How to manage LED heat? From static cooling and transient cooling

LEDs are complex devices. LEDs not only have common problems associated with semiconductor design and operation, but LEDs are primarily used for illumination. Therefore, optical coatings, beam management devices such as reflectors and lenses, wavelength converting phosphors, and the like, have further systemic complexity. Still, thermal management is critical to reliable solid-state lighting (SSL) products. In addition, you need to know how to cool the LEDs in both static and transient backgrounds.

For LEDs, two thermal management parameters need to be observed. One is the required operating temperature and the other is the highest operating temperature. Usually, the required operating temperature needs to be as low as possible. Achieving this ensures high electro-optic efficiency, good spectral quality and long device life. Operating at high temperatures not only reduces the amount of light produced by the LED, but also reduces the quality and quantity, which ultimately triggers many failure mechanisms.

LED manufacturers are well versed in these defects and are able to design products with junction temperatures 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. If it is higher than the rated junction temperature, the LED lifetime will be reduced by about half for every 10 °C rise.

Converting electrons into phonons, LED efficiency is relatively low. High-brightness white LEDs can achieve 40% efficiency, while UVC LEDs can have 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

A conventional method of keeping the LEDs cool is to mount the LED device on the heat sink. Heat from the LEDs is conducted into the heat sink and then dissipated into the air. If heat is removed by water or other fluids, the heat sink is sometimes referred to as a cold plate because the associated heat sink system often has to design the working fluid to be at a fixed temperature below the indoor environment.

The ability to efficiently transport heat from LEDs to heat sinks depends on materials with high thermal conductivity. For example, it can be seen from the graph of Figure 1 that copper is superior to aluminum and brass and superior to stainless steel.

figure 1. The materials have varying degrees of thermal conductivity.

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

Dielectric and airflow

For example, medium to high power LED arrays are typically built on thermally conductive PCBs. On the top side, there is a copper plate that is electrically connected to the LED, and a piece of aluminum underneath to conduct heat. There is a dielectric layer between copper and aluminum to prevent electrical shorting of the copper to aluminum. Manufacturers have adopted 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 lowest thermal resistance is almost an order of magnitude, and the thinnest dielectric material can be applied while still providing the required insulation isolation.

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

However, Figure 2 does not describe all of them. Assuming 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 adhesive, and others will be pressed together (for example using screws). These joints present additional obstacles to heat transfer, which can be large, unpredictable, and change over time.

The series/parallel addition of all thermal resistance and interface resistance in the system is called thermal impedance and the conduction path is designed to keep the LEDs cool. The calculation is similar to a resistor network. In Figure 3, the voltage is essentially the temperature, the current is the heat flux, and the resulting resistance is the thermal resistance.

image 3. In development work, you can rely on the equivalent resistance of the heat conduction path. In order to obtain a complete thermal impedance system model, thermal interface resistance must be added at each transition between materials.