Where Vs, V^, Vo are the source, Thevenin's, output voltages respectively, Zth, Zs, Zp are the Thevenin's, series and parallel impedances and, fSN is the no-load source current. From equations (3.19)—(3.22) 70= Fs at any load and ZSN = 0 if and only if Zs = 0 and Zp= x (3.25) From equations (3.23) and (3.24), the constraints of equation (3.25) will be satisfied if Xls Xcs and ^lp -^cp (3.26) Equation (3.26) reveals that the series branch and parallel branch of the inverter of Fig. 11 should be tuned to the operating frequency of the inverter in order to achieve an ideal voltage source. Since both the series and parallel branches of the output resonant network of the inverter are tuned to the inverter operating frequency, this inverter system, therefore, can be named as a ‘Double Tuned Resonant Inverter System'. Assuming that XLS=XCP=XS and XCP=XLP—Xp, the worst harmonic distortion of the output voltage is given by: Equation (3.27) indicates that the harmonic distortion of the output voltage can be kept at an acceptably low level by choosing the value of Xs/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 (3.28) shows that the voltage gain of the hybrid resonant 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 series and parallel branches such that they are tuned to the operating frequency of the inverter. (2) Select a value of Xs/Xp such that the total harmonic distortion is always equal or lower than the acceptable level. (3) For the selected value in step no. 2, choose a value of Xp such that the ratings of resonant components are optimum with respect to the circulating current losses of the parallel branch. These losses are to be minimized as they contribute a major portion of the no-load losses. Using the above selection procedure, the performance of a typical hybrid resonant inverter topology is shown in Fig. 13. In studying the performance it is assumed that the output load varies from 100% to 10% and the output voltage is kept constant under
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