1. Field of the Invention
The present invention relates to a switching power supply apparatus for deriving a DC power output of desired voltage by use of a switching regulator constructed as a DC-DC converter which converts a DC power obtained by rectifying an AC power input, and more particularly to a switching power supply apparatus designed to have a function of protection from an overcurrent flowing at the time of starting of the AC power input or overcurrent flowing at the time of recovery from temporary outage of the AC power input.
2. Description of the Related Art
In a switching power supply apparatus, an AC power input is converted into a DC power by a rectifier circuit, the thus obtained DC power is switched, rectified and smoothed by a switching regulator and then supplied to a load as a desired DC power output. The switching regulator is constructed as a DC-DC converter to convert a stabilized DC input into a DC output of desired voltage.
A power failure or outage may occur in the commercial power source, that is, in the AC power source provided by the electric company by various causes. Further, the AC power source may be sometimes subject to the instantaneous power failure or temporary outage which is a phenomenon that the power supply is temporarily interrupted only for a brief period of time. In various types of electric, electronic or electromechanical devices which require high reliability, it is necessary to suppress the influence by the power failure or temporary outage.
Among the conventional switching power supply apparatus, an apparatus as shown in FIG. 1 is provided. The switching power supply apparatus of FIG. 1 includes an AC power source 2, switching circuit 4, diodes D1, D2, capacitors C1, C2, resistor R1, transformer TF and thyristor TH.
An AC power input from the AC power source 2 is supplied to a rectifier circuit constructed by a series circuit of the diode D1, resistor R1 and capacitor C1 and a DC output is derived between two ends of the capacitor C1. The DC output between two ends of the capacitor C1 is switched by the switching circuit 4 and supplied to the primary side of the transformer TF as an AC power. When the gate of the thyristor TH is supplied with a signal derived from the primary side of the transformer TF and the thyristor is turned ON by the gate signal, it short-circuits the resistor R1. An output of the secondary side of the transformer TF is rectified and smoothed by a rectifier circuit constructed by the diode D2 and capacitor C2 and a DC power of desired voltage is derived between two ends of the capacitor C2.
In the switching power supply apparatus of FIG. 1, in order to prevent the breakage thereof even if the period of the temporary outage is relatively long, a capacitor of large capacitance is often used as the smoothing capacitor C1 of the rectifier for the AC power input. When input of an AC power from the AC power source 2 is interrupted by the temporary outage, charges stored on the smoothing capacitor C1 are discharged. As a result, since the DC voltage applied to the switching circuit 4 is kept at a value within substantially a range in which the normal operation can be effected within a short period of time, the switching circuit 4 is normally operated within a short period of time. If the AC power source 2 is recovered and a power input is supplied again within the short period of time, the DC power output between the two ends of the capacitor C2 on the secondary side of the transformer TF is not influenced by the temporary outage.
However, if the large capacitance capacitor is used as the smoothing capacitor C1 as described above, there occurs a possibility that an excessively large charging current (that is, rush current) temporarily flows into the smoothing capacitor C1 when the smoothing capacitor C1 is not sufficiently charged, for example, when the AC power source 2 is turned ON (when the power source is normally turned ON) while the temporary outage is not occurring. In order to prevent such an overcurrent, the resistor R1 is inserted between the AC power source 2 and the smoothing capacitor C1 as shown in FIG. 1 to suppress a current flowing when the AC power source 2 is turned ON. Then, it is determined that the smoothing capacitor C1 is sufficiently charged after the operation of the switching circuit 4 is continued for a short period of time and the thyristor TH which is a switching element for short-circuiting the resistor R1 is turned ON. When the thyristor TH is turned ON, the power input from the AC power source 2 is supplied to the smoothing capacitor C1 via the thyristor TH without passing the resistor R1. Therefore, in the steady state, unwanted power consumption in the current limiting resistor R1 can be prevented.
As described above, a temporary overcurrent flowing when the power source is turned ON can be limited even if the smoothing capacitor C1 is formed with large capacitance.
However, in the above switching power supply apparatus, an excessively large charging current at the time of turn-ON of the power source can be prevented, but an excessively large charging current at the time of recovery from the temporary outage cannot be prevented.
That is, since the smoothing capacitor C1 is discharged during the temporary outage, most of the charges stored on the smoothing capacitor C1 will be discharged unless the period of the temporary outage is extremely short. Therefore, when the AC power source 2 is recovered so as to supply the AC power again after occurrence of the temporary outage, an excessively large charging current flows into the smoothing capacitor C1. However, in this case, since the resistor R1 is already short-circuited by the thyristor TH, the overcurrent cannot be suppressed.
Therefore, in the conventional art, the smoothing capacitor C1 cannot be formed with excessively large capacitance, and as a result, the switching power supply apparatus can cope with only the temporary outage of short period of time.