Abstract:
The present invention provides a power source protection device comprising a switching circuit and an overload circuit coupled between the switching circuit and power source for driving the switching circuit to limit current stopping the increase in the current output by the power source. Thus, the device is protected against overcurrent damage and maintains stable system operation.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention is relates in general to a protection device for a power source, and more specifically to the use of a switching circuit, an overload protection circuit, and a short protection circuit for protecting an electronic device. 
   2. Description of the Related Art 
   The simplest current limiting component is a fuse, and it can be used alone or with a protection component. The fuse is capable of preventing over current impact, hence it is essential to a system as it is the final defense against system breakdown. 
   Presently, there are two electron protection components for power supplies in common use. One is a general fuse, which has the drawback of only one time use, and requiring replacement change after burning out. Thus, it used a chance exists that must be returned to the manufacturer for maintenance. 
   Another type protection component is a poly switch, which possesses the advantage of continuous use after providing the protection function, with the defect of high operational temperature.  FIG. 1  is a conventional temperature and current diagram of poly switch. A represents the characteristic curve of model nanoSMD, microSMD or miniSMD, B represents the characteristic curve of the model SMDH160. The protection current of poly switch increases with a temperature in the surrounding area causing the working range of voltage to be limited by the temperature in the  FIG. 1 . A poly switch, however, can be used continuously and only requires periodic replacement, thus it offers an advantage over the standard fuse. 
   The description of a poly switch is as follows. The poly switch can protect against circuit damage by adjusting itself resistance when high current occurs. Once the current goes back to predetermined level, the poly switch automatically reverts to the normal level of resistivity. These operational characteristics of the poly switch make it rational the ideal choice for power supply and communication applications, due to the fact that changing a battery or connecting device with USB plug may generate instant high current. The poly switch typically replaces traditional glass fuse in some circuits, but is used for mainly in USB applications. 
   The general fuse stops current by melting and the poly switch limits current by adjusting its impedance according to current passing. Both must be triggered by over current heat to implement the protection function in the induction circuit. 
   The general fuse and poly switch solutions are in widespread use, however, there is limitation above. Therefore, the present invention provides better temperature characteristics than a poly switch for limiting the current and stopping the output of a power source, and without replacement every time over current occurs. 
   SUMMARY OF THE INVENTION 
   Therefore, the object of the present invention is to provide a power source protection device, and more specifically to use a switching circuit, an overload protection circuit, and a short protection circuit for protecting an electronic device. 
   The present invention achieves the above-indicated objects by providing a power source protection device comprising a switching circuit, an overload protection circuit, and a short protection circuit. 
   The switch circuit comprises an input terminal coupled to a power source and an output terminal for outputting the power from the power source. When the current outputs from the power source equal or greater than a first predetermined current, the switching circuit is driven to limit the current stopping the increase in the current output by the power source. Thus, the device is protected against overcurrent damage and maintains stable system operation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention can be more fully understood by reading the subsequent detailed description and examples with reference made to the accompanying drawings, wherein: 
       FIG. 1  is a characteristic diagram of temperature and current of a conventional; 
       FIG. 2  is a block diagram of a power source protection device according to first embodiment of the present invention; 
       FIG. 3  is a block diagram of a power source protection device according to second embodiment of the present invention; 
       FIG. 4  is a block diagram of an electronic device with protection device according to third embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   First Embodiment 
     FIG. 2  is a block diagram of a power source protection device according to first embodiment of the present invention. A switching circuit  20  comprises a switching transistor  202  and a bias circuit  204  in  FIG. 2 . The input port of switching circuit  20  is coupled to a power source  50  and the output port outputs the power Vout. An overload protection circuit  10  is coupled between the switching circuit  20  and power source  50  comprising a detection unit  102  and a first protection circuit  104 . A short protection circuit  30  comprising a short detection unit  302  and a second protection unit  304  is coupled to the output terminal of switching circuit  20 . 
   The overload protection circuit  10  comprises the detection unit  102  and the first protection unit  104 . The detection unit  102  is coupled between the input pole of switching transistor  202  and power source  50  for detecting the output current of the power source, when the current equal or greater than the first current level, an overload signal V 1  is output. The first protection unit  104  is coupled to the output of detection unit  102 . When the first protection unit  104  receives the overload signal V 1 , the first protection unit  104  can output a first control signal S 1 . The first control signal S 1  passes through the bias circuit  204  to control the bias of the control pole of switching transistor  202  limit the current output by power source  50 , to limit it in an acceptable current range. 
   The short protection circuit  30  comprises the short detection unit  302  and the second protection unit  304 . The short detection unit  302  is coupled to the output terminal of switching circuit  20 , when the output terminal of the switch circuit  20  is short, a short signal V 2  is then output. The second protection unit  304  is coupled between the short detection unit  302  and the switching circuit  20 , when the second protection unit  304  receives the short signal V 2 , a second control signal S 2  is output. The second control signal S 2  will change the bias that provides by the bias circuit for closing the switching transistor  202  that output the power Vout with the breakup for operating the protection of power source. 
   Second Embodiment 
     FIG. 3  is a block diagram of a power source protection device according to second embodiment of the present invention. The detection unit  102  comprises a starting resistor RI, first protection unit  104  comprises a first transistor Q 1 , switching transistor  202  comprises a second transistor Q 2 , short detection unit  302  comprises a schottky diode D 1 , and the bias circuit  204  comprised a first bias resistance R 1 , a second bias resistance R 2 , and a third bias resistance R 3 . The bias circuit  204  couples to the power source  50  and the control pole of second transistor Q 2  of for providing a bias voltage to turn on the second transistor Q 2 , then passing the power through the emitter pole of second transistor Q 2 . 
   The starting resistance RI is coupled between the emitter of second transistor Q 2  and power source  50  in  FIG. 3 . When the flow of current  11  of starting resistance RI satisfies a first current, wherein the product of current  11  and starting resistance RI is equal to a particular voltage, for example 0.6 volts. At this time, the starting resistance RI outputs an overload signal V 1  (0.6 volts) to turn on the first transistor Q 1 . When the first transistor Q 1  receives the overload signal V 1  (0.6 volts), the first transistor Q 1  then outputs a first control signal S 1 . The first control signal S 1  passes thorough the first bias resistance R 1  to control the bias of second transistor Q 2  for driving the second transistor Q 2  to limit the current output by power source  50  to within an acceptable current range. 
   The schottky diode D 1  is coupled between the second transistor Q 2  and the base of third transistor Q 3 . When the power Vout is short, the schottky diode D 1  outputs a short signal V 2  (0.2 volts) to the base of third transistor Q 3 , wherein the short signal V 2  (0.2 volts) cuts off the third transistor Q 3 . The second control signal S 2  passes through the second bias circuit directing the bias circuit  204  to close the second transistor Q 2  outputting power Vout by breakup. 
   Third Embodiment 
     FIG. 4  is a block diagram of an electronic device with protection device according to third embodiment of the present invention. The difference being a power protection device  70  in an electronic device  80  requires an internal circuit  40 . The second protection unit  304  can also output a state signal L 3  to display device  60  for displaying operational status on indication, current limiting or breakup output of power from switching circuit  20 . 
   The present invention provides the following advantages over the related art. As the system or equipment functions only a short circuit or over current occurs, present invention protect presents damage to the system or equipment. The present invention has better temperature characteristics and provides improved over efficiency a poly switch. 
   While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.