The single phase online UPS integrate with the three leg type converter which operate with battery charger and the inverter. The first leg controls the ability of charging battery. The third leg controls the output voltage. The common leg is used to control the line frequency. The battery charger has the ability of making the correction of power factor during the battery charging. The inverter is used to maintain the output voltage and to limit the output current. The main feature of the three leg converter is used to reduce the number of switching devices. So that the system has the low value of power loss and low cost.
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