On the control and design issues of single phase transformerless inverters for photovoltaic applications

Design and development details of a high power density, low cost, single phase transformerless PV inverter with reduced losses are reported. H-4 bridge topology is adopted to implement the proposed design where count of power devices is limited to four. One leg of the bridge operates at high switching frequency while the other leg operates at line frequency, this modulation technique reduces the switching losses. This also obviates the requirement of fast recovery anti parallel diodes across power devices for low frequency leg. Use of fewer devices and digital control (instead of analog control) has led to reduced cost and size. Switching and conduction losses are further reduced by using MOSFETs with very low on state resistance. A novel control scheme has been implemented to eliminate the DC offset at the output. A highly reliable over current and shoot-through protection is implemented through digital control. The digital control algorithm implemented enhances the reliability of operation at high power level. Optimal design of PCB and high frequency filters with SMD components contribute to high power density. A 3 kW prototype has been developed and tested for various reactive loads. All the analysis, simulation and experimental details of this work are presented.