Photography – With object illumination for exposure – With power supply detail
Reexamination Certificate
2003-06-09
2004-03-30
Perkey, W. B. (Department: 2851)
Photography
With object illumination for exposure
With power supply detail
C348S371000, C315S24100S
Reexamination Certificate
active
06714735
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to a capacitor charging device for a flash, and more particularly, to a capacitor charging device that finishes charging the flash capacitor in a gradual manner.
BACKGROUND OF THE INVENTION
In accordance with prior art, a capacitor charging device for a flash is usually disposed in a digital camera. It is used to charge the flash capacitor of the digital camera so as to provide electricity to the flash. As shown in
FIG. 1
, the charging device includes a flash capacitor
10
, voltage transforming circuit
20
, current mode pulse width modulation (PWM) control circuit
30
and charging control circuit
40
. The voltage transforming circuit
20
further includes a transformer T
10
, diode D
10
, resistor R
100
and metal-oxide-semiconductor field-effect transistor (MOSFET) Q
10
.
When the charging device functions initially, the current mode PWM control circuit
30
outputs a pulse current I to control the gate-to-source voltage V
GS
. The charging control circuit
40
controls the pulse width of the current I with a time control capacitor (not shown). Since the capacitance of the time control capacitor is small, the soft start period (i.e. the period for reaching the maximum pulse width of the current I) of the current mode PWM control circuit
30
is very short. Hence, the current mode PWM control circuit
30
can output a pulse current I with maximum pulse width in a very short time.
When the pulse current I is “on” (i.e. a state with an output current), the gate-to-source voltage VGS of the MOSFET Q
10
is positive and results in an increase of the current I
D
passing through the MOSFET Q
10
. While the current ID increases, the transformer T
10
generates an induced current I
0
to charge the flash capacitor
10
. When the pulse current I is “off” (i.e. a state with no output current), the gate-to-source voltage V
GS
of the MOSFET Q
10
is zero and results in the decrease of the current I
D
. While the current I
D
decreases, due to the tremendous reverse resistance of the diode D
10
, the induced current I
0
also reduces to zero. Thereby, the flash capacitor
10
can be charged by the induced current I
D
in the on-off circle of the pulse current I.
However, since the soft start period of the conventional charging device is very short, the induced current I
D
reaches its maximum value in a very short time. Hence, the conventional charging device easily damages the external circuit, flash capacitor
10
and the rechargeable battery of the digital camera so that the life of the digital camera is decreased. Further, the conventional current mode PWM control circuit
30
is expensive (comparing with the voltage mode PWM control circuit) and makes the digital cameral costly.
Accordingly, as discussed above, the conventional charging device has some drawbacks that could be improved. The present invention aims to resolve the drawbacks in the prior art.
SUMMARY OF THE INVENTION
An objective of the present invention is to provide a capacitor charging device for a flash that can charge the flash capacitor completely within the soft start period so as to prevent the induced current from increasing abruptly. Thereby, the prevent invention can reduce the damage of the external circuit, flash capacitor and rechargeable battery and increase the life of the digital camera.
Another objective of the present invention is to provide a capacitor charging device for a flash that uses a voltage mode PWM control circuit to replace the current mode PWM control circuit to reduce the cost of the digital camera.
Still another objective of the present invention is to provide a capacitor charging device for a flash that gradually increases the pulse width of the pulse voltage output from the voltage mode PWM control circuit to prevent instantaneously large current. Still another objective of the present invention is to provide a capacitor charging device for a flash that use a time control capacitor ranging from 3.3 uF to 22 uF to control the length of the soft start period.
For reaching the objective above, the present invention provides a capacitor charging device for a flash that includes a flash capacitor, voltage transforming circuit, voltage mode PWM control circuit and charging control circuit. The charging control circuit has a time control capacitor ranging from 3.3 uF to 22 uF to control the voltage mode PWM control circuit to increase the pulse width of the pulse voltage gradually within the soft start period. Then, by using the pulse voltage, the voltage mode PWM control circuit can drive the voltage transforming circuit to provide an induced current that increases gradually to charge the flash capacitor. In this manner, the present invention can reduce the damage of the external circuit, flash capacitor and rechargeable battery, and charge the flash capacitor completely within the soft start period.
Numerous additional features, benefits and details of the present invention are described in the detailed description, which follows.
REFERENCES:
patent: 5250978 (1993-10-01), Ogawa
Hsu Chien-Kuo
Lin Wen-Hwa
Browdy and Neimark
Perkey W. B.
Premier Image Technology Corporation
LandOfFree
Capacitor charging device for a flash does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Capacitor charging device for a flash, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Capacitor charging device for a flash will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3220518