To provide a switching power supply unit that is configured in that a stand-by power supply does not stop before a main power supply does at the time of cutting an AC input without having excesses of capacity and a power-factor improvement circuit that is not fundamentally necessary, provided is a back-up circuit 140 that supplies the direct current output obtained by a main rectifying and smoothing circuit 111 of a main power supply circuit 110 to a sub converting circuit 122 of a stand-by power supply circuit 120.

A power supply control method is adapted to a current-to-voltage conversion circuit which has a transformer for converting and outputting an input power. The power supply control method stops a power supply to the transformer when an output side of the current-to-voltage conversion circuit is in a no-load state, and starts a power supply to the transformer when an external voltage is applied to the output side of the current-to-voltage conversion circuit.

In a system having multiple power supplies with outputs connected in parallel, the number of supplies providing current is controlled to improve the overall system efficiency. For example, when the output current of individual power supplies falls below a threshold, one or more supplies may be placed into a standby mode. This increases the output current of the supplies that are in an operational mode, improving the efficiency of the supplies that are in an operational mode.

An electronic circuit which has a voltage terminal, a switch, a voltage stabilizer and a computer. In addition, an interface terminal is provided, via which the computer can exchange data via an interface. After a supply voltage is applied at the voltage terminal, the switch (11) is initially closed so that the circuit has an extremely low quiescent current. A signal at the interface terminal causes the switch to close and the voltage stabilizer and the computer are activated.