Abstract:
In a capacitor charger including a transformer to transform a primary coil voltage to a secondary coil voltage to charge through a charging node a capacitor that is connected to an output to approach a predetermined voltage thereon, a voltage sense apparatus and method comprise sensing the voltage on the capacitor with a voltage divider or a sense current flowing through a resistor 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 applying prevention of an inverse current flowing from the capacitor to the charging node for the capacitor from leakage through the voltage sense apparatus.

Description:
FIELD OF THE INVENTION  
       [0001]     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  
       [0002]     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. Conventionally, as shown in  FIG. 1 , a capacitor charger  100  for a flash lamp has a transformer  102  including a primary coil L 1  and a secondary coil L 2  with 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  switches the transistor M 1  connected between the coil L 1  and ground GND by the driver  112  controlled by the control circuit  110  to control the power delivery of the transformer  102 . To sense the capacitor voltage Vout, resistors R 1  and R 2  are connected between the output Vout and ground GND to divide the voltage Vout to generate a feedback signal V FB  to the integrated circuit  108  that has a comparator  114  to compare the feedback signal V FB  with a reference Vref to generate a comparison signal S for the control circuit  110 . Subsequently, the charger  100  will stop charging the capacitor C O  when the capacitor voltage Vout reaches the predetermined level.  
         [0003]     For the power delivery, the operations of the charger  100  shown in  FIG. 1  are illustrated by  FIG. 2  and  FIG. 3 . When the transistor M 1  conducts a current I 1 , as shown in  FIG. 2 , the voltage V S  and the current I 2  both are zero. When the transistor M 1  is turned off, the capacitor C O  is charged by the current I 2 , as shown in  FIG. 3 . Once the capacitor voltage Vout reaches or exceeds the predetermined level, the feedback signal V FB  is equal to or larger than the reference Vref, 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, as shown in  FIG. 4 , by which a leakage current I Loss  flows from the capacitor C O  to ground GND through the resistors R 1  and R 2 , resulting in voltage drop of the capacitor voltage Vout and power loss from the capacitor C O .  
         [0004]     To reduce such 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 to stop charging the capacitor. Even this art removes the mentioned power loss from the voltage sense apparatus, it also has the whole circuit to be complicated and huge.  
         [0005]     Therefore, it is desired a simple and lossless capacitor charge sensing apparatus and method for capacitor charger.  
       SUMMARY OF THE INVENTION  
       [0006]     One object of the present invention is to provide a voltage sense apparatus and method for a capacitor charger, which can prevent the charged capacitor from leakage through the voltage sense apparatus.  
         [0007]     In a capacitor charger including a transformer to transform a primary coil voltage to a secondary coil voltage to charge through a charging node a capacitor that is connected to an output to approach a predetermined voltage thereon, 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.  
         [0008]     Alternatively, according to the present invention, a voltage sense apparatus and method comprise sensing the voltage on the capacitor to generate a sense current to flow through a resistor to generate the feedback signal for the capacitor charger, and preventing an inverse current flowing from the capacitor to the charging node by a rectifier circuit to prevent the capacitor from current leakage and power loss through the voltage sense apparatus.  
         [0009]     In another embodiment, according to the present invention, a voltage sense apparatus and method comprise transforming the primary coil voltage to a second secondary coil voltage, generating the feedback signal for the capacitor charger by dividing the second secondary coil voltage or by generating a sense current from the second secondary coil voltage to flow through a resistor, and preventing an inverse current flowing from the capacitor to the charging node by a rectifier circuit.  
     
    
     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 the circuit diagram of a conventional capacitor charger for a flash lamp;  
         [0012]      FIG. 2  illustrates the status when the transistor M 1  in the charger shown in  FIG. 1  is conducted;  
         [0013]      FIG. 3  illustrates the status when the transistor M 1  in the charger shown in  FIG. 1  is turned off;  
         [0014]      FIG. 4  illustrates the leakage occurred in the charger shown in  FIG. 1 ;  
         [0015]      FIG. 5  shows the first embodiment of the present invention;  
         [0016]      FIG. 6  shows the second embodiment of the present invention;  
         [0017]      FIG. 7  shows the third embodiment of the present invention;  
         [0018]      FIG. 8  shows the fourth embodiment of the present invention;  
         [0019]      FIG. 9  shows the fifth embodiment of the present invention;  
         [0020]      FIG. 10  shows the sixth embodiment of the present invention;  
         [0021]      FIG. 11  shows the seventh embodiment of the present invention; and  
         [0022]      FIG. 12  shows the eighth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     The present invention will be illustrated by various embodiments which either employ voltage divider to sense the capacitor voltage and to generate a feedback signal by a feedback apparatus in the voltage divider, or a sense current to flow through a resistor to generate a feedback signal, for a capacitor charger to stop charging the capacitor when the capacitor voltage reaches a predetermined value. However, the detailed circuits in these embodiments are designed to illustrate the present invention, but not desired to be limitations to the present invention.  
         [0024]      FIG. 5  shows the first embodiment of 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 a flash lamp module  208 , an integrated circuit  210  switches a transistor  212  connected between the coil L 1  and ground GND by a driver  216  through a control circuit  214  to control the power delivery of the transformer  202 . To sense the capacitor voltage Vout, resistors R 1  and R 2  are connected in series between the charging node  204  and ground GND to divide the charging voltage V S  on the charging node  204 , to generate a feedback signal V FB  for the integrated circuit  210  that has a comparator  218  to compare the feedback signal V FB  with a reference Vref to generate a comparison signal S to signal the control circuit  214  to stop charging the capacitor C O  when the capacitor voltage Vout reaches a predetermined level. A diode  206  is further connected between the charging node  204  and output Vout to prevent an inverse current flowing from the output Vout to the charging node  204 .  
         [0025]     Still referring to  FIG. 5 , when the transistor  212  conducts a current I 1 , the secondary coil voltage V S  of the transformer  202  is  
               V   S     =       (     -     V   bat       )     ×         N   S       N   P       .               [     EQ   ⁢     -     ⁢   1     ]             
 
 Due to the negative value of V S , a current I 2  flows from ground GND to the transformer  202  through the resistors R 1  and R 2 , thereby generating the feedback signal by voltage dividing theory  
                 V   FB     =         V   S     ×     R1     R1   +   R2         =       -     V   bat       ×       N   S       N   P       ×     R1     R1   +   R2             ,           [     EQ   ⁢     -     ⁢   2     ]             
 
 and therefore, the feedback signal V FB  also has a negative value. Latch-up is easily occurred to most integrated circuits formed on P-type substrates if the voltages on their pins are lower than −0.3V, and therefore, the turns ratio N P :N S  of the coils L 1  and L 2  and the resistance ratio of the resistors R 1  and R 2  are preferably selected to have the feedback signal V FB  not lower than −0.3V. On the other hand, when the transistor  212  is turned off, the current I 2  flows from the transformer  202  to the capacitor C O , thereby charging the capacitor C O , and the secondary coil voltage is 
 
 V   S   =Vout+V   f ,  [EQ-3]
 
 where V f  is the forward bias of the diode  206 . Likewise, by the voltage dividing theory, the feedback signal is  
               V   FB     =       (     Vout   +     V   f       )     ×       R1     R1   +   R2       .               [     EQ   ⁢     -     ⁢   4     ]             
 
 When the feedback signal V FB  is equal to or larger than the reference Vref, the output S of the comparator  218  will signal the control circuit  214  to stop charging-the capacitor C O . The diode  206  prevents the capacitor C O  from the leakage through the resistors R 1  and R 2  to ground GND. 
 
         [0026]      FIG. 6  shows the second embodiment of the present invention, which capacitor charger  300  is a modification of the charger  200  shown in  FIG. 5  with a voltage clamping circuit  302  inserted between the resistors R 1  and R 2 . In the voltage clamping circuit  302 , a resistor R 3  is connected between the resistors R 1  and R 2 , and a diode D 1  is connected between a clamping node  304  and ground GND. When the transistor  212  is turned on, due to the forward bias of the diode D 1  of about 0.7V, the voltage on the clamping node  304  is clamped at −0.7V, and thus, according to voltage dividing theory, the feedback signal is  
               V   FB     =       (     -   0.7     )     ×       R1     R1   +   R3       .               [     EQ   ⁢     -     ⁢   5     ]               
 By selecting the resistances of the resistors R 1  and R 3 , the feedback signal V FB  can be determined to be not lower than −0.3V. In further modified embodiments, the diode D 1  can be replaced by several diodes connected in series, or the positive electrode of the diode D 1  connected to ground GND in  FIG. 6  can be alternatively connected to a reference voltage, thereby having the voltage on the clamping node  304  to be clamped over a desired level. 
 
         [0027]     Still referring to  FIG. 6 , when the transistor  212  is turned off, the current I 2  flows from the transformer  202  to the capacitor C O , and thus charges the capacitor C O . The secondary coil voltage V S  also follows the equation EQ-3, and according to voltage dividing theory, the feedback signal is  
                 V   FB     =       (     Vout   +     V   f       )     ×     R1     R1   +   R2   +   R3           ,           [     EQ   ⁢     -     ⁢   6     ]             
 
 where V f  is the forward bias of the diode  206 . When the feedback signal V FB  is equal to or larger than the reference Vref, the output S of the comparator  218  will have the control circuit  214  to stop charging the capacitor C O . Likewise, the diode  206  prevents the capacitor C O  from leakage through the resistors R 1 , R 2  and R 3  to ground GND. 
 
         [0028]      FIG. 7  shows the third embodiment of the present invention, which capacitor charger  400  is also a modification of the charger  200  shown in  FIG. 5  with a diode D 1  inserted between the charging node  204  and resistor R 2 . In the charger  400 , the diode D 1  prevents an inverse current flowing from ground GND to the charging node  204 . However, apparently the location of these three elements D 1 , R 1  and R 2  are interchangeable, without departing from their operations. When the transistor  212  is turned on, the diode D 1  blocks the path between the charging node  204  and ground GND, thereby no current flowing through the resistors R 1  and R 2 , and the feedback signal V FB  is equal to zero. When the transistor  212  is turned off, the current I 2  flows from the transformer  202  to the capacitor C O  to charge the capacitor C O . The secondary coil voltage V S  still follows the equation EQ-3, and according to voltage dividing theory, the feedback signal is  
               V   FB     =       (     Vout   +     V   f     -     V   D1       )     ×     R1     R1   +   R2                 [     EQ   ⁢     -     ⁢   7     ]               
 where V D   1  is the forward bias of the diode D 1 . When the feedback signal V FB  is equal to or larger than the reference Vref, the output S of the comparator  218  signals the control circuit  214  to stop charging the capacitor C O . The diode  206  still prevents the capacitor C O  from leakage through the resistors R 1  and R 2  and diode D 1  to ground GND. 
 
         [0029]      FIG. 8  shows the fourth embodiment of the present invention. In a capacitor charger  500 , a transformer  502  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  504  to charge a capacitor Co connected to an output Vout to supply a flash lamp module  508 , an integrated circuit  510  switches a transistor  512  connected between the coil L 1  and ground GND by a driver  516  through a control circuit  514  to control the power delivery of the transformer  502 . To sense the capacitor voltage Vout, a servo amplifier  520  has an operational amplifier  526  with its two inputs connected to a reference voltage VB and a servo node  524 , respectively, for the servo node  524  to be at the reference voltage VB, and a transistor  522  connected between the servo node  524  and a feedback node V FB  with its gate connected with the output of the operational amplifier  526 , and a resistor R 2  is connected between the charging node  504  and servo node  524 , to generate a sense current I 3  flowing therethrough and through the transistor  522  in the servo amplifier  520  to provide to the feedback node V FB  connected with a resistor R 1 . The sense current I 3  is determined by the resistance of the resistor R 2  and the voltage drop thereacross, i.e., the voltage difference between the nodes  504  and  524 , and the feedback signal V FB  provided for the integrated circuit  510  is determined by the product of the resistance of the resistor R 1  and the sense current I 3 . The comparator  518  in the integrated circuit  510  compares the feedback signal V FB  with a reference Vref to generate a comparison signal S for the control circuit  514  to stop charging the capacitor C O  when the capacitor voltage Vout reaches a predetermined value. To prevent an inverse current flowing from the output Vout to the charging node  504 , a diode  506  is connected between the charging node  504  and output Vout.  
         [0030]     Still referring to  FIG. 8 , when the transistor  512  conducts a current I 1 , the secondary coil voltage Vs of the transformer  502  is at a negative level, and the transistor  522  is thus turned off, and the feedback signal V FB  is equal to zero. When the transistor  512  is turned off, the current I 2  charges the capacitor C O , the servo voltage on the servo node  524  is VB, and the voltage V S  of the secondary coil follows the equation EQ-3. Through the resistor R 2  the sense current is  
               I3   =         Vs   -   VB     R2     =       Vout   +     V   f     -   VB     R2         ,           [     EQ   ⁢     -     ⁢   8     ]             
 
 and therefore the feedback signal is  
               V   FB     =         R1   ×     (     Vout   +     V   f     -   VB     )       R2     .             [     EQ   ⁢     -     ⁢   9     ]             
 
 Likewise, when the feedback signal V FB  is equal to or larger than the reference Vref, the output S of the comparator  518  has the control circuit  514  to stop charging the capacitor C O . The diode  506  prevents the capacitor C O  from leakage through the resistors R 1  and R 2  and the transistor  522  to ground GND. 
 
         [0031]      FIG. 9  shows the fifth embodiment of the present invention, which capacitor charger  600  is a modification of the charger  500  shown in  FIG. 8  with the reference voltage VB for the servo amplifier  520  to be the primary coil voltage V bat , which can be done by connecting the input of the operational amplifier  526  to the input of the coil L 1  of the transformer  502 . By this manner, in the capacitor charger  600 , the servo voltage on the servo node  524  will follow the battery voltage V bat .  
         [0032]     When the transistor  512  conducts a current I 1 , the secondary coil voltage V S  of the transformer  502  is at a negative level, and the transistor  522  is thus turned off, and the feedback signal V FB  is equal to zero. When the transistor  512  is turned off, the current I 2  charges the capacitor C O , and the servo voltage on the servo node  524  is V bat . By substituting the voltage V bat  into the equation EQ-9 for the voltage V B , the feedback signal is  
               V   FB     =         R1   ×     (     Vout   +     V   f     -   Vbat     )       R2     .             [     EQ   ⁢     -     ⁢   10     ]             
 
 Likewise, when the feedback signal V FB  is equal to or larger than the reference Vref, the output S of the comparator  518  will signal the control circuit  514  to stop charging the capacitor C O . Also, the diode  506  prevents the capacitor C O  from leakage through the resistors R 1  and R 2  and the transistor  522  to ground GND. 
 
         [0033]      FIG. 10  shows the sixth embodiment of the present invention, which capacitor charger  700  is a further modification of the charger  600  shown in  FIG. 9  with a resistor R 3  connected between the reference voltage V bat  for the servo amplifier  520  and the feedback node V FB , and the resistor R 3  has a resistance 
   R   3 = R   2 − R   1 .  [EQ-11] 
 Since the resistors R 1  and R 3  are connected in series between the voltage V bat  and ground GND, the current flowing through the resistor R 3  is  
               I   R3     =         V   bat       R1   +   R3       .             [     EQ   ⁢     -     ⁢   12     ]               
 Substituting the equation EQ-11 into the equation EQ-12, it has  
               I   R3     =         V   bat       R1   +   R2   -   R1       =         V   bat     R2     .               [     EQ   ⁢     -     ⁢   12     ]               
 When the transistor  512  conducts a current I 1 , the secondary coil voltage V S  of the transformer  502  is at a negative level, and the transistor  522  is thus turned off, and the feedback signal V FB  is equal to zero. When the transistor  512  is turned off, the current I 2  charges the capacitor C O , and the servo voltage on the servo node  524  is V bat . In addition to the current I 3 , the current I R3  is also supplied to the resistor R 1 , and thus the total current flowing through the resistor R 1  is 
   I   R1   =I   3   +I   R3 .  [EQ-14] 
 Substituting the voltage V bat  into the equation EQ-8 for the voltage V B , it is obtained the sense current  
             I3   =         Vout   +     V   f     -     V   bat       R2     .             [     EQ   ⁢     -     ⁢   15     ]               
 According to the equations EQ-12, EQ-13, and EQ-14, the total current flowing through the resistor R 1  becomes  
                 I   R1     =           Vout   +     V   f     -     V   bat       R2     +       V   bat     R2       =       Vout   +     V   f       R2         ,           [     EQ   ⁢     -     ⁢   16     ]               
 and therefore, the feedback signal is  
               V   FB     =         R1   ×     (     Vout   +     V   f       )       R2     .             [     EQ   ⁢     -     ⁢   17     ]               
 Likewise, when the feedback signal V FB  is equal to or larger than the reference Vref, the output S of the comparator  518  has the control circuit  514  to stop charging the capacitor C O , and the diode  506  prevents the capacitor C O  from leakage through the resistors R 1  and R 2  and the transistor  522  to ground GND. 
 
         [0034]     It is shown by the equation EQ-17, the introduction of the resistor R 3  eliminates the effect from the primary coil voltage V bat  to the feedback signal V FB . Battery is typically used for the power source (V bat ) of a capacitor charger, and the supplied voltage of the battery drops down gradually as the time goes by. This embodiment shown in  FIG. 10  prevents the capacitor charger  700  from operating in error resulted from the decline or exhaustion of the battery power. In addition, this embodiment also shows the excellent operations adaptive to various battery voltage V bat .  
         [0035]      FIG. 11  shows the seventh embodiment of the present invention. In a capacitor charger  800 , a transformer  802  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 L   2  to charge a capacitor C O  connected to an output Vout to supply a flash lamp module  806 , an integrated circuit  808  switches a transistor  810  connected between the coil L 1  and ground GND by a driver  814  through a control circuit  812  to control the power delivery of the transformer  802 . To sense the capacitor voltage Vout, another secondary coil L 3  is employed to transform the primary coil voltage V bat  to another secondary coil voltage V L   3 , and resistors R 1  and R 2  are connected in series between the secondary coil voltage V L   3  and ground GND to divide the secondary coil voltage V L   3  to generate a feedback signal V FB  for the integrated circuit  808  that has a comparator  816  to compare the feedback signal V FB  with a reference Vref to determine a signal S for the control circuit  812  to stop charging the capacitor C O  when the capacitor voltage Vout reaches a predetermined value. A diode  804  is connected between the coil L 2  and output Vout to prevent an inverse current flowing from the output Vout to the transformer  802 .  
         [0036]     When the transistor  810  conducts a current I 1 , the secondary coil voltage of the coil L 3  is  
                 V   L3     =       (     -     V   bat       )     ×     Ns2   Np         ,           [     EQ   ⁢     -     ⁢   18     ]             
 
 and therefore, according to voltage dividing theory, the feedback signal is  
                 V   FB     =       (     -     V   bat       )     ×     Ns2   Np     ×     R1     R1   +   R2           ,           [     EQ   ⁢     -     ⁢   19     ]             
 
 which is negative value. To prevent the integrated circuit  808  from latch-up, the turns ratio N P :N S  of the coils L 1  and L 2  and the resistance ratio of the resistors R 1  and R 2  are selected to have the feedback signal V FB  not lower than −0.3V. On the other hand, when the transistor  810  is turned off, the capacitor C O  is charged by a current I 2 , and the feedback signal is  
                 V   FB     =       V   L3     ×     R1     R1   +   R2           ,           [     EQ   ⁢     -     ⁢   20     ]             
 
 and due to the turns ratio N S   1 :N S   2  between the coils L 2  and L 3 , it is obtained  
               V   L3     =       V   L2     ×       Ns2   Ns1     .               [     EQ   ⁢     -     ⁢   21     ]             
 
 Substituting the equation EQ-21 into the equation EQ-20, the feedback signal becomes  
               V   FB     =       V   L2     ×     Ns2   Ns1     ×       R1     R1   +   R2       .               [     EQ   ⁢     -     ⁢   22     ]             
 
 From this equation EQ-22, it is shown that the feedback signal V FB  is proportional to the secondary coil voltage V L   2  of the coil L 2 . Since the voltage sense apparatus to sense the voltage Vout on the capacitor C O  in this embodiment  800  is coupled to ground GND through the coil L 3 , there is no leakage consideration for the capacitor C O . 
 
         [0037]      FIG. 12  shows the eighth embodiment of the present invention, which capacitor charger  900  is a modification of the charger  800  shown in  FIG. 11  with one terminal of the secondary coil L 3  connected to the input V bat  of the primary coil L 1  and a servo amplifier  818  connected between the resistors R 1  and R 2 . The servo amplifier  818  has an operational amplifier  824  with its two inputs connected to the primary coil voltage V bat  and a servo node  822 , respectively, for the servo node  822  to be at the primary coil voltage V bat , and a transistor  820  connected between the servo node  822  and a feedback node V FB  with its gate connected with the output of the operational amplifier  824 . Since the servo voltage on the servo node  822  is V bat  and the secondary coil L 3  is also connected to V bat , the voltage drop across the resistor R 2  is the secondary coil voltage VL 3 , by which a sense current I 3  is generated to provide to the feedback node V FB  through the transistor  820 , thereby generating the feedback signal V FB  by the transistor R 1  for the integrated circuit  808  that has a comparator  816  to compare the feedback signal V FB  with a reference Vref to generate a comparison signal S for the control circuit  812  to stop charging the capacitor C O  when the capacitor voltage Vout reaches a predetermined value. Likewise, a diode  804  is connected between the coil L 2  and output Vout to prevent an inverse current flowing from the output Vout to the transformer  802 .  
         [0038]     When the transistor  810  conducts a current I 1 , the voltage V L   3  has a negative value, and the transistor  820  is therefore turned off, and the feedback signal V FB  is equal to zero. When the transistor  810  is turned off, the capacitor C O  is charged by a current I 2 , and the charging voltage is 
 
 V   L   2 = Vout+V   f ,  [EQ-23]
 
 where V f  is the forward bias of the diode  804 . Due to the turns ratio of the coils L 2  and L 3  is N S   1 :N S   2 , it has  
                 V   L     ⁢   3     =         V   L2     ×     Ns2   Ns1       +       V   bat     .               [     EQ   ⁢     -     ⁢   24     ]             
 
 Since the servo voltage on the servo node  822  is maintained at V bat  by the servo amplifier  818 , the sense current flowing through the resistor R 2  is  
               I3   =       V   L3     R2       ,           [     EQ   ⁢     -     ⁢   25     ]             
 
 and the feedback signal will be  
               V   FB     =       I3   ×   R1     =       R1   R2     ⁢       V   L3     .                 [     EQ   ⁢     -     ⁢   26     ]             
 
 Combined with the equation EQ-24, it is obtained  
               V   FB     =       R1   R2     ×     (         V   L2     ×     Ns2   Ns1       +     V   bat       )               [     EQ   ⁢     -     ⁢   27     ]             
 
 From the equation EQ-27, it is shown that the feedback signal V FB  is proportional to the secondary coil voltage V L   2 . Likewise, there is no leakage consideration resulted from the voltage sense apparatus for the capacitor C O  since the voltage sense apparatus is coupled to the coil L 3  to sense the capacitor voltage Vout. 
 
         [0039]     Briefly, the leakage from the charged capacitor through the voltage sense apparatus is prevented either by a rectifier circuit such as a diode inserted between the capacitor and voltage sense apparatus, or by a second secondary coil to remove the voltage sense apparatus from direct connection to the first secondary coil to charge the capacitor. The effect to the operation resulted from the power exhaustion of battery is further eliminated.  
         [0040]     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.