Abstract:
An input power stabilizing circuit, adapted to be disposed between a DC power source and a load to which DC power from the source is to be supplied. Voltage on a capacitor in the input power stabilizing circuit is prevented as much as possible from causing reverse current to flow when the DC power source develops a short circuit momentarily. A current detecting unit detects reverse current from the capacitor in an input power supply line, and a current interrupting unit interrupts a current line connecting the load with the DC power source. When reverse current flow is detected, the current line is interrupted by the current detecting unit and the current interrupting unit. The reverse current from the capacitor is held to a minimum value in the input power stabilizing circuit.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to an input power stabilizing circuit, in which a reverse current, discharged from a capacitor in the input power stabilizing circuit, is prevented as much as possible, at the time when an input power supply line has some trouble, such as a momentary short circuit. 
   DESCRIPTION OF THE RELATED ART 
   Recently, the power consumption of apparatuses has been increased, and solutions have been required when troubles occur by outside causes. One of the troubles may be that an input power supply line becomes a short circuit momentarily. A diode is generally used in an input power stabilizing circuit as a solution to prevent a reverse current from flowing in the input power stabilizing circuit at the time when the input power supply line has some trouble such as a momentary short circuit. 
     FIG. 1  is a circuit diagram in which a conventional input power stabilizing circuit is connected to a direct current (DC) power source and a load. As shown in  FIG. 1 , a DC power source  1  is connected to a load  2  via an input power stabilizing circuit  15 . The input power stabilizing circuit  15  includes a capacitor  3 , an inductor  16 , and a diode  14 . A DC voltage from the DC power source  1  is charged on the capacitor  3 , and the capacitor  3  supplies the charged voltage to the load  2  when impedance in the load  2  is changed. The inductor  16  filters out noise from the current inputted from the DC power source  1 . If the input power supply line between terminals “a” and “b” becomes a short circuit, the diode  14  prevents a reverse current from the “b” terminal “b” to the “d” terminal “d”, which was discharged from the capacitor  3 . 
   In an input power stabilizing circuit using a diode, when a large current flows, heating caused by the large current becomes a big problem. This problem is explained by using the circuit diagram shown in FIG.  1 . Since a current always flows in the diode  14 , electric power, being the product of forward voltage and forward current of the diode  14 , is always consumed. This electric power is wasteful and also causes a large amount of heating in the input power stabilizing circuit  15  when the load  2  draws a large current. Consequently, there is a problem that a structure for dissipating the heat in the input power stabilizing circuit  15  must have a large size, and its manufacturing cost becomes high. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the present invention to provide an input power stabilizing circuit, to be disposed between a DC power source and an apparatus that is powered by the DC power from the source, that can prevent a reverse current from flowing from a capacitor in the input power stabilizing circuit as much as possible when the DC power source has some trouble, such as the DC input terminals developing a short circuit momentarily, and that also can reduce the power consumption of the input power stabilizing circuit as much as possible at the normal operating time of the input power stabilizing circuit. Moreover, the input power stabilizing circuit of the present invention significantly reduces the amount of heat generated in the circuit, and its heat radiating structure is of a small size, and also its manufacturing cost is made to be low. 
   According to a first aspect of the present invention, for achieving the object mentioned above, there is provided an input power stabilizing circuit, which is to be disposed between a DC power source and a load to which the DC power from the source is supplied. The input power stabilizing circuit includes a voltage storing means which stores a voltage supplied from the DC power source, supplies the stored voltage to the load when a trouble has occurred at the side of the DC power source, and stabilizes the voltage input to the load, a current detecting means which detects a reverse current from the voltage storing means in an input power supply line through which a current from the DC power source to the load is supplied, and a current interrupting means which interrupts a current line connecting the load with the DC power source, when the reverse current has been detected by the current detecting means. 
   According to a second aspect of the present invention, in the first aspect, the current detecting means includes a current transformer whose primary side is connected to the input power supply line, and a first transistor whose gate is made to be on or off by a voltage generated at the secondary side of the current transformer. 
   According to a third aspect of the present invention, in the second aspect, the current detecting means further provides a diode which rectifies the voltage generated at the secondary side of the current transformer, a capacitor which smoothes the voltage rectified at the diode, and a first voltage divider which divides the voltage smoothed at the capacitor and takes out a first designated voltage. The first transistor becomes “on” when the voltage divided at the first voltage divider is applied to the gate of the first transistor, and this “on” state of the first transistor is applied to the current interrupting means. 
   According to a fourth aspect of the present invention, in the third aspect, the current interrupting means includes a second voltage divider which divides the voltage inputted from the DC power source and takes out a second designated voltage, and a second transistor which is disposed in the current line connecting the load with the DC power source. The gate of the second transistor is made to be on or off by the voltage taken out from the second voltage divider. The second transistor is made to be “off” when the voltage from the second voltage divider has not been applied to the gate of the second transistor, and the current line is interrupted. 
   According to a fifth aspect of the present invention, in the fourth aspect, the first transistor short circuits the ends of one of resistors of which the second voltage divider is composed, and stops applying the voltage from the second voltage divider to the gate of the second transistor. 
   According to a sixth aspect of the present invention, in the first aspect, the input power stabilizing circuit further includes an inductor which delays the reverse current from the voltage storing means in the input power supply line. 
   According to a seventh aspect of the present invention, in the second aspect, the current detecting means includes plural current transformers in the input power supply line in parallel, and detects the reverse current from the voltage storing means. 
   According to an eighth aspect of the present invention, in the second aspect, the current detecting means includes plural current transformers in the input supply line in series, and detects the reverse current from the voltage storing means. 
   According to a ninth aspect of the present invention, in the first aspect, the voltage storing means discharges the reverse current to the input power supply line when the output terminals of the DC power source became a short circuit. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The objects are features of the present invention will become more apparent from the consideration of the following detailed description taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a circuit diagram in which a conventional input power stabilizing circuit is connected to a DC power source and a load; 
       FIG. 2  is a circuit diagram in which an input power stabilizing circuit in accordance with a first embodiment of the present inventions is connected to a DC power source and a load; 
       FIG. 3  is a circuit diagram showing a reverse current detecting section in an input power stabilizing circuit in accordance with a second embodiment of the present invention; and 
       FIG. 4  is a circuit diagram showing a reverse current detecting section in an input power stabilizing circuit in accordance with a third embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now to the drawings, embodiments of the present invention are explained in detail. In the embodiments of the present invention, the reference number of each function has the same number that the conventional example has, when the function in the embodiments of the present invention is almost the same as the conventional example. The input power stabilizing circuit in the embodiments of the present invention has also the same reference numbers, but its structure is different from that of the conventional example. 
     FIG. 2  is a circuit diagram in which an input power stabilizing circuit in accordance with a first embodiment of the present inventions is connected to a DC power source and a load. As shown in  FIG. 2 , a DC power source  1  is connected to a load  2  via an input power stabilizing circuit  15 . The input power stabilizing circuit  15  includes a first capacitor  3 , a current transformer  4 , first and second transistors  5  and  6 , a second capacitor  7 , a diode  8 , first, second, third, fourth, and fifth resistors  9 , 10 , 11 , 12 , and  13 , an inductor  16 , and a parasitic diode  17 . 
   The first capacitor  3  stores a DC voltage from the DC power source  1 , and supplies the stored voltage to the load  2  when the impedance in the load  2  is changed. 
   The current transformer  4  detects a reverse current in the input power supply line. Since a current I 1  is generally flowing from the “a” terminal to the “c” terminal at the primary side of the current transformer  4 , the current transformer  4  is in a magnetic saturation state. Consequently, a current is not generated at the secondary side of the current transformer  4 . 
   The diode  8  rectifies a voltage generated at the secondary side of the current transformer  4 . The second capacitor  7  holds the peak value of the voltage rectified at the diode  8  and smoothes the voltage. 
   The second resistor  10  and the third resistor  11  are connected in series, and work as a first voltage divider. A voltage that is a desired ratio of the voltage rectified and smoothed at the diode  8  and the second capacitor  7  is taken out at the tap between the second resistor  10  and the third resistor  11 . 
   The voltage divided at the first voltage divider (the second and third resistors  10  and  11 ) is applied to the gate of the first transistor  5 . Since a voltage is not being applied to the gate of the first transistor  5  during normal operation, the first transistor  5  is kept in an “off” state. 
   The fourth resistor  12  and the fifth resistor  13  are connected in series, and work as a second voltage divider. A voltage that is a desired ratio of the voltage Vin is taken out at the tap between the fourth resistor  12  and the fifth resistor  13 . 
   The voltage divided at the second voltage divider (the fourth and fifth resistors  12  and  13 ) is applied to the gate of the second transistor  6 . Since a voltage is being applied to the gate of the second transistor  6  during normal operation, the second transistor  6  is kept in an “on” state. 
   The inductor  16  has an inductance value equivalent to the inductance value from the terminal “a” to the terminal “c”, and filters out noise from the current inputted from the DC power source  1 . The inductor  16  also works to delay the reverse current from the first capacitor  3 . Therefore, the characteristics of the inductor  16  are based on the time from when the current transformer  4  has detected the reverse current to when the second transistor  6  is made to be “off”. 
   The parasitic diode  17  works to supply the inputted voltage to the load  2 , irrespective of whether the second transistor  6  is “on” or “off”. However, during normal operation, the inputted current flows through the second transistor  6  and does not flow through the parasitic diode  17 . 
   In the embodiments of the present invention, a field effect transistor (FET) is used for both of the first and second transistors  5  and  6 , for the following reason. It is desirable that the time, from when the current transformer  4  detects the reverse current to when the second transistor  6  is made to be “off” is as close to zero possible, in order to make the reverse current discharged from the first capacitor  3  be a minimum value. Therefore, in order to make the time be the minimum value, the FET is the most suitable, because the FET can realize a high speed switching by controlling the voltage of the gate. However, the first and second transistors  5  and  6  are not limited to a FET; a bipolar transistor can be used for the first and second transistors  5  and  6 . 
   Next, operation of the input power stabilizing circuit of the first embodiment of the present invention is explained. A case in which the input terminals “a” and “b” of the input power stabilizing circuit  15  became a short circuit momentarily is explained. 
   First, when the terminals “a” and “b” become a short circuit, electric charge stored on the first capacitor  3  starts to flow to the input side terminal “a” via the current transformer  4 . 
   During the time for the current transformer  4  to reach a magnetic saturation state in the reverse direction, a voltage of Ve is generated by the first resistor  9  at the secondary side of the current transformer  4  at the tap “e”, “e” due to this reverse current from the first capacitor  3 . This generated voltage Ve is rectified by the diode  8  and is smoothed at the second capacitor  7 . This smoothed voltage is divided by the first voltage divider (the second and third resistors  10  and  11 ), and the divided voltage is applied to the gate of the first transistor  5 . When the first transistor  5  becomes “on” due to the gate voltage, the ends of the fifth resistor  13  are short circuited. 
   The input voltage Vin, divided by the second voltage divider (the fourth and fifth resistors  12  and  13 ), has been applied to the gate of the second transistor  6 . When the ends of the fifth resistor  13  are short circuited by the first transistor  5 , the gate voltage of the second transistor  6  becomes zero, and the second transistor  6  becomes “off”. 
   By the operation mentioned above, the reverse current from the terminal “b” to the terminal “d” is prevented by the “off” state of the second transistor  6 , and the discharge from the first capacitor  3  becomes small enough. 
   The length of time required for the current transformer  4  to reach the magnetic saturation state in the reverse direction, depends on the characteristics of the inductor  16 . When the short circuit has been corrected, that is, the input voltage has recovered, the power is supplied to the load  2  via the parasitic diode  17 , irrespective of the “on” or “off” state of the second transistor  6 . However, soon after the recovery, the second transistor  6  becomes “on” “on,” and current flows through the second transistor  6 . 
   Further, when a reverse voltage is applied to the input power stabilizing circuit  15 , caused by an error in wiring at the input power supply line, the gate of the second transistor  6  is reverse biased and keeps transistor  6  in the “off” state. Therefore, the input power stabilizing circuit  15  also works as a protection circuit for a circuit following the input power stabilizing circuit  15 . 
   Next, referring to the drawing, a second embodiment of the present invention is explained.  FIG. 3  is a circuit diagram showing a reverse current detecting section in an input power stabilizing circuit of the second embodiment of the present invention. As shown in  FIG. 3 , in the second embodiment, plural current transformers are provided in the input power stabilizing circuit. That is, in the second embodiment, a current transformer  4  and a current transformer  4 ′ are provided. The primary sides of the current transformers  4  and  4 ′ are connected in parallel, and the secondary sides of the current transformers  4  and  4 ′ are connected in series. 
   In the second embodiment of the present invention, a large current can be detected, and even when the reverse current flowing in the current transformers  4  and  4 ′ is unequal, the reverse current can be detected by the total amount of the reverse currents. 
   Next, referring to the drawing, a third embodiment of the present invention is explained.  FIG. 4  is a circuit diagram showing a reverse current detecting section in an input power stabilizing circuit of the third embodiment of the present invention. As shown in  FIG. 4 , at the third embodiment, plural current transformers are also provided in the input power stabilizing circuit. That is, in the third embodiment, a current transformer  4  and a current transformer  4 ′ are provided, as in the second embodiment. However, the connection between the current transformers  4  and  4 ′ is different from the second embodiment. That is, the primary sides of the current transformers  4  and  4 ′ are connected in series, and the secondary sides of the current transformers  4  and  4 ′ are connected in series. 
   In the third embodiment of the present invention, since the primary sides of the current transformers  4  and  4 ′ are connected in series, the detecting voltage can be twice as great as in the first and second embodiments, and the current sensitivity can easily be made to be high. With this embodiment, even when the reverse current discharged from the first capacitor  3  is very small, the voltage applied to the gate of the first transistor  5  is of a high value, within the operation characteristics of the first transistor  5 , controlling the on/off state of the first transistor  5 . 
   As mentioned above, according to the present invention, when a reverse current is detected in the input power supply line, the current line is made to be “off”. With this, a discharge from a capacitor in the input power stabilizing circuit, caused by, for example, a momentary short circuit of the input power supply line, can be made to be a minimum value. 
   In the conventional input power stabilizing circuit used a diode, since current always flowing in the diode, electric power being the product of forward voltage and forward current of the diode is always consumed. This electric power is wasteful and causes significant heating in the input power stabilizing circuit when the load current is large. Consequently, there is a problem that a structure for radiating the heat must be of a large size, and its manufacturing cost becomes high. However, according to the present invention, the loss is only caused by “on” resistance of the second transistor, and in the normal operating state a current does not flow through a parasitic diode of the second transistor. Therefore, the amount of heating is significantly reduced, compared with the case in which a diode was used, as mentioned at the conventional example, and its heat radiating structure can be small, and the manufacturing cost is low in the present invention. 
   While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by those embodiments. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention.