Simplified analysis of a PWM-controlled parallel resonant inverter for electronic ballast applications

In this paper, the fundamental approximation, combined with a time-varying phasor transformation is applied to obtain the envelope model of an LC_p resonant inverter, controlled by the pulse width modulation (PWM) technique. 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 duty cycle parameter. The explicit form of the small-signal transfer function from the duty cycle, D, to the output variable is obtained by neglecting the effect of the parallel capacitor C_p. The proposed PWM 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.