Abstract
A series of experimental investigations with stringent measurement methods on the studies related to transient- and steady-state thermal behavior for switching power supplies have been successfully performed. The power-on transient/steady and steady/power-off transient thermal performances of switching power supplies under various input AC voltages and output DC loads have been explored. During the experiments, it is found that the air temperature distribution inside the power supply is significantly non-uniform. Consequently, some hot spots can be found at the high-power components, which are located in the region nearby the solid case walls without perforation. This problem may be overcome with proper redesigns in the hardware design of electronic circuits, fan quality, and openings of the case walls. As for determining the fan airflow rate, the total system flow resistance of the tested switching power supply has been properly predicted; and the prediction of airflow rate is quite consistent with the experimental data. In addition, the experimental data for the steady-state temperature rises of the components of interest are quite consistent with those reported by Delta Electronics, Inc. for the tested switching power supply operating at standard output DC loads. Furthermore, in order to interpret the transient thermal behaviors of switching power supplies, two dimensionless formulas of in terms of for the power-on transient/steady period and for the steady/power-off period have been successfully proposed. For exploring the effects of output DC loads on the thermal behavior of the switching power supply, the steady-state temperature rises of all the components of interest for the cases with 230V/60Hz input increase with increasing currents at output voltages of +12V, +5Vsb and –12V; while a minimum temperature rise of any specific components can be found at a specific current for output voltage of +5V or +3.3V. Contrary to the conclusion drawn for the cases with 230V/60Hz input, no minimum temperature rise of any specific components for the cases with 115V/60Hz input can be observed for all the variations of output DC loads. Finally, during the power-off transient period, the transient temperatures at some specific components, e.g., C4, internal air, and case temperatures, suddenly increase in the beginning short time of power-off transient period; and then gradually decrease in the period. This extraordinary phenomenon may be due to the following two factors: (1) Fan will be stopped and no cooling air supplied from the fan when the power-off period starts; and (2) the stored energy in the circuit will simultaneously be discharged when the power is switched off.