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In this paper, the fundamental approximation combined with a time-varying phasor transformation is applied to obtain an envelope model of a phase-controlled triple LCpCs resonant inverter. The resulting model removes the high-frequency component while keeping the useful information contained in the waveform envelopes. The method provides large and small-signal Spice compatible models in a straightforward manner. The inverter is controlled at constant switching frequency by the phase displacement, Psi, of the midpoint voltage of one class D section referred to the others. An approximated explicit form of the small-signal transfer function from the control angle, Psi, to the load current is obtained by using a reduced order model. The proposed overlap phase control exhibits a wide bandwidth suitable for applications such as control of gaseous discharges. Experimental results are given using a 400 W high-pressure sodium lamp as load.