Abstract:
An over current protection method for a power supply includes steps as follows. Firstly, an output current value of the power supply is received. Secondly, whether the power supply has an over current status is determined in accordance with the received output current value. Thirdly, if the over current status occurs, the over current status is monitored to check whether the over current status continues for more than a setting period of time. If the over current status continues for more than the setting period of time, the power supply is disabled. On the other hand, if the over current status is eliminated with the setting period of time, a process flow returns to the first step of receiving an output current value of the power supply.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates in general to an over current protection method for a power supply and apparatus thereof, and more particularly to a method to detect an over current status of a power supply and apparatus thereof. 
         [0003]    2. Description of the Related Art 
         [0004]    Limited Power Sources (LPS) regulation is set by safety administration to provide a safety standard for a load to avoid an over current that may cause a fire accident. When a power supply apparatus passes the LPS safety regulation&#39;s test, the power supply apparatus is allowed to use inferior fireproofing material. 
         [0005]    In order to pass the LPS safety regulation&#39;s test, the conventional measurements are as follows. 
         [0006]    Firstly, a fuse is used as over current protection. With reference to  FIG. 6 , a fuse  71  is coupled to an output terminal of a power supply  70 . When an output current of the power supply  70  is larger than a standard short current of the LPS safety regulation, the fuse  71  must melt within one minute to secure. Although this method can achieve the objective to protect the over current, once the fuse  71  melts, the fuse  71  can not recover automatically. Hence the power supply is unable to work until the fuse  71  is replaced. 
         [0007]    Secondly, a constant current protection method is used. This method is to set a constant value as the maximum output current of the power supply. With reference to  FIG. 7 , a constant current circuit  80  is coupled to a power supply  70 . The constant current circuit  80  checks an output current of the power supply  70  and determine whether the output current is larger than a setting constant current value. If determining result is positive, the constant current circuit  80  disables the power supply  70 . In this way, the output current of the power supply  70  is limited to the constant value, so that the power supply  70  is not suitable to be used for an electronic device with variable large loads. 
       SUMMARY OF THE INVENTION 
       [0008]    An objective of the present invention is to provide an over current protection method for a power supply and apparatus thereof. The present invention is aimed to effectively overcome the disadvantages of the conventional over current protection measurements that the fuse can not auto-recover and the power supply can not provide the larger current than the setting constant value. 
         [0009]    In order to achieve the above objective, the over current protection method for a power supply and apparatus thereof is provided. The over current protection method includes multiple steps as follows. 
         [0010]    Firstly, an output current value of the power supply is received. Secondly, whether the power supply has an over current status is determined in accordance with the received output current value. Thirdly, if the over current status occurs, the over current status is monitored to check whether the over current status continues for more than a setting period of time. If the over current status continues for more than the setting period of time, the power supply is disabled. On the other hand, if the over current status is eliminated with the setting period of time, a process flow returns to the first step of receiving an output current value of the power supply. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a block diagram of an over current protection apparatus coupled to a power loop; 
           [0012]      FIG. 2  is a flow diagram of an over current protection method in accordance with the present invention; 
           [0013]      FIG. 3  is a current curve diagram of a power supply; 
           [0014]      FIGS. 4A to 4C  are parts of a detailed circuit diagram of the over current protection apparatus in accordance the present invention; 
           [0015]      FIG. 5  is a detailed circuit diagram of a control unit in accordance with the present invention; 
           [0016]      FIG. 6  is a block diagram of a first conventional over current protection device coupled to a power loop; and 
           [0017]      FIG. 7  is a block diagram of a second conventional over current protection device coupled to the power loop in accordance with the prior art. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    With reference to  FIG. 1 , a power supply  20  is connected to a load to establish a power loop. An over current protection apparatus  100  is coupled to the power loop to detect a present output current of the power supply  20  and executes an over current protection method in accordance with the present invention. Further, the over current protection apparatus  100  presets a standard value and a target time, and is connected to the power supply  20   
         [0019]    With reference to  FIG. 2 , the over current protection method has following steps of: 
         [0020]    (a) receiving a present output current of the power supply to obtain the value of the present output current  201 ; 
         [0021]    (b) determining whether the value of the present output current is larger than standard value  202 , wherein if a determining result is negative, return to step (a), and if the determining result is positive an over current status is existed; 
         [0022]    (c) counting the target time  203 ; 
         [0023]    (d) receiving a present output current of the power supply to compare with the standard value to re-determine the over current status is still existed  204 ; wherein if a determining result is negative, stop counting  207  and return to the step (a) and if the determining result is positive, the over current status is continuously existed; 
         [0024]    (e) determining whether the target time is ended  205 , wherein if a determining result is negative, go to the step (d) and if the determining result is positive, turn off the power supply  206 . 
         [0025]    The aforesaid working operation principle in accordance with the present invention can be illustrated as a characteristic curve figure in  FIG. 3 . When the power supply works, an output current lo is undulated that may be higher than the standard value of a dotted line along a transverse axle. Once the output current Io is larger than the standard value, which occurs the over current status, the target time (td) is started to count. If the over current status continues for a period of time t which is shorter than the target time (td), the counting step is stopped. On the contrary, if the over current status continues for a period of time t which is longer than the target time td, the power supply is turned off. With the aforesaid power on delay design, the large number of current value can be supplied to a variable load and also the LPS safety requirement can be met. 
         [0026]    With reference to  FIGS. 4A to 4C , a detailed circuit diagram of a preferred embodiment of an over current protection apparatus  100  in accordance with the present invention is shown. The over current protection apparatus  100  has a first over current protection circuit  10  and a second over current protection circuit  10 ′. A structure and an operation method of the two over current protection circuits  10  and  10 ′ are exactly the same. The purpose to prepare two identical over current protection circuits  10  and  10 ′ is to make the first over current protection circuit  10  works normally and the second over current protection circuits  10 ′ works as a backup circuit. If the first over current protection circuits  10  is out of order, the second over current protection circuits  10 ′ can replace the first over current protection circuits  10  to provide a protection measurement. Since the two over current protection circuits  10  and  10 ′ are identical, the following description takes the first over current protection circuit  10  as the example to illustrate. 
         [0027]    The first over current protection circuit  10  has a control unit  11 , a delay unit  12  and a turning off signal generator  13 . 
         [0028]    In this preferred embodiment, the control unit  11  is made up by a number NCP  4300  controller IC 3  together with other peripheral components. With reference to  FIG. 5 , an internal part of the controller IC 3  has two OP amplifiers, which is defined as a first comparator  111  and a second comparator  112  in accordance with the present invention. A positive input terminal +IN 2  of the second comparator  112  forms an output current detection terminal CS 1  for coupling with an output terminal of a power supply via an output resistor (not shown in the diagram), so as to receive an output current value of the power supply. The positive input terminal +IN 2  compares the received current with a reference electric potential of a negative terminal −IN 2 . In accordance with a comparison result, the electric potential of an output signal of the second comparator  112  is determined. A negative terminal −IN 1  of the first comparator  111  is coupled with an output terminal of the delay unit  12  for determining whether the electric potential of an output signal of the delay unit  12  is larger than the electric potential of a positive terminal +IN 1 . In accordance with a comparison result, the electric potential of an output signal of the first comparator  111  is determined. 
         [0029]    The delay unit  12  includes a transistor Q 1  and a charging and discharging circuit. The charging and discharging circuit is made up by a resistor R 4  and a capacitor C 3 , which are series-connected between a power source VO and the ground terminal. Fully charged time T is determined by a resistance of the resistor R 4  and a capacitance of the capacitor C 3 . The resistor R 4  and the capacitor C 3  are coupled to the negative terminal −IN 1  of the first comparator  111  of the control unit  11  by a serial-connected node. Moreover, in this preferred embodiment, the transistor Q 1  is an NPN transistor. Abase of the NPN transistor is coupled to the output terminal of the second comparator  112  of the control unit  11 , so as to be a control terminal. A collector and an emitter of the transistor Q 1  are cross-coupled on two terminals of the capacitor C 3 . In other words, when the transistor Q 1  is cutoff, the capacitor C 3  is charged via the resistor R 4 . When the capacitor C 3  is charged for a period of time, the capacitor C 3  is fully charged and generates an electric potential on the negative terminal −IN 1  larger than the positive terminal +IN 1  of the first comparator  111 . Hence the first comparator  111  outputs a low electric potential. On the contrary, when the capacitor C 3  is charged, if the transistor Q 1  is conductive, the two terminals of the capacitor C 3  is short and the capacitor C 3  stops to be charged. At this moment, the first comparator  111  outputs a high electric potential. 
         [0030]    The turning off signal generator  13  has an optical isolator  131  and an electric switch  132 . The electric switch  132  is made up by a silicon controlled rectifier SCR. A gate of the silicon controlled rectifier SCR is coupled with an output terminal of the optical isolator  131 . A positive pole and a negative pole of the silicon controlled rectifier SCR are used for coupling with the power supply to provide a turning off signal. An input terminal of the optical isolator  131  is coupled to the output terminal of the first comparator  111  of the control unit  11  for determining whether the turning off signal generator  13  should send out the turning off signal or not in accordance with the output electric potential of the first comparator  111 . 
         [0031]    The detailed circuit structure of the preferred embodiment in accordance with the present invention can be clearly understood from the above description. With reference to  FIGS. 4A to 4C , an operation method of the present invention for a power supply is further illustrated as follows. When a power supply (not shown in the diagram) outputs current normally, the second comparator  112  of the control unit  11  outputs a high electric potential to make the transistor Q 1  of the delay unit  12  conductive. At this moment, the capacitor C 3  is short and not charged. When the power supply outputs the current larger than the setting value, the second comparator  112  outputs a low electric potential to make the transistor Q 1  of the delay unit  12  cut-off. At this moment, the capacitor C 3  starts to be charged and also achieves fully charged within a setting charged time in accordance with the resistance of the resistor R 4  and the capacitance of the capacitor C 3 . In this preferred embodiment, the setting charged time is of 45 seconds. The setting charged time also provides delay buffer duration. During the delay buffer duration, if the power supply outputs the current that is lower than the setting value, the transistor Q 1  is conductive again and the turning off signal generator  13  is not enabled. At this moment, the power supply still operates normally. 
         [0032]    On the other hand, if the output current of the power supply continues to be larger than the setting value until the capacitor C 3  is fully charged, the output terminal of the first comparator  111  of the control unit  11  outputs the low electric potential and the turning off signal generator  13  is enabled. Then the electric switch  132  provides a disconnection action to the power supply to disable the power supply, so as to achieve the objective of the over current protection. 
         [0033]    It can be clearly understood from the above circuit design that the over current protection apparatus in accordance with the present invention allows the power supply to output the undulated current. With the over current protection method and apparatus thereof in accordance with the present invention, even when the power supply outputs the current larger than the setting value, the power supply will not be disabled immediately. With the delay buffer duration, the large number of current can be acquired and also the LPS safety requirement can be met, which is very useful for large tools. 
         [0034]    While the invention has been described by way of example and in terms of a preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.