Two stage resonant inverter for AC distributed power supply-full control of output voltage magnitude and phase angle

Paralleling of multiple resonant inverters is necessary for AC distributed power supply system. However, the multiple module operation (MMO) requires tight regulation of the output voltage of both amplitude and phase angle. Voltage control mode (VCM) has effective control over the amplitude of the output only. Perturbation, component tolerance and other forms of uncertainties cause phase discrepancy in the output voltages of the modules. Such inverters when connected in parallel produce huge circulating currents because the ac bus impedance is very small. In this paper, a new concept of phase angle control mode (PCM) is proposed which is simple but effective for resonant inverter phase angle regulation. A two stage resonant inverter with a buck converter as front-end and a half bridge resonant Inverter in cascade is proposed with dual loop control (OLC) over the outputs of inverters. The half bridge resonant inverter regulates the voltage phase angle by PCM loop, while the buck converter regulates the voltage magnitude by a VCM loop respectively. Thermal losses are distributed in two stages and wide range of zero voltage switching (ZVS) is possible for wide range load and any kinds of load impedance. Because of the half bridge inverter operating with 0.5 duty ratio and the series parallel resonant network has high quality factor and high order, the output voltage has the best waveform quality. Since all the phase angles and magnitudes of the modules are tightly controlled, circulating current is minimized in MMO, therefore the system efficiency is improved. Simulations for both steady state and transients verify the proposed control and the analysis.