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input, P i , and that the efficiency defined by h = Po>Pi cannot be greater than 1. The factor
missing from the discussion above that permits an ac power output greater than the input ac
power is the applied dc power. It is the principal contributor to the total output power even
though part of it is dissipated by the device and resistive elements. In other words, there is an
“exchange” of dc power to the ac domain that permits establishing a higher output ac power.
In fact, a conversion efficiency is defined by h = Po(ac)>Pi(dc), where Po(ac) is the ac power
to the load and Pi(dc) is the dc power supplied.
Perhaps the role of the dc supply can best be described by first considering the simple
dc network of Fig. 5.1 . The resulting direction of flow is indicated in the figure with a plot
of the current i versus time. Let us now insert a control mechanism such as that shown in
Fig. 5.2 . The control mechanism is such that the application of a relatively small signal to
the control mechanism can result in a substantial oscillation in the output circuit.