Patent Publication Number: US-2011069423-A1

Title: Protection circuit for over-current and short protection

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to protection circuits and, particularly, to a protection circuit for over-current protecting and short protecting. 
     2. Description of Related Art 
     Nowadays, electronic devices usually include short protection circuits and over-current protection circuits. When an electronic device is short-circuited, a short protection circuit of the electronic device will disconnect the power source to protect the electronic device. When the current of the electronic device is over-current, an over-current protection circuit of the electronic device will disconnect the power source to protect the electronic device. However, it is usually costly to design and manufacture the short protection circuit and the over-current protection circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram of a first exemplary embodiment of a protection circuit connected to an electronic device. 
         FIG. 2  is a circuit diagram of a second exemplary embodiment of a protection circuit connected to an electronic device. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a first exemplary embodiment of a protection circuit  1  includes a thyristor SCR, a transistor Q 1 , and four resistors R 1 , R 2 , R 3 , R 4 . The protection circuit  1  can disconnect a power source V from an electronic device RL when the electronic device RL is short-circuited or over-current. 
     An anode of the thyristor SCR functions as an input of the protection circuit  1 , and is connected to the power source V. A cathode of the thyristor SCR is connected to the electronic device RL via the resistor R 1 . A gate of the thyristor SCR is connected to the anode of the thyristor SCR via the resistor R 2 . 
     The cathode of the thyristor SCR is also connected to a base of the transistor Q 1  via the resistor R 3 . The base of the transistor Q 1  is grounded via the resistor R 4 . A collector of the transistor Q 1  is connected to the anode of the thyristor SCR. An emitter of the transistor Q 1  is connected to a node N between the resistor R 1  and the electronic device RL. In the embodiment, a node M is connected by the cathode of the thyristor SCR, the resistor R 3 , and the resistor R 1 . The node N functions as an output of the protection circuit  1  and is connected by the resistor R 1 , the emitter of the transistor Q 1 , and the electronic device RL. 
     When there is no over-current and no over-voltage in the electronic device RL, the thyristor SCR is turned on. The power source V provides the voltage for the electronic device RL via the thyristor SCR and the resistor R 1  in turn. At this time, the electronic device RL operates normally. 
     When over-current happens in the electronic device RL, a voltage difference between the voltage at the node M divided by the resistors R 3 , R 4  and the voltage at the node N is more than 0.7 volts, that is to say, a voltage difference between the base and the emitter of the transistor Q 1  is more than 0.7 volts. The transistor Q 1  is turned on. The voltage between the anode and the cathode of the thyristor SCR is less than an operating voltage of the thyristor SCR. As a result, the thyristor SCR turns off. Therefore, the transistor Q 1  turns off. The power source V becomes disconnected from the electronic device RL, to protect the electronic device RL. It can be understood that over-current means that the current of the electronic device RL is more than a rated current I Max  of the electronic device RL. 
     The mechanism that the protection circuit  1  protects the electronic device RL when the electronic device RL is short-circuited will be described as follow. 
     Based on the structure of the protection circuit  1 , the voltage at the node M V M  can be obtained via a first equation: V M =I L R 1 +V OUT . It can be understood that I L  denotes the current flowing the electronic device RL. R 1  denotes the resistance of the resistor R 1 . V OUT  denotes the voltage at the output of the protection circuit  1 . 
     The voltage at the base of the transistor Q 1  can be obtained via a second equation: 
     
       
         
           
             
               V 
               B 
             
             = 
             
               
                 
                   
                     R 
                      
                     
                         
                     
                      
                     4 
                   
                   
                     
                       R 
                        
                       
                           
                       
                        
                       3 
                     
                     + 
                     
                       R 
                        
                       
                           
                       
                        
                       4 
                     
                   
                 
                 · 
                 
                   V 
                   M 
                 
               
               = 
               
                 
                   
                     R 
                      
                     
                         
                     
                      
                     4 
                   
                   
                     
                       R 
                        
                       
                           
                       
                        
                       3 
                     
                     + 
                     
                       R 
                        
                       
                           
                       
                        
                       4 
                     
                   
                 
                 · 
                 
                   
                     ( 
                     
                       
                         
                           I 
                           L 
                         
                          
                         
                           R 
                           1 
                         
                       
                       + 
                       
                         V 
                         OUT 
                       
                     
                     ) 
                   
                   . 
                 
               
             
           
         
       
     
     The voltage at the emitter of the transistor Q 1  can be obtained via a third equation: V E =V OUT . As a result, the voltage between the base and the emitter of the transistor Q 1  can be obtained via a fourth equation: 
     
       
         
           
             
               V 
               BE 
             
             = 
             
               
                 
                   V 
                   B 
                 
                 - 
                 
                   V 
                   E 
                 
               
               = 
               
                 
                   
                     
                       R 
                        
                       
                           
                       
                        
                       4 
                     
                     
                       
                         R 
                          
                         
                             
                         
                          
                         3 
                       
                       + 
                       
                         R 
                          
                         
                             
                         
                          
                         4 
                       
                     
                   
                   · 
                   
                     ( 
                     
                       
                         
                           I 
                           L 
                         
                          
                         R 
                          
                         
                             
                         
                          
                         1 
                       
                       + 
                       
                         V 
                         OUT 
                       
                     
                     ) 
                   
                 
                 - 
                 
                   
                     V 
                     OUT 
                   
                   . 
                 
               
             
           
         
       
     
     It can be understood that R 3  denotes the resistance of the resistor R 3 . R 4  denotes the resistance of the resistor R 4 . 
     Supposing that 
     
       
         
           
             
               K 
               = 
               
                 
                   R 
                    
                   
                       
                   
                    
                   4 
                 
                 
                   
                     R 
                      
                     
                         
                     
                      
                     3 
                   
                   + 
                   
                     R 
                      
                     
                         
                     
                      
                     4 
                   
                 
               
             
             , 
           
         
       
     
     the voltage between the base and the emitter of the transistor Q 1  can be obtained via a fifth equation: 
         V   BE   =K ( I   L   R 1 +V   OUT )− V   OUT   =KI   L   R 1 +V   OUT ( K− 1).
 
     As a result, the current flowing through the electronic RL can be obtained as a sixth equation: 
     
       
         
           
             
               I 
               L 
             
             = 
             
               
                 
                   
                     V 
                     B 
                   
                   - 
                   
                     
                       V 
                       OUT 
                     
                      
                     
                       ( 
                       
                         K 
                         - 
                         1 
                       
                       ) 
                     
                   
                 
                 
                   KR 
                    
                   
                       
                   
                    
                   1 
                 
               
               . 
             
           
         
       
     
     When the electronic device RL is short-circuited, the voltage at the output of the protection circuit  1  equals zero. At this time, the current I SL  flowing through the protection circuit  1  can be obtained by applying a seventh equation: 
     
       
         
           
             
               I 
               SL 
             
             = 
             
               
                 
                   V 
                   BE 
                 
                 
                   KR 
                    
                   
                       
                   
                    
                   1 
                 
               
               . 
             
           
         
       
     
     As a result, when the electronic device RL is short-circuited, the maximum current I SL (Max) flowing through the protection circuit  1  can be obtained as an eighth equation: 
     
       
         
           
             
               I 
               
                 SL 
                  
                 
                   ( 
                   Max 
                   ) 
                 
               
             
             = 
             
               
                 
                   
                     V 
                     BE 
                   
                    
                   
                     ( 
                     Max 
                     ) 
                   
                 
                 
                   KR 
                    
                   
                       
                   
                    
                   1 
                 
               
               . 
             
           
         
       
     
     It can be understood that V BE (Max) denotes the maximum voltage at the output of the protection circuit  1 . 
     When the electronic device RL is not short-circuited, the current I L  flowing through the protection circuit  1  can be obtained as a ninth equation: 
     
       
         
           
             
               I 
               L 
             
             = 
             
               
                 
                   
                     V 
                     BE 
                   
                   - 
                   
                     
                       V 
                       OUT 
                     
                      
                     
                       ( 
                       
                         K 
                         - 
                         1 
                       
                       ) 
                     
                   
                 
                 
                   KR 
                    
                   
                       
                   
                    
                   1 
                 
               
               . 
             
           
         
       
     
     As a result, when the electronic device RL is not short-circuited, the maximum current I L (Max) flowing through the protection circuit  1  can be obtained as a tenth equation: 
     
       
         
           
             
               I 
               
                 L 
                  
                 
                   ( 
                   Max 
                   ) 
                 
               
             
             = 
             
               
                 
                   
                     
                       V 
                       BE 
                     
                      
                     
                       ( 
                       Max 
                       ) 
                     
                   
                   - 
                   
                     
                       V 
                       OUT 
                     
                      
                     
                       ( 
                       
                         K 
                         - 
                         1 
                       
                       ) 
                     
                   
                 
                 
                   KR 
                    
                   
                       
                   
                    
                   1 
                 
               
               . 
             
           
         
       
     
     As described above, the maximum current I L (Max) flowing through the protection circuit  1  when the electronic device RL is not short-circuited is more than the maximum current I SL (Max) flowing through the protection circuit  1  when the electronic device RL is short-circuited. 
     In the protection circuit  1 , the maximum current I L (Max) flowing through the protection circuit  1  when the electronic device RL is not short-circuited is set to be equal to the rated current I MAX  of the electronic device RL. As a result, the rated current I Max  of the electronic device RL can be obtained as an eleventh equation: 
     
       
         
           
             
               I 
               Max 
             
             = 
             
               
                 
                   
                     
                       V 
                       BE 
                     
                      
                     
                       ( 
                       Max 
                       ) 
                     
                   
                   - 
                   
                     
                       V 
                       OUT 
                     
                      
                     
                       ( 
                       
                         K 
                         - 
                         1 
                       
                       ) 
                     
                   
                 
                 
                   KR 
                    
                   
                       
                   
                    
                   1 
                 
               
               . 
             
           
         
       
     
     As a result, even if the electronic device RL is short-circuited, the current flowing through the protection circuit  1  is less than the rated current I Max  of the electronic device RL. Therefore, the protection circuit  1  can protect the electronic device RL when the electronic device RL is short-circuited. 
     Referring to  FIG. 2 , a second exemplary embodiment of a protection circuit  2  includes two transistors Q 10  and Q 20 , four resistors R 10 , R 20 , R 30 , and R 40 . 
     A collector of the transistor Q 20  functions as an input of the protection circuit  2 , and is connected the power source V. An emitter of the transistor Q 20  is connected to the electronic device RL via the resistor R 10 . A base of the transistor Q 20  is connected to the collector of the transistor Q 20  via the resistor R 20 . 
     The emitter of the transistor Q 20  is also connected to a base of the transistor Q 10  via the resistor R 30 . The base of the transistor Q 10  is grounded via the resistor R 40 . A collector of the transistor Q 10  is connected to the base of the transistor Q 20 . An emitter of the transistor Q 10  is connected to a node between the resistor R 10  and the electronic device RL. 
     When there is no over-current and no over-voltage in the electronic device RL, the second transistor Q 20  is turned on. The power source V provides the voltage for the electronic device RL via the second transistor Q 20  and the resistor R 10  in turn. At this time, the electronic device RL operates normally. 
     When over-current happens in the electronic device RL, a voltage difference between the voltage at a node between the resistors R 30 , R 40  and the voltage at the emitter of the transistor Q 10  is more than 0.7 volts, that is to say, a voltage difference between the base and the emitter of the transistor Q 10  is more than 0.7 volts. The transistor Q 10  is turned on. The voltage between the base and the emitter of the transistor Q 20  is less than an operating voltage of the transistor Q 20 . As a result, the transistor Q 20  turns off. Therefore, the transistor Q 10  turns off. The power source V becomes disconnected from the electronic device RL, to protect the electronic device RL. 
     The protection circuit  2  in the second embodiment protects the electronic device RL from short-circuiting using the same mechanism the protection circuit  1  in the first embodiment does in protecting the electronic device RL from short-circuiting. 
     The foregoing description of the exemplary embodiments of the disclosure has been presented only for the purposes of illustration and description and is not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. Many modifications and variations are possible in light of the above everything. The embodiments were chosen and described in order to explain the principles of the disclosure and their practical application so as to enable others of ordinary skill in the art to utilize the disclosure and various embodiments and with various modifications as are suited to the particular use contemplated. Alternative embodiments will become apparent to those of ordinary skills in the art to which the present disclosure pertains without departing from its spirit and scope. Accordingly, the scope of the present disclosure is defined by the appended claims rather than the foregoing description and the exemplary embodiments described therein.