Why Would You Use a Constant Power LED Driver?

When to use a constant power LED driver
Applications

In 2018 MEAN WELL released the first of their constant power LED drivers, the LDC series. The range has now been expanded to include the XLG series.

MEAN WELL LDC-80
MEAN WELL XLG-75

Typically, an LED lighting application will require either a constant voltage LED driver, or a constant current LED driver.  

Constant voltage LED drivers are used to power LED strip and light fittings, which have built in electronics to regulate the current. Constant CURRENT LED drivers are used, where there is no in-built current regulation, so why would you use a constant power LED driver? 

The Benefits of Constant Power LED Drivers 

Reduced Inventory Costs 

Constant power LED drivers offer lighting designers and manufacturers a flexible and efficient method of powering different configurations of LEDs, with only one model of LED driver. 

This has the benefit of reducing the number of different models of LED drivers that need to be stocked / managed for in order to maintain production. 

Constant Power LED Drivers Offer a Wider Output Voltage and Current Range 

The key feature of a constant power LED driver is the wider range of the output current and output voltage.  

Up until now it has been necessary to drive LEDs with a constant current. When the forward voltage of a LED driver increases by 2.5%, the corresponding output current rises by 16%.  

Ratio between LED driver current output and voltage output

The forward voltage of the LED can be affected by the junction temperature. Temperature fluctuations can cause the forward voltage to vary by up to 20% or more, causing the current to jump. The brightness of the LED is proportional to the forward current, causing the brightness of the LED to intensify.  

Therefore, to maintain a constant brightness you need to maintain a constant current.  

The problem with constant current LED drivers is the limitation of the forward voltage range required to ensure that the current is delivered at the required power output. Lighting manufacturers usually end up having to manage an inventory of several different models of LED drivers to power different luminaires, due to the fact that they cannot find a LED driver flexible enough to provide all of their power requirements.  

MEAN WELL’s constant power LED drivers overcome this limitation

To show how a constant power LED driver provides more flexibility for lighting manufacturers and designers we will compare the MEAN WELL ELG-75-48 with the XLG-75-H. We should point out at this stage that the XLG-75 has yet to be released, although smaller models are now available. 

The maximum power output of ELG-75-48 is achieved at 48V/1.56A as depicted in the following chart.  

MEAN WELL ELG-75-48 output curve

When the LED voltage is much lower than 48V, the output current cannot be increased accordingly to maintain the power output of 75 WATTS, due to the limited forward voltage range of the LED driver.  

The XLG-75-H easily avoids this issue. With the constant power design, the XLG-75-H provides full power within a voltage range between 36V to 58V. Customers are able to the adjust output current within the area shaded in the following chart allowing them to use this LED driver to power luminaires with different power requirements. 

MEAN WELL XLG output power curves

To provide such a wide range of full power, the XLG series uses higher rated components and verification requirements, to achieve higher reliability and a longer lifetime. The wide adjustment range characteristic of XLG makes it possible for manufacturers and lighting designers to adjust the output characteristics of the LED driver for use with many different luminaire designs. 

Other Applications 

Constant power LED drivers could also prove to be very useful for the replacement market. It is often very difficult to match a LED driver to a light fitting. With the wider output ranges offered by constant power LED drivers, it should be easier to find a suitable LED driver.  

However, the electrical characteristics of the LEDs would need to be known in order to select a suitable LED driver.  

Constant power LED drivers also throw up a challenge for the replacement market, which currently exists for some of the MEAN WELL adjustable LED drivers.  

It won’t be enough for an electrician to simply check the part number of a LED driver that has failed and swap it out. The electrical characteristics of the LEDs would need to be known, so that the electrician can set the LED driver to the correct output current / voltage, which is required for the LEDs to operate at an optimum level without being overdriven. 

Further Information 

Call ADM on 1300 236 467 if you would like further information on MEAN WELL’s constant power LED drivers. A member of our expert team will gladly answer any questions that you may have. 

Acknowledgments: 
Technical data provided by Tony Hsieh, Technical Service Manager at MEAN WELL 

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30 April 2019