Abstract:
An overvoltage protection circuit protects a portable electronic device against overvoltage. The overvoltage protection circuit includes an input unit for receiving an input voltage supplied by a voltage source; a voltage-divider module for dividing the input voltage and outputting a divided voltage; a voltage-regulator module for comparing a comparison voltage with the divided voltage and generating a first control signal; a first switch unit being controllably switched by the first control signal to a short-circuit state or an open-circuit state and generating a second control signal; and a second switch unit being controllably switched by the second control signal to a circuit state reverse to that of the first switch unit, and stopping supplying the input voltage to the portable electronic device when the input voltage is no less than a rated voltage of the portable electronic device. Therefore, a temperature-independent overvoltage protection can be achieved.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No(s). 100134825 filed in Taiwan, R.O.C. on Sep. 27, 2011, the entire contents of which are hereby incorporated by reference. 
       FIELD OF TECHNOLOGY 
       [0002]    The present invention relates to overvoltage protection circuits, and more particularly to a temperature-independent overvoltage protection circuit for effectively protecting a portable electronic device against overvoltage. 
       BACKGROUND 
       [0003]    According to the prior art, a conventional portable electronic device is connected to an external power supply via an adapter, so that an input voltage from the external power supply is supplied to the portable electronic device via the adapter to, for example, charge or power the portable electronic device. 
         [0004]    Following the constantly increased types of portable electronic devices being carried about by a user, the number of different adapters for the portable electronic devices is also increased. These adapters are different in their electrical properties, such as having different rated input/output voltages and currents. In the event of connecting the portable electronic device to an adapter of wrong specification, direct damage to the portable electronic device will occur. For instance, when the maximum rated voltage for the batteries, electronic elements and electronic circuits in the portable electronic device is 12V, the direct supply of a voltage higher than 12V to the portable electronic device would dangerously cause damage to the batteries, electronic elements and electronic circuits in the portable electronic device, making the latter inoperative. 
         [0005]    Moreover, as disclosed in the prior art, diodes, transistors or comparator units are used to form a conventional overvoltage protection circuit. However, these electronic components are possibly affected by ambient temperature variation to result in a change of electric properties thereof and an error in performing the overvoltage protection. 
         [0006]    Therefore, the inventor of the present invention develops an improved overvoltage protection circuit to eliminate the drawbacks in the prior art. 
       SUMMARY 
       [0007]    A primary object of the present invention is to provide an overvoltage protection circuit, which is arranged between a voltage source and a portable electronic device to ensure that the portable electronic device is not damaged due to the use of an input voltage higher than an acceptable rated voltage of the portable electronic device. 
         [0008]    Another object of the present invention is to provide a temperature-independent overvoltage protection circuit, so that the overvoltage protection circuit can always stably and effectively isolate a portable electronic device from an improper input voltage without being affected by ambient temperature variation. 
         [0009]    A further object of the present invention is to provide an overvoltage protection circuit that utilizes a voltage divider module, which can be dynamically set to a rated voltage acceptable by a portable electronic device. 
         [0010]    To achieve the above and other objects, the overvoltage protection circuit according to the present invention is provided between a voltage source and a portable electronic device for determining whether an input voltage supplied by the voltage source is higher than an acceptable rated voltage of the portable electronic device and providing overvoltage protection for the latter. The overvoltage protection circuit includes an input unit, a voltage divider module, a voltage regulator module, a first switch unit, and a second switch unit. The input unit receives the input voltage supplied by the voltage source. The voltage divider module is connected to the input unit and divides the input voltage to output a divided voltage. The voltage regulator module is connected to the input unit and the voltage divider module and has a comparison voltage. The voltage regulator module compares the comparison voltage with the divided voltage, and generates a first control signal according to a comparison result. The first switch unit is connected to the voltage regulator module and is controlled by the first control signal to generate a corresponding second control signal. The second switch unit is connected to the input unit and the first switch unit, and is controlled by the second control signal to supply or stop supplying the input voltage to the portable electronic device. When the input voltage is no less than the divided voltage, the voltage regulator module controls the first switch unit and the latter in turn controls the second switch unit to stop supplying the input voltage to the portable electronic device. 
         [0011]    Compared to the prior art, the overvoltage protection circuit of the present invention employs a simple voltage divider module and voltage regulator module to achieve the object of overvoltage protection. In particular, the voltage regulator module is characterized by a resistance to temperature variation and would not cause erroneous control due to ambient temperature variation. Further, the voltage divider module can be easily set to the maximum rated voltage that can be accepted by the portable electronic device. With this setting, it is ensured the portable electronic device can be protected against damage caused by receiving an input voltage higher than the rated voltage. Therefore, the present invention has the advantages of low manufacturing cost, stable function, easy circuit design and low power consumption. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The structure and the technical means adopted by the present invention to achieve the above and other objects can be best understood by referring to the following detailed description of the preferred embodiment and the accompanying drawings, wherein 
           [0013]      FIG. 1  is a block diagram of an overvoltage protection circuit according to a preferred embodiment of the present invention; 
           [0014]      FIG. 2  is a detailed circuit diagram of a voltage divider module included in the overvoltage protection circuit of  FIG. 1 ; 
           [0015]      FIG. 3  is a detailed circuit diagram of a voltage regulator module included in the overvoltage protection circuit of  FIG. 1 ; 
           [0016]      FIG. 4  is a circuit diagram showing the operation of a first switch unit included in the overvoltage protection circuit of  FIG. 1 ; and 
           [0017]      FIG. 5  is a circuit diagram showing the operation of a second switch unit included in the overvoltage protection circuit of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    The present invention is hereunder described with a preferred embodiment thereof and with reference to the accompanying drawings. 
         [0019]    Please refer to  FIG. 1  that is a block diagram of an overvoltage protection circuit according to a preferred embodiment of the present invention. As shown in  FIG. 1 , the overvoltage protection circuit  10  is provided between a voltage source  2  and a portable electronic device  4  for determining whether an input voltage V in  supplied by the voltage source  2  is higher than an acceptable rated voltage of the portable electronic device  4  and providing overvoltage protection for the latter. The input voltage V in  supplied by the voltage source  2  is a direct current (DC) voltage or an alternating current (AC) voltage. In an embodiment of the present invention, the rated voltage may be a voltage that can be withstood by all circuit units contained in the portable electronic device  4 . 
         [0020]    Further, the overvoltage protection circuit  10  is composed of an input unit  12 , a voltage divider module  14 , a voltage regulator module  16 , a first switch unit  18 , and a second switch unit  20 . The input unit  12  is connected to the voltage source  2  for receiving the input voltage V in  supplied by the voltage source  2 . In an embodiment of the present invention, the input unit  12  may include a rectification circuit, so that an input AC voltage V in  supplied by the voltage source  2  and received by the input unit  12  can be rectified by the rectification circuit to a DC voltage. 
         [0021]    The voltage divider module  14  has two terminals, one of which is connected to the input unit  12  and the other one of which is connected to a ground GND. With these arrangements, the input voltage V in  across the input unit  12  is divided by the voltage divider module  14  and a corresponding divided voltage V vd  is produced. 
         [0022]    Please refer to  FIG. 2  along with  FIG. 1 . The voltage divider module  14  may consist of a first resistor unit R 1  and a second resistor unit R 2  that are connected in series. The input voltage V in  is applied across the first resistor unit R 1  and the second resistor unit R 2  to produce the divided voltage V vd , and there are a first voltage drop V R1  and a second voltage drop V R2  across the first resistor unit R 1  and the second resistor unit R 2 , respectively. The divided voltage V vd  is directly output from the second resistor unit R 2 . In other words, the divided voltage V vd  is equal to the second voltage drop V R2 . Therefore, the relation between the divided voltage V vd , the input voltage V in , the resistance of the first resistor unit R 1 , and the resistance of the second resistor unit R 2  can be expressed by the following equation: 
         [0023]    V vd =(R 2 /(R 1 +R 2 ))×V in , where R 1 , R 2  denote the resistance of the first resistor unit R 1  and the resistance of the second resistor unit R 2 , respectively. 
         [0024]    The voltage regulator module  16  is connected to the input unit  12  and the voltage divider module  14 , and has a comparison voltage V cmp  (shown in  FIG. 3 ). The voltage regulator module  16  generates a first control signal FCS according to a comparison result obtained in a comparison of the comparison voltage V cmp  with the divided voltage V vd  received by the voltage regulator module  16 . 
         [0025]    In an embodiment of the present invention, when the divided voltage V vd  is lower than the comparison voltage V cmp , the voltage regulator module  16  generates the first control signal FCS for controllably switching the first switch unit  18  to an open-circuit state. On the other hand, when the divided voltage V vd  is higher than or equal to the comparison voltage V cmp , the voltage regulator module  16  generates the first control signal FCS for controllably switching the first switch unit  18  to a short-circuit state. 
         [0026]      FIG. 3  is a detailed circuit diagram of the voltage regulator module  16 . Please refer to  FIG. 3  along with  FIG. 1 . In an embodiment of the present invention, as shown in  FIG. 3 , the voltage regulator module  16  further includes a third resistor unit R 3 , a comparator unit  162 , and a third switch unit  164 . The third resistor unit R 3  has two terminals, one of which is connected to the input unit  12  and the other one of which is connected to the third switch unit  164 . That is, the third resistor unit R 3  and the third switch unit  164  are connected in series. The comparator unit  162  has a divided-voltage terminal  1622 , a comparison-voltage terminal  1624 , and a comparison output terminal  1626 . The divided-voltage terminal  1622  receives the divided voltage V vd . The comparison-voltage terminal  1624  receives the comparison voltage V cmp . 
         [0027]    The comparison output terminal  1626  compares the divided voltage V vd  with the comparison voltage V cmp  and outputs a comparison result CR. The third switch unit  164  includes a third input terminal  1642 , a third output terminal  1644 , and a third control terminal  1646 . The third control terminal  1646  is connected to the comparison output terminal  1626 . The third input terminal  1642  is connected to the third resistor unit R 3 . The third output terminal  1644  is connected to the ground GND. Further, the third switch unit  164  can be switched to a short-circuit state according to the comparison result CR. That is, the input voltage V in  can be connected to the ground GND via the third resistor unit R 3 , the third input terminal  1642 , and the third output terminal  1644 . Since the ground GND has a potential of zero, a first control signal FCS is generated at the third input terminal  1642  for controllably switching the first switch unit  18  to a short-circuit state. Alternatively, when the third switch unit  164  is switched to an open-circuit state according to the comparison result CR, the input voltage V in  cannot be supplied to the third switch unit  164  but is directly output to the first switch unit  18 , and a first control signal FCS is generated at the third input terminal  1642  for controllably switching the first switch unit  18  to an open-circuit state. 
         [0028]    Please refer back to  FIG. 1 . The first switch unit  18  is connected to the input unit  12  and the voltage regulator module  16 , and is controlled by the first control signal FCS to generate a corresponding second control signal SCS. The first switch unit  18  includes a first input terminal  182 , a first output terminal  184 , and a first control terminal  186 . The first control terminal  186  is connected to the third resistor R 3  and the third switch unit  164 . The first input terminal  182  is connected to the input unit  12 . The first output terminal  184  is connected to the second switch unit  20 . In addition, the first switch unit  18  selectively generates the second control signal SCS at the first output terminal  184  for controllably switching the second switch  20  to an open-circuit state or a short-circuit state, depending on the first control signal FCS. 
         [0029]    The second switch unit  20  is connected to the input unit  12  and the first switch unit  18 , and controls the output of the input voltage V in  to the portable electronic device  4  according to the second control signal SCS. The second switch unit  20  includes a second input terminal  202 , a second output terminal  204 , and a second control terminal  206 . The second control terminal  206  is connected to the first switch unit  18 . The second input terminal  202  receives the input voltage V in . The second output terminal  204  is connected to the portable electronic device  4 . 
         [0030]    In other words, the first switch unit  18  and the second switch unit  20  are controllably switched by the first control signal FCS and the second control signal SCS to two reverse circuit states, namely, an open-circuit state and a short-circuit state, respectively. 
         [0031]      FIG. 4  is a detailed circuit diagram of the first switch unit  18 . Please refer to  FIG. 4  along with  FIG. 1 . In  FIG. 4 , the first switch unit  18  is illustrated as a p-type metal-oxide-semiconductor field-effect transistor (MOSFET) with the first input terminal  182  corresponding to the source, the first output terminal  184  corresponding to the drain, and the first control terminal  186  corresponding to the gate. According to the property of the p-type MOSFET, when the voltage applied across the gate is lower than the voltage across the source (or the drain), a short-circuit state (also referred to as a turn-on state) is formed between the source and the drain. On the other hand, when the voltage applied across the gate is higher than the voltage across the source (or the drain), an open-circuit state (also referred to as a cut-off state) is formed between the source and the drain. 
         [0032]    The first switch unit  18  includes a first gate, a first source and a first drain. The first gate is connected to the voltage regulator module  16 , the first source is connected to the input unit  12 , and the first drain is connected to the second switch unit  20 . 
         [0033]    When the first control signal FCS is a zero voltage, since the voltage across the first source is higher than the voltage across the first gate, a short-circuit state is formed between the first source and the first drain of the first switch unit  18 , allowing the input voltage V in  to be input to the first source and output from the first drain. Therefore, a second control signal SCS having the input voltage V in  is generated for controllably switching the second switch unit  20  to an open-circuit state. And, when the first control signal FCS is the input voltage V in , since the voltage across the first source is equal to the voltage across the first gate, an open-circuit state is formed between the first source and the first drain of the first switch unit  18 , and the input voltage V in  could not be output via the first drain, and the second control signal SCS has a zero-volt voltage at this time. 
         [0034]      FIG. 5  is a detailed circuit diagram of the second switch unit  20 . Please refer to  FIG. 5  along with  FIG. 1 . In  FIG. 5 , the second switch unit  20  is illustrated as a p-type MOSFET with the second input terminal  202  corresponding to the source, the second output terminal corresponding to the drain, and the second control terminal  206  corresponding to the gate. 
         [0035]    The second switch unit  20  includes a second gate, a second source and a second drain. The second gate is connected to the first drain of the first switch unit  18  for receiving the second control signal SCS. The second source is connected to the input unit  12 . The second drain is connected to the portable electronic device  4 . 
         [0036]    When the second control signal SCS is the input voltage V in , the voltage across the second source of the second switch unit  20  is equal to the voltage across the second gate, bringing an open-circuit state to form between the second source and the second drain of the second switch unit  20 , and the input voltage V in  could not be supplied to the portable electronic device  4  via the second switch unit  20 . And, when the second control signal SCS is a zero volt voltage, since the voltage across the second source is higher than the voltage across the second gate, a short-circuit state is formed between the second source and the second drain of the second switch unit  20 , allowing the input voltage V in  to be input to the second source and output from the second drain to the portable electronic device  4 . 
         [0037]    In brief, the overvoltage protection circuit of the present invention employs the simple voltage divider module and voltage regulator module to achieve the purpose of overvoltage protection. Wherein, the voltage regulator module is characterized by having a resistance to temperature variation and would not cause erroneous control due to ambient temperature variation. Further, the voltage divider module can be easily set to the maximum rated voltage acceptable by the portable electronic device. With such setting, it is ensured the portable electronic device can be protected against damage due to receiving an input voltage higher than the rated voltage. Therefore, the present invention has the advantages of low manufacturing cost, stable function, simple circuit design, and low power consumption. 
         [0038]    The present invention has been described with a preferred embodiment thereof and it is understood that many changes and modifications in the described embodiment can be carried out without departing from the scope and the spirit of the invention that is intended to be limited only by the appended claims.