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.