How to carry out LED thermal management?

LED is a complicated device. LEDs not only have common problems associated with semiconductor design and operation, but LEDs are primarily used to emit light. Therefore, there is further system complexity for optical coatings, beam management devices such as reflectors and lenses, wavelength conversion phosphors, and the like. However, thermal management is crucial to reliable solid state lighting (SSL) products. In addition, you need to understand how to cool LEDs in static and transient conditions.

For LEDs, two thermal management parameters need to be followed. One is the desired operating temperature and the other is the maximum operating temperature. In general, the required operating temperature needs to be as low as possible. Achieving this ensures high electro-optic efficiency, good spectral quality and long device lifetime. Operating at high temperatures not only reduces the amount of light generated by the LEDs, but also decreases in quality and quantity, eventually triggering many failure mechanisms.

LED manufacturers are adept at these flaws and can design products with junction temperatures of up to 130 ° C. Due to the thermal resistance of the LED package, the temperature of the printed circuit board (PCB) is approximately 10 ° C. With a rise of 10 ° C above the rated junction temperature, the LED life is reduced by about half.

Converting electrons into phonons, LED efficiency is relatively low. High-brightness white LED can achieve 40% efficiency, and UVC LED may only 5% efficiency. In both cases, the remaining heat must be removed by conduction to prevent overheating. This is the responsibility of the LED light source or lighting designer.

Static cooling LED

The usual way to keep the LED cool is to mount the LED device on the heat sink. The heat from the LEDs is conducted into the heat sink and then dissipated into the air. Radiators are sometimes referred to as cold plates if the heat is removed by water or other fluids because the associated cooling system is often designed with the working fluid at a fixed temperature below the indoor environment.

The efficient transport of heat from LEDs to heat sinks depends on the high thermal conductivity of the material. For example, as can be seen from the graph of FIG. 1, copper is superior to aluminum and brass, again superior to stainless steel.

 

figure 1. The material has different degrees of thermal conductivity.

Although copper is the best thermal conductor in these metals, the thermal conductivity is independent of the thickness of the material. The ability to transfer heat through the material is mainly related to the thermal resistance, the thicker the thickness, the greater the thermal resistance.

Dielectric and air flow

For example, medium and high power LED arrays are typically built on thermally conductive PCBs. On the top, there is a copper plate that is electrically connected to the LED, while below it there is an aluminum to conduct heat. There is a dielectric layer between copper and aluminum to prevent electrical shorting of the copper plate to aluminum. Various manufacturers have taken different approaches in the selection of dielectric materials, from organic materials to inorganic compounds, covering the entire spectrum. As shown in Figure 2, the dielectric material with the smallest thermal resistance is almost an order of magnitude, allowing the thinnest dielectric material to be applied while still providing the required isolation.

 

figure 2. The thickness of the dielectric material affects the heat resistance.

However, FIG. 2 does not describe all. Assuming that the device is air-cooled, there will be many interfaces in the thermal path between the LED and the heat sink. Some are bridged by solder, some are bridged by adhesives, others are pressed together (using screws, for example). These junctures create additional obstacles to heat conduction, which can be large, unpredictable, and variable over time.

The series / parallel addition of all thermal and interface resistances in the system is called thermal impedance and the turn-on path is designed to keep the LED cool. Calculated similar to resistor network. In Figure 3, the voltage is essentially the temperature, the current is the heat flux, the resulting resistance is the thermal resistance.

Transient cooling LED

The previous discussion assumed that at steady state, the LED was permanently energized and the heat sink dissipated heat continuously to the surrounding air. This thermal model will fail in both cases. One is when turning on the LEDs, and more usually in pulsed operation. Surprisingly, a hot path can be designed to keep the LED cool while operating continuously but overheating when switched on. When this is done, the associated thermal excursions may cause the LEDs to suddenly fail, just as suddenly when the tungsten filament is turned on. Therefore, LED thermal solution design needs to consider transient operation, and include time and space variables.

Time dependent

The temporal component of transient cooling is due to the specific heat capacity of the material in the thermal path. This can be added as a capacitor to the electrical model of the RTD (Figure 4). Heat capacity refers to the nature of the material that absorbs (or emits) heat when it is heated (or cooled). Heat capacity with the specific heat capacity (referred to as specific heat) said.

 

Figure 4. The time dependence of heat conduction is due to the heat capacity of the material in the system, and the electrical equivalent model is an RC low-pass filter.

The electrical model analogy means that the thermal impedance is sometimes used to describe the time-dependent thermal properties of a material. Please note that at this time to pay attention to the distinction, because the thermal impedance can also be used to describe the entire system static thermal resistance.

Spatial dependence

Transient cooling of the spatial component of the heat from which direction to spread more. For example, an LED mounted on a large thin metal plate. Initially, the entire board is at ambient temperature. LED as a point of heat. When switched on, the LED generates heat, which transfers heat to the board. Heat quickly through the metal plate, raising the temperature of the area under the LED. So, first, a small portion of the metal plate was used to cool the LEDs. The conductivity of the metal plate means that some of the heat from the LED will expand laterally within the plate and eventually appear on the surface (Figure 5). As a result, the volume of the metal plate involved in cooling the LEDs increases over time, resulting in significant changes in thermal resistance and heat capacity.

 

Figure 5. A hot body is on a thin metal plate. This simple finite element thermal model shows spatial dependence by varying the volume of the sheet involved in cooling. The calculations for these models are made by increasing the time from top left to bottom right.

Spatial dependence is particularly important when there is a high thermal resistance interface or layer in the path. By taking measures, the heat is spread over the largest possible area before the barrier so that the LED can achieve better cooling in both steady-state and pulsed operation.

Convection and radiation

Any material above ambient temperature loses heat by convection and radiation. Although these are the main mechanisms for the cooling of tungsten lamps, they play a minor role in the thermal management of LEDs. However, convection and radiation should be included in any model in order to ensure the closest match to reality.

In short, the LED must be cooled for optimum efficiency and to ensure the stability and longevity of its light output. Electrical model-based models can be used to construct a simple, thermally-conductive, steady-state model. However, in order to correctly understand the thermal path, especially under transient conditions, it is best to use tools that can adapt to changes in time, space and temperature.

The temporal and spatial dependence of heat conduction explains why there is a hierarchy in material selection. The high specific heat capacity or thermal conductivity will vary with the material's location in the thermal path and the LED's intended mode of operation.

 

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