Heat dissipation technology
For a light-emitting diode composed of a PN junction, when a forward current flows from the PN junction, the PN junction has a heat loss, which is radiated into the air via a bonding glue, a potting material, a heat sink, etc., in the process. Some materials have thermal impedance that blocks heat flow, that is, thermal resistance, which is a fixed value determined by the size, structure, and material of the device.
Let the thermal resistance of the LED be Rth (°C/W) and the heat dissipation power be PD(W). At this time, the PN junction temperature rise due to the heat loss of the current is:
The PN junction temperature is:
Where TA is the ambient temperature. As the junction temperature rises, the probability of PN junction luminescence recombination decreases, and the brightness of the LED decreases. At the same time, due to the increase in temperature rise due to heat loss, the brightness of the LED will no longer continue to increase proportionally with the current, indicating thermal saturation. In addition, as the junction temperature rises, the peak wavelength of the luminescence will also drift toward the long wavelength, about 0.2-0.3 nm/°C, which is for the white LED obtained by mixing the YAG phosphor coated with the blue chip. Drift causes a mismatch with the excitation wavelength of the phosphor, thereby reducing the overall luminous efficiency of the white LED and causing a change in the white color temperature.