Fig. 2 shows the simulation result for junction temperature rise with time under DC and AC mode. When LED was driven at 1 W of DC power, the transient temperature was raised. At constant input power, the junction temperature was reached into a saturation point. In DC mode, the junction temperature of LED was 63 °C with the input power of 1 W and the ambient temperature of 20 °C. From Eq. (1), the thermal resistance of the LED package was calculated to be 43 °C/W ((63 À 20 °C)/1 W) under DC operation. The junction temperature was reached into a saturation in a band after 360 s by the effective input power of 0.92 W. The calculation of the effective input power can be found in the further discussion and
Eq. (3) below. Under the AC operation, the input power periodically exhibits forward and reverse half-wave. When LED is driven at forward half-wave, LED is heated with time and its junction temperature was raised. On the other hand, LED experiences a cooling process during the time of reverse half-wave as is clearly shown in Fig. 2. Therefore, the heating and cooling process is repeated in LED with a general increase of temperature (due to the much faster heating speed than the cooling speed) and saturation with time. Insert graph of Fig. 2 shows the junction temperature variation under AC mode in the saturation region. From Fig. 2, it can be seen that the junction temperature was increased in fluctuation
way and then reached saturation in band with time. It was confirmed that the saturated temperature rise at AC mode is lower