(i.e. value of Xp decreases) to obtain a load independent output voltage with low THD, the voltage gain of the inverter decreases. This results in the reduced output power for the total volt-ampere rating of the inverter components. The output voltage of the inverter as a function of pulse width angle d is given by: The open-loop output voltage of the inverter does not need to be load independent as it can be kept constant by varying the pulse width angle of the inverter under closeloop control for any value of output load RL. However, it is desirable to keep the openloop voltage variation to a minimum to simplify the requirements of control circuit. Therefore, the selection of resonant components of the inverter should be such that it results in an optimum design. The selection of these components is outside the scope of this paper, however, the following are some suggested steps: (1) Select a value of Xs/Xp such that the worst THD requirement is satisfied. (2) Select a value of Xp such that the open-loop voltage regulation is approximately 10%. (3) Re-iterate the value of Xs/Xp such that the total volt-ampere rating of the resonant component is optimized with respect to the no-load losses of the inverter while satisfying the step no. 1. Using the above selection criteria the performance of a typical parallel resonant inverter topology is illustrated in Fig. 7. In studying the performance it is assumed that the output load varies from 100% to 10% and the output voltage is kept constant under the close-loop control with maximum THD below 4%. The following points are derived from Fig. 7: (1) The converter efficiency suffers adversely as the load decreases from full-load to 10% of the load. This is because the output current of the inverter (7S) and therefore conduction losses are relatively constant. (2) A narrow dynamic range of pulse width angle (<5) is required to keep the output voltage constant for varying output load. (3) The voltage gain of the inverter is low (0.42).
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