Patent Publication Number: US-2017364116-A1

Title: Discharge circuit for diode reverse leakage current

Description:
FIELD OF THE INVENTION 
     The present invention relates to a discharge circuit for a diode reverse leakage current, and more particularly to a discharge circuit capable of reducing loss. 
     BACKGROUND OF THE INVENTION 
     In the DC/DC converter design, the input terminal may meet the specification of “Reverse Reverse Protection”; therefore, an active switch such as a BJT, MOSFET or even a relay is used to perform input turn off for protecting. However, to achieve this protection, the additional cost is quite high. So in the design of the premise of the economy, usually the Schottky diode is used to achieve the aforementioned protection. Schottky diode has some advantages such as low bias voltage, easy to use and quick response; therefore, Schottky diode is widely used in the input-terminal reverse protection. As shown in  FIG. 1 , by reversely coupling the two diodes D 1  and D 2  with the terminals A and B, respectively, the output terminals C and D can be completely isolated thereby achieving the protection. 
     However, the approach of  FIG. 1  has a drawback. Because the Schottky diode has a certain reverse leakage current I R  which is usually tens to hundreds of uA, so when voltages across the terminals C and D exists, the reverse leakage current I R  would continue to charge across the terminals A and B to maintain at a high voltage if the input voltage is removed, which is not allowed in some applications. 
     To solve the above problem, a discharge resistor may be added between the terminals A and B to form a current path for the reverse leakage current I R  as shown in  FIG. 2 . Because the voltage across the terminals A and B is V AB =I R ×R 1 , therefore the voltage across the terminals A and B can be limited within a certain range. However, in some applications, it is necessary to limit the voltage across the terminals A and B to a very low voltage level (less than 1 to 2 V), so a low resistance value of the resistor R 1 , such as several KΩ to 10KΩ, is required to limit the voltage across the terminals A and B. But when the circuit is in the normal operation, a high voltage may be across the terminals A and B; therefore, when the resistor R 1  is continuously coupled between the terminals A and B, an unnecessary but great power loss up to 1 W or more may occur across the resistor R 1  according to P=V2/R. 
     SUMMARY OF THE INVENTION 
     In order to achieve the aforementioned or other objectives, the present invention provides a discharge circuit for a diode reverse leakage current. The discharge circuit includes an input positive terminal, an input negative terminal, an output positive terminal, an output negative terminal, a first diode and a current source device. The output positive terminal is coupled to the input positive terminal. The output negative terminal is coupled to the input negative terminal. The first diode has an anode terminal coupled to the input positive terminal and a cathode terminal coupled to the output positive terminal. The current source device has a first end coupled to the anode terminal of the first diode and a second end coupled between the input negative terminal and the output negative terminal. When the input voltage is turned off, since the current of the current source device may be larger than the diode reverse leakage current, the voltage difference across the current source device may be reduced by less than about 1 V. The current source device continues to operate when the input voltage is in normal operation, and the loss would be proportional to the input voltage due to P=VI and the objective of low loss is achieved accordingly. 
     The present invention further provides a discharge circuit for a diode reverse leakage current. The discharge circuit includes an input positive terminal, an input negative terminal, an output positive terminal, an output negative terminal, a first diode and a current source device. The output positive terminal is coupled to the input positive terminal. The output negative terminal is coupled to the input negative terminal. The first diode has an anode terminal coupled to the output negative terminal and a cathode terminal coupled to the input negative terminal. The current source device has a first end coupled between the input positive terminal and the output positive terminal and a second end coupled to the cathode terminal of the first diode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The present disclosure will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which: 
         FIG. 1  is a schematic circuit diagram in prior art; 
         FIG. 2  is another schematic circuit diagram in prior art; 
         FIG. 3  is a schematic circuit diagram of the first embodiment of the present invention; 
         FIG. 4  is a schematic circuit diagram of the second embodiment of the present invention; 
         FIG. 5  is a schematic circuit diagram of the third embodiment of the present invention; 
         FIG. 6  is a schematic circuit diagram of the fourth embodiment of the present invention; 
         FIG. 7  is a schematic circuit diagram of the fifth embodiment of the present invention; 
         FIG. 8  is a schematic circuit diagram of the sixth embodiment of the present invention; 
         FIG. 9  is a schematic circuit diagram of an implementation of the sixth embodiment; 
         FIG. 10  is a schematic circuit diagram of another implementation of the sixth embodiment; 
         FIG. 11  is a schematic circuit diagram of the seventh embodiment of the present invention; and 
         FIG. 12  is a schematic circuit diagram of the eighth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention will now be described more specifically with reference to the following embodiments. It is to be noted that the following descriptions of preferred embodiments of this invention are presented herein for purpose of illustration and description only. It is not intended to be exhaustive or to be limited to the precise form disclosed. 
     Referring to  FIG. 3 , which is a schematic circuit diagram of the first embodiment of the present invention. As shown in  FIG. 3 , the present invention provides a discharge circuit for a diode reverse leakage current, which includes an input positive terminal  10 , an input negative terminal  11 , an output positive terminal  12 , an output negative terminal  13 , a first diode  20  and a current source device  30 . The output positive terminal  12  is coupled with the input positive terminal  10 ; and the output negative terminal  13  is coupled with the input negative terminal  11 . The anode terminal of the first diode  20  is coupled to the input positive terminal  10  and the cathode terminal thereof is coupled to the output positive terminal  12 . A first end of the current source device  30  is coupled to the anode terminal of the first diode  20  and a second end thereof is coupled between the input negative terminal  11  and the output negative terminal  13 . The current source device  30  may be a constant current diode or other equivalent element, and the present invention is not limited thereto. 
     In the present embodiment, the current source device  30  is disposed between the input positive terminal  10  and the input negative terminal  11 . When the input voltage is turned off, since the current of the current source device  30  may be larger than the reverse leakage current I R  flowing through the first diode  20 , the voltage difference across the current source device  30  may be reduced by less than about 1 V. The current source device  30  continues to operate when the input voltage is in normal operation, and the loss would be proportional to the input voltage due to P=VI; so that when the voltage difference V is at a relatively low level, the loss resulted by the current source device  30  differs from that resulted by a resistor in prior art. 
     Please refer to  FIG. 4 , which is a schematic circuit diagram of the second embodiment of the present invention. As shown in  FIG. 4 , the second embodiment differs from the first embodiment of  FIG. 3  in that the discharge circuit further includes a second diode  21 . The anode terminal of the second diode  21  is coupled to the second end of the current source device  30  and the cathode terminal thereof is coupled between the input negative terminal  11  and the output negative terminal  13 . The second diode  21  is mainly for protecting the current source device  30  from damage. 
     Please refer to  FIGS. 5 and 6 , which are schematic circuit diagrams of the third and fourth embodiments of the present invention, respectively. As shown in  FIGS. 5 and 6 , the third and fourth embodiments differ from the previous first and second embodiments of respective  FIGS. 3 and 4  in that they each further include a third diode  22 . The anode terminal of the third diode  22  is coupled to the output negative terminal  13  and the cathode terminal thereof is coupled to the input negative terminal  11 . The third diode  22  is mainly for protecting the integrated circuit from damage. In the third and fourth embodiments as shown in respective  FIGS. 5 and 6 , the second end of the current source device  30  and the cathode terminal of the second diode  21  are coupled to the cathode terminal of the third diode  22 . However, in other embodiments, the second end of the current source device  30  and the cathode terminal of the second diode  21  may be coupled to the anode terminal of the third diode  22 , and the present invention is not limited thereto. 
     Please refer to  FIG. 7 , which is a schematic circuit diagram of the fifth embodiment of the present invention. As shown in  FIG. 7 , the seventh embodiment differs from the previous second embodiment of  FIG. 4  in that the second diode  21  is replaced by a light-emitting element  23 . The first end of the light-emitting element  23  is coupled to the second end of the current source device  30  and the second end of the light-emitting element  23  is coupled between the input negative terminal  11  and the output negative terminal  13 . The light-emitting element  23  may be an LED. Because needing to be in a constant current state to emit light, the LED may be used to indicate whether or not the current source device  30  is in operation in a protective function state. 
     Please refer to  FIG. 8 , which is a schematic circuit diagram of the sixth embodiment of the present invention. As shown in  FIG. 8 , the sixth embodiment differs from the previous embodiments in that the current source device  30  may be an adjustable circuit; wherein the current of the current source device  30  may be regulated by an external signal S. In  FIG. 8 , the adjustable circuit is a current mirror.  FIG. 9  is a schematic circuit diagram of an implementation of the sixth embodiment. As shown in  FIG. 9 , the current mirror is composed of a double BJT element.  FIG. 10  is a schematic circuit diagram of another implementation of the sixth embodiment. As shown in  FIG. 10 , the current mirror is composed of a double FET element. The aforementioned implementations are for an exemplary purpose only, and the present invention is not limited thereto. 
     Please refer to  FIGS. 11 and 12 , which are schematic circuit diagrams of the seventh and eighth embodiments of the present invention, respectively. As shown in  FIGS. 11 and 12 , the seventh and eighth embodiments differ from the previous first and second embodiments in the position of the first diode  20 , and the other functions are completely identical. As shown in  FIGS. 11 and 12 , the anode terminal of the first diode  20  is coupled to the output negative terminal  13  and the cathode terminal thereof is coupled to the input negative terminal  11 , thereby achieving the same effect as in the first and second embodiments. 
     While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.