OUTPUT VOLTAGE AND CURRENT RATINGS
DUTY CYCLE LOADING
In some applications the load current varies periodically from a minimum to a maximum value. At first it might seem that a regulated power supply having a current rating in excess of the average load requirement (but less than the peak load value) would be adequate for such applications. However, the current limit or constant current circuit within a semiconductor power supply limits the output current on an instantaneous not an average basis, because extremely rapid protection is necessary to provide adequate safeguard against burnout of the regulating elements.
The first question which must be answered when powering a dc load that draws a large current during some portion of its operating cycle is whether (1) the power supply need only withstand the peak load condition, or whether (2) the power supply must continue to deliver its full value of regulated output voltage during the peak load interval.
Examples of the first category are dc motors and filaments for large vacuum tubes. While the starting resistance of these loads is very low compared to the normal operating value, it is not necessary that the power supply be able to deliver this peak
As for the second category, if it is desired to meet a duty cycle requirement similar to that illustrated in Figure 63 while retaining the full value of regulated output voltage during peak load conditions, then a power supply must be selected which has a current rating equal to or greater than the peak load requirement. However, if the peak load condition is of relatively short duration, then the stored energy in the power supply output capacitor may prevent an excessive output voltage sag.
Thus for peak loads of either category (1) or (2), it is of interest to know how much the output voltage will drop for a peak load condition in excess of the power supply current rating, and how long it will take for the supply to recover to its normal output voltage following the removal of the overload. Figure 63 illustrates the equivalent circuit and output voltage waveform which are characteristic of a power supply experiencing a short term overload. When the overload condition is first imposed, the power supply goes into the current limit mode and is, therefore, equivalent to a constant current generator IL feeding the output capacitor Co (already charged to ENORM) in parallel with the lowered value of load resistance RL PEAK. Thus the capacitor begins discharging exponentially toward the final output voltage value which would result if the overload condition were retained,
namely ILRL PEAK. The amount of voltage sag ∆ V depends upon the output time constant and the duration of the overload peak load condition; the equation for this voltage sag is given in Figure 63. When the peak load condition is removed, RL is restored to its normal value and the supply continues in the current limiting mode, charging the output capacitor on another exponential curve. This time the asymptotic level approached by the exponential curve is ILRL NORM. However, this charging action stops when the voltage level has risen to the normal level, and the regulator changes from the current limit mode to the normal constant voltage mode. Figure 63 also gives the equation for the time required for this voltage recovery following the removal of the
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