Switched mode power supply

Claim 1

In a switched mode power supply including a rectifier having input terminals for connection to an alternating voltage supply; a separating transformer having at least one main primary winding and at least one secondary winding; an output current regulated oscillator circuit connected to d.c. terminals of said rectifier, with said oscillator including a switching transistor having its emitter-collector path connected in series with said main primary winding and serving to periodically turn the primary current in said primary winding on and off; first protective circuit means connected to said switching transistor for normally limiting said primary current to a first predetermined value; and circuit means, including at least one rectifier and connected to said at least one secondary winding, for providing a d.c. operating voltage at an output terminal of said power supply; the improvement comprising: a second protective circuit means for limiting said primary current to a second predetermined value, which is greater than said first predetermined value, upon nonoperation of said first protective circuit means, said second protective circuit means including a thyristor connected between the base of said switching transistor and a circuit point providing a blocking voltage for said switching transistor, and second circuit means, including a threshold device, for connecting the control electrode of said thyristor to a point of reference voltage proportional to said primary current via said threshold device; further comprising: a third protective circuit means connected to said secondary winding for limiting said d.c. operating voltage, when said operating voltage rise to a limit value.