Abstract: A switching power circuit with a redundant structure comprises a back-current obstructing element provided at an output side of the switching power circuit, error amplifiers provided at an output side (the load side) and internal side (the switching power circuit side) of the back-current obstructing element, respectively, to control both the voltages at the output side and inside of the back-current obstructing element, a field effect transistor used as the back-current obstructing element so that a source electrode and a drain electrode are connected in a forward direction with respect to an output current, and a voltage detecting circuit for detecting an output voltage of the error amplifier for inside of the switching circuit to produce an output signal on the basis of which a gate voltage of the field effect transistor is controlled.

Abstract: A parallel type switching power source circuit wherein reliable energization is achieved by individual power source switches provided for power source units, without common start-up switch, is provided. The start-up signal input terminals I.sub.1 and I.sub.2 and the drive stop signal input terminals R.sub.1 and R.sub.2 of power source units 12, 22 become active when they are connected to the negative side of an input power source 31. The power source unit 12 (or 22) starts only if the start-up signal input terminal I.sub.1 (or I.sub.2), is active and the drive stop signal input terminal R.sub.1 (or R.sub.2) is non-active. When the power source switch 16 is turned on at first, I.sub.1 becomes active, while R.sub.1 retains in active. Then the start-up condition is not satisfied and both of the first and second switching power sources 15 and 25 do not start. When the power source switch 26 is turned on next, I.sub.2 becomes active and both of R.sub.1 and R.sub.2 become non-active.