Patent Publication Number: US-6714735-B1

Title: Capacitor charging device for a flash

Description:
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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various objects and advantages of the present invention will be more readily understood from the following detailed description when read in conjunction with the appended drawing, in which: 
     FIG. 1 is a circuit diagram of a conventional capacitor charging device for a flash; 
     FIG. 2 is a block diagram of a capacitor charging device for a flash according to the present invention; 
     FIG. 3A is a time diagram of the pulse voltage V 1  according to the present invention; 
     FIG. 3B is a time diagram of the induced current I 1  according to the present invention; and 
     FIG. 4 is circuit diagram of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Reference is made to FIG. 2, which is a block diagram of a capacitor charging device for a flash according to the present invention. The charging device includes a flash capacitor  1 , voltage transforming circuit  2 , voltage mode PWM control circuit  3  and charging control circuit  4 . 
     When the voltage across the flash capacitor  1  is low, the charging control circuit  4  provides a low voltage to activate the voltage mode PWM control circuit  3  and make it output a pulse voltage V 1 . Then, the voltage mode PWM control circuit  3  provides a constant current to charge a time control capacitor (not shown) of the charging control circuit  4 . At the same time, the voltage mode PWM control circuit  3  gradually increases the pulse width of the pulse voltage V 1  according to the voltage across the time control capacitor. The pulse voltage V 1  drives the voltage transforming circuit  2  to provide an induced current I 1  to charge the flash capacitor  1 . When the flash capacitor  1  is charged completely, the voltage transforming circuit  2  provides a high voltage V 2  to stop the voltage mode PWM control circuit  3  outputting the pulse voltage V 1 . 
     Reference is made to FIG. 3A, which is a time diagram of the pulse voltage V 1 . In the soft start period, the pulse width of the pulse voltage V 1  increases gradually according to the voltage across the time control capacitor. Reference is made to FIG. 3B, which is a time diagram of the induced current I 1 . In the soft start period, the induced current I 1  increases gradually with the pulse width of the pulse voltage V 1 . 
     Furthermore, the time control capacitor of the charging control circuit  4  relates to the length of the soft start period. If the capacitance of the time control capacitor is increased, the time control capacitor will need more charging time and the length of the soft start period will also increase. On the contrary, the soft start period will decrease if the capacitance of the time control capacitor is decreased. In accordance with the present invention, the time control capacitor should be large enough so that the flash capacitor  1  can be charged completely within the soft start period. In practice, the capacitance of the time control capacitor can be 3.3 uF-22 uF. 
     Reference is made to FIG. 4, which is a circuit diagram of the present invention. The voltage transforming circuit  2  includes a diode D 1 , transformer T 1 , resistors R 1  and R 2 , capacitor C 2  and MOSFET Q 1 . The charging control circuit  4  includes capacitors C 1 , C 5  and C 7 , resistors R 6 - 8 , R 10  and R 12 , transistors Q 2  and Q 3  and a time control capacitor C 8 . 
     When the voltage across the flash capacitor  1  is low, the external circuit  3  (not shown) provides a high voltage to the transistor Q 3  of the charging control circuit  4 . At this time, the transistor Q 3  is “on” and the voltage of the pin BR/CTL of the voltage mode PWM control circuit  3  is low. Thereby, the voltage mode PWM control circuit  3  is activated to output the pulse voltage V 1 . 
     Then, the voltage mode PWM control circuit  3  outputs a constant current to charge the time control capacitor C 8  of the charging control circuit  4 . At the same time, the voltage mode PWM control circuit  3  gradually increases the pulse width of the pulse voltage V 1  according to the voltage across the time control capacitor C 8 . 
     The pulse voltage V 1  drives the MOSFET Q 1  of the voltage transforming circuit  2  to make the current passing through the MOSFET Q 1  change with the pulse voltage V 1 . Hence, via the transformer T 1 , the voltage transforming circuit  2  provides an induced current I 1  to charge the flash capacitor  1 . 
     When the flash capacitor  1  is charged completely, the voltage across the flash capacitor  1  is high so as to provide a high voltage V 2  to the pin IN of the voltage mode PWM control circuit  3  via the resistors R 1  and R 2 . Then, the voltage mode PWM control circuit  3  stops outputting the pulse voltage V 1 . 
     In conclusion, the charging device according to the present invention can improve the drawbacks of the conventional charging device indeed. It can charge the flash capacitor completely within the soft start period so as to reduce the damage of the external circuit, flash capacitor and rechargeable battery. Further, the charging device of the present invention employs the low-price voltage mode PWM control circuit to lower the cost of the digital camera. 
     Although the present invention has been described with reference to the preferred embodiment thereof, it will be understood that the invention is not limited to the details thereof. Various substitutions and modifications have been suggested in the foregoing description, and other will occur to those of ordinary skill in the art: Therefore, all such substitutions and modifications are embraced within the scope of the invention as defined in the appended claims.