March 29, 2024

Consider constant current source requirements and features when designing street lamps or when selecting LED modules

All LEDs must be powered by a constant current source, but many street lamp manufacturers are currently looking for a suitable constant current module when the LED module has been designed. As everyone knows, this design method is problematic, at least making this design not optimal. It is possible to redesign the LED 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 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 (see Figure 1).


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