closed-loop control with maximum THD lower than 4%. The following points are derived from this figure: (1) The output voltage of the inverter is independent of output load as the pulse width angle (6) is relatively constant for a changing load. (2) The converter maintains excellent efficiency from full-load to reduced load. This is because the output current of the inverter decreases as the load is lowered. (3) The voltage gain of the inverter is high (0.9). 3.4 A Discussion on DC/AC Resonant Inverter Topologies The DC/AC resonant inverter topologies have been analyzed and their performance presented in Section 3.3. This section presents a comparative discussion for selecting a particular inverter topology for a given application. 3.4.1 Parallel-resonant Inverter. The parallel-resonant inverter topology (Fig. 5) has the main disadvantage that its output voltage is highly load dependent. In order to make the output voltage relatively constant under open-loop control, the size of output capacitor Cp should be large. The large value of capacitor Cp results in low output voltage and, therefore, the low voltage gain of the inverter. For a given output power, a lower voltage gain will result in higher current drawn from the input DC source. This results in higher conduction losses and volt-amperes rating of the inverter switches. The voltage gain of the inverter can be improved by selecting a low value of capacitor Cp, and output voltage of the inverter can be kept constant by varying the pulse-width angle (S) under closed-loop control for a changing output load. Figure 14 shows the voltage gain and the range of pulse width angle (J) for a value of capacitor Cp which results in the same voltage gain as either series-parallel or hybrid resonant inverters. It can be seen from this figure that a wide change in pulse width angle is required to control the output voltage from full-load to no-load. A wide change in angle results in a highly non-linear transfer function of this control circuit. This makes it more difficult to design a control circuit with a fast dynamic response.
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