LED driver power of the correct selection method and device configuration

LEDs are rapidly replacing incandescent and fluorescent lamps in indoor and outdoor lighting due to their energy saving, long life, durability and design flexibility. But 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 with 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 precise 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.

safety standard

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.

Harmonic current

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 40-50% above the full load rating of the power supply, the ITE power supply may reach its limit.

Here to give an example, XP Power's IP67-class DLE series is designed for LED lighting applications designed power series. The series includes 15, 25, 35 and 60 watts models, in line with EN61347 and UL8750 safety regulations.

LED lighting drive power how to choose? Reading this can help you

figure 1. An example LED power supply from XP Power's 15 - 60 Watt DLE series.

LED configuration

Some lighting applications may use only one LED. The power used for this type is typically about 1W, the forward voltage is in the range of 2-3V and the forward current is about 350mA. While this will produce a bright light source, it is more likely that LEDs will be used in a single light fixture or some array of fixtures to produce a brighter and more uniform light source. LEDs are commonly used in one of four configurations. The LED series, parallel or matrix (in series and parallel combination) configuration to drive from a single power supply. The fourth configuration uses multiple channels and requires more than one power supply.

Tandem configuration

LED lighting drive power how to choose? Reading this can help you

Figure 2 - LEDs are connected in series

In a series configuration, the individual LEDs are arranged in series. The advantage of doing this is that the same current passes through each of them, resulting in the same brightness of light emitted. Another advantage is that if one LED fails in the short circuit, the other LEDs are not affected and remain lit. The disadvantage is that if one LED fails in an open circuit, the current will be interrupted and all other LEDs will be off. Another disadvantage is that if many LEDs are needed to produce the desired amount of light, the sum of the forward voltages may require a power supply that uses a rather high output voltage.

Parallel configuration

LED lighting drive power how to choose? Reading this can help you

Figure 3 - LEDs connected in parallel

When connected in parallel, the LEDs can still be arranged in series in two or more LED strings. The advantage is that for the same number of LEDs that is the same brightness, the power supply can have a lower output voltage because the number of LEDs in each string can be reduced. Another advantage is that if one LED in one string becomes open, the other strings are unaffected, the luminaire can still emit light, but the brightness decreases. The downside is that because there is slightly different forward voltage in each string, the current in each string can not be precisely controlled by a single supply, a current balancing device may be required per string, which reduces overall efficiency.

Matrix configuration

LED lighting drive power how to choose? Reading this can help you

Figure 4 - LEDs connected in a matrix configuration

In a matrix configuration, the LEDs can be similarly configured in a parallel configuration, but each LED between strings is connected. The big advantage of this configuration is that if a single LED becomes open, there is still a path for current to flow through all the other LEDs in the string, so the light output is hardly diminished. The main disadvantage is that it is more difficult to control the current in each string because current balancing devices can not be used. This means that the LEDs used must have closely matched forward voltages, which may increase costs.

Multi-channel configuration

LED lighting drive power how to choose? Reading this can help you

Figure 5 - LEDs are connected via multi-channel

Using this method, the LEDs are arranged in series with multiple strings, similar to the side-by-side and matrix configurations. The advantage is that for any given brightness, the total string voltage can be reduced, and since each string is supplied by a single power supply, the failure of any one string will not affect the other strings in any way. The disadvantage is that the power supply will be more expensive because each string has a separate output but is more flexible in the application.

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