PowerPeg thermal management and efficiency

[TEMProducts]

Everyone loves the Cree XML. This past week I did some long overdue applications testing based on the Cree XML with PowerPeg thermal management.

http://www.cree.com/LED-Components-and-Modules/Products/XLamp/Discrete-Directional/XLamp-XML2

About Cree XML LED

Over the past four years the Cree XML series LED has been my favorite LED. It is a very popular component in DIY projects and commercial products around the world.

Cree XML series is one of many new-generation light emitting diodes which are designed to be surface-mounted on a PCB. This small LED packs big power into a package only 5mm X 5mm in size.

The thing that makes the XML so special is it’s especially large die. The semiconductor inside the XML is 3mm X 3mm in size, providing nine square millimeters for light emission. In LED physics size does matter. You may be able to pipe more power into a smaller LED, but efficiency will undoubtedly be lower when emission area is limited.

The 3mm die in XML LEDs is connected to a large thermal pad on the bottom side of the chip. The LED itself is rated at 10 watts maximum, but this rating is very limited by how effectively heat can dissipate from the LED.

Efficiency in lighting

Efficiency in lighting systems is important. As greenhouse gasses are continuously accumulating in the earth’s atmosphere, all the big names in lighting seek to produce the world’s most efficient light bulbs. LEDs are green because they use less electricity, and they are also green because they last for more years than old technologies.

The especially large die in the Cree XML means it can:

A) handle huge power for it’s size. Many of the new ULTRABRIGHT flashlights use the XML technology because it is just so powerful! The LED can be driven at up to 3 Amps if cooled properly.

Or

B) The XML can be driven at a moderate power level to achieve high energy efficiency!

Ultimately the energy savings equates to less fossil fuel consumption, and a healthier planet.

Calculations

Calculating the energy efficiency of an LED light can be tricky. In resistive circuits P = VI, but LEDs are semiconductors which means V is a function of the temperature and other variables.

The best way to characterize the efficiency of an LED light is by conducting a lab test.

The goal of this experiment is to test Cree XML LED at various power levels, and with multiple heatsinks. The effect of each heatsink on energy efficiency will be observed.

The Heatsinks

After four years of research and development into PowerPeg Thermal Management, I have a beautiful assortment of standardized heatsinks to test.

Once PowerPeg is soldered to the LED, any of these heatsinks can be added or removed.



The Experiment

The experiment is designed to monitor:

the temperature of the thermal pad
the forward voltage of the LED
the light output of the LED
A benchtop power supply maintains constant-current through the LED during each test. Four heatsink arrangements were tested:

PowerPeg with no heatsink attached
GP-43-A heatsink
GP-43-A-FC1 forced convection cooler
WC-43-A water cooler
The experiment was repeated at three power levels (350mA, 700mA, 1500mA) for a total of twelve trials.

The Apparatus

An Arduino Mega served as the controller. The KCA-259 thermocouple shield easily attached to the Arduino for temperature measurements.



The LED was installed on a PCB with a TCAP-4325 PowerPeg.

Tiny 40 AWG thermocouple wires were sandwiched between the LED and PCB for measuring the temperature. I started by welding the thermocouple wires to the copper pad on the PCB using a capacitor, then covered the pad with solder, then reflowed the LED on top.



Test leads were soldered to the PCB as close as possible to the LED for measuring the forward voltage.

The LED was mounted at the top end of an optical tube approximately 3” long. The optics tube contained a series of light diffusers and filters. The filters were necessary in order to bring the light intensity down to a measureable level.

The light intensity is measured at the bottom end of the tube by a TSL230 light intensity sensor. The bottom half of the assembly was positioned inside a cardboard box to block out ambient light.



Results

The experiment produced a comprehensive data set.

When the LED kicks on the temperature of the system rises until thermal equilibrium is reached. Each heatsink reaches a different temperature for a given power level.



Temperature affects light output. The measured light intensity was identical at the start of each test. Light output decreased as temperature increased.



The forward voltage of the LED decreased as temperature increased.



The power supply delivers constant current despite changes in voltage. The decrease in forward voltage means that power input also decreased proportionally (P=VI).

Coefficient of relative power input was calculated by dividing the forward voltage at each point by the initial voltage.

Then efficiency was calculated by dividing relative Lux by the relative power input.







The experiment showed that efficiency was significantly improved by incorporating a thermal management system.



More Pictures
https://plus.google.com/photos/105142970227332631428/albums/6180263645545718161

PowerPeg Applications Test PDF
http://www.tem-products.com/PowerPeg%20Application%20Test%20Cree%20XML.pdf

Hackaday 2015
https://hackaday.io/project/6874-powerpeg-thermal-management-system