Patent Publication Number: US-2012044602-A1

Title: Regulating system having overvoltage protection circuit and overcurrent protection circuit

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to regulating systems and, particularly, to a regulating system having an overvoltage protection circuit and an overcurrent protection circuit. 
     2. Description of Related Art 
     Power circuits are widely used in various electronic products such as computers notebooks, and LCD monitors. Most current power circuits have a complicated configuration to include a regulating system with an overvoltage protection circuit and an overcurrent protection circuit for regulating output voltage of the power circuits. 
     Therefore, it is desirable to provide a new regulating system which can overcome the above-mentioned limitations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Many aspects of the present disclosure should be better understood with reference to the following drawing. The components in the drawing are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present disclosure. 
       The FIGURE shows a circuit diagram of a regulating system according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure will now be described in detail with reference to the drawing. 
     The FIGURE shows a regulating system  20 , according to an exemplary embodiment of the present disclosure. The regulating system  20  includes an input port  21  and an output port  22 . The regulating system  20  further includes an overvoltage protection circuit  220 , a regulating circuit  240 , and an overcurrent protection circuit  260 . 
     The overvoltage protection circuit  220  includes a fuse R 0 , a semiconductor controlled rectifier (SCR)  207 , a first resistor R 1 , and a voltage regulator Q 1 . The fuse R 0  includes a first end  201  and a second end  202 . The SCR  207  includes an anode  203 , a cathode  204 , and a gate  205 . When a working voltage is added on the gate  205 , the anode  203  will be electronically connected to the cathode  204 . The voltage regulator Q 1  includes a cathode  161  and an anode  162 . The first end  201  of the fuse R 0  is connected to the input port  21 . The second end  202  of the fuse R 0  is connected to the anode  203  of the SCR  207 . The cathode of the SCR  207  is connected to the ground. The gate  205  of the SCR is connected to the anode  162  of the voltage regulator Q 1  through the first resistor R 1 . The cathode of the voltage regulator Q 1  is connected to the output port  22 . 
     The regulating circuit  240  includes a metal oxide semiconductor (MOS) transistor Q 2 , a second resistor R 2 , a third resistor R 3 , a fourth resistor R 4 , and a regulating unit Q 3 . The MOS transistor Q 2  includes a drain electrode  124 , a gate electrode  125 , and a source electrode  126 . The regulating unit Q 3  includes an anode  121 , a cathode  122 , and a reference terminal  123 . The regulating unit Q 3  is capable of automatically adjusting a voltage of the cathode  122  according to a voltage of the reference terminal  123 . In one embodiment, the voltage of the cathode  122  increases following a voltage decrease of the reference terminal  123  and decreases following a voltage increase of the reference terminal  123 . The regulating unit Q 3  is a three-terminal adjustable voltage regulator. 
     The drain electrode  124  of the MOS transistor Q 2  is connected to the second end  202  of the fuse R 0 . The gate electrode  125  of the MOS transistor Q 2  is connected to the cathode  122  of the regulating unit Q 3  and is connected to the second end  202  of the fuse R 0  via the fourth resistor R 4 . The second resistor R 2  and the third resistor R 3  are connected in series between the output port  22  and the ground. The anode  121  of the regulating unit Q 3  is grounded. The reference terminal  123  of the regulating unit Q 3  connects to a node “a” between the first second resistor R 2  and the third resistor R 3 . 
     The overcurrent protection circuit  260  includes a first bipolar transistor Q 4  and a fifth resistor R 5 . An emitter of the bipolar transistor Q 4  is connected to the output port  22 . A collector of the bipolar transistor Q 4  is connected to the gate electrode  125  of the MOS transistor Q 2 . A base of the bipolar transistor Q 4  is connected to the source electrode  126  of the MOS transistor Q 2  and is also connected to the output port  22  via the fifth resistor R 5 . In one embodiment, the bipolar transistor Q 4  is an NPN bipolar transistor. 
     The node “a” between the first resistor R 1  and the second resistor R 2  is defined to be a first reference point. A node “b” between the gate electrode  125  of the MOS transistor Q 2  and the cathode  122  of the regulating unit Q 3  is defined to be a second reference point. 
     In operation, the input port  21  receives a power supply from an external circuit (not shown). The regulating system  20  generates an output voltage which is output from the output port  22 . 
     When the output voltage of the output port  22  decreases, a first reference voltage of the first reference point “a”, which is a divided voltage of the output voltage, is correspondingly decreased. Since the reference terminal  123  of the regulating unit Q 3  is connected to the first reference point “a”, the voltage of the second reference point “b” increases following a voltage decrease of the reference terminal  123 . Therefore, a voltage of the source electrode of the MOS transistor Q 2  correspondingly increases based on the characteristic of the MOS transistor Q 2 , so as to compensate for the voltage decrease of the first output terminal  14   a.    
     On the contrary, when the output voltage of the output port  22  increases, the first reference voltage of the first reference point “a” is correspondingly increased. The voltage of the cathode  122  of the regulating unit Q 3  correspondingly decreases and the voltage of the source electrode of the MOS transistor Q 2  correspondingly decreases to compensate for the voltage increase of the first output terminal  14   a.    
     Because voltage between the base and the emitter of the bipolar transistor Q 4  is approximately equal to 0.7V, the bipolar transistor Q 4  turns on when the current flowing through the fifth resistor R 5  increases to reach 0.7V divided by a resistance value “r5” of the fifth resistor R 5 , namely 0.7V/r5. That is, the maximum voltage across the fifth resistor R 5  is limited to be 0.7V by the bipolar transistor Q 4 , a maximum current flowing through the fourth resistor R 4  is approximately equal to 0.7V/r5. Therefore, the maximum current output from the output port  14  is also limited to 0.7V/r5 to achieve overcurrent protection function. 
     When the output voltage of the output port  22  increases to a breakdown voltage of the voltage regulator Q 1 . The voltage regulator Q 1  will then be broken down and the output voltage will load on the gate  205  of the SCR  207 , thereby turning on the SCR  207 . In this case, the second end  202  of the fuse R 0  is connected to the ground, which leads to the fuse R 0  to perform an overvoltage protection function. 
     It is to be understood, however, that even though numerous characteristics and advantages of certain inventive embodiments have been set out in the foregoing description, together with details of the structures and functions of the embodiments, the disclosure is illustrative only; and that changes may be made in detail, especially in matters of arrangement of parts within the principles of present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.