Precautions Observed In Thermal Design

Junction Temperature Rises, Light Emitted Diminishes

What precautions should be observed in my thermal design? The amount of light emitted by the LED lamp diminishes as junction temperature rises. This is because of an increase in the recombination of holes and electrons that make no contribution to light emission. Additionally, the failure rate of semiconductor devices is greatly accelerated as operating temperatures increase. Therefore, observe the following precautions concerning the thermal design to achieve optimum reliability:

• Keep the ambient temperature (Ta) as low as possible.
• If the device's dynamic power dissipation is relatively large, select the most appropriate circuit board material.
• Such measures will help lower the thermal resistance of the package.
• Derate the device's absolute maximum ratings to minimize thermal stress from power dissipation.

As shown in the figure below, the internal thermal stress on a device is the sum of the ambient temperature and the temperature rise due to power dissipation in the device.

In Which

• θja = thermal resistance between junction and surrounding air (°C/W)
• θjc = thermal resistance between junction and package surface, or internal thermal resistance (°C/W)
• θca = thermal resistance between package surface and surrounding air, or external thermal resistance (°C/W)
• Tj = junction temperature or chip temperature (°C)
• Tc = package surface temperature or case temperature (°C)
• Ta = ambient temperature (°C)
• W = power dissipation (W)

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