  This paper documents a design and modelling of a grid-connected emergency
back-up power supply for medium power applications. There are a rectifier-link
boost derived battery charging circuit and a 4-switch push-pull power inverter
circuit which are controlled by pulse width modulation (PWM) signals. This
paper presents a state averaging model and Laplace domain transfer function of
the charging circuit and a switching converter model of the power inverter
circuit. A changeover relay based transfer switch controls the power flow
towards the utility loads. During off-grid situations, loads are fed power by
the proposed inverter circuit and during on-grid situations, battery is charged
by an ac-link rectifier-fed boost converter. There is a relay switching circuit
to control the charging phenomenon of the battery. The proposed design has been
simulated in PLECS and the simulation results corroborate the reliability of
the presented framework.
