The correct selection of drive power LED lighting method and device configuration

Because the LED energy saving, long service life, durable and flexible design, the LED is rapidly replacing the incandescent lamp and fluorescent lamp in the indoor and outdoor lighting equipment. But a part of the choice of the LED equation is designed. In order to make solid-state lighting design of your overall efficiency, durability and service life, you need to select the appropriate power to your application requirements and the use of the LED match. This paper will provide some useful suggestions for you to consider in the selection process of local power supply.

Related background

When the power supply voltage of LED is equal to or greater than the forward voltage drop of the diode (usually in the region of 2-3V), LED began to produce light. The current needed for full brightness depending on the device, but for 1W LED (usually the smallest size of lighting applications) is usually 350mA. Unlike incandescent lamp, LED is a nonlinear device. This means that when the supply voltage exceeds the diode forward voltage, current and voltage with the power supply through the exponential growth. No current is a form of regulation, the LED chip will be expensive, monostable flash lamp.

In order to prevent this, the power supply must provide appropriate voltage at the right current. The easiest way is to select a power supply output voltage is greater than the selected LED positive voltage, and a current limiting resistor will limit the current in the maximum value specified by the manufacturer LED. The disadvantage of this method is that one of the major advantages of high efficiency LED lighting will be affected by the power dissipation of the device.

Another problem with this approach is that the LED junction temperature affects the forward voltage. Because the output voltage of the power supply is fixed, which in turn means that if the change of voltage limiting device at both ends, so that the current will change. The current changes will affect the emission of light, reducing the reliability of LED. The best way is used to drive the LED constant current source. In this way, can be set to the maximum current value LED specified by the manufacturer, in order to achieve maximum efficiency and reliability, or achieve the desired brightness, and can also eliminate the influence of junction temperature on LED or when the ambient temperature change.

In lighting applications, one of the benefits of using LED is easy to change the brightness. This can be achieved by changing the current through the LED, but with less than the maximum operating current LED will reduce the efficiency and may cause slight color change. Therefore, it is better in the current between zero and maximum pulse to change the average light emission. As long as this is done at a high enough frequency, in order to avoid being watched as a pulse flash, this is the best way to achieve the dimming. The current pulse usually at a fixed frequency, the ratio of total current and zero change. This is the pulse width modulation (PWM) method.

Select the power

Select the power for lighting applications will be based on the type of several factors. First, consider the application running environment. The application in indoor or outdoor? Whether the power supply need waterproof or need special IP level? Whether the power supply can use conduction cooling or only using convection cooling?

Next, to define the overall power requirements. A single lamp may require only a small power, but a complex system may need to provide hundreds of watts of power. In addition, also need other functions? For example, if the power supply is still need to work under the mode of constant current constant voltage mode is simple, if the application needs light?

Laws and regulations is very important

So, now is the time to consider regulations. Do you want to run the whole system of harmonic current in a certain range of? Does it need to comply with the safety standards of lighting, or ITE power supply is enough? In this era of energy sensitive power, how to effectively meet the needs of local or regional standards?

It is also important, in some local government agencies to meet the specific efficiency and power factor correction level products offer discounts or other subsidies, products are sold in these places? Similarly, you know whether the design criteria to meet the requirements, it is also very important, including the power supply when the power supply is switched off the power consumption of any requirements.

safety standards

Applicable to various standard lighting system. IEC61347 international, the first part covers the general safety requirements of lighting control device, and the second part of the thirteenth section (2) for LED power module. The United States UL8750, European EN61347, are based on the section named IEC format.

harmonic current

Lighting applications usually require the harmonic current emission to meet the requirements of EN61000-3-2, and the lighting category C. In this category, for more than 25W active power input with a set of constraints for 25W and below for another. However, this standard only mentioned 25W and following discharge lighting.

In order to meet the 25W above the limit, usually due to the limitation of power factor correction, and as a percentage of amperage calculation rather than absolute value, so it is best to use a special power supply for lighting applications, rather than the ITE type power supply. However, as long as the lighting load is higher than the rated full load power 40-50%, ITE power may reach the limit.

Here give an example of IP67 DLE series XP Power power series is designed for LED lighting applications. This series of products including 15,25,35 and 60 watt models, with EN61347 and UL8750 security specification.

Figure 1. an example of LED power, XP Power from 15 - 60 watts DLE series.

led configuration

Some lighting applications may use only one LED. The use of power is about 1W, the forward voltage is in the range of 2-3V, the forward current is about 350mA. Although this will produce bright light, but more likely, L