LED lights with its unique advantages: energy saving, long life, durability and design flexibility. And then has gradually replaced incandescent and fluorescent lamps, but you know how to choose the correct LED driver power?
Choosing the right LED is only part of the equation. To achieve full efficiency, durability, and lifetime for your solid-state lighting design, you need to choose the right power source to match your application requirements to the LED you are using. This article will provide you with some useful advice in the choice of power supply process needs to be considered.
Related background introduction
Once the LED's supply voltage is equal to or greater than the diode's forward voltage drop (typically in the region of 2-3V), the LED begins to produce light. The current required for full brightness varies by device, but is typically 350mA for a 1W LED, which is usually the smallest size in lighting applications. Unlike incandescent lamps, LEDs are non-linear devices. This means that once the supply voltage exceeds the forward voltage of the diode, the current passing exponentially increases with the supply voltage. Without some form of current regulation, LED chips will become expensive, monostable flash bulbs.
To prevent this, the power supply must provide the proper voltage at the right current. The easiest way to do this is to select a source that has an output voltage greater than the forward voltage of the selected LED and use a current-limiting resistor to limit the current to the maximum specified by the LED manufacturer. The disadvantage of this method is that one of the main advantages of LED lighting is that the high efficiency is affected by the power dissipated by the current limiting device.
Another problem with this approach is that the LED junction temperature affects the forward voltage. Since the output voltage of the power supply is fixed, this in turn means that the current will change if the voltage across the current limiter changes. The changing current will affect the amount of light emitted, reducing the reliability of the LED. The best way is to drive the LED with a constant current source. This allows you to set the current to the maximum specified by the LED manufacturer for maximum efficiency and reliability, or to achieve the exact brightness you require, and to eliminate the effects of junction temperature as LED or ambient temperature changes.
One benefit of using LEDs in lighting applications is the ease of changing the brightness. This can be done by changing the current through the LED, however, running the LED at less than its maximum current can reduce efficiency and can result in slight changes in color. Therefore, a better approach is to pulse current between zero and maximum to change the average light emitted. As long as this is done at a sufficiently high frequency to avoid the pulse being seen by the human eye as a flash, this is the best way to achieve dimming. Current pulses are usually at a fixed frequency, zero and full current ratio changes. This is a pulse width modulation (PWM) method.
Select the power
The type of power supply chosen for lighting applications will be based on several factors. First, consider the environment in which the application is running. Application is indoors or outdoors? Does the power supply need to be waterproof or need a special IP rating? Is the power supply capable of conducting cooling or using only convection cooling?
Next, define the overall power requirements. A single fixture may only require a small power supply, but a complex system may need to provide hundreds of watts of power. Also, do you need other features? For example, should the power supply be operated in a simple constant voltage mode or a constant current mode, does the application require dimming?
Regulations and regulations are important
So now is the time to think about regulation. Does the entire system need to operate within a certain range of harmonic currents? Does it need to meet lighting safety standards or is ITE power sufficient? In this energy-sensitive era, how does the power supply effectively meet local or regional standards?
Equally important, some local government agencies offer discounts or other subsidies to products that meet certain levels of efficiency and power factor correction, where are the products sold? Similarly, it is also important to understand that your design standards meet the requirements, including any requirements for power consumption when the power is turned off.
There are various standards for lighting systems. Internationally, IEC61347 Part 1 covers the general safety requirements for luminaire controls and Part 2, Section 13 (2) applies to LED module power supplies. The United States has UL8750, Europe has EN61347, are all in the chapter named IEC format.
Lighting applications usually require harmonic current emissions to meet the requirements of EN61000-3-2, while lighting class C. In this category there is a set of limits for active input power above 25W, another for 25W and below. However, this standard only refers to discharge lighting of 25W and below.
In order to meet the 25W and above limitations, power factor correction is usually required, and because the limit is calculated in percent of amperes rather than absolute, it is best to use a power supply dedicated to lighting applications instead of an ITE type power supply. However, as long as the lighting load is above 40-50% of the full load rating of the power supply, the ITE power supply may reach its limit.