Dual feedback control for a high-efficiency class-d power amplifier circuit

A ballast for a high intensity discharge lamp of the type having an excitation coil situated about an arc tube for exciting an arc discharge in an ionizable fill contained in said arc tube, comprising:

• a Class-D power amplifier including two switching devices coupled in series in a half-bridge configuration and having an output resonant circuit coupled at the junction between said two switching devices;
• a phase control feedback loop, including phase detecting means for detecting the phase angle between the resonant circuit voltage and resonant circuit current, said phase control feedback loop varying the phase angle of the resonant circuit current to tune said resonant circuit under different load conditions;
• and
• an amplitude control feedback loop , including amplitude comparator means for comparing the amplitude of the resonant circuit impedance with a reference amplitude, said amplitude control feedback loop varying the amplitude of the resonant circuit current to tune said resonant circuit under different load conditions.