ABSTRACT
There is no exact definition for the term “secondary power supplies” and different authors give different definitions of this term. We shall understand this term as the power supplies intended for transformation of a main voltage of the electrical network (DC or AC) into voltages of other classes, intended for the direct power supplies of electronic devices. Secondary power supplies can be external (in the form of separate modules) or built in-that is, integrated in electronic devices and being their integral part. Such a secondary power supply is the principal component of any electronic device, including relay protection devices and specially digital protective relays (DPRs) upon which the reliability of the device’s working capacity depends. Today, a DPR is used exclusively for switch-mode power supplies.
