Analysis of static and transient cooling cooling to LED thermal management

In order to ensure the development of reliable solid-state lighting products, GILES HUMPSTON in this paper to explain the complexity of LED and cooling hot path simulation modeling.

LED is a complex equipment. LED not only exist with the semiconductor design and operation related problems, and LED is mainly used for lighting. Therefore, optical coatings, light management devices such as reflector and lens, wavelength converting phosphor has the complexity of the system further. Nevertheless, the heat management for reliable solid-state lighting (SSL) products is crucial. In addition, you need to understand how the cooling LED in static and transient background.

For LED, the need to comply with two parameters of thermal management. One is the required working temperature, the other is the highest working temperature. Usually, the work required temperature as low as possible. To achieve this point can ensure high electro-optic efficiency, good quality spectra and long service life of the device. Under high temperature operation will not only reduce the production of LED light, and the quality and quantity will be reduced, will eventually trigger many failure mechanism.

LED manufacturer is very proficient in these defects can be designed as high as 130 DEG C junction temperature products. Due to the thermal resistance of LED package, printed circuit board (PCB) small temperature of about 10 DEG C. If higher than the rated junction temperature, every increase of 10 degrees C, half life of about LED.

Will be converted to LED electron phonon, the efficiency is relatively low. High brightness white LED can reach 40% efficiency, while the UVC LED may be only 5% efficiency. In these two cases, must be to prevent overheating through removal of residual heat conduction. This is a LED light source or lighting designer's responsibility.

Static cooling LED

LED will maintain a regular method of cooling is LED devices will be mounted on the radiator. From the LED heat by conduction into the radiator, and then emitted into the air. If the heat is water or other fluid is removed, the radiator is sometimes called the cold plate, because the associated cooling system often design working fluid at fixed temperature is lower than the indoor environment.

From the LED to the heat sink can effectively transport heat depends on high thermal conductivity materials. For example, from the graph in Figure 1 can be seen, copper is better than aluminum and brass, stainless steel is better than that of.

Figure 1. materials with different thermal conductivity.

Although copper is the best conductor of heat in these metals, but the thermal conductivity and the thickness of the material. The ability to transfer heat conduction through the material with thermal resistance, the thicker the thickness of rezu greater.

The dielectric and air flow

For example, in high power LED arrays are usually based on the heat conduction PCB. At the top, there are electrically connected with the copper plate and the LED, and in the following is a piece of aluminum heat conduction. Between copper and aluminum with a dielectric layer to prevent short circuit copper to aluminum. The manufacturer has taken a different approach in the choice of dielectric material, from organic materials, inorganic compounds, covering the whole spectrum. As shown in Figure 2, the minimum thermal resistance of dielectric material is almost an order of magnitude, the thin dielectric material can be applied at the same time, still can provide the required insulation.

Figure 2. the thickness of the dielectric material will affect the heat resistance.

However, figure 2 does not show all. If the device is used for air cooling, heat in the path between the LED and the heat sink has many interface. Some of the solder bridging by adhesive bridge, the other will be pressed together (such as screw). The joint of the heat conduction poses an additional barrier, its size may be large, it is difficult to predict, and change over time.

All of the thermal resistance and interface resistance in series / parallel addition is called heat conduction path impedance, designed to keep the LED cooling. The calculation is similar to the resistance network. In Figure 3, the voltage is essentially the current temperature, heat flux, thermal resistance is obtained.

Figure 3. in development work, the equivalent resistance you can depend on the heat conduction path. In order to get a complete thermal resistance model of the system, each must be in transition between material added thermal interface resistance.

Transient cooling LED

The previous discussion is assumed to be in a stable state, i.e. LED permanent electricity and heat radiator will continuously dissipated into the surrounding air. The thermal model of failure in two cases. One is to connect LED, more often in pulse operation. Surprisingly, you can design a hot path, keep the LED cooling in continuous operation, but when activated will overheat. When this operation, the associated thermal offset may make LED sudden failure, similar sudden breaks open as to tungsten filament. Therefore, the solution design needs to consider the transient operation of LED and heat, including time and space variables.

Time dependent

The time component of transient cooling is due to thermal path of specific heat capacity. This can be added to the electrical model of thermal resistance as the capacitor (Figure 4). Heat capacity is the amount of material heating (or cooling) when the absorption properties of heat (or released). The heat capacity of the size of the specific heat (the heat) said.

Figure 4. heat conduction time dependence is due to the heat capacity of materials in the electrical system, the equivalent model is RC low pass filter.

The electrical model analogy implies that thermal impedance is sometimes used to describe the thermal properties of materials related to time. Please note, then we must distinguish, because the static thermal resistance thermal impedance can also be used to describe the whole system.

Spatial dependence

The space component source transient cooling in heat diffusion direction more. For example, installed in a thin metal plate on a large LED. Initially, the entire plate in ambient temperature