A direction and a magnitude of a current flowing between an external alternating current power source (20) and a load (30) are detected by a current transformer (11). A voltage generated to a secondary winding (11B) of the current transformer as a result of detection is superimposed upon a voltage of the alternating current power source (20), and is supplied to the synchronous power source (S). The synchronous power source (S) generates a voltage having an instantaneous value which is the same as that of the voltage supplied thereto, and applies the generated voltage to the load (30) in parallel. Since the voltage applied by the synchronous power source (S) has a value for lowering the current detected by the current transformer (11), there is substantially no current flowing between the alternating current power source (20) and the load (30).

The synchronous switching power supply can be employed in multiple output power supply systems requiring extremely high efficiency and low number of components. The input signal may be AC or pulsating DC. A single switch is switched at zero voltage and current. The short circuit protection is inherent. A switching power supply with boost or flyback converter includes an inductor coupled to a node for attaining a current, a switch coupled to the node for selectively applying the input signal of one polarity across the inductor, wherein the inductor and switch are separately coupled to the input and ground in either order, a capacitor providing the output signal, rectifier coupled between the node and capacitor for conducting the current, a pair of comparators for comparing the input and output signals against respective reference signals, a circuit for determining an optimum value of the current as to minimize an overshoot of the output signal, and a circuit controlling the switch.

A power source apparatus has a power factor improving circuit which uses switching elements (4) as buildup transistors. According to the power source apparatus of the present invention, a current carrying ratio of the switching elements is decided to be a value which is obtained by subtracting a number of proportional value of the power source current waveform from the maximum current carrying ratio. Since the device of the present invention does not necessitate a detecting circuit of the power source voltage waveform for outputting an instruction waveform, the circuit thereof is simple and is not affected by the voltage change of the power source and noise generated at the power source.

A battery backup system using a power factor correction (PFC) front end as a DC-to-DC converter of a power supply for supplying a regulated DC input signal to a main power converter. The battery backup system includes a battery backup for supplying power, a switch circuit for connecting the battery in the event of loss of the AC input signal and a detection circuit for detecting the presence or absence of AC power for controlling the switch circuit. The PFC front end preferably receives either a pulsing DC voltage from a bridge rectifier or the battery voltage and switches this voltage through a series inductor for providing the regulated DC input voltage to the main power converter. The battery is switched in to replace the AC source and rectifier in the event of loss of the AC signal. In the preferred embodiment, the PFC circuit includes a series inductor and diode, a switching transistor and a control circuit for providing the regulated DC input voltage. The backup battery is coupled through a diode and a controlled switch to the input of the PFC circuit.

A device includes an active rectifier (14) having control gates controllable to produce an output voltage on a DC bus (20), a gate control circuit (16) for producing gate control signals for controlling the active rectifier control gates, a first circuit (18) connected to the gate control circuit (16) for producing a command current magnitude signal and a power factor signal for use by the gate control circuit (16), a current line (30) providing a signal related to the DC load current to the first circuit (18), and a voltage line (32) providing a signal related to the DC bus voltage to the first circuit (18). The first circuit (18) includes a command current magnitude signal generator (34) producing the command current signal based on the DC load current and a power factor controller (44, 48) producing the power factor signal. The power factor controller (44, 48) includes a feed forward circuit (52, 56) for increasing the power factor signal in response to current fluctuations on the current line (30). Also a method of controlling a voltage on a DC bus with the device.