Abstract:
In a capacitor charger including a transformer to transform a primary coil voltage to a secondary coil voltage to charge a capacitor at an output through a charging node to approach a predetermined voltage, a voltage sense apparatus and method comprise sensing the voltage on the capacitor with a voltage divider to generate a feedback signal to stop charging the capacitor when the capacitor voltage is sensed to be equal to or higher than the predetermined voltage, and preventing an inverse current flowing from the capacitor to the charging node for no leakage occurred from the capacitor to the voltage sense apparatus.

Description:
RELATED APPLICATIONS  
       [0001]     This application is a Divisional patent application of co-pending application Ser. No. 11/166,133, filed on 27 Jun. 2005. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention is related generally to a capacitor charger and more particularly to a voltage sense apparatus and method for a capacitor charger. BACKGROUND OF THE INVENTION  
         [0003]     Capacitor charger receives more and more attentions due to the gradually popular portable apparatus. A typical application of capacitor charger is for the power supply of flash lamp module. Conventionally, as shown in  FIG. 1 , a capacitor charger  100  for a flash lamp module has a transformer  102  including a primary coil L 1  and a secondary coil L 2  with the turns ratio of N P :N S , to transform the primary coil voltage V bat  to a secondary coil voltage V S , to charge a capacitor C O  through a diode  104 , to supply the electric power for a flash lamp module  106  connected to an output Vout. An integrated circuit  108  has a transistor M 1  connected between the primary coil L 1  and ground GND and a driver  112  controlled by a control circuit  110  to switch the transistor M 1  for the power delivery of the transformer  102  to the output Vout. To sense the capacitor voltage Vout, two resistors R 1  and R 2  are connected between the output Vout and ground GND to divide the capacitor voltage Vout to generate a feedback signal V FB  to a comparator  114  in the integrated circuit  108  to compare with a reference V ref  to generate a comparison signal S for the control circuit  110  to switch the transistor M 1 . When the capacitor voltage Vout reaches a predetermined level, the charger  100  will stop charging the capacitor C O .  
         [0004]     For the power delivery, the operations of the charger  100  shown in  FIG. 1  are illustrated by  FIGS. 2 and 3 . When the transistor M 1  conducts a current I 1 , as shown in  FIG. 2 , energy is stored into the primary coil L 1 , both the voltage V S  and current I 2  of the secondary coil L 2  are zero. When the transistor M 1  turns off, as shown in  FIG. 3 , the secondary coil L 2  releases the stored energy to produce a current I 2  flowing through the diode  104  to charge the capacitor C O . Once the capacitor voltage Vout reaches or exceeds the predetermined level, the feedback signal V FB  is equal to or larger than the reference V ref , and the output S of the comparator  116  signals the control circuit  110  to stop charging the capacitor C O . However, since the resistors R 1 , and R 2  are connected between the output Vout and ground GND, there is always a leakage path therewith, as shown in  FIG. 4 , through which a leakage current I Loss  flows from the capacitor C O  to ground GND, resulting in a voltage drop of the capacitor voltage Vout and power loss from the capacitor C O .  
         [0005]     To reduce such leakage power loss, Schenkel et al. proposed a capacitor charger circuit in U.S. Pat. No. 6,518,733, by sensing the primary coil voltage to determine when to stop charging the capacitor. Even this art removes the mentioned power loss from the voltage sense apparatus, it has the whole circuit to be complicated and huge.  
         [0006]     Therefore, it is desired a simple and lossless voltage sense apparatus and method for a capacitor charger.  
       SUMMARY OF THE INVENTION  
       [0007]     One object of the present invention is to provide a lossless voltage sense apparatus and method for a capacitor charger.  
         [0008]     In a capacitor charger including a transformer to transform a primary coil voltage to a secondary coil voltage to charge a capacitor at an output through a charging node to approach a predetermined voltage, according to the present invention, a voltage sense apparatus and method comprise sensing the voltage on the capacitor with a voltage divider to generate a feedback signal for the capacitor charger to stop charging the capacitor when the capacitor voltage is sensed to be equal to or higher than the predetermined voltage, and preventing an inverse current flowing from the capacitor to the charging node by a rectifier circuit. As a result, the capacitor is prevented from current leakage and power loss through the voltage sense apparatus.  
         [0009]     Alternatively, in a capacitor charger including a transformer to transform a primary coil voltage to a secondary coil voltage to charge a capacitor at an output through a charging node to approach a predetermined voltage, a voltage sense apparatus and method according to the present invention comprise drawing a taper from the secondary coil, dividing the voltage on the taper with a voltage divider to generate a feedback signal for the capacitor charger to stop charging the capacitor when the capacitor voltage is sensed to be equal to or higher than the predetermined voltage, and preventing an inverse current flowing from the capacitor to the charging node by a rectifier circuit. As a result, the capacitor is prevented from current leakage and power loss through the voltage sense apparatus. This voltage sense apparatus and method allow the resistors used for the voltage divider to have smaller resistance and volume. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0010]     These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which:  
         [0011]      FIG. 1  shows a circuit diagram of a conventional capacitor charger for a flash lamp module;  
         [0012]      FIG. 2  shows the capacitor charger of  FIG. 1  when the transistor M 1  turns on;  
         [0013]      FIG. 3  shows the capacitor charger of  FIG. 1  when the transistor M 1  turns off;  
         [0014]      FIG. 4  shows the leakage occurred in the capacitor charger of  FIG. 1 ;  
         [0015]      FIG. 5  shows a first embodiment of a voltage sense apparatus and method applied for a capacitor charger according to the present invention; and  
         [0016]      FIG. 6  shows a second embodiment of a voltage sense apparatus and method applied for a capacitor charger according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]      FIG. 5  shows a first embodiment of a voltage sense apparatus and method according to the present invention. In a capacitor charger  200 , a transformer  202  has a primary coil L 1  and a secondary coil L 2  with a turns ratio of N P :N S  to transform the primary coil voltage V bat  to a secondary coil voltage V S , through a charging node  204  to charge a capacitor C O  connected to an output Vout to supply for a flash lamp module  208 , an integrated circuit  210  has a transistor  212  connected between the primary coil L 1  and ground GND and a driver  216  controlled by a control circuit  214  to switch the transistor  212  for the power delivery of the transformer  202  to the output Vout. To sense the capacitor voltage Vout, resistors R 1 , R 3  and R 4  are connected between the charging node  204  and ground GND in such a manner that the resistor R 1 , is connected between a feedback node V FB  and ground GND to generate a feedback signal V FB , and the other resistors R 3  and R 4  are connected in series between the charging node  204  and feedback node V FB . A small voltage drop across the forward-biased diode  206  is present between the charging node  204  and output Vout, and may be neglected. The feedback signal V FB  is compared with a reference V ref  by a comparator  218  in the integrated circuit  210  to produce a comparison signal S for the control circuit  214 . Once the capacitor voltage Vout reaches or exceeds a predetermined level, the feedback signal V FB  will be equal to or larger than the reference V ref , and the output S of the comparator  218  will signal the control circuit  214  to stop charging the capacitor C O . The diode  206  between the charging node  204  and output Vout prevents the capacitor C O  from leakage to the charging node  204 , and the resistors R 1 , R 3  and R 4  for the voltage sense will not cause any leakage or power loss of the capacitor C O  since they are connected to the charging node  204 .  
         [0018]     Referring to  FIG. 5 , when the transistor  212  conducts a current I 1 , it is determined the output voltage  
                 V   ⁢           ⁢   out     =       (     -     V   bat       )     ×       N   S       N   P           ,           [     EQ   ⁢     -     ⁢   1     ]             
 
 which is a negative voltage, and therefore the current I 2  flows from ground GND to the transformer  202  through the resistors R 1 , R 3  and R 4 , thereby generating the feedback signal by voltage dividing theory  
               V   FB     =     Vout   ×         R   1         R   1     +     R   3     +     R   4         .               [     EQ   ⁢     -     ⁢   2     ]             
 
 By substituting the equation EQ-1 to the equation EQ-2, it is obtained  
                 V   FB     =       -     V   bat       ×       N   S       N   P       ×       R   1         R   2     +     R   3     +     R   4             ,           [     EQ   ⁢     -     ⁢   3     ]             
 
 which is a negative voltage. When the transistor  212  turns off, the current I 2  flows from the transformer  202  to the capacitor C O , thereby charging the capacitor C O , and the feedback signal V FB  is as shown in the equation EQ-2. Once the capacitor C O  is charged to a predetermined level, the feedback signal V FB  will be equal to or larger than the reference V ref , and therefore the output S of the comparator  218  will signal the control circuit  214  to stop charging the capacitor C O . Even the capacitor voltage Vout is charged to a high level, with the diode  206  between the charging node  204  and output Vout, the capacitor C O  is prevented from leakage to ground GND through the resistors R 1 , R 3  and R 4 . 
 
         [0019]     Referring to  FIGS. 1 and 5 , the combination of the resistors R 3  and R 4  in the charger  200  is equivalent to the resistor R 2  in the charger  100  in their resistance, however, the parasitic capacitance is reduced in the charger  200 . Each resistor has a parasitic capacitance, which is proportional to the resistance of the resistor, and therefore the capacitance C 1  parasitic to the resistor R 2  is larger than the capacitance C 2  parasitic to the resistor R 3  and the capacitance C 3  parasitic to the resistor R 4 . The larger a capacitance is, the significant the capacitive effect will be. More significant capacitive effect is easier to produce error operations. For example, with a predetermined threshold of 300V for the capacitor voltage Vout to stop charging the capacitor C O , a significant capacitive effect may result in earlier stop of charging the capacitor C O  before the capacitor voltage Vout reaches 300V. In the charger  200 , the resistors R 3  and R 4  are used to replace the resistor R 2 , and therefore the equivalent parasitic capacitance C 4  will have a value determined by  
                 1     C   4       =       1     C   2       +     1     C   3           ,           [     EQ   ⁢     -     ⁢   4     ]             
 
 and it is obtained  
               C   4     =           C   2     ⁢     C   3           C   2     +     C   3         .             [     EQ   ⁢     -     ⁢   5     ]             
 
 From the equation EQ-5, the equivalent capacitance C 4  is smaller than the capacitances C 2  and C 3 , and is therefore smaller than the capacitance C 1 . In other words, the charger  200  will have less significant capacitive effect. 
 
         [0020]      FIG. 6  shows a second embodiment of a voltage sense apparatus and method according to the present invention. In a capacitor charger  300 , a transformer  302  has a primary coil L 1  and a secondary coil L 2  with a turns ratio of N P :N S  to transform the primary coil voltage V bat  to a secondary coil voltage V L2 , through a diode  304  to charge a capacitor C O  connected to an output Vout to supply for a flash lamp module  306 , an integrated circuit  308  has a control circuit  312  to control a driver  314  to switch a transistor  310  connected between the primary coil L 1  and ground GND for the power delivery of the transformer  302  to the output Vout. To sense the capacitor voltage Vout, a taper  3022  is drawn from the secondary coil L 2 , with which the secondary coil L 2  is separated to a segment of one turn and a segment of N S -1 turns, and two resistors R 1 , and R 2  are connected between the taper  3022  and ground GND to divide the voltage Vout′ on the taper  3022  to generate a feedback signal V FB  on a feedback node V FB . In the integrated circuit  308 , a comparator  316  compares the feedback node V FB  with a reference V ref  to generate a comparison signal S to signal the control circuit  312  to stop charging the capacitor C O  when the capacitor voltage Vout is equal to or larger than a predetermined threshold.  
         [0021]     When the transistor  310  turns off, the capacitor C O  is charged by the current I 2 , resulting in the feedback signal  
               V   FB     =     V   ⁢           ⁢     out   ′     ×         R   1         R   1     +     R   2         .               [     EQ   ⁢     -     ⁢   6     ]             
 
 Since the two segments of the secondary coil L 2  have the turns ratio of 1:N S -1, by neglecting the small voltage drop across the forward-biased diode  304 , it is obtained  
               V   ⁢           ⁢     out   ′       =         V   ⁢           ⁢   out       N   S       .             [     EQ   ⁢     -     ⁢   7     ]             
 
 By substituting the equation EQ-7 to the equation EQ-6, it is obtained  
               V   FB     =         V   ⁢           ⁢   out       N   S       ×         R   1         R   1     +     R   2         .               [     EQ   ⁢     -     ⁢   8     ]             
 
 From the equation EQ-8, it is shown that the feedback signal V FB is proportional to the capacitor voltage Vout. With the diode  304  between the output Vout and transformer  302 , the capacitor C O  is prevented from leakage to the voltage sense apparatus. 
 
         [0022]     While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.