Photography – With object illumination for exposure – With power supply detail
Reexamination Certificate
1999-08-18
2001-04-17
Perkey, W. B. (Department: 2851)
Photography
With object illumination for exposure
With power supply detail
Reexamination Certificate
active
06219493
ABSTRACT:
The present invention relates to an electronic flash device which controls an illuminating unit for a camera (electronic flash) in the course of photographing.
When a type of the electronic flash device is classified roughly, it includes the following.
Classification by an oscillating type
Self-excitation oscillating type: A type wherein a circuit which oscillates by it self is structured in a charging circuit.
Separate excitation oscillating type: A type wherein oscillation control signals are given to an oscillating element from the outside.
Classification by a type of voltage step up
Forward Type:
A type wherein voltage impressed on a primary coil of a transformer is stepped up in accordance with the turns ratio of the primary coil to a secondary coil, and is outputted on the secondary coil.
Flyback Type:
A type wherein backward voltage which is generated when an electric current that is running through the primary coil is interrupted, is generated on the secondary coil.
Therefore, those wherein the oscillating type one and the step-up type one stated above are combined can be considered as the electronic flash device.
Incidentally, conventional electronic flash devices employ a step up circuit which is of a self-excitation oscillating type and of a forward type (hereinafter referred to as self-exciting forward type).
In the electronic flash device of this self-exciting forward type, oscillating operations are stable despite a relatively simple circuit structure, because of the structure wherein a circuit for conducting oscillating operations is formed in the electronic flash device, and a charging current to a main capacitor flowing through the secondly coil is fed back to a base current for an oscillation transistor (bipolar transistor) connected to the primary coil in series.
In the step up circuit of the self-exciting forward type, the circuit automatically conducts the step up control wherein when the charging voltage is low, a large amount of currents are made to flow through the primary coil to conduct rapid step up operation, while when the charging voltage comes to the level which is close to the supply voltage multiplied by the turns ratio, the current to be consumed is changed to one which is marginally sufficient to maintain oscillating operations. Therefore, the step up circuit of the self-exciting forward type is generally used frequently for the electronic flash device which is not provided with CPU.
Even after being provided on a CPU-controlled camera, the step up circuit of the self-exciting forward type has been used generally because of the reason of the simple control such as changing a signal for start charging from OFF to ON in the case of start charging, and just changing a signal for start charging from ON to OFF in the case of inputting a charging completion signal and thereby stopping the charging.
On the other hand, a step up circuit of a separate excitation flyback type which is a combination of separate excitation oscillating and a flyback type is provided with a characteristic that an electric current to be consumed can be controlled, because behavior of an electric current flowing through the primary coil is constant independently of charging voltage due to the flyback type actions wherein a current is made to flow through the primary coil to accumulate energy in a transformer, and a flyback pulse generated in the secondly coil instantaneously when the current flowing through the primary coil is cut off is accumulated in the main capacitor.
Further, since a switching element connected to the primary coil can be turned on and turned off from the outside, there is provided a characteristic of separate excitation oscillating control that it is possible to prevent release of waste energy by controlling a switching element to turn it on or turn it off so that heat generation caused by current saturation of the switching element, magnetic saturation of the transformer and by over-current.
Since both characteristics stated above are provided, it is possible to charge an electronic flash at high conversion efficiency in a stable way from the start of charging to the end thereof, namely to make the battery life longer, only by supplying relatively simple oscillation control signals having constant duty and frequency, which is an advantage.
In this flyback system, however, the time for flyback pulses to be generated varies greatly depending on charging voltage of the main capacitor connected. Therefore, it is not possible to input a large amount of electric power into the primary coil by utilizing the feed back and thereby to enhance the charging speed, which is different from the forward type. Further, in this flyback system, it is not possible, due to the repeated operations wherein energy accumulated while an oscillation transistor is kept to be on is released when the oscillation transistor is turned off, to enhance the duty of the time through which the charging current flows, resulting in a disadvantage that a period of time for charging is longer, which is different from the forward type.
It is not preferable that the charging time for electronic flash of a camera is long in spite of an advantage that a current consumed is constant and a conversion efficiency is high as stated above. The charging circuit for electronic flash of this separate excitation flyback type is disclosed in TOKKAIHEI No. 7-22189, for example, but there is no consideration to shorten the charging time. Therefore, the charging circuit of the separate excitation flyback type has not been used.
SUMMARY OF THE INVENTION
The invention has been achieved in view of the technical problems stated above, and its first object is to provide an electronic flash device and camera which can charge at high speed while conducting step up operations at high conversion efficiency.
Now, if the time for an oscillation transistor to be turned off is set to the time for a flyback pulse to be generated which is near the completion of charging where charging voltage is high and the time for the flyback pulse to be generated is short, for the purpose of shortening the charging time, when charging voltage of the main capacitor immediately after the start of charging is low, the oscillation transistor is undesirably turned on while a charging current is still flowing to the main capacitor where flyback pulses are being generated, and a current is forced to start flowing to the primary coil. Under this condition, there is generated phenomena that an unexpected excessive current flows through the primary coil, and conversion efficiency for charging is lowered.
Therefore, the time for the oscillation transistor to be turned off needs to be set in synchronization with the timing wherein charging voltage of the main capacitor is low and the time for flyback pulses to be generated is long, resulting in generation of the so-called “idle time” which contributes neither to accumulation of energy of a transformer nor to charging on the main capacitor caused by generation of flyback pulses, on the point of completion of charging. For this reason, there has been disadvantage that the charging time is made longer.
It is not preferable that the charging time for electronic flash of a camera is long in spite of the merit that a current to be consumed for charging can be controlled and conversion efficiency is high. The electronic flash device of this separate excitation flyback type is disclosed in TOKKAIHEI No. 7-22189, for example, but there is no consideration to shorten the charging time. Therefore, the charging circuit of the separate excitation flyback type has not been put to practical use.
It is known that an electromotive force of a battery with less remaining life and of a battery under low temperature is lowered. If it is tried to draw out a large amount of current for shortening the charging time when charging an electronic flash device with such battery, it results in a rapid drop of battery voltage, causing longer charging time on the contrary and a problem of shortening of battery life. In addition t
Aoki Toshiyuki
Yasui Teruhiko
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Konica Corporation
Perkey W. B.
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