Abstract:
A charging protection circuit with an overcurrent protector circuit takes input from a power supply, and provides output power to a power storage device, such as a battery. A fuse is connected between an input and an output of the charging protection circuit to protect the power storage device from a current spike, or overcurrent, that would damage the power storage device. When the overcurrent is above a threshold of the fuse, the fuse melts, effectively creating an open circuit and blocking the overcurrent from reaching the power storage device. To speed up the fuse meltdown, the overcurrent protector circuit is connected to the output of the fuse, and draws an extra current through the fuse when the overcurrent is detected.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a charging protection circuit, and more specifically, to a charging protection circuit with a circuit-level enhanced overcurrent protection mechanism.  
         [0003]     2. Description of the Prior Art  
         [0004]     With the development and progress of electronic technologies, the size and weight of electronic devices such as mobile phones, personal digital assistants (PDA), digital cameras, portable media players, portable computers, and so on, have been greatly reduced, making them easily portable. Most of these portable devices are powered by batteries, and therefore battery chargers are essential to keeping these devices functioning. One example of such a charger is a mobile phone charging base, which converts alternating current (AC) into direct current (DC) for charging the batteries.  
         [0005]     In the charging process, however, the charging devices may malfunction, or short-circuit, for many reasons, such as rust in a conducting metal, charging an out-of-spec battery, alternating the polarity of the battery, damaged battery circuits, and so forth. The malfunction or short circuit pulls a large current from the power supply, and the excess charge current can damage the battery and even cause an explosion that destroys the device and could harm users.  
         [0006]     The prior art teaches some overcurrent protection mechanisms in the mobile phone charging process which are realized through a fuse that prevents the danger of malfunction or short-circuit in the battery. In these techniques, the fuses are installed in the charger and coupled to the power supply and the battery so as to conduct the charging current. When the charging current exceeds a predetermined value, the fuses melt down and the batteries are disconnected from the power supply. Other techniques are also available, most of which employ software-controlled protection mechanisms in the mobile phone. For instance, some specific software may be loaded into a processor of the mobile phone to monitor current and voltage values during the charging process.  
         [0007]     Nevertheless, there are disadvantages to the above techniques. One of the main drawbacks is that it takes time to melt down the fuse and to disconnect the batteries from the power supply when the charging current exceeds the predetermined value. The batteries are still charged with the excess current before the fuse disconnects, and the damage caused by the malfunction or the short-circuit is not avoided. Furthermore, modern electronic devices usually operate at low voltages to conserve power, such that the maximum tolerable current is consequently lower than in higher voltage topologies. In this situation, the charging current might exceed the maximum tolerable current, but may not melt the fuse and engage the overcurrent protection mechanism. As for software-based protection mechanisms, there are more high-level operations involved, so that the reliability and response time are still unsatisfactory. In other words, the prior art overcurrent protection mechanisms are not sensitive or responsive enough to adequately protect the battery being charged.  
       SUMMARY OF THE INVENTION  
       [0008]     It is therefore an objective of the present invention to provide a charging protection circuit with an overcurrent protection circuit to address the above-mentioned problems. The charging protection circuit comprises a fuse, an output and an overcurrent protector. The fuse has a first end and a second end. The first end of the fuse is coupled to a power supply. The second end of the fuse is connected to the output end, which outputs a current transmitted from the second end of the fuse. The overcurrent protector is coupled to the second end of the fuse to increase the current flowing through the fuse, and thus speed up meltdown of the fuse, when the current transmitted from the second end of the fuse is greater than a predetermined value.  
         [0009]     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a diagram of a charging protection circuit of the present invention.  
         [0011]      FIG. 2  is an electronic circuit connection diagram of the charging protection circuit of the present invention.  
         [0012]      FIG. 3  is a graph illustrating a current-voltage characteristic when the charging protection circuit of the present invention performs an overvoltage protection.  
         [0013]      FIG. 4  is a graph illustrating a current-voltage characteristic when the charging protection circuit of the present invention performs an overcurrent protection.  
         [0014]      FIG. 5  is a diagram of the charging protection circuit of the present invention as utilized in an electronic device. 
     
    
     DETAILED DESCRIPTION  
       [0015]     Please refer to  FIG. 1 .  FIG. 1  is a diagram of a charging protection circuit  30 . The charging protection circuit  30  comprises a fuse  21 , a current detector  36 , a switch  32 , an auxiliary fuse  22 , a current limiter  38 , a charging controller  46 , an auxiliary switch  34 , an overcurrent protector  40 , a voltage limiter  42 , and an auxiliary charging controller  48 . The first end of the fuse  20  is connected to a node Na and receives a current from a power supply. The second end of the fuse is connected to a node Nb, a branch  203 , and a branch  205 , and transmits the current from the power supply. The current detector  36 , the switch  32 , and the auxiliary fuse  22  are coupled at the branch  203 . A charging current Ic flows through the branch  203  and the charging current is output to the battery at a node Nc, namely the output end of the charging protection circuit  30 .  
         [0016]     The switch  32  is used to control the charging current Ic. The current detector  36  measures the magnitude of the charging current Ic. The current limiter  38  amplifies the measurement from the current detector  36  to the charging controller  46 . The charging controller  46  then controls the switch  32  by a voltage according to the amplified measurement. When the charging current Ic approaches a predetermined value, the current limiter  38  amplifies the signal from the current detector  36  and transmits the signal to the charging controller  46 . The charging controller  46  makes the switch  32  clamp the charging current Ic. The interoperation of the current detector  36 , the current limiter  38 , the charging controller  46 , and the switch  32  achieves current-limiting protection.  
         [0017]     The magnitude of the current flowing through the switch  32  serves as an overcurrent signal for controlling the overcurrent protector  40 . The auxiliary switch  34  is used to control the overcurrent protector  40  based on the overcurrent signal from the switch  32  for conducting an auxiliary current loc over the branch  205 . When the charging current Ic is under the predetermined value, the overcurrent protector  40  does not conduct the auxiliary current loc. The whole charging current goes through the fuse  21  and the branch  203  to the node Nc of the output end. However, when the magnitude of the charging current Ic on the branch  203  approaches or exceeds the predetermined value, the switch  32  decreases the output current based on the amplified measurement transferred from the current limiter  38  and the charging controller  46  to realize the current-limiting protection.  
         [0018]     The higher the current flows over the branch  203 , the more the switch  32  decreases the output current. When the charging current Ic exceeds the predetermined value, the overcurrent protector starts to conduct the auxiliary current loc. As shown in  FIG. 1 , the conduction of loc increases the current load of the fuse  21  in order to burn down the fuse when the current at the second end of the fuse is greater than the predetermined value, and melts down the fuse  21  to accelerate disconnection of the batteries from the power supply. The overcurrent protection is thus achieved.  
         [0019]     From the above description, the charging protection circuit  30  can rapidly burn down the fuse  21  by conducting an auxiliary current loc through the overcurrent protector  40 . Therefore, we can adjust the sensitivity and response time of the overcurrent protection mechanism by modifying the design parameters. For example, we can speed up the time for melting down the fuse by increasing the current conducted through the overcurrent protector  40 .  
         [0020]     Furthermore, additional overcurrent protection is realized by adopting an auxiliary fuse  22 . The auxiliary fuse  22  can be thermo-coupled with the switch  32  in the circuit layout. The switch  32  heats up to melt down the auxiliary fuse  22  when current flowing through the switch  32  exceeds the predetermined value.  
         [0021]     In addition to current limiting and overcurrent protection, the charging protection circuit  30  further provides an overvoltage protection through a voltage limiter  42 . The voltage limiter  42  detects the voltage at the node Nb and signals the charging controller  46  and the auxiliary charging controller  48  to control the switch  32  and the auxiliary switch  34 , respectively. When the voltage at the node Nb exceeds a predetermined value, the voltage limiter  43  makes the charging controller  46  turn off the switch  32  to disconnect the branch  203  and causes the auxiliary charging controller  48  to turn off the auxiliary switch  34  to disconnect the branch  205 . Therefore, overvoltage protection is accomplished.  
         [0022]     Please refer to  FIG. 1  and  FIG. 2 .  FIG. 2  is an exemplificative circuit implementation according to the present invention in  FIG. 1 . As shown in  FIG. 2 , the current detector  36  includes one or several resistors connected in parallel. Because the current detector is connected in series with the branch  203 , if one resistor does not draw sufficient current in the current detector  36 , further resistors can be connected in parallel to increase the maximum current through the current detector  36 , such as the configuration shown in  FIG. 2 . The current limiter  38  can be realized with two bipolar junction transistors (BJTs)  5  and  6 . The base-emitter voltage of the BJT  5  is controlled by the current detector  36 . The BJT  6  amplifies the collector current of the BJT  5  and transmits the amplified current to the charging controller  46  via a branch  204 . The charging controller  46  can be realized with a capacitor  7  and a resistor  9 , and the voltage at a node Nd serves as the current-limiting or voltage-limiting signal. The switch  32  can be realized as a Metal-Oxide-Semiconductor (MOS) transistor Q 1  with the source and the drain coupled to the branch  203 . The gate of MOS transistor Q 1  is coupled to the node Nd to adjust the conduction current according to the signal from the charging controller  46 . In addition, the MOS transistor Q 1  can be thermo-coupled with the auxiliary fuse  22  for additional overcurrent protection.  
         [0023]     On the branch  205 , the overcurrent protector  40  can by realized by one or several controlled current sources. As shown in  FIG. 2 , the overcurrent protector  40  comprises two controlled current sources, wherein one is formed by a BJT  14  and a resistor  18  and the other is formed by a BJT  15  and a resistor  19 . The sum of the controlled current sources is equal to the auxiliary current loc. The bases of the BJTs in the controlled current sources are coupled to the drain of the MOS transistor Q 1  with a diode  17  and a resistor  16 . The drain voltage of the MOS transistor Q 1  serves as the overcurrent signal to control the conduction of the controlled current sources. The auxiliary switch  34  can be realized with a MOS transistor Q 2  with a gate controlled by the auxiliary charging controller  48 , which can be realized with a resistor  8 . The voltage at a node Ne is the auxiliary voltage-limiting signal.  
         [0024]     The voltage limiter  42  can be implemented with a BJT transistor  1 , a Zener diode  2 , and a resistor  3 . The base of the BJT transistor  1  is coupled to the Zener diode  2  and the resistor  3 . The emitter of the BJT transistor  1  is coupled to the node Nb and the collector of the BJT transistor  1  is coupled to the node Ne for generating the auxiliary voltage-limiting signal with the auxiliary charging controller  48 . The collector of the BJT transistor  1  is further coupled to the node Nd with a diode  4  for generating a voltage-limiting signal with the charging controller  46 . Because the current limiter  38  is coupled to the node Nd, the diode  4  can also prevent the current in the branch  204  from flowing to the voltage limiter  42 .  
         [0025]     The operation of the circuit in  FIG. 2  can be described as follows. The charging current Ic generates a voltage across the current detector  36 , which is the base-emitter voltage of the transistor  5 . Therefore, the charging current Ic effectively controls the magnitude of the current flowing through the transistor  5 . The current flowing through the collector and the emitter of the transistor  5  is amplified by the transistor  6  and transmitted to the charging controller  46  to generate a current-limiting signal at the node Nd and further control the switch  32 . Therefore, when the charging current Ic increases, the voltage across the current detector  36  increases, and accordingly, the current flowing through the transistor  5  increases. The transistor  6  amplifies the current on the transistor  5  and transmits the amplified current to the charging controller  46  to rapidly increase the voltage at the node Nd and decrease the gate-source voltage of the transistor Q 1 , effectively limiting the current conducted to the output. Hence, the current through Q 1  decreases and the charging current Ic on the branch  203  is constrained to the predetermined value. Therefore, the proposed current-limiting function is realized by a control circuit loop comprising the current detector  36 , the current limiter  38 , the charging controller  46 , and the switch  32 .  
         [0026]     When the charging current is under the predetermined value, the difference the collector-emitter voltage of the transistor  14  and  15  are not large enough for the transistors  14  and  15  to conduct, such that the majority of the charging current flows through the branch  203 . As the charging current Ic through the branch  203  increases, the transistors  5  and  6  conduct more current, and accordingly, the voltage at the node Nd increases. Therefore, the gate-source voltage of the transistor Q 1  and the current flowing through the transistor Q 1  both decrease, and consequently, the drain voltage of the transistor Q 1  and the base voltage of the transistors  14  and  15  decrease. Therefore, when the current exceeds the predetermined value, the transistors  14  and  15  rapidly conduct currents on the branches  206  and  207 , respectively. The overcurrent protector  40  starts to conduct the auxiliary current loc on the branch  205 , which equals to the sum of the currents on the branch  206  and  207 . Hence, the charging current Ic and auxiliary current loc can quickly burn the fuse  21  to achieve the proposed overcurrent protection. When the charging current is large enough to melt the transistor Q 1 , the thermo-coupled fuse  22  can also be burned at the same time to achieve auxiliary overcurrent protection.  
         [0027]     The overvoltage protection mechanism is described in the following. In the voltage limiter  42 , the Zener diode  2  and the resistor  3  establish a reference voltage at the gate of the transistor  1 . When the voltage at the node Nb is within a normal operating range, the transistor  1  does not draw current, and thus does not establish a voltage at the resistor  8  of the auxiliary charging controller  48 . Thus, the transistor Q 2  functions normally, and the voltage at the node Nd is controlled by the current limiter  38 . When the voltage at the node Nb exceeds the normal operating range, the transistor  1  starts to push current into the auxiliary control  48  and the charging controller  46 . The voltage at the node Ne, i.e. the gate voltage of the transistor Q 2 , increases, thereby decreasing the gate-source voltage of the transistor Q 2 , and eventually turning off the transistor Q 2 . In the same manner, the voltage at the node Nd, i.e. the gate voltage of the transistor Q 1 , also increases, such that the gate-source voltage of the transistor Q 1  decreases, and the transistor Q 1  is turned off. Therefore, a voltage higher than the predetermined value at the node Nb is not transmitted to the output through the branches  203  and  205 , and voltage-limiting protection is achieved.  
         [0028]     For the embodiment shown in  FIG. 2 , the activation conditions and the response time of the overcurrent and overvoltage protection circuits can be easily modified by changing the circuit design parameters. For example, the activation condition and the sensitivity of the voltage-limiting protection circuit can be modified by changing the resistance of the resistor  3  and/or changing the breakdown voltage of the Zener diode  2 . The response time of the current limiter  38  is characterized by the resistance of the resistors  11 - 13  and/or the driving capability of the transistors  5  and  6 . By changing the value of the resistor  16  and/or the diode  17 , the time delay when the overcurrent protection turns on can be modified. Improving the drive capability of the transistors  14  and  15  and/or adding more controlled current sources can also speed up the response time of the overcurrent protection mechanism. For instance, the controlled current sources in the overcurrent protector  40  can be replaced with other kinds of controlled current sources or current mirrors. The voltage limiter  42  can also be realized with a comparator and a controlled current source. The current limiter  38  can also be regarded as a controlled current source and thus is interchangeable with the current source structure in the overcurrent protector  40 . Based on the above descriptions, the diagram in  FIG. 2  is one of the possible embodiments of  FIG. 1  and can be easily modified to apply to many electronic devices.  
         [0029]     Please refer to  FIG. 2 ,  FIG. 3 , and  FIG. 4 .  FIG. 3  is an IV (current-voltage) plot of the overvoltage protection response of the circuit in  FIG. 2 . In  FIG. 2 , when the input voltage Vin exceeds the predetermined value, the transistors  1  and  2  stop conducting and disconnect the battery from the power supply for overvoltage protection. In  FIG. 4 , when the charging current approaches or exceeds the predetermined value at the point Pa, the current-limiting protection turns on and the charging current is maintained at a fixed value. When the current increases to the point Pb, the overcurrent protection turns on and melts down the fuse  21  to quickly disconnect the battery from the power supply.  
         [0030]     Please refer to  FIG. 1 ,  FIG. 2 , and  FIG. 5 . FIG. 5  is a diagram in which the charging protection circuit  30  is used in an electronic device  50 . A processing circuit  52  controls the operations of the electronic device  50  and a battery  54  is used to store power for the processing circuit  52 . The charging protection circuit  30  can be installed at the input end of the battery  54 . In the charging process, the charging protection circuit  30  is coupled to a power supply  56  and the battery  54  to provide the current-limiting, overcurrent, and overvoltage protections. The electronic device  50  can be a mobile phone. The processing circuit can include antennas, wireless communication circuits, microphones, speakers, man-machine interfaces, microprocessors, memories, and so on. The electronic device  50  can also be a digital camera, a PDA, a portable computer, a portable media player, etc.  
         [0031]     In conclusion, compared to the known techniques of the prior art, the present invention can realize an electronic-circuit-based overcurrent protection mechanism to supplement a fuse, providing high sensitivity, a fast response, and a robust charging protection mechanism both for electronic devices and users.  
         [0032]     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.