A design of single-switch two-stage DC-DC converters with PWM and PFM for off-grid solar power system

In this paper, a single-switch two-stage DC-DC conversion circuit is proposed for an off-grid solar power application. A photovoltaic(PV) panel powers the load and a storage unit(battery) via the proposed circuit. The battery is designed to balance the supply and the demand of power under different irradiation situations. Based on conventional cascaded DC-DC converters, the proposed design is developed with the single switch technique reducing size, cost and power loss. The control scheme in this design is pulse width modulation(PWM) with pulse frequency modulation(PFM). The PWM module is similar to conventional design except its ramp signal with a variable frequency is provided by a resettable integrator. As a result, the PWM and PFM modules regulate the two stages of the proposed circuit separately with the same switching control signal. In this paper, the modes of operation of the circuit are discussed as well as the control schemes. The design process is described along with the circuit analysis and comparisons with conventional design are made as well. A prototype has been built to verify the proposed circuit with simulation and experimental results.