Patent Publication Number: US-6982882-B2

Title: High voltage charging circuit for charging a high voltage capacitor

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a high voltage charging circuit and, more particularly, to a high voltage charging circuit for charging a high voltage capacitor. 
   2. Description of the Related Art 
   For cameras, when a user needs to take a picture in insufficient light, the camera utilizes a flash to provide extra light to obtain better exposure effects. However, a working voltage (such as 300 volts) of the flash is much higher than a voltage (such as 5 volts) of a direct current used in the camera; therefore, a high voltage charging circuit is utilized to raise the voltage of the direct current by using a transformer with a high transformation ratio to charge a capacitor. When a voltage on the capacitor reaches the working voltage of the flash, the capacitor can be used to provide current to the flash. 
   As shown in  FIG. 1 , a conventional high voltage charging circuit  10 , also known as a ring chock converter (RCC), comprises: a direct current power source  12  (for example, a 5 volt supply), three resistors  14 ,  16 , and  30 , a power transistor  18 , a transformer  20 , a diode  22 , a high voltage capacitor  24  (for example, a 300 volt capacitor), a Zener diode  26  (with, for example, a breakdown voltage of 300 volts), a capacitor  28 , and an idle control circuit  32 . The transformer  20  has a primary side winding N 1 , a secondary side winding N 2 , and an auxiliary winding N 3 . The primary side winding N 1  and the auxiliary winding N 3  induct with the secondary side winding N 2 ; the primary side winding N 1  and the secondary side winding N 2  are oppositely polarized, and the number of coils on the secondary side winding N 2  is N times the number of coils on the primary side winding N 1  (e.g. 60 times). 
   When the direct current power source  12  provides current to the transformer  20 , the resistor  14  and the primary side winding N 1  are inductive with each other, the power transistor  18  is in a saturated state, and the auxiliary winding N 3  and the resistor  14  are inductive with each other. A current passes through the primary side winding N 1 , which is a magnetically induced current that stores its energy in the transformer  20  instead of charging the high voltage capacitor  24 . 
   When the current passing through the resistor  14  increases, the power transistor  18  moves to an active state, which reduces the current passing through the primary side winding N 1 , and so the polarities of the primary side winding N 1  and the auxiliary winding N 3  are reversed. Then, the power transistor  18  transitions into a cut-off state, and the secondary side winding N 2  and the diode  22  become inductive with each other to perform a charging process on the high voltage capacitor  24  with a charging current. When the secondary side winding N 2  transfers the energy stored in the transformer  20  to the high voltage capacitor  24 , the primary side winding N 1  goes back to its original state, and becomes inductive with the resistor again. 
   When the voltage on the high voltage capacitor  24  reaches a predetermined value (for example, 300 volts), this voltage will cause the Zener diode  26  to breakdown and short, which activates the idle control circuit  32  to stop the transformer  20  from charging the high voltage capacitor  24 . 
     FIG. 2  is a graph of a charging current with time. As shown in  FIG. 2 , only when the charging current drops back to zero, the transformer  20  starts to provide the charging current to the high voltage capacitor  24  for charging. 
   Accordingly, the conventional high voltage charging circuit  10  has the following disadvantages: 
   (1) The power transistor  18  is a BJT, which needs a base-polar driving current, and has a saturation voltage V CE  of about 300 mV. Therefore, the power transistor  18  itself wastes a lot of energy and thus leads to a poor charging efficiency of the high voltage charging circuit  10 . 
   (2) Since the transformer  20  needs the auxiliary winding N 3 , and as the switching frequency of the power transistor  18  is usually 100 kHz, the size of the transformer  20  is not easy to minimize. 
   (3) Since the conventional high voltage charging circuit  10  does not operate under a continuous conduction mode, it has a low efficiency. 
   Therefore, it is desirable to provide a high voltage charging circuit for charging a high voltage capacitor to mitigate and/or obviate the aforementioned problems. 
   SUMMARY OF THE INVENTION 
   A main objective of the present invention is to provide a high voltage charging circuit that can provide fast charging. 
   Another objective of the present invention is to provide a high voltage charging circuit that can reduce the size of the transformer. 
   In order to achieve the above-mentioned objectives of the present invention, the circuit comprises: a direct current power source for outputting a large current with a low voltage value; a transformer for transforming the large current with a low voltage value into a small current with a high voltage value and outputting the high voltage value; a diode for inputting the small current with a high voltage value and then outputting the small current with a high voltage value to the high voltage capacitor to charge the high voltage capacitor; a power transistor; wherein when the power transistor is turned on, the transformer transforms the large current with a low voltage value but does not output the large current with a low voltage value; when the power transistor is turned off, the transformer stops transforming the large current with a low voltage value and outputs the large current with a low voltage value; an activation control circuit for controlling an ON time period of the power transistor; and an inactivation control circuit for controlling an OFF time period of the power transistor; wherein the activation control circuit and the inactivation control circuit control the ON time period or the OFF time period of the power transistor by controlling a positive output signal from a positive end of a flip flop. 
   Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a circuit diagram of a conventional high voltage charging circuit  10 ; 
       FIG. 2  is a graph of a charging current versus time; 
       FIG. 3  is a circuit diagram of a high voltage charging circuit in a first embodiment according to the present invention; 
       FIG. 4  is a graph of a primary side current, a secondary side current, a charging current and a voltage value of a high voltage capacitor under a continuous charging mode; 
       FIG. 5  is a graph of a primary side current, a secondary side current, a charging current and a voltage value of a high voltage capacitor under a boundary charging mode; and 
       FIG. 6  is a circuit diagram of a high voltage charging circuit in a second embodiment according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Two embodiments are disclosed in the following detailed description. 
   First Embodiment: 
   Please refer to  FIG. 3 . In this first embodiment, a high voltage charging circuit  40  comprises three circuits: an inactivation control circuit  50 , an activation control circuit  60  and an idle detection circuit  70 , as well as certain other electronic elements. The activation control circuit  60  outputs a reset signal with a high voltage value so that a positive output signal Q from the positive end of a flip-flop  46  has a high voltage value to control an ON time period for a power transistor  44 . The inactivation control circuit  50  outputs a setting signal with a high voltage value so that a positive output signal Q from the positive end of the flip-flop  46  has a low voltage value to control an OFF time period for the power transistor  44 . The idle detection circuit  70  is used for detecting a voltage value on a charging capacitor  36 , and if the voltage value on the charging capacitor  36  reaches a predetermined voltage value (for example, 300 volts), the idle detection circuit  70  outputs a detection signal to activate an idle control circuit  32  to stop the high voltage charging circuit  40 . Subsequently, the high voltage charging circuit  40  stops charging a high voltage capacitor  24 . An inductance of a primary side winding N 1  of a transformer  42  is L p . An inductance of a secondary side winding N 2  is L S . The number of coils on the secondary side winding N 2  is N times the number of coils on the primary side winding N 1 . 
   After the high voltage charging circuit  40  is switched on, a current of secondary side is Is, and the voltage at two ends of the resistor  52  is zero; since a comparer  52  outputs a setting signal with high voltage value, the setting signal sent into the flip-flop  46  has a high voltage value, and the positive output signal Q from the positive end of the flip-flop  46  has a high voltage value. After being driven by a driver  48 , the power transistor  44  is conductive (in a saturated status). By utilizing a current mirror  62 , a charging current begins charging an internal capacitor  66  in the activation control circuit  60 , and the charging current is equal to the current passing through resistor  64 . The capacitor  66  is charged from zero volts. While the voltage across the capacitor  66  is less than a voltage V 3 , the power transistor  44  is conductive; when the voltage across the capacitor  66  exceeds the voltage V 3 , a comparer  69  outputs a reset signal with a high voltage value to the reset signal end of the flip-flop  46 , and so the positive output signal Q from the positive end of the flip-flop  46  has a low voltage value. After being driven by a driver  48 , the power transistor  44  is cutoff (in a cutoff state). Hence, the activation control circuit  60  controls the ON time of the power transistor  44  by controlling a charging time of the capacitor  66 . When the power transistor  44  is in the conductive state, the direct current power source  12  provides a primary side current I P  to the transformer  42 . Due to the polarity of the secondary side of the transformer  42 , the diode  28  does not conduct. Therefore, the energy generated by the primary side current I P  is stored as magnetic energy in the transformer  42 . A maximum value I P,max  of the primary side current I P  is represented as: 
               I     P   ,   max       =           V     i   ⁢           ⁢   n       -     V   ds         L   P       ×     t   ON               (   1   )             
 
   wherein V in  is an input voltage provided by the direct current power source  12 , and V ds  is a voltage difference of the power transistor  44 . When the power transistor  44  conducts, V ds  is so small that it can be ignored; so the formula (1) can be simplified as: 
               I     P   ,   max       =         V     i   ⁢           ⁢   n         L   P       ×     t   ON               (   2   )             
 
   wherein V in /L P  is a current increasing slope of the primary side current I P , and t ON  is an ON time period of the power transistor  44  (which is identical with an ON time of the primary side winding N 1 , also known as an active time). t ON  can be determined by the resistance of the resistor  64  and the capacitance of the capacitor  66 , and a value of the charging current for the capacitor  66  is given by a voltage V 1  divided by the resistance of the resistor  64 . The period t ON  can be changed by adjusting the resistance of the resistor  64 , which further changes the maximum value I P,max  of the primary side current I P . Furthermore, the period t ON  can also be changed by having different charging currents. When a user selects the resistance of the resistor  64 , the period t ON  is thereby fixed as well; according to the formula (2), the smaller V in  is, the smaller the maximum value of I P,max  for the primary side current I P  is, too. When V in  is low, the charging current is low too, and so the high voltage charging circuit  40  can provide different charging modes with different charging currents to increase the life time of the direct current power source  12  with a lower V in . Furthermore, in accordance with formula (2), the user can select a smaller Lp to keep constant the maximum value I P,max  for the primary side current I P . Therefore, the period t ON  can be made smaller as well, which means that a transformer  14  with a smaller volume can be utilized to reduce the entire size of the high voltage charging circuit  40 . 
   When the positive output signal Q from the positive end of the flip-flop  46  has a low voltage value, a negative end of the flip-flop  46  outputs a reserved output signal ˜Q with a high voltage value and provides a discharge path by causing the transistor  68  to conduct to remove the energy stored in the capacitor  66 . 
   Accordingly, the capacitor  41  is used as a voltage stabilized capacitor. The power transistor  44  is preferably a N-type field effect transistor, and the gate is connected to the driver  48 . This provides the advantages of a fast reaction speed and a lower conductive resistance (R DS ON ). Of course, the power transistor  44  can also be a P-type field effect transistor, and the gate would then be connected to an inverted driver. 
   In addition, a counter  67  increments a value by one every microsecond (μS). When the value of the counter  67  exceeds a predetermined value (such as 10), the counter  67  outputs a high voltage value to the reset end of the flip-flop  46  and clears the value. According to the value of the counter  67 , the ON time period controlled by the active control circuit  60  is limited, and so the primary side current I P  is also limited to prevent from increasing when the resistor  64  is open. 
   During the ON time, due to the polarity of the secondary side of the transformer  42 , the diode  28  does not conduct. Therefore, the secondary side current Is of the transformer  42  is zero, and the voltage across the resistor  52  is zero, too. When the voltage across the capacitor  66  exceeds the voltage V 3 , the flip flop  46  turns off the power transistor  44  (placing it in the cutoff state). After the power transistor  44  is in the cutoff state, the excited-magnetic current of the transformer  42  conducts with the diode  28 , and the excited-magnetic energy stored in the transformer  42  charges the high voltage capacitor  24 . Therefore, the secondary side current Is of the transformer  42  has a decreasing current. A decreasing rate of the secondary side current Is is equal to Vout (the high voltage value of the high voltage capacitor  24 ) divided by L S ; Ls is the inductance of the secondary side of the transformer  42 . Since Vout is slowly increasing during the entire charging process, the decreasing rate of the secondary side current Is will change. When the voltage across the resistor  52  exceeds a voltage V 2 , the comparer  54  outputs a setting signal with a low voltage value to a setting signal end of the flip flop  46 , and the power transistor  44  is placed in the cutoff state. The inactivation control circuit  50  keeps the power transistor  44  in the cutoff state, until the voltage across the resistor  52  is less than the voltage V 2 ; the comparer  54  outputs a setting signal with a high voltage value to the setting signal end of the flip flop  46 . Then, the driver  48  causes the power transistor  44  to conduct. This period of non-conduction time of the power transistor  44  caused by the inactivation control circuit  50  is called the OFF time. 
   The maximum value I S,max  of the secondary side current I S  can be represented by the following: 
               I     S   ,   max       =         N   1       N   2       ×     I     P   ,   max                 (   3   )             
 
   When the secondary side current I S  is equal to the voltage V 2  divided by the resistance of the resistor  52  (the secondary side current I S  is given as I min /N, where I min  is a minimum current), the comparer  54  outputs a setting signal with a high voltage value to the setting signal end of the flip flop  46 , and the power transistor  44  is in a saturated state, which causes the primary side winding N 1  to conduct and the primary side current I P  starts from the minimum current I min . By repeating the above-mentioned steps, the transformer  42  can keep charging the high voltage capacitor  24  until the voltage value of the high voltage capacitor  24  reaches the desired 300 volts. 
   Concerning the idle detection circuit  70 , when the voltage value of the high voltage capacitor  24  reaches 300 volts, which causes the Zener diode  26  to break down and charges the capacitor  28  until the voltage drop on the capacitor  28  is larger than the voltage V 3 , then, the comparer  72  outputs a detection signal to the idle control circuit  32  to stop the high voltage charging circuit  40 . 
   Since the minimum current I min  can be set up by the user, for example, when the minimum current I min  is larger than zero, the high voltage charging circuit  40  continuously charges the high voltage capacitor  24 , which is called a continuous conduction charging mode.  FIG. 4  shows the differences of the primary side current I P , secondary side current I S , and Vout during the times when the power transistor  44  is on and off. Since the high voltage charging circuit  40  continuously charges the high voltage capacitor  24 , a charging efficiency of the high voltage charging circuit  40  is higher than the prior art charging efficiency. Furthermore, the minimum current I min  can be set to zero, and the high voltage charging circuit  40  can continuously/discontinuously charge the high voltage capacitor  24 , which is called a boundary charging mode. Please refer to  FIG. 5 .  FIG. 5  is a graph of a primary side current, a secondary side current, a charging current and a voltage value of a high voltage capacitor under a boundary charging mode. 
   Second Embodiment: 
   A high voltage charging circuit  80  in the second embodiment of the present invention is shown in  FIG. 6 , which has a structure similar to the high voltage charging circuit  40 . The high voltage charging circuit  80  comprises the inactivation control circuit  50 , an activation control circuit  65  for replacing the activation control circuit  60  and the idle detection circuit  70 . A main difference between the high voltage charging circuit  80  and the high voltage charging circuit  40  is that the current mirror  62  of the activation control circuit  60  is replaced by a current mirror  63  of the activation control circuit  65 . The inactivation control circuit  50  and the idle detection circuit  70  operate in a manner similar to the first embodiment, and so require no further description. The charging current for the capacitor  66  is equal to the input voltage V in  divided by the resistance of the resistor  64 . Therefore, the higher the input voltage V in  is, the faster the capacitor  66  will be fully charged, and the shorter the period t ON  is. Accordingly, the time period t ON  is determined by the resistor  64  and the capacitor  66 . On the other hand, the lower the input voltage V in  is, the slower the capacitor  66  will be fully charged, and the longer the time period t ON  is; in other words, a value of Vin*t ON  can be a constant value. 
   Therefore, the user can change the value of Vin*t ON  by changing the resistance of the resistor  64 , to change the maximum value I P,max  of the primary side current I P . When the resistance of the resistor  64  is fixed, the value of Vin*t ON  is fixed as well. According to formula (2), the maximum value I P,max  of the primary side current I P  will not be changed by the input voltage V in , and so the high voltage charging circuit  80  in the second embodiment of the present invention can provide a almost constant charging current. 
   Accordingly, the high voltage charging circuits of the present invention have the following advantages: 
   (1) There is no need for the auxiliary winding N 3  of the prior art, and a smaller inductance Lp of the primary side winding N 1  will lead to a smaller t ON  value, so a small transformer  14  can be utilized to reduce the size of the high voltage charging circuit. 
   (2) The high voltage charging circuit can be operated in the continuous conduction charging mode, which makes for a higher power transfer efficiency and a shorter charging time. 
   (3) The high voltage charging circuit can provide a constant current (the charging current will not change with different input voltages V in ) control or a variable current (the charging current will change with different input voltages V in ) control, which the user may select. 
   Although the present invention has been explained in relation to its preferred embodiment, it is to be understood that many other possible modifications and variations can be made without departing from the spirit and scope of the invention as hereinafter claimed.