not change when the load changes. The constant current module is a type of power supply that maintains a constant output current when the input voltage changes. Before using it, you must understand its various characteristics.

1. Current setting: Generally, the output current of the constant current module can be adjusted according to the user's requirements in a large range. Simply change the output current setting resistor.

2. Type: The constant current module has three types: boost type, step-down type and buck-boost type. Which one is chosen is entirely determined by the relationship between the required input voltage and the output voltage. But if you want to get the most efficiency, then you should choose the buck type. The step-down type is that the input voltage is relatively high. Therefore, the input current is relatively small, so the loss caused by the copper loss is relatively small, and the buck type constant current module usually has higher efficiency. Therefore, it is also necessary to minimize the number of LEDs connected in series in the load so that a step-down constant current driving module can be used.

3. Relationship between input voltage and output voltage

Regardless of whether it is a step-up or step-down constant current module, the closer the boost ratio or the step-down ratio is to 1, the higher the efficiency. Of course, it cannot be equal to 1, but 2-3V must be set aside for the constant current module to consume. This is entirely determined by the overall lamp design of the street light. Therefore, in the overall design of the street lamp, this factor must be considered to select the number of LEDs in series with the power supply voltage and load. Rather than designing it, how much efficiency the constant current module must provide.

4, dimming ability: Constant current modules usually have dimming ability, and this dimming ability is not simply to adjust its output current, but a dimming method called pulse width modulation (PWM), It takes advantage of the fast switching capability of the LEDs and the visual residuals of the human eye, causing the perceived brightness to change. This avoids changes in the chromatogram due to the regulation of the current. This dimming capability is very important in the design of street lamps, because the use of this performance can make the brightness of the street lamp change according to the traffic flow, thereby further achieving the purpose of energy saving.

Below we discuss the solar LED street light and the city LED street light.

Third, solar street lights

The biggest feature of a solar LED street light is that it is usually powered by a battery, and the battery has some features that need to be considered:

3.1 The battery has only a few specified voltages, and 12V is the most commonly used because it is the standard voltage for automotive batteries. 24V requires two series connections. 36V is even less common. 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.

3.2 Another issue to consider in solar streetlights is the change in input voltage.

For the output power and efficiency of the constant current source, to get the highest efficiency, keep the ratio of the input and output voltages closer to 1. But if the input voltage is unstable. Then it is difficult to maintain the best state of maximum efficiency. Solar LED street lighting systems typically use batteries as energy storage units. The voltage of the battery will change greatly when it is fully charged and discharged quickly. 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 constant current modules, such changes are completely affordable. That is to say, it is possible to keep its output current constant over such a large range of variation. But this also comes at a price, that is, it can't work at its best. The best state is the most efficient state. Or the state with the highest output power.

For constant current modules, whether it is step-up or step-down, 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 is rarely changed. So when the input voltage changes within a range, its efficiency also changes. In order to ensure that you can work in the worst case, you can't work at the best!

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! The longer the life and reliability.

As can be seen from the figure, the chip package is also very important. The chip package using QFN is obviously better than the chip package using TSSOP because its thermal resistance is much lower. Therefore, when purchasing the module, you should also understand the package of the chip used (when possible, use QFN package, try not to use TSSOP package).

Now give a concrete example to illustrate: If the solar LED street light uses a 12V battery, the load is 8 strings 3 and 1W LED, then the load voltage at the output is roughly 26, 4V, the current is 1, 05A, the output power For the 27, 7W. If a 12V battery is used, the voltage is 14 and 8V at the highest voltage, and the boost ratio is 1, 78. At that time, the case temperature is about 60 degrees when the QFN package is not used. It is also acceptable, but when the battery voltage drops to 10, 8V, the boost ratio becomes 2, 44 times, when the case temperature rises to 70 degrees or more, and if the TSSOP package is used, the case temperature It will rise to about 85 degrees, which is not allowed!

In addition to considering the case temperature, the maximum input current is also considered. Because the maximum input current of the SLM2842 is 3A. When calculating the maximum input current, consider the efficiency of the constant current module itself. 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, no more than the allowed 3A; but when the battery voltage When the voltage is reduced to 10 or 8V, the input current reaches 3, 28A, which is more than 3A, and the chip will automatically stop working.

So if you do not consider the battery voltage change is a very dangerous design!

So is it possible to use two batteries in series? It is also not possible here, because in this case, the input voltage is just the same as the output voltage, which is around 24V. At this time, it is necessary to use the lift-down type SLM2842SJ.

The buck-boost constant current module is characterized by an output current that is constant regardless of the input voltage, such that it is below or above the output voltage. That is, it can automatically change from boost to buck based on the relationship between the input voltage and the output voltage. For example, the buck-boost constant current module SLM2842SJ can maintain the output current constant at 1A when the output current is 1A and the output voltage is 13V. The input voltage can be kept constant from 7 to 5V. Is it possible to use a lift type? This is not the case, because the output power of the buck-boost type is relatively small, and its efficiency is low, the price is expensive, and it is not necessary to use it as little as possible.

Can you use three batteries in series, and use a buck-type constant current module? In theory, it is ok, and higher efficiency can be obtained at this time. However, because three batteries are connected in series, it is possible to use excessive capacity (Ah), which increases the cost and increases the probability of replacing the battery during maintenance. Overall it is not cost effective.

So for the design of solar LED street lights, it can be summarized as follows:

As mentioned above, three battery series are not recommended, so if the total power is greater than 60W, then 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 brightness. Light time and brightness. This can save about 40-50% of the capacity. In this way, it is still possible to adopt a scheme in which two 12V batteries are connected in series.

Fourth, the city's LED street lights

In the case of utility power, it is of course preferable to use an AC-DC direct constant current output scheme.

However, most of these solutions can only provide a small power output (<40W). Currently only the US PowerIntegration TOP250YN can provide a 75W power output solution. But its circuit is very complicated, expensive, and low flexibility.

Although the domestic Maoshuo company also provides direct constant current output power, it usually only has a single output. If you want to multi-channel, you must customize (number of channels and each current). On the one hand, the cost is increased, and any changes in the future will be very troublesome.

Therefore, in the case of commercial power, it is best to use a constant voltage power supply and then use multiple constant current modules for constant current. The biggest benefit of this is that you can get the highest efficiency 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 match. And with high flexibility, it's easy to change its combination.

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

1. Wide input voltage range, can be applied to different voltage specifications in different countries.

2, high efficiency, usually can achieve more than 90%.

3, high power factor, usually can achieve 0, 99 or more

4, with anti-surge measures, can resist lightning 4kv or more, can protect the circuit behind.

5, with perfect overcurrent, overvoltage, short circuit, over power protection

6, low cost

7. The biggest advantage is that it is easy to choose its output voltage to get close to the load voltage for maximum efficiency.

Here's an example:

This is a street lamp with a power of 112W. The LED modules are divided into 4 groups, each group is 7 strings and 4, 28W.

We use 150W of Hong Kong Yuda Electronics Co., Ltd. to output 28V power supply regulated power supply VP28SA150U.

Because the load is 7 strings 4 and. The voltage is about 23-24V, so the switching power supply with 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, 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.

Moreover, the biggest advantage of this constant pressure and constant current separation scheme is that it can be inserted into the program dimming system. This can further save more than 40% of electrical energy, which is not possible with any direct constant current output scheme.

And this solution can be combined with a variety of different LED connection architecture. For example, assume that the LED is 10 strings 3 and. Then just change the output of the regulator source in the above picture to 36V, everything else will be 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 thorough understanding of the characteristics of various constant current power supplies in order to design a high-performance, highly reliable street light system!