Stabilizing CCT in pcW-LEDs by self-compensation between excitation efficiency and conversion efficiency of phosphors

A novel method to stabilize the correlated color temperature in pcW-LEDs from their initial turn-on state to thermal equilibrium is proposed and demonstrated. Under the normal operation condition, it can stabilize the CCT of a pcW-LED by the positive matching of the blue LED peak wavelength to the phosphor excitation spectrum. When the operating temperature unavoidably becomes higher in the LED die quickly after the initial turn-on, the phosphor conversion efficiency degrades and the LED blue light performs red shift. With the positive matching, the red shift actually helps enhance the excitation efficiency of the phosphor to compensate the thermal quenching and efficiency degradation. Therefore, the ratio of the blue light to the yellow light can keep almost constant, as well as the CCTs. In the experiments, the CCT variation could be as small as from 7 K to 83 K in different cases. Finally, we introduce a new factor, the so-called guide number, which is used to count the total change of the enhancement in equivalent excitation efficiency and the relative reduction of the phosphor light emission. The guide number essentially helps in designing the matching blue LED die and phosphor pair for good CCT stabilization.