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

LEDs are complex devices that go beyond traditional semiconductor challenges. While they share common issues related to semiconductor design and operation, LEDs are primarily used for lighting, which adds layers of complexity such as optical coatings, reflectors, lenses, wavelength-converting phosphors, and more. Thermal management is especially crucial for reliable solid-state lighting (SSL) systems. Understanding how to cool LEDs under both static and dynamic conditions is essential for optimal performance. Two key thermal parameters must be considered when managing LED temperatures: the operating temperature and the maximum allowable temperature. Keeping the operating temperature as low as possible ensures higher electro-optic efficiency, better light quality, and longer device life. Operating at high temperatures, however, reduces light output and can lead to premature failure due to various degradation mechanisms. LED manufacturers typically design products with junction temperatures up to 130°C. However, due to the thermal resistance of the LED package, the PCB temperature usually remains around 10°C below the junction. If the junction temperature exceeds its rating, the LED lifespan can be cut in half for every 10°C increase. The conversion of electrons into phonons makes LED efficiency relatively low. High-brightness white LEDs can reach up to 40% efficiency, while UVC LEDs may only achieve 5%. In both cases, the excess heat must be efficiently removed through conduction to avoid overheating, a critical responsibility of the LED light source or lighting designer. **Static Cooling for LEDs** A standard approach to cooling LEDs is mounting them on a heat sink. Heat generated by the LED is conducted into the heat sink and then dissipated into the air. When water or another fluid is used, the heat sink is often referred to as a cold plate, as the system requires the working fluid to maintain a temperature lower than the ambient environment. The effectiveness of heat transfer from LEDs to heat sinks depends on materials with high thermal conductivity. For example, copper has superior thermal properties compared to aluminum, brass, and stainless steel. As shown in Figure 1, the thickness of the material affects thermal resistance—thicker materials tend to have higher resistance, reducing heat transfer efficiency. **Dielectric and Airflow** Medium to high-power LED arrays are often built on thermally conductive PCBs. These boards typically feature a copper layer on top for electrical connection to the LED and an aluminum base for heat dissipation. A dielectric layer separates the two to prevent electrical shorts. The choice of dielectric material varies widely, from organic to inorganic compounds, each offering different levels of thermal resistance. As illustrated in Figure 2, thinner dielectric layers provide lower thermal resistance while still maintaining insulation. However, the thermal path also includes multiple interfaces, such as solder joints, adhesives, and mechanical connections, which can introduce additional thermal resistance that changes over time. Thermal impedance, which combines all thermal resistances in the system, is critical for designing effective cooling solutions. This concept is similar to an electrical resistor network, where temperature corresponds to voltage, heat flux to current, and thermal resistance to electrical resistance. **Transient Cooling for LEDs** While previous discussions focused on steady-state operation, real-world LED applications often involve pulsed or transient conditions. A thermal path that works well under continuous operation may fail during startup or in pulsed modes. This is because the thermal response of materials depends on their specific heat capacity, leading to time-dependent behavior. In an electrical analogy, this time dependence can be modeled using capacitors in an RC circuit, as shown in Figure 4. Thermal impedance, which accounts for both static and dynamic thermal characteristics, is essential for accurate modeling of transient cooling scenarios. **Spatial Dependence** Heat diffusion in LEDs is also spatially dependent. For example, an LED mounted on a thin metal plate initially acts as a point heat source. Over time, the heat spreads across the plate, increasing the area involved in cooling. This change in thermal resistance and heat capacity over time must be accounted for in transient models. Figure 5 illustrates this effect, showing how the cooling area expands as heat spreads laterally. Spatial dependence becomes even more important when there are high-resistance interfaces in the thermal path. By spreading heat over a larger area before encountering barriers, cooling efficiency can be improved in both steady and pulsed operations. **Convection and Radiation** Although convection and radiation play a smaller role in LED cooling compared to traditional incandescent bulbs, they should still be included in thermal models to improve accuracy. These mechanisms help dissipate heat but are less effective in high-performance LED systems. In conclusion, proper thermal management is vital for maximizing LED efficiency and ensuring stable, long-lasting light output. While a simple steady-state model can be useful, understanding transient and spatial thermal behavior requires advanced tools that account for time, space, and temperature variations. The hierarchy of material selection is determined by these factors, with thermal conductivity and specific heat capacity playing key roles depending on the application.

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