High speed power supply circuit with circuit with positive feedback

Abstract

A self-oscillating power supply circuit comprises a series arrangement of the primary winding (N.sub.1) of a transformer (TR), the main current path of a first semiconductor switch (T.sub.1) and a current measuring resistor (R.sub.1). The first semiconductor switch is driven by a second semiconductor switch (T.sub.2) which in turn is controlled by a comparator (CP). The voltage across the current measuring resistor is applied to the non-inverting input (6) of the comparator and, a reference voltage obtained from a reference voltage source (RS) is applied to the comparator inverting input (5). By arranging a capacitor (C.sub.2) between the output of the second semiconductor switch and the inverting input (5) of the comparator, the first semiconductor switch rapidly turns-off at the end of the forward phase at an accurately determined current through the primary winding. By applying a lower reference voltage to the comparator at a higher mains voltage, an output current is obtained from the transformer secondary winding (N.sub.2) for the load L, which is substantially independent of the mains voltage.