Abstract:
A programmable power discharge circuit and a method of discharging power are provided. The programmable power discharge circuit includes a programmable voltage controller, a detect circuit, and a discharge circuit. The programmable voltage controller selects and provides a threshold voltage by a voltage divider including a plurality of impedance components. The detect circuit detects a difference between the threshold voltage and a working voltage to decide whether the working voltage is discharged.

Description:
FIELD OF THE INVENTION 
     The invention is about discharge circuits. To be more specific, the invention is about a programmable quick discharge circuit and a method of discharging power. 
     DESCRIPTION OF RELATED ART 
     In the application of a power supply, a circuit inside a voltage regulator is usually provided with a capacitor having huge capacitance, so as to obtain a nice load regulation rate and low ripple. However, when the power supply is powered off, the output capacitor is equivalent to a parallel huge capacitor, and thus the voltage of the capacitor drops quite slowly. As such, even if the power supply is powered off, the capacitor still continuously outputs a voltage for a long time until the power stored thereon is discharged. Therefore, when the power supply is powered on next time, a power-on reset circuit may not be successfully activated by a control chip inside the circuit due to that the voltage may still remain in the capacitor, thereby resulting in an operation error. 
     In order to discharge the power stored in the capacitor in the circuit inside the voltage regulator when the power supply is powered off, a capacitor discharge circuit is usually employed in the art. With the discharge circuit, once the voltage of the power supply is less than to a threshold voltage, the power stored in the capacitor in the circuit inside the voltage regulator can be discharged through the discharge circuit. However, the conventional discharge circuit only provides a predetermined threshold voltage, such that if the threshold voltage needs to be adjusted, transistors or resistors in the discharge circuit have to be replaced to satisfy the desired threshold voltage. This not only increases the cost, but also brings great inconvenience. 
     Accordingly, how to find a way to provide a programmable quick discharge circuit, which not only quickly discharges the power stored in the capacitor in the circuit inside the voltage regulator when an AC power source is powered off, but also allows a user to conveniently adjust the threshold voltage for activating the quick discharge circuit becomes the objective being pursued by persons skilled in the art. 
     SUMMARY OF THE INVENTION 
     Given abovementioned defects of the prior art, the present invention provides a programmable discharge circuit to conveniently adjust the threshold voltage for activating the quick discharge circuit. 
     In order to achieve abovementioned and other objectives, the present invention provides a programmable power discharge circuit, comprising: a programmable voltage controller, a detect circuit, and a discharge circuit. The programmable voltage controller selects and provides a threshold voltage by a voltage divider including a plurality of impedance components. The detect circuit detects a difference between the threshold voltage and a working voltage to decide whether the working voltage is discharged. The discharge circuit includes a MOS transistor and a resistor-capacitor (RC) circuit, and discharges the working voltage when the working voltage is less than the threshold voltage. 
     In an embodiment, the programmable voltage controller selects and provides a threshold voltage by a voltage divider including a plurality of impedance components. 
     The present invention also provides a method of discharging power, comprising: selecting a threshold voltage, detecting a difference between the threshold voltage and a working voltage, and discharging the working voltage when the working voltage is less than the threshold voltage. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The present invention can be more fully understood by reading the following detailed description of the preferred embodiments, with reference made to the accompanying drawings, wherein: 
         FIG. 1  is a system structure view of a programmable power discharge circuit according to the present invention; 
         FIG. 2  is a scheme view of the programmable power discharge circuit according to an embodiment of the present invention; 
         FIG. 3  is a simulation graph illustrating a simulation result of the programmable power discharge circuit shown in  FIG. 2 ; 
         FIG. 4  is a scheme view of the programmable power discharge circuit according to an embodiment of the present invention; and 
         FIG. 5  is a scheme view of the programmable power discharge circuit according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the following, specific embodiments are provided to illustrate the detailed description of the present invention. Those skilled in the art can easily conceive the other advantages and effects of the present invention, based on the disclosure of the specification. The present invention can also be carried out or applied by other different embodiments. 
     As shown in  FIG. 1 , a system structure view of a programmable power discharge circuit  100  according to the present invention is provided. The programmable power discharge circuit  100  comprises a programmable voltage controller  102 , a detect circuit  104  electrically connected to the programmable voltage controller  102 , and a discharge circuit  106  electrically connected to the detect circuit  104 . The programmable voltage controller  102  has N threshold voltages that can be selected by a user through a selector with N bits D 1  to D N . A selected threshold voltage V r  is then utilized by the detect circuit  104  to compare with a working voltage VCC provided by, for example, a power supply, such that the discharge circuit  106  is activated when it is detected that the working voltage VCC is less than the selected threshold voltage V r . Once the discharge circuit  106  is activated, a discharge path is provided to the working voltage VCC, so as to quickly discharge the working voltage VCC of the power supply until the working voltage VCC drops to 0 volt. 
       FIG. 2  is a scheme view of a programmable power discharge circuit  200  according to an embodiment of the present invention. As shown in  FIG. 2 , the programmable power discharge circuit  200  comprises a programmable voltage controller  202 , a detect circuit  204  and a discharge circuit  206 . In this embodiment, the programmable voltage controller  202  is implemented by a voltage divider, for example. The voltage divider includes three charging paths having a PMOS switch transistor and a group of impedance component(s) such as diode(s) in series and a second RC circuit. In an embodiment, switch transistors M s1  to M s3  are controlled by a selector with three bits D 1  to D 3 , such that only one switch transistor is on to select the group of diode(s) for charging a reference capacitor C r  of the second RC circuit to reach the threshold value. 
     Typically, the voltage drop of one diode is about 0.7 volt, such that the number of the diode(s) can determine the threshold voltage V r . For example, the threshold voltage determined by one diode is approximately VCC-0.7 volt, which should be higher than the threshold value determined by five diodes, which is approximately VCC-3.5 volt, since each diode provides a voltage drop. Also, it should be appreciated that the bits of the selector and corresponding number of the charging paths is not limited to three, and can be modified upon the actual need. In addition, the PMOS transistors employed herewith can be replaced with NMOS transistors and the NMOS transistor can also be replaced with PMOS transistor through a suitable modification, such modification is conceivable to persons skilled in the art and thus is omitted. 
     In an embodiment, the detect circuit includes two PMOS transistors M p1  and M p2 , such that when the working voltage VCC is less than the selected threshold voltage V r , the source of the transistor M p2  provides a voltage sufficient to activate the discharge circuit  206 . 
     The discharge circuit  206  includes a NMOS transistor M n  and a first RC circuit having an output capacitor C o  coupled to an equivalent resistor. For example, as shown in  FIG. 2 , a resistor R o  is presented to simulate an equivalent resistor of a standby power when an AC power source is powered off and the circuit is not in operation. Accordingly, it should be appreciated that the output capacitor C o  is not limited to be coupled to the resistor R o , but can be coupled to any suitable equivalent resistor to discharge the power. Therefore, when the power supply normally provides power, the NMOS transistor M n  is switched off to avoid consuming additional power. When the power supply is powered off, the working voltage VCC drops slowly, and once the working voltage VCC is less than the reference voltage V r , the NMOS transistor M n  is switched on to activate the discharge circuit  206 , so as to quickly discharge the working voltage VCC of the power supply until the working voltage VCC drops to 0 volt. 
     In an embodiment, the second RC circuit includes the reference capacitor C r  and a PMOS resistor M r  in parallel. Preferably, a RC constant of the first RC circuit, i.e., the resistance of the equivalent resistor coupled to the output capacitor C o  times the capacitance of the output capacitor C o , is smaller than a RC constant of the second RC circuit, i.e., the equivalent resistance of the PMOS resistor M r  times the capacitance of the reference capacitor C r . Therefore, when the working voltage VCC is less than the reference voltage V r , the discharging speed of the second RC circuit is slower than the first RC circuit, so as to continuously provide a relatively high voltage to keep the discharge circuit  206  in activation. 
     The properties of electronic components such as transistors, resistors and capacitors can be selected by persons skilled in the art upon the actual need. For example, the switch transistors M s1  to M s3  and the PMOS transistor M p1  may be PMOS transistors having a width/length (W/L) ratio of 10μ/0.5μ, the PMOS transistor M p2  may have a W/L ratio of 0.3μ/5μ, the NMOS transistor M n  may have a W/L ratio of 1000μ/0.5μ, the output capacitor C o  may be 10 μF, the reference capacitor C r  may be 5 μF, and the MOS resistor M r  may include at least two, such as six, serially connected PMOS transistors each having a W/L ratio of 0.3μ/20μ. 
       FIG. 3  is a simulation graph illustrating a simulation result of the programmable power discharge circuit shown in  FIG. 2 , in which three scenarios corresponding to the selections of the three charging paths of the programmable voltage controller  202  shown in  FIG. 2 , respectively. Scenario (a) refers to the charging path controlled by the bit D 1 , where only one diode is arranged. It can be seen that the maximum of the threshold voltage V r  is about 4.57 volt, such that when the working voltage VCC drops to 3.85 volt which is about 0.5 volt below the reference voltage V r , the NMOS transistor M n  is switched on to discharge the working voltage VCC to 0 volt. Also, since the RC constant of the first RC circuit is smaller than the RC constant of the second RC circuit, the threshold voltage discharged by the second RC circuit drops significantly slower than the working voltage VCC discharged by the first RC circuit, so as to keep the discharge circuit  206  in activation. Similarly, scenario (b) refers to the charging path controlled by the bit D 2 , where three diodes are arranged. It can be seen that the maximum of the threshold voltage V r  is about 3.76 volt, such that when the working voltage VCC drops to 3.05 volt which is about 0.5 volt below the reference voltage V r , the NMOS transistor M n  is switched on to discharge the working voltage VCC to 0 volt. Further, scenario (c) refers to the charging path controlled by the bit D 3 , where five diodes are arranged. It can be seen that the maximum of the threshold voltage V r  is about 3.01 volt, such that when the working voltage VCC drops to 2.23 volt which is about 0.5 volt below the reference voltage V r , the NMOS transistor M n  is switched on to discharge the working voltage VCC to 0 volt. 
     In addition, the dashed line in the simulation graph of  FIG. 3  represents the working voltage VCC without using the programmable power discharge circuit of the present invention. A comparison of abovementioned three scenarios and the scenario without using the programmable power discharge circuit is provided in the Table 1 below. Apparently, the working voltage VCC without using the programmable quick power discharge circuit drops much slower than that in the scenarios (a) to (c). 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Duration time 
               
               
                   
                   
                 Required time for voltage VCC 
               
               
                   
                 Scenarios 
                 discharging from 5 volt to 0.5 volt 
               
               
                   
                   
               
             
             
               
                   
                 Without using a quick 
                 23 sec  
               
               
                   
                 discharge circuit 
               
               
                   
                 D1 = 0, D2 = 1, D3 = 1 
                 2 sec 
               
               
                   
                 D1 = 1, D2 = 0, D3 = 1 
                 4.5 sec   
               
               
                   
                 D1 = 1, D2 = 1, D3 = 0 
                 7 sec 
               
               
                   
                   
               
             
          
         
       
     
       FIG. 4  is a scheme view of the programmable power discharge circuit according to an embodiment of the present invention. As shown in  FIG. 4 , the programmable power discharge circuit  400  comprises a programmable voltage controller  402 , a detect circuit  404  and a discharge circuit  406 . In this embodiment, the diodes shown in  FIG. 2  are replaced with MOS resistors M pr1  to M pr3  providing different equivalent resistances, such that the working voltage VCC is divided by a selected MOS resistor M pr1 , M pr2  or M pr3  with the MOS resistor M r  to determine the threshold voltage V r . 
     Accordingly, when the power supply normally provides power, the reference capacitor C r  is charged through the charging path determined by the bits D 1  to D 3  of the selector to reach the threshold voltage V r , and the reference capacitor C r  is prevented from discharging. Also, when the power supply normally provides power, the NMOS transistor M n  is switched off to avoid consuming additional power. When the power supply is powered off, the working voltage VCC drops slowly, and once the working voltage VCC is less than the reference voltage V r , the NMOS transistor M n  is switched on to activate the discharge circuit  406 , so as to quickly discharge the working voltage VCC of the power supply until the working voltage VCC drops to 0 volt. 
       FIG. 5  is a scheme view of the programmable power discharge circuit according to an embodiment of the present invention. As shown in  FIG. 5 , the programmable power discharge circuit  500  comprises a programmable voltage controller  502 , a detect circuit  504  and a discharge circuit  506 . In this embodiment, the MOS resistors M pr1  to M pr3  shown in  FIG. 4  are replaced with resistors R 1  to R 3  having different resistances and the MOS resistor M r  shown in  FIG. 4  is replaced with a reference resistor R r , such that the working voltage VCC is divided by a selected resistor R 1 , R 2  or R 3  with the reference resistor R r  to determine the threshold voltage V r . Moreover, the PMOS transistor M p2  shown in  FIG. 4  is also replaced with a resistor R p , such that when the PMOS transistor M p1  is switched on, a current will flow through the resistor R p  and provide a voltage sufficient to switch on the NMOS transistor M n . 
     Accordingly, when the power supply normally provides power, the reference capacitor C r  is charged through the charging path determined by the bits D 1  to D 3  of the selector to reach the threshold voltage V r , and the reference capacitor C r  is prevented from discharging. Also, when the power supply normally provides power, the NMOS transistor M n  is switched off to avoid consuming additional power. When the power supply is powered off, the working voltage VCC drops slowly, and once the working voltage VCC is less than the reference voltage V r , the NMOS transistor M n  is switched on to activate the discharge circuit  506 , so as to quickly discharge the working voltage VCC of the power supply until the working voltage VCC drops to 0 volt. 
     From the foregoing, the present invention provides a programmable quick discharge circuit and method thereof, which not only quickly discharge the power stored in the capacitor in the circuit inside the voltage regulator when an AC power source is powered off, but also allow a user to conveniently adjust the threshold voltage for activating the quick discharge circuit. 
     The above examples are only used to illustrate the principle of the present invention and the effect thereof, and should not be construed as to limit the present invention. The above examples can all be modified and altered by those skilled in the art, without departing from the spirit and scope of the present invention as defined in the following appended claims.