Abstract:
An apparatus for controlling a charging circuit is provided. The apparatus includes a first detector, a second detector, and a controller. The first detector detects a voltage level at a first time and generates a first indication value corresponding to the voltage level at the first time, where the voltage level corresponds to an output voltage of the charging circuit. The second detector detects the voltage level at a second time after the first time and generates a second indication value corresponding to the voltage level at the second time. The controller receives the first and second indication values, and generates a control signal according to the first and second indication values for turning the charging circuit on and off.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This divisional application claims the benefit of co-pending U.S. patent application Ser. No. 11/306,857, filed on Jan. 13, 2006 and included herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a charger, in particular, an apparatus which prevents a charger from overcharging a capacitor. 
     2. Description of the Prior Art 
     Please refer to  FIG. 1 .  FIG. 1  shows a prior art charger  100 . The charger  100  includes a charging circuit  110 , a voltage divider  120 , a capacitor  130 , a comparator  140 , and a controller  150 . The voltage divider  120  is not restricted to connect to the cathode of the rectifying diode  118 , but could connect to the anode of the rectifying diode. The charging circuit  110  contains a transformer  112 , which receives an input voltage V in . A low level input voltage V in  is stepped up to a high level voltage by the transformer  112 , and the high voltage passes through the rectifying diode  118  to charge the capacitor  130 . The voltage at the terminal of the capacitor  130  is used as the output voltage for the charger  100 . The on/off state of the transformer  112  is controlled by a MOSFET  116 , which is driven by a driver  114 . The driver  114  receives a control signal from the controller  150  and provides the MOSFET  116  with a driving voltage. The charging circuit  110  is of a Flyback configuration. The voltage divider  120  includes two resistors R 1  and R 2  connected in series. The voltage drop on the resistor R 2  is a fraction of the output voltage V out , and is regarded as a feedback voltage V FB . The feedback voltage V FB  is compared with a predetermined reference voltage V ref  by the comparator  140 . The controller  150  generates the control signal based on the result given by the comparator  140 . 
     The example below further illustrates the functions of the charger  100 . For the flash operation of a camera, an input voltage of 3V is transformed into a high voltage to charge the capacitor  130 . Since the flash requires a driving voltage as high as 300V, the target voltage of the capacitor  130  is set to 300V. The ratio of R 2 /R 1  is set to 1/299, and the reference voltage V ref  is set to 1V. After receiving an enable signal, the controller  150  sends the control signal to the driver  114 , and the charging circuit  110  starts charging the capacitor  130 . As the voltage of the capacitor  130  increases and moves towards the target voltage of 300V, the feedback voltage V FB  moves towards a voltage of 1V, which is equal to the reference voltage V ref . When the voltage of the capacitor  130  is charged to 300V, the feedback voltage V FB  is therefore equal to 300×[1/(1+299)]=1 V. Once the feedback voltage V FB  reaches the reference voltage V ref , the comparator  140  sends an indication signal to the controller  150 . As soon as the indication signal is received, the controller  150  sends the control signal to stop the driver  114  and turn off the charging circuit  110 . A ready signal indicating the full charge of the capacitor  130  is also sent out. As a result, the capacitor  130  can be carefully charged to capacity. However, if the resistor R 1  is open-circuited or the resistor R 2  is shorted, the controller  150  will never receive the indication signal from the comparator  140  because the feedback voltage V FB  will never reach 1 V. Therefore, the capacitor  130  will be overcharged and may face potential risks of high voltage damage to other components. 
     SUMMARY OF THE INVENTION 
     One objective of the claimed invention is to therefore provide an apparatus for controlling a charging circuit and a method to solve the above-mentioned problems. 
     According to one aspect of the present invention, an apparatus for controlling a charging circuit is provided. The apparatus includes a first detector, a second detector, and a controller. The first detector detects a voltage level at a first time and generates a first indication value corresponding to the voltage level at the first time, where the voltage level corresponds to an output voltage of the charging circuit. The second detector detects the voltage level at a second time after the first time and generates a second indication value corresponding to the voltage level at the second time. The controller receives the first and second indication values, and generates a control signal according to the first and second indication values for turning the charging circuit on and off. 
     According to another aspect of the present invention, a method for controlling a charging circuit is provided. The method includes: detecting a voltage level at a first time and generating a first indication value corresponding to the voltage level at the first time, the voltage level corresponding to an output voltage of the charging circuit; detecting the voltage level at a second time after the first time and generating a second indication value corresponding to the voltage level at the second time; and generating a control signal according to the first indication value and the second indication value for turning the charging circuit on and off. 
     According to yet another aspect of the present invention, an apparatus for controlling a charging circuit is provided. The apparatus includes a first detector, a comparator, and a controller. The first detector detects a voltage level of a feedback voltage at a first time and generates a first indication signal corresponding to the voltage level at the first time, where the feedback voltage represents an output voltage of the charging circuit. The comparator detects whether the feedback voltage has reached a target level and outputs a terminal signal accordingly. The controller receives the first indication signal and the terminal signal to generate a control signal for turning the charging the charging circuit on and off. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a charger according to the prior art. 
         FIG. 2  shows a charger according to a first embodiment of the present invention. 
         FIG. 3  shows a waveform of the control signal within one period (T). 
         FIG. 4  shows a charger according to a second embodiment of the present invention. 
         FIG. 5  shows waveforms of the output voltage V out  and enable and ready signals during charging. 
     
    
    
     DETAILED DESCRIPTION 
     Please refer to  FIG. 2 .  FIG. 2  shows a charger  200  according to a first embodiment of the present invention. The charger  200  includes a charging circuit  110 , a voltage divider  120 , a capacitor  130 , a first comparator  140 , a controller  210 , a second comparator  220 , and a third comparator  230 . The charging circuit  110 , the voltage divider  120 , the capacitor  130 , and the first comparator  140  are functionally identical to those components corresponding to the same schematic symbol as shown in  FIG. 1 . The comparators  220  and  230  can be implemented through operational amplifiers. In this embodiment, the controller  210  not only generates a control signal to control the charging circuit  110 , but also provides a control value D M . The control value D M  is generated by monitoring the duty cycle of the control signal. The second comparator  220  compares the control value D M  with a predetermined threshold value D S , and generates a corresponding indication signal S 2 . The third comparator  230  compares the feedback voltage V FB  with a predetermined threshold voltage level V S  and generates a corresponding indication signal S 3 . The controller  210  sends out the control signal to control the charging circuit  110 , according to indication signals S 1 , S 2 , and S 3 . The indication signal S 1  is generated by the first comparator  140 . 
     The function of the controller  210  is described below. Because the charger  200  is of a Flyback configuration, the time-average voltage of the transformer  112  within an entire period is equal to zero when under a stable operating condition. Please refer to  FIG. 3 .  FIG. 3  shows a waveform of the control signal within one period (T). The output voltage V out  and the input voltage V in  satisfy Eq (1). 
     
       
         
           
             
               
                 
                   
                     
                       
                         ( 
                         
                           
                             
                               V 
                               out 
                             
                             + 
                             
                               V 
                               
                                 d 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 1 
                               
                             
                           
                           N 
                         
                         ) 
                       
                       × 
                       
                         T 
                         off 
                       
                     
                     = 
                     
                       
                         V 
                         in 
                       
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                         T 
                         on 
                       
                     
                   
                   , 
                 
               
               
                 
                   Eq 
                   . 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
             
               
                 
                   DutyCycle 
                   = 
                   
                     Ton 
                     
                       Ton 
                       + 
                       Toff 
                     
                   
                 
               
               
                 
                   Eq 
                   . 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     2 
                     ) 
                   
                 
               
             
           
         
       
     
     where V d1  is the voltage drop on the rectifying diode  118 , N is the transformer turn ratio (N=Ns/Np, Ns:Secondary turns, Np:Primary turns), and T on  and T off  are the respective on time and off time of the control signal within one period. V d1  is about 0.5V˜1 V and is usually neglected. From Eq. (1) and Eq. (2), it is obvious that the higher the output voltage V out , the higher the duty cycle of the control signal. Moreover, in  FIG. 3  the product of the on time T on  and the voltage V in , i.e. the area A 1 , must be equal to product of the off time T off  and the voltage (V out +V d1 )/N, i.e. the area A 2 . Assuming that N=20, V in =3V, T on =3.3 μs, when the output voltage V out  is charged to an amount of 100V, the off time T off  is equal to 1.98 μs. This implies that the duty cycle (i.e., the control value D M ) of the control signal is equal to 62.5%. If the control value D M  is equal to 62.5%, the feedback voltage V FB  should be 100×[1/(1+299)]=0.333V with the ratio of R 2 /R 1  equal to 1/299. Consequently, the threshold voltage level V S  is set to 0.333V. Therefore, when the indication signal S 2  generated by the second comparator  220  indicating that the control value D M  has reached the threshold value D S , and the indication signal S 3  generated by the third comparator  230  indicating that the feedback voltage V FB  has not reached the threshold voltage level V S , the controller  210  will send the control signal to disable the charging circuit  110 . However, sometimes the input voltage V in  is smaller than its normal voltage level, and hence the threshold voltage level V S  should be modified. The threshold voltage level V S  should obey the following equation. 
     
       
         
           
             
               
                 
                   
                     
                       V 
                       S 
                     
                     &lt; 
                     
                       
                         ( 
                         
                           
                             R 
                             2 
                           
                           
                             
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                               1 
                             
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                               2 
                             
                           
                         
                         ) 
                       
                       × 
                       N 
                       × 
                       
                         
                           V 
                           in 
                         
                         ⁡ 
                         
                           ( 
                           min 
                           ) 
                         
                       
                       × 
                       
                         ( 
                         
                           
                             D 
                             S 
                           
                           
                             1 
                             - 
                             
                               D 
                               S 
                             
                           
                         
                         ) 
                       
                     
                   
                   , 
                 
               
               
                 
                   Eq 
                   . 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     3 
                     ) 
                   
                 
               
             
           
         
       
     
     where V in (min) is the possible minimum voltage of the input voltage V in . If the V in (min) is equal to 1.8V, and the threshold value D S  is set to be 62.5%, then V S  should be set to smaller than 0.2V. A reasonable value of 0.075V is selected as the threshold voltage level V S . As described in this example, once the control value D M  reaches the threshold value D S  (62.5% in this example) while the feedback voltage V FB  is still smaller than the threshold voltage level V S  (0.075V in this example), operation of the voltage divider  120  is regarded as abnormal, and the charging circuit  110  is disabled. Consequently, by monitoring the feedback voltage V FB  and the duty cycle D M  of the control signal, the capacitor  130  can avoid being overcharged if R 1  is open-circuited, or R 2  is shorted, or even both. 
     Please refer to  FIG. 4 .  FIG. 4  shows a charger  400  according to a second embodiment of the present invention. In addition to having a charging circuit  110 , a voltage divider  120 , a capacitor  130 , and a comparator  140 , which are all functionally similar to those components having the same schematic symbol in  FIG. 1 , the charger  400  includes a controller  410 , a first detector  420 , and a second detector  430 . Please refer to  FIG. 5 .  FIG. 5  shows a waveform plot of the output voltage V out , and the enable and ready signals during charging. At time t 1 , when the enable signal (dashed line) received by controller  410  rises up, the charging circuit  110  will be turned on by the control signal output from the controller  410 . The output voltage V out  (thin solid line) will then start increasing. At time t 2 , when the output voltage V out  reaches a target level, the ready signal (thick solid line) goes from a low logic level to a high logic level, and the controller  410  sends out the control signal to disable the charging circuit  110  according to the indication signal S 1  generated by the comparator  140 . 
     From inspection of the output voltage V out , it is obvious that the curve is a monotonic increasing curve, meaning that if the output voltage V out  is sampled, a latter-sampled value is definitely larger than a previously-sampled value. According to this characteristic, an abnormal charging process can be detected by monitoring the output voltage V out . Referring back to  FIG. 4 , the feedback voltage V FB  is fed into the first detector  420  and the second detector  430 . The first detector  420  detects the feedback voltage V FB  at a first time and generates a first indication value. The first indication value represents the detected feedback voltage V FB  at the first time. Afterwards, The second detector  430  detects the feedback voltage V FB  at a second time, which is behind the first time, and generates a second indication value. The second indication value represents the detected feedback voltage V FB  at the second time. The first indication value and the second indication value are sent to the controller  410 . The controller  410  then compares these two indication values and generates the control signal accordingly. According to the characteristic of the output voltage V out , the second indication value is typically larger than the first indication value. Therefore, if the second indication value is smaller than the first indication value, the controller  410  sends a control signal to disable the charging circuit  110 . Consequently, by detecting and comparing the feedback voltage V FB , the capacitor  130  can avoid overcharging if R 1  is open-circuited, or R 2  is shorted, or even both. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.