Envelope modeling and small-signal 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 load current and power are obtained. 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.