Power supply operable on varying inputs

Abstract

A power supply provides the stable DC voltages needed for a computer terminal from a wide range of line voltages and frequencies. The line is rectified and fed to a flyback transformer wherein primary current is controlled in duration for providing the desired energy transfer to the secondary winding. The outputs from the flyback transformer are rectified and filtered. A separate start-up circuit uses a transformer across the line voltage, and a positive coefficient resistor provides a time limit to allow the use of a small transformer even though the line voltage may be high. The start-up circuit must produce a minimum voltage for the switching transistor in the flyback arrangement to allow operation of the power supply, and the start-up transformer is disconnected from the circuit after operation to prevent electromagnetic interference within the terminal. The output from the power supply is used in the control of the switching transistor so that, once the start-up circuit has achieved the needed voltage level, the power supply can assist in maintaining the operation.