Thermal management of high power HB LED

The efficiency of transformation of electricity into the light is a key point for HB LEDs. Efficiency of the typical LEDs achieves 40% to 50%. That means that more than 50% of the electrical power transforms into heat. The reason is a numerous of non-radiative recombination channels in the LEDs, because of high density of dislocations and point defects, and light extraction losses.

The strategic fight for LED high efficiency is the fight for high quality of the material and, in particular, for low dislocation density. Therefore, GaN or AlN substrates should be used to get high efficient HB LEDs or low threshold LDs.

However, the shortest way to get HB LEDs today is to increase electrical power applied to the LEDs and to improve LEDs thermal management at the same time.

Our research shows that LEDs can be separated on two groups: low power (less 200 mW) and high power (more 500 mW) devices, according to different ways of heat extraction from the device. The main conclusions are:

  1. No additional heat radiator is needed for lower power LEDs (up to 200 mW). Heat can be dissipated by mounting surface with 2 cm2 surface area for 60 mW and 8 cm2 for 200 mW. In this case, the copper lead cross section area should be 0.7 mm2.

Remark for Customers: the distance between lower power HB LEDs in an array or matrix should be not less then 3 cm.

  1. An additional heat radiator is needed for high power LEDs (0.5÷10 W). The needed radiator surface area for each power can be determined from Fig.1.2.3. For instance, to keep LED overheating not more then 35° the radiator surface area should be ~ 40 cm2 for 1 Watt dissipated power.

  1. Heat transfer from radiator to air is main problem of LED thermal management. The reason is low temperature difference between the active layer and ambient. In this case the contributions into the heat transfer of natural air convection is low and comparable with infrared radiation. Therefore, the radiator surfaces should be painted in any color to enhance in ~ 3 times infrared-radiation from heat sink. Any color of paint looks nearly black in infrared range of light.

  1. To reduce radiator size dramatically it make sense to develop HB LEDs operating at high temperatures 150-170°C, keeping the reliability issue in mind. This is a challenging task for future.

Thermal management: Experiment

Experiments show that improving thermal management of  standard LED-chips increases dramatically light output:

LED light output at differenct cooling
Light output as a function of current via standard chip at forced Peltier cooling on different radiators.

Thus, the thermal management is nowadays task to get HB LEDs.

Thermal management: Modeling.

Modern LED structures allow ~35° overheating of the active layer. The overheating ΔT is determined by power W applies to LED and total heat resistance RT of all surroundings of LED chip and air.

ΔT = W · RT

Heat resistance RT depends on the three main factors:

1. Heat resistance of a substrate Rsubstrate

2. Radiator heat spreading resistance Rspreading

3. Heat transfer from radiator to air

1. Heat resistance of the substrate. Heat transfers from active layer to a mounting surface or radiator through a substrate (in junction up technology). Heat resistance of the substrate Rsubstrate can be determined from the Table, if substrate thickness is 80 µm:

Material

Al2O3

GaN

AlN

Thermal conductivity, Wm-1K-1

25

260

290

Rsubstrate K/W for 80 µm

35.56

3.42

3.06

ΔT, °С (60 mW)

2.13

0.21

0.18

AlN and GaN substrates are much better then Al2O3 from thermal management point of view.

2. Radiator heat spreading resistance Rspreading determines dissipation of heat in mounting surface or radiator. Heat spreading resistance depends on thermal conductivity of the mounting surface material and chip aspect ratio (AR = a/b, where a – is a length and b – is a width of the chip). If chip has a square shape the chip aspect ratio is equal 1. From the Figure we can see that to improve heat spreading resistance in 3 times we should increase AR up to 70. The value of the Rspreading for square chip is 9 K/W for Cu and 17 K/W for Al mounting surface. Heat spreading resistance is not a limiting factor for LEDs.

LED Heat spreading on aspect ratio.
Heat spreading resistance as a function of chip aspect ratio.

3. Heat transfer from radiator to air by natural convection and IR-radiation is main obstacle in extraction of heat from the LEDs. For instance, for low power 60 mW LED and overheating ΔT = 30°, the radiator surface area should be as much as ~ 2.5 cm2,

LED radiator area
Radiator surface area needed to dissipate low heat power at overheating ΔT = 30°
high power LED radiator area
Radiator surface area needed to dissipate high heat power at overheating ΔT = 30°

Heat transfer from a heat sink (mounting surface or radiator) to air is more effective at large temperature difference dT, Fig. 1.2.4. Therefore, a large surface of heat sink is needed for small temperature difference, 35°, as it is in modern LEDs. For instance, for 1-2 Watts LEDs the surface of the heat sink should be 50-100 cm2

LED Radiator surface on junction temperature
Radiator surface can be reduced dramatically, if LEDs p-n junction could work at high overheating (ΔT =70-100°)

Conclusion.

LEDs can be separated on two groups: low power (less 200 mW) and high power (more 500 mW) devices, according to different ways of heat extraction from the device.

  1. No additional heat radiator is needed for lower power LEDs (up to 200 mW).

Heat can be dissipated by mounting surface with 2 cm2 surface area for 60 mW and 8 cm2 for 200 mW. In this case, the copper lead cross section area should be 0.7 mm2.

Remark for Customers: distance between HB LEDs in an array or matrix should be not less then 3 cm.

  1. An additional heat radiator is needed for high power LEDs (0.5÷10 W). The needed radiator surface area for each power can be determined from Fig.1.2.3. For instance, to keep LED overheating not more then 35° the radiator surface area should be ~ 40 cm2 for 1 Watt dissipated power.

  1. Heat transfer from radiator to air is main problem of LED thermal management. The reason is low temperature difference between the active layer and ambient. In this case the contributions into the heat transfer of natural air convection is low and comparable with

infrared radiation. Therefore, the radiator surfaces should be painted in any color to enhance in ~ 3 times infrared-radiation from heat sink. Any color of paint looks nearly black in infrared range of light.

  1. To reduce radiator size dramatically it make sense to develop HB LEDs operating at high temperatures 150-170°C, keeping the reliability issue in mind. This could be a challenging task for future.