1. Field of the Invention
The present invention relates generally to an automatic voltage switching power source and more specifically to a power source apparatus capable of supplying a constant output with different input ac voltages by automatically switching between voltage doubler rectification and full-wave rectification.
2. Description of the Background Art
Generally, two types of so-called commercial ac power sources, namely the 100V system utilized in U.S.A. and Japan and the 200V system utilized in Europe and others, are available. In order to allow the same electric device or the like to be operated by either of the commercial power source systems, a switching power source or the like is demanded which can always generate a constant output dc voltage even if different ac voltages are supplied. Usually, the switching power source or the like incorporated in the device is designed for the 200V system and performs full-wave rectification as the ordinary operation if the power source is operated by the 200V system but performs voltage doubler rectification to double the output if the power source is operated by the 100V system. The switching between the ordinary and doubler rectifications is typically carried out manually with a mechanical switch.
However, not only the switching operation is troublesome but also a problem of destructing the succeeding electric device and the power source circuit itself arises if the switching operation is incorrectly performed, particularly if the voltage doubler rectification is carried out with 200V to supply a doubled output voltage system instead of the full-wave rectification. Also, if the switching is reversely operated with 100V system, only a half of the normal output voltage is generated, causing a problem of insufficient power which disables the electric device or the like from being driven normally.
Consequently, an automatic switching power source capable of automatically switching the rectification functions through detecting whether the supplied ac voltage is high or low, has been proposed that has structures as shown in FIG. 1. In FIG. 1, first and second capacitors C1 and C2 are connected in series between the output terminals of a bridge rectifying circuit 2 composed of four diodes D, and the output terminals of the bridge rectifying circuit 2 is connected to a switching power source 3. The connection point of the first and the second capacitors C1 and C2 is connected to the input ground terminal of the bridge rectifying circuit 2 through a switch means 4.
The switch means 4 is associated with a control circuit 6 which is connected between the input terminals of the bridge rectifying circuit 2, and is so designed as to open if the voltage between the input terminals is high and to close if the voltage is low. That is, if the voltage is high, the switch means opens and the ordinary full-wave rectification is performed. If the voltage is low, the switch means closes and the first and the second capacitors C1 and C2 are individually connected to the output of the bridge rectifying circuit 2, so that the voltage between the terminals of the capacitors C1 and C2 is made to be two times as large as the voltage provided by the full-wave rectification.
However, since the switch is simply closed when the input voltage is lower than 100V (effective value) and opened when the voltage is higher than 100V (effective value) in the known apparatus described above, various problems are caused. Namely, assuming that the apparatus is just connected to the 200V system, the voltage applied to the apparatus does not become 200V at the moment it is connected to the commercial power source but reaches 200V after a given transient period has passed. In the earlier region of the transient period, the voltage doubler rectification is carried out with the switch closed since the voltage to the apparatus is lower than 100V. Even if the voltage increases further and exceeds 100V, the switch cannot alters instantly and therefore the voltage doubler rectification continues for a certain period of time after exceeding 100V. As a result, the electric device or the like may be damaged.
A problem similar to the one described above is also caused while the apparatus is being driven by the 200V system, if an instantaneous drop, or voltage decrease for a certain period of time, takes place to decrease the voltage from the commercial power source once to a level lower than 100V and then the voltage returns to the normal level.