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Figure 30. Output Power Plot
The triac control circuit also monitors the unregulated dc to provide ac line compensation. Variations in the
amplitude or frequency of the ac line modify the amplitude of the unregulated dc voltage which, in turn, alter
the position of the IOD1 and IOD2 decision lines. For example, both IOD lines decrease (move to the left) if the
ac line voltage increases. The reverse occurs if the ac line voltage decreases. This action prevents excessive
regulator dissipation during periods of high line yet assures an adequate voltage drop across the series regulator
during periods of low line.
Noise Reduction. Triacs, as well as any other switching devices, can generate a considerable amount of EMI.
Because this interference is proportional to the turn-on dv/dt, the triacs in this supply are fired only when the
voltage across them is low, thus minimizing EMI. The current through the triacs, however, must be higher than
the minimum latching current that is required to sustain conduction. Thus, triac firing must occur late enough in
the ac input cycle for the current to be above the latching level yet early enough in the cycle so that the dv/dt
will not generate excessive EMI.
Faster Down-Programming Speed
As mentioned previously, the programming speed of this unit is considerably better than that of the preregulated
power supplies that exist at this power level. Up-programming is not a particular problem in either type of unit
because it is aided by the conduction of the series regulator. Down-programming, however, is much slower in a
preregulated unit because: (1) an active element is not normally available to aid in the discharge of the output
capacitor and (2) any decrease in the charge across the output capacitor must be accompanied by a decrease in
the voltage across the input filter capacitor to avoid damaging the series regulator. This problem exists because
the series regulator, which normally operates with a Vce of only a few volts, does not have the heat sink
capability to absorb the extra energy dissipated when discharging the filter capacitor during
down-programming.