Simplifying the design of LED heat dissipation with innovative ceramic method

This paper will explain the theory and method of proof of concept, and how to use these ceramic radiator and ultimately improved. Simulation process based on computational fluid dynamics (CFD) for thermal optimization and product process design.

Light-emitting diode (LED) for being restricted by the heating problem with it as an ideal light source is understandable. We have given a lot of attention to the radiator, but the LED and the heat dissipation between the surface and the barrier is not much consideration.

Concept and the change of material removal can simplify the system implementation, but also can significantly improve the thermal management capability and reliability. The use of ceramic as a radiator, circuit carrier and part of the product design not only need new ideas, but also have the will to overcome the traditional model.

What is the heat

LED is energy efficient as everyone knows, the light source, and because of its small size by the designer's favorite. But only when they are not involved in thermal management can they be called "small"". Although the temperature of the LED light source is much lower than that of the incandescent light source at 2500 DEG C. Therefore, many designers finally realized that heat dissipation is a problem that can not be ignored. Although LED is still hot, but the temperature is relatively low, so it will not be a big problem. However, the operating temperature of semiconductor devices based on LED should be less than 100.

According to the law of conservation of energy, heat (energy) must be transmitted to nearby areas. LED can only work at 25 DEG C ambient temperature and maximum temperature of 100 degrees, the temperature range is only about 75 degrees C. Therefore, the need for a larger heat dissipation surface and very effective thermal management.

Two optimization block

As shown in Figure 1, the Group 1 is LED itself, still generally can't touch. The center position is a LED bare chip and a heat radiating copper strip which is connected with the bottom of the LED. From a thermal point of view, the ideal solution is to directly connect the LED chip with the radiator. As a result of mass production, the concept is not up to business. We see LED as a standardized "directory" product that cannot be changed. It's a black box.

Figure 1: in the definition of optimized block, three Group to construct a thermal management system.

Group 2 includes a radiator, radiator function is to transfer heat from the heat source to the heat source. Usually, ambient air is either free or forced convection. The more material the radiator is, the more it needs to be hidden. However, the deeper it hides, the lower the cooling efficiency. Of course, you can also choose the appearance and performance are suitable materials. These materials can be directly exposed to the air and become visible parts of the product design.

In Group 1 and Group 2 is Group 3, which provides mechanical connection and electrical isolation and heat conduction. This may seem contradictory, because most materials with good thermal conductivity can also conduct electricity. On the other hand, almost every kind of electrical insulation material is insulated.

Best compromise is the LED soldered to a printed circuit board onto metal on the radiator (PCB). PCB as the original function of the circuit board can be retained. Although PCB has a variety of thermal conductivity, but they all play a role in the heat conduction barrier.

Comparison of effective thermal resistance system

LED can be obtained from the manufacturer (die to heat plate) and the thermal resistance between the radiator. However, little attention has been paid to Group 3 and its significant impact on the overall thermal performance. The total thermal resistance (RTT) can be obtained by adding all the thermal resistances outside the LED (Group 1) itself (Figure 2). True thermal comparison can be performed by RTT.

Figure 2:RTT pointed out from the LED heat sink to the total resistance of the surrounding environment.

: a ceramic material to achieve two functions

Optimization approach is common only radiator. At present, there are hundreds of kinds of radiator design. However, to further improve performance, it is necessary to upgrade or even cancel Group 3. Electrical isolation must be obtained from the radiator itself by other materials. We think this material should be ceramic. Ceramic materials, such as Rubalit (aluminum oxide) or Alunit (AlN), combine two key features: electrical insulation and thermal conduction.

Rubalit thermal conductivity than aluminum low, while the Alunit is slightly higher than that of aluminum. On the other hand, Rubalit is not as expensive as Alunit (Figure 3). Their thermal expansion coefficient can meet the requirements of semiconductor. In addition, they are hard, corrosion resistant, and meet the EU's Hazardous Substances Directive (RoHS). Ceramics are completely inert substances, they are the most durable parts of the system.

The simplified structure (no adhesive insulating layer, etc.) will be a large power LED ceramic radiator directly and permanently bound together, creating the ideal working conditions for the whole assembly. This brings excellent long-term stability, safe thermal management and high reliability. We have applied for a patent named CeramCool for this method.

Theory

CeramCool ceramic radiator is effectively integrated circuit board and a radiator, the heat sensitive element and the circuit can be reliably heat. It supports direct and permanent connections between devices. In addition, the ceramic itself is not conductive, it can be provided by the use of metal gasket surface. If necessary, it may even provide a three-dimensional conductor track structure for a customer.

For power electronic applications,