1. Field of the Invention
The present invention relates to a power supply device which charges a main capacitor for use in a flash apparatus or the like.
2. Description of Related Art
In the field of cameras having electronic flash apparatuses, it has become necessary to reduce the sizes of component parts of flash apparatuses as the sizes of cameras have become smaller. Batteries used as power sources for the flash apparatuses also have come to be in a smaller size to have a lower voltage. As a result, it has become necessary to use a step-up circuit (a power supply device or a converter) which is made compact and arranged to stably oscillate.
To meet this requirement, for example, a step-up circuit disclosed in Japanese Laid-Open Patent Application No. SHO 54-102521 is arranged as shown in FIG. 6. Referring to FIG. 6, a protective resistor 102 is connected to a power supply 101. A switch 103 is connected in series to the protective resistor 102. A primary coil 104a and a feedback coil 104b of an oscillation transformer 104 are connected to the protective resistor 102 and the switch 103. The primary coil 104a is connected to the collector of an oscillation transistor 105, which is composed of a silicon transistor. The feedback coil 104b is connected to the base of the oscillation transistor 105.
An oscillation capacitor 106 is connected to the base circuit of the oscillation transistor 105 in parallel therewith. To a secondary coil 104c of the oscillation transformer 104 are connected a capacitor 107, a protective resistor 108 and a diode 110. A neon tube 109 is connected to the protective resistor 108.
A main capacitor 111 is connected to the diode 110. In addition, there are provided output terminals 112a and 112b.
With the step-up circuit arranged in the above manner, the electric charge of the power supply 101 is supplied to the oscillation transformer 104 when the switch 103 is turned on. Then, a current flows to the base of the oscillation transistor 105, which is connected to the oscillation transformer 104. Thus, an oscillation circuit is formed by the flow of current to begin oscillation.
The switch 103 is arranged either to immediately turn off upon completion of its turn-on action or to turn off after the lapse of, for example, three seconds.
The oscillation circuit operates to continue oscillation, because energy stored in the inductance of the oscillation transformer 104 is electromagnetically induced to the feedback coil 104b and the secondary coil 104c for dissipating the energy and a current necessary for the oscillation is allowed to flow to the base circuit as the input resistance and oscillation frequency of the oscillation transistor 105 are high.
The signal which has been supplied to the oscillation transformer 104 and has been boosted there in the above manner is supplied to a load part which includes the main capacitor 111. The capacitor 107, which is connected to the load part, corrects the waveform of the signal by absorbing a surge voltage.
Since the thus-corrected signal is supplied to the neon tube 109 through the protective resistor 108, the on-state of the oscillation circuit can be verified by a lighted-up state of the neon tube 109 and the off-state of the oscillation circuit can be verified by a put-out state of the neon tube 109.
Further, the above signal is rectified by the diode 110 and is, then, stored in the main capacitor 111. The signal stored in the main capacitor 111 is supplied from the output terminals 112a and 112b, for example, to a discharge circuit of a flash apparatus.
The oscillation circuit is arranged to automatically turn off in the following manner. Before the main capacitor 111, which is a load, is charged, a current of, for example, 50 mA is caused to flow to the main capacitor 111. The current comes to decrease accordingly as the main capacitor 111 is gradually charged with electric charge. When the current decreases to a value of, for example, 50 .mu.A, the base current of the base circuit also decreases accordingly. Then, the oscillation transistor 105 comes to have only a current of less than its operating point flowing there. As a result, the oscillating function of the oscillation transistor 105 comes to a stop, so that the supply of power is cut off.
In the conventional step-up circuit described above, the oscillation transistor 105 is connected in series to the primary winding (primary coil 104a) of the oscillation transformer 104. The base current of the oscillation transistor 105 is controlled by the charging current flowing to the main capacitor 111. The step-up circuit is thus arranged to have a current feedback system. Further, to make the start-up of the oscillation transistor 105 reliable, the feedback winding (feedback coil 104b) of the oscillation transformer 104 is also used.
However, in order to charge the main capacitor 111 up to a necessary voltage level, the arrangement for using the oscillation transistor 105 for controlling the oscillation of the oscillation transformer 104 has necessitated selecting and setting, as the oscillation transistor 105, such a transistor that has a large current-amplification factor proportional to the transformer winding and has a low emitter-to-collector voltage during oscillation.
Another problem with the conventional step-up circuit also lies in that, depending on the setting of arrangement of the oscillation transistor 105, the size of the transistor element becomes large.
Further, a transistor has such an element that the current-amplification factor thereof varies with temperature. For example, at a low temperature, the current-amplification factor of the oscillation transistor becomes smaller and the oscillation comes to a stop before the charging voltage reaches a setting voltage value.
Further, when the load of the main capacitor becomes lighter in the last stage of a charging process, the charging current becomes smaller in proportion to the decrease of the load. Then, the base current of the oscillation transistor also becomes smaller to make the oscillation difficult. Therefore, in order to ensure a stable operation, the oscillation must be caused to be carried on by using the feedback winding. Otherwise, it is hardly possible to have a stable operation and to have the charging voltage reach the setting voltage value.