Active and Passive Cooling Techniques for Achieving Desired Junction Temperature

Jon DomingoDistinguishing quality high power LEDs from inferior technologies most frequently centers on identifying products designed with superior thermal management to avoid loss in efficacy, decreased life hours, and color degradation.

As thermal management is a key element in ensuring reliable quality high power LED performance, be sure to find a supplier who is expert in controlling junction temperature via thermal conduction, convection and radiation techniques and who is very familiar with the role considerations such as thermal resistance, substrate material and ambient temperature play in thermal management. Specifically, make sure they can guide you through the following leading active and cooling passive techniques to identify the solution that works best for your particular application.

Active and Passive Cooling Techniques
Junction temperature can be addressed by a number of different passive and active cooling techniques. Passive techniques pertain largely to the basic composition of the high power LED technology whereas active cooling techniques require additional power.

Passive Cooling Techniques
For some applications with mild environmental concerns, passive cooling techniques alone may be sufficient to address heat dissipation requirements. For more complex applications, oftentimes passive cooling techniques are combined with active cooling techniques to achieve desired results. Because they do not require additional power – passive cooling options are exhausted before implementing active cooling methods.

The four most commonly used passive cooling techniques are thermal paste, metal core PCBs (MCPCB), heat sinks and passive convection.

a) Thermal paste
Thermally conductive pastes are made with a specific chemical composition designed to reduce thermal resistance. Thermal pastes are applied between the high power LED and the PCB, increasing the surface area the LED-generated heat comes in contact with to the entire surface area of the PCB.

The four most commonly used types of thermal paste in order of least costly, to most costly are:
• Ceramic
Ceramic-based thermal paste is normally made of a ceramic powder suspended in a liquid silicone compound. The most commonly used are: beryllium oxide, aluminum nitride, aluminum oxide, zinc oxide, and silicon dioxide. These are the most ineffective, yet most cost effective option.
• Metal based
This type of paste is filled with metal particulate matter, typically silver or aluminum and has better conductivity than ceramic paste.
• Carbon based
Carbon fibers or diamond powder make up the carbon based thermal conductors and are considered the most efficient thermal paste
• Liquid metal based
Some thermal pastes have exotic metals in them, like gallium. These options are not as effective as the carbon based varieties, but also tend to be a bit pricier as the exotic metals can be quite cost prohibitive.

b) Metal core PCB (MCPCB)
Using a metal core PCB board is another passive cooling technique for controlling junction temperature. Metal core PCBs can dissipate heat more efficiently than the FR4 PCBs used in many general electronics applications because materials in metal core PCB has better thermal conductivity.

Commonly used metal core PCBs include aluminum and copper alloy. Copper has better heat conductivity than aluminum, however, copper is heavier and higher cost than aluminum, making, aluminum the most popular metal core PCB substrate for high power LED applications.

c) Heat Sinks
Heat sinks are a commonly used passive cooling technology that help conduct heat away from the LED die. Their primary task is to create more surface area to be cooled by convection and radiation. Having a larger surface area can help reduce heat more effectively. So by adding fins in the heat sink not only helps add surface area, but also helps cool the heat sink.

Passive cooling techniques: Heat sinks often have imperfections or small dips which can reduce surface area. Using a thermal paste helps increase surface area to fill in those dips and small holes. Th

d) Passive Convection
Using a passive convection technique is the simplest way of transferring heat away from the heat sink. Natural air flow around the fins has a cooler temperature which in turns moves the heat away from the heat sink. This is a cost effective way on helping diffusing heat.

Passive Convection uses ambient air to flow in between the heat sink fins. The heat sink transfers the heat from the metal fins to the passing ambient air. This in turn lowers the heat sink thermal re

Active Cooling Techniques
Active cooling techniques require additional power input. Three of the most common active cooling techniques include fans, water cooling, and thermoelectric cooling.

a) Fans
Fans use active convection to dramatically change the ambient temperature by forcing air into the environment. Using a fan on top of a heat sink brings in air from a distance and actively pushes it across the heat sink fins. This has a far better performance than passive convection. A typical setup can reduce thermal resistance to .5C/watt, which is far better than then 10C/watt typically achieved with passive convection. 

Fan and Heat Sink- The fan will force ambient air into the heat sink creating a much greater force in cooling off the fins.

b) Water cooling
Water cooling is another active cooling technique that relies on active convection. This is a more efficient, but a significantly more expensive active convection technique since additional components are needed. There are three main components in a water cooling system; the heat sink, pump, and radiator (heat exchanger). Moving fluid is used to absorb the heat from the heat sink and carry it away from the heat source and back into the heat exchanger which is cooled by ambient temperature. 

Fig. 4:  Water Cooling: Using fluid (like water) can transfer heat into the ambient air.

c) Thermoelectric cooling
A thermoelectric cooler is a solid state device that uses P-type and N-type semiconductors sandwiched between the 2 plates. This setup will alternate charge flow to transfer the heat creating a temperature difference in which heat transfers from one side of the plate to another. The effect can be for both cooling and heating depending on the polarity of the voltage applied to the unit. Adequate ambient temperature is still needed to create an efficient cooler. This technique has several noteworthy disadvantages including energy consumption, heat generation and inefficiency. 

Fig. 5:  Diagram of a Thermoelectric Cooler

Both active and passive cooling techniques play a critical role in thermal management of the junction temperature and can make a significant difference in the efficacy, life hours and color integrity of your LED solution.

A quality supplier will provide both large and small customers with expert design support on a variety of active and passive cooling techniques designed to minimize junction temperature. They will also be able to integrate high power LED technology and any needed cooling techniques with LCDs, light pipes, switches, connectors and other key components to create solutions that maximize performance and efficiency in each individual application.

Author´s Biography:
Jon Domingo is a Product Development Engineer at Lumex. He has 6 years expertise in the design and creation of custom LCD and LED technologies.

Lumex Contact Information
For additional information or engineering assistance:

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