Switch-mode power supply

Abstract

A switch-mode power supply has first and second switching transistors connected in series together and in parallel with the primary of a power transformer which alternately connect the primary winding of the power transformer across a DC potential. The bases of the switching transistors are each connected to a separate secondary winding of a control transformer the primary of which receives a control signal. Simultaneous conductance of the switching transistors is prevented. A control circuit providing the control signal is powered initially from charge derived from the power supply primary circuit and stored on a capacitor. A Schmitt trigger connects the capacitor to power the control circuit when the voltage across the capacitor rises to above a first predetermined level. Once the power supply begins to operate, semi-regulated power from the secondary circuit of the supply is connected via the Schmitt trigger to power the control circuit output. Current limiting is provided to reduce the duty cycle of the switching transistors when an overcurrent occurs. When the voltage applied to power the control circuit falls below a second predetermined level, the Schmitt trigger disconnects the control circuit from the capacitor to shut down the power supply. The control circuit restarts automatically, providing a 'burp mode' of operation. Supply output voltage is monitored by a comparator which is connected to the control circuit through an opto-coupler operated in a digital mode to provide high primary-to-second circuit isolation without requiring special circuit compensation.