The purpose of the DC UPS is to generate disturbance-free uninterrupted DC power for critical loads. Available input is 3 phase AC supply and a battery bank is employed to generate output during brownouts and blackouts of AC input, lasting upto 30 minutes. The high frequency switching technique is chosen to minimize size and weight of magnetic components (transformers and chokes) that happens to be a desired feature for on-board application.

AC input supply is available through two independent feeders NORMAL and ALTERNATE via corresponding MCBs. The contactor-based SELECTOR circuit selects the supply that is healthy, i.e. no single phasing and voltages within permissible limits. Preference is given to NORMAL supply.

The LINE FILTER functions to make the equipment meet EMI/EMC requirements. A three phase fullwave rectifier generates DC voltage VDC1 of the order of 600 V. Following filter L1, C1, C2 absorbs ripple and also generates a center tap of DC voltage.

IGBTs VT1 and VT2 form a half bridge converter that generates a controlled dc voltage VDC2.
The control circuit functions as follows: