Equations (3.15) and (3.16) reveal that for output voltage to be load independent either the parallel capacitor Cp should be very large or the series branch should be tuned at the operating frequency of the inverter. As seen during the discussion of the parallel resonant inverter in Section 3.3.1, the first solution (Cp is large) is undesirable as it results in reduced voltage gain of the inverter and lower conversion efficiency. Therefore, the load independent output voltage of the inverter is achieved by tuning the series branch at the operating frequency. Assuming that XLS=Xcp=Xs, the worst harmonic distortion of the output voltage is given by: Equation (3.17) indicates that the harmonic distortion of the output voltage can be kept at an acceptably low level by choosing the value of Ars/Xp to be high. For a load independent output voltage with low total harmonic distortion, the voltage gain of the inverter is given by: Equation (3.18) shows that the voltage gain of the series-parallel inverter is constant. The selection of resonant components is outside the scope of this paper, however, the following steps are suggested: (1) Choose the components of the series branch such that they are tuned to the operating frequency of the inverter. (2) Select a value of Xs/X such that the total harmonic distortion is always below or equal to the acceptable level. (3) For the selected value of As/Ap in step no. 2, choose a value of Xf which results in the optimum rating of the resonant components with respect to the no-load losses. Using the above selection criteria the performance of a typical series-parallel resonant inverter topology is shown in Fig. 10. In studying the performance it is assumed that the output load varies from 100% to 10% and the output voltage is kept constant under closed-loop control with maximum THD lower than 4%. The following predictions are made from this figure: (1) The output voltage of the inverter is independent of output load as the pulse width angle 8 is relatively constant for a changing load. (2) The converter maintains excellent efficiency from full-load to half-load and moderate efficiency at lower loads. (3) The voltage gain of the inverter is high (0.9). 3.3.3 Hybrid Resonant Inverter Topology. This section investigates another resonant inverter topology which truly resembles the characteristics of an ideal voltage source.
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