Abstract:
A power supply device supplying power to various electric devices or inverters of a DC motor. The power supply device can detect an over-voltage which is applied to various electric devices or inverters of a DC motor from an external power source or an over-voltage caused by defects of power lines. The power supply device can break power, such that power supply circuits or systems can be protected against damage and malfunction thereof. The power supply device includes a rectifier rectifying power from an external AC power source, a smoothing unit, and an over-voltage protection circuit determining whether the rectified voltage includes an over-voltage, and breaking the over-voltage before the over-voltage is applied to the smoothing unit if the determination is positive.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of Korean Patent Application No. 10-2006-0006862, filed on Jan. 23, 2006 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a power supply technology, and, more particularly, to a power supply device supplying power to various electric devices or inverters of a DC motor, which is capable of detecting an over-voltage which is applied to the electric devices or inverters, from an external power source, or an over-voltage caused by defects in power lines, and breaking power, such that power supply circuits or systems can be protected against damage and malfunction thereof. 
         [0004]    2. Description of the Related Art 
         [0005]    Generally, a power supply circuit rectifies a single phase or three phase power input from an external power source through rectifiers including a plurality of diodes, smoothes the rectified power through a capacitor to transform the power to a DC voltage, and then supplies the DC voltage to inverters or electric devices, etc. 
         [0006]      FIG. 1  is a block diagram illustrating a conventional three-phase power supply device. 
         [0007]    As shown in  FIG. 1 , the three-phase power supply device includes a three-phase AC power source  2 , a first rectifier  4 , a first inrush current limiter  6 , a first smoothing unit  8 , a first inverter  10 , a first motor  12 , a second rectifier  14 , a second smoothing unit  16 , a first switching mode power supply (SMPS)  18 , a first controller  20 , and a phase detector  22 . 
         [0008]    The three-phase AC power source  2  supplies AC power of three phases, R-, S-, and T-phases, thereto, and to supply a single phase voltage thereto, in which the single phase voltage is generated between a T-phase power line and a neutral power line N. The first rectifier  4  converts the AC power supplied from the three-phase power source  2  to a full wave rectified voltage. The first inrush current limiter  6  limits inrush current generated when the AC power is initially applied thereto. 
         [0009]    The first smoothing unit  8  smoothes the full wave rectified voltage rectified in the first rectifier  4  to transform the rectified voltage to a DC voltage. The first inverter  10  converts the DC voltage of the first smoothing unit  8  to an AC voltage driving the first motor  12 . The first inverter  10  includes a plurality of switching elements operated by control signals of the first controller  20 . The first motor  12  is driven by the three-phase power supplied from the first inverter  10 . The first motor  12  is implemented with a brush less direct current (BLDC) motor, etc. 
         [0010]    The second rectifier  14  rectifies the AC power supplied between the T-phase power line of the three-phase AC power source  2  and the neutral power line N. The second smoothing unit  16  smoothes the full wave rectified voltage from the second rectifier  14 . 
         [0011]    The first SMPS  18  transforms the DC voltage from the second smoothing unit  16  to DC voltages operating the first controller  20  and the phase detector  22  and then supplies the DC voltages thereto. 
         [0012]    The phase detector  22  detects position and speed of a rotor of the first motor  12  and transmits this information to the first controller  20 , such that the first controller  20  can control the first inverter  10  using information transmitted from the phase detector  22 . 
         [0013]    When over-voltage is applied to the second rectifier  14  as the two-phase power lines (R- and S-) are connected to one another, power is not supplied to the second rectifier  14  through the T-phase power line and the neutral power line N, and the first SMPS  18  malfunctions or breaks down. Also, when over-voltage is applied to the first rectifier  4  by such a malfunction of the three-phase AC power source  2 , the circuit malfunctions or breaks down. 
         [0014]    Therefore, an over-voltage protection circuit is used in the power supply device of  FIG. 1  in order to prevent malfunctions of the system from breaking down, due to the over-voltage. 
         [0015]      FIG. 2  is a conventional over-voltage protection circuit. 
         [0016]    As shown in  FIG. 2 , the conventional over-voltage protection circuit  40 , which is located between the second rectifier  14  and the second smoothing unit  16 , includes a capacitor C 4 , a current detector  42 , a breaking unit  44  and an over-voltage detector  46 . 
         [0017]    The capacitor C 4  absorbs an over-voltage, which is generated for a short period when AC power is initially input thereto, and the current detector  42  prevents a transient current from flowing. 
         [0018]    The over-voltage detector  46  detects voltage between both leads of a capacitor C 3  of the second smoothing unit  16  and compares the detected voltage with a predetermined voltage to control ON/OFF operations of the breaking unit  44 . On the other hand, the breaking unit  44  is turned on/off according to the control of the over-voltage detector  46 , such that power supply is controlled from the second rectifier  14  to the first SMPS  18 . 
         [0019]      FIG. 3  illustrates waveforms according to operations of the conventional over-voltage protection circuit, in which horizontal and vertical axes denote time and voltage, respectively. 
         [0020]    More specifically, the upper waveform of  FIG. 3  illustrates a voltage input to the over-voltage protection circuit, and the lower waveform of  FIG. 3  illustrates a voltage Vin output when the over-voltage protection circuit is operated as a voltage V is input thereto. 
         [0021]    As shown in  FIG. 3 , since the conventional over-voltage protection circuit  40  is operated as voltage of both ends of the capacitor C 3  of the second smoothing unit  16  is detected and compared with the predetermined voltage Vdet, when an over-voltage exceeding the predetermined voltage Vdet is input thereto, the over-voltage detector  46  controls the breaking unit  44  so as not to supply power thereto. 
         [0022]    When power is cut off, charge of the capacitor C 4  is discharged on the basis of a time constant such that the potential of the capacitor is decreased. Therefore, the over-voltage detector  46  controls the breaking unit  44  such that power can be re-supplied thereto. However, since over-voltage is still input thereto after the power is supplied, the over-voltage detector  46  controls the breaking unit  44  again so as not to supply power thereto. Namely, such power supply and break operations are repeatedly performed in the over-voltage detector  46  and the breaking unit  44 . 
         [0023]    Therefore, when the over-voltage is input thereto, the conventional over-voltage protection circuit does not completely break power, instead a pulsating voltage is output as shown in the lower graph of  FIG. 3 . When such a pulsating voltage is input to the system, the system operates unstably or malfunctions. 
       SUMMARY OF THE INVENTION 
       [0024]    Accordingly, it is an aspect of the present invention to provide a power supply device which is capable of breaking power supplying a system when over-voltage from an AC power source is applied or an over-voltage caused by a defect of power lines, etc., is applied. 
         [0025]    Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the invention. 
         [0026]    The foregoing and/or other aspects of the invention are achieved by providing a power supply device comprising: a rectifier rectifying a voltage from an external AC power source; a smoothing unit; and an over-voltage protection circuit determining whether the rectified voltage includes an over-voltage, and breaking the over-voltage before the over-voltage is applied to the smoothing unit if determined that the rectified voltage includes an over-voltage. 
         [0027]    The over-voltage protection circuit may include: a voltage divider performing voltage division of the rectified voltage; a voltage comparator comparing the divided voltage from the voltage divider with a predetermined reference voltage and outputting signals for an over-voltage or a normal voltage, in which the predetermined reference voltage is included in the voltage comparator; a switching element which is turned on/off according to the signals output from the voltage comparator; and a voltage generator generating a driving voltage of the switching element. The over-voltage protection circuit may be located between the rectifier and the smoothing unit. 
         [0028]    The over-voltage protection circuit determines whether the rectified voltage output from the rectifier includes an over-voltage, based on comparing the rectified voltage with a predetermined reference voltage. 
         [0029]    The over-voltage protection circuit may perform circuit connection such that a normal voltage is supplied to the smoothing unit, if the rectified voltage is determined as the normal voltage, when the over-voltage has been broken. 
         [0030]    The foregoing and/or other aspects may also be achieved by providing a power supply device comprising: a rectifier rectifying a voltage from an external AC power source; a smoothing unit; and an over-voltage protection circuit. Here, the over-voltage protection circuit includes: a voltage divider performing voltage division of the rectified voltage; a voltage comparator comparing the divided rectified voltage with a predetermined reference voltage and to determine an over-voltage and outputting an over-voltage signal or a normal voltage signal, in which the predetermined reference voltage is included in the voltage comparator; a switching element which is turned on/off according to the output of the voltage comparator; and a voltage generator generating a driving voltage for the switching element, wherein the over-voltage protection circuit breaks the over-voltage before the over-voltage is supplied to the smoothing unit, if determined that the divided rectified voltage includes the over-voltage. 
         [0031]    The over-voltage protection circuit may be located between the rectifier and the smoothing unit. The voltage comparator may output the over-voltage signal if the rectified voltage is greater than the reference voltage and the normal voltage signal if the rectified voltage is less than the reference voltage. The switching element may be turned off if the voltage comparator outputs the over-voltage signal, and is turned on if the voltage comparator outputs the normal voltage signal. 
         [0032]    The switching element is turned on if the voltage comparator outputs a normal voltage signal when the switching element has been turned off. The rectifier and the smoothing unit are electrically cut off from one another if the switching element is turned off, and connected to one another if the switching element is turned on. 
         [0033]    The foregoing and/or other aspects may also be achieved by providing a device comprising: a three-phase four-wire power supply device comprising three phase power lines and a neutral power line; a three-phase rectifier rectifying three-phase AC power to a rectified voltage; a first smoothing unit smoothing the rectified voltage of the three-phase rectifier; a single-phase rectifier rectifying a single-phase AC power to a rectified voltage, in which the single-phase AC power is generated between one of the three phase power lines and the neutral power line; a second smoothing unit smoothing the rectified voltage of the single-phase rectifier; and first and second over-voltage protection circuits. Here, the first over-voltage protection circuit determines whether the rectified voltage of the three-phase rectifier includes an over-voltage, and breaks the over-voltage before the over-voltage is supplied to the first smoothing unit, if the rectified voltage of the three-phase rectifier includes the over-voltage. The second over-voltage protection circuit determines whether the rectified voltage of the single-phase rectifier includes an over-voltage, and breaks the over-voltage before the over-voltage is supplied to the second smoothing unit, if the rectified voltage of the single-phase rectifier includes the over-voltage. 
         [0034]    Each of the first and second over-voltage protection circuits may include: a voltage divider performing voltage division of the respective rectified voltage; a voltage comparator comparing the divided voltage with a predetermined reference voltage and outputting signals for the over-voltage or a normal voltage, in which the predetermined reference voltage is included in the voltage comparator; a switching element which is turned on/off according to the output of the voltage comparator; and a voltage generator generating a driving voltage for the switching element. 
         [0035]    The first over-voltage protection circuit determines whether the rectified voltage of the three-phase rectifier includes the over-voltage, based on comparing the rectified voltage of the three-phase rectifier with a first predetermined reference voltage. The second over-voltage protection circuit determines whether the rectified voltage of the single-phase rectifier includes the over-voltage, based on comparing the rectified voltage of the singe-phase rectifier with a second predetermined reference voltage. 
         [0036]    If the rectified voltage of the three-phase rectifier is greater than the first predetermined reference voltage, the first over-voltage protection circuit determines that the rectified voltage of the three-phase rectifier includes the over-voltage, and if the rectified voltage of the three-phase rectifier is less than the first predetermined reference voltage, determines that the rectified voltage of the three-phase rectifier is the normal voltage. 
         [0037]    If the rectified voltage of the single-phase rectifier is greater than the second predetermined reference voltage, the second over-voltage protection circuit determines that the rectified voltage of the single-phase rectifier includes the over-voltage and if the rectified voltage of the single-phase rectifier is less than the second predetermined reference voltage, determines that the rectified voltage of the single-phase rectifier is the normal voltage. 
         [0038]    The first over-voltage protection circuit performs circuit connection such that a normal voltage is supplied to the first smoothing unit, if the rectified voltage of the three-phase rectifier is determined as the normal voltage, when the over-voltage has been broken. 
         [0039]    The second over-voltage protection circuit performs circuit connection such that a normal voltage is supplied to the second smoothing unit, if the rectified voltage of the single-phase rectifier is not an over-voltage, when the over-voltage has been broken. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0040]    These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
           [0041]      FIG. 1  is a block diagram illustrating a conventional three-phase power supply device; 
           [0042]      FIG. 2  is a conventional over-voltage protection circuit; 
           [0043]      FIG. 3  illustrates waveforms according to operations of the conventional over-voltage protection circuit; 
           [0044]      FIG. 4  is a block diagram illustrating a three-phase power supply device including an over-voltage protection circuit according to an embodiment of the present invention; 
           [0045]      FIG. 5  is a block diagram illustrating a single-phase power supply device including an over-voltage protection circuit according to another embodiment of the present invention; 
           [0046]      FIG. 6  is a detailed illustration of the second over-voltage protection circuit of  FIG. 4 ; 
           [0047]      FIG. 7  is a circuit diagram of a voltage detector included in an over-voltage protection circuit of  FIG. 4 ; and 
           [0048]      FIG. 8  illustrates simulation waveforms when an over-voltage is applied to the over-voltage protection circuit of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0049]    Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below to explain the present invention by referring to the figures. 
         [0050]      FIG. 4  is a block diagram illustrating a three-phase power supply device including an over-voltage protection circuit according to an embodiment of the present invention. 
         [0051]    As shown in  FIG. 4 , the three phase power supply device according to the embodiment of the present invention includes a three-phase AC power source  102 , a first rectifier  104 , a first over-voltage protection circuit  105 , a first inrush current limiter  106 , a first smoothing unit  108 , a first inverter  110 , a first motor  112 , a second rectifier  114 , a second over-voltage protection circuit  115 , a second smoothing unit  116 , a first switching mode power supply (SMPS)  118 , a first controller  120 , and a phase detector  122 . 
         [0052]    The three-phase AC power source  102 , which includes three power lines for R-, S- and T-phases, and a neutral power line N, supplies 380V thereto among the three power lines, and 220V thereto between the T-phase power line and a neutral power line N. 
         [0053]    The first rectifier  104 , which includes 6 diodes, D 1  to D 6 , converts the AC power from the three-phase power source to  102  to a full wave rectified voltage. The first inrush current limiter  106  may be implemented with a relay and an inrush current limitation resistor, etc. to limit inrush current generated when the AC power is initially applied thereto. 
         [0054]    The first over-voltage protection circuit  105 , which is located between the first rectifier  104  and the first smoothing unit  108 , inputs a voltage output from the first rectifier  104  and compares the output voltage with a predetermined reference voltage, such that power supply can be controlled on the basis of the comparison result so as not to apply an over-voltage to a system connected thereto. 
         [0055]    The first smoothing unit  108 , which includes two capacitors C 1  and C 2 , and resistors R 1  and R 2  which are connected, in parallel, to the two capacitors C 1  and C 2  smooth the full wave rectified voltage rectified in the first rectifier  104  to transform the rectified voltage to a DC voltage. Here, the resistors R 1  and R 2  function as a voltage divider, such that the respective capacitors C 1  and C 2  can be biased by an equal voltage to compensate for characteristics of the capacitors C 1  and C 2 . 
         [0056]    The first inverter  110  converts the DC voltage of the first smoothing unit  108  to an AC voltage to drive the first motor  112 . The first inverter  110  includes a plurality of switching elements operated by control signals of the first controller  120 . The first motor  112  is driven by the three-phase power, such that the three-phase power is supplied to the first motor  112  as the switching elements of the first inverter  110  are turned on/off. The first motor  112  is implemented with a brush-less direct current (BLDC) motor, etc. 
         [0057]    The second rectifier  114 , which includes four diodes, serves to rectify the AC power supplied between the T-phase power line of the three-phase AC power source  102  and the neutral power line N. 
         [0058]    The second over-voltage protection circuit  115 , which is located between the second rectifier  114  and the second smoothing unit  116 , receives a voltage output from the second rectifier  114  and compares the output voltage with a predetermined reference voltage, such that power supply to the first SMPS  118  can be controlled on the basis of the comparison result so as not to apply an over-voltage to the first SMPS  18 . 
         [0059]    The second smoothing unit  116 , which is implemented with a capacitor C 13 , smoothes the full wave rectified voltage from the second rectifier  114 . The first SMPS  118  transforms the DC voltage from the second smoothing unit  116  to DC voltages operating the first controller  120  and the phase detector  122  and then supplies the DC voltages thereto. 
         [0060]    The first controller  120  and the phase detector  122  receive the power from the first SMPS  118 . The phase detector  122  detects position and speed of a rotor of the first motor  112  and transmits this information to the first controller  120 , such that the first controller  120  can control the switching elements of the first inverter  110  using information transmitted from the phase detector  122 , thereby driving the first motor  112 . 
         [0061]      FIG. 5  is a block diagram illustrating a single-phase power supply device including an over-voltage protection circuit according to another embodiment of the present invention. 
         [0062]    As show in  FIG. 5 , the single-phase power supply device includes a single phase AC power source  24 , a third rectifier  26 , a third over-voltage protection circuit  27 , a second inrush current limiter  28 , a third smoothing unit  30 , a second inverter  32 , a second motor  34 , a second SMPS  36 , and a second controller  38 . 
         [0063]    The single phase AC power source  24  supplies a single phase AC power thereto. The third rectifier  26 , which is implemented with four diodes D 7  to D 10 , converts the single phase AC power from the single phase AC power source  24  to a full wave rectified voltage. 
         [0064]    The third over-voltage protection circuit  27 , which is located between the third rectifier  26  and the third smoothing unit  30 , receives a voltage output from the third rectifier  26  and compares the output voltage with a predetermined reference voltage, such that power supply to the second SMPS  36  can be controlled on the basis of the comparison result so as not to apply an over-voltage to the second SMPS  36 . 
         [0065]    The second inrush current limiter  28  limits inrush current generated when the AC power is initially applied thereto. The third smoothing unit  30 , which is implemented with a capacitor C 5 , o smoothes the full wave rectified voltage rectified in the third rectifier  26  to convert the smoothed voltage to a DC voltage. The second inverter  32  converts the DC voltage from the third smoothing unit  30  to an AC voltage operating the second motor  34 . The second inverter  32  includes switching elements operated by control signals of the second controller  38 . 
         [0066]    The second SMPS  36  transforms the DC voltage from the third smoothing unit  30  to DC voltages operating the second controller  38  and then supplies the DC voltages thereto. The second controller  38  inputs the DC voltages from the second SMPS  36  and then controls the second inverter  32 , which drives the second motor  34 . 
         [0067]      FIG. 6  is a detailed illustration of the second over-voltage protection circuit  115  of  FIG. 4  in detail. 
         [0068]    As shown in  FIG. 6 , the over-voltage protection circuit  115  includes resistors R 3  and R 4 , which are connected to one another in series between both ends a and c of the second rectifier  114 , and a capacitor C 6  which is connected to both ends of the resistor R 4  in parallel. Also, the over-voltage protection circuit  115  includes a resistor R 5 , one end of which is connected to the end c of the second rectifier, and a voltage detector IC whose cathode lead K is connected to another end of the resistor R 5 , whose reference lead R is connected to the node which is commonly connected to ends of R 3 , R 4 , and C 6 , and whose anode lead A is connected to the end a of the second rectifier. 
         [0069]    In addition, the over-voltage protection circuit includes a Zener diode Vz whose one end is connected to a node between the cathode K of the voltage detector IC and the resistor R 5 , and whose other end is connected to the anode lead A of the voltage detector IC, a capacitor C 7  which is connected to the Zener diode in parallel Vz, a resistor R 6  whose one end is connected to the end of the capacitor C 7 , a switching transistor SW whose gate is connected to another end of the resistor R 6 , whose collector is connected to one end b of the second smoothing unit  116 , and whose emitter is connected to the end a of the second rectifier, and a resistor R 7  whose both ends are connected to the gate and the emitter of the switching transistor SW, respectively. 
         [0070]    The resistors R 3  and R 4  function as a voltage divider, which divides the voltage output from the second rectifier  114  into a proper voltage to be input to the voltage detector IC. The capacitor C 6  removes noise included in the voltage to be input to the voltage detector IC. 
         [0071]    The voltage detector IC compares the input voltage with a predetermined reference voltage, and then outputs the comparison result, such that the switching transistor SW can be turned on/off. The resistor R 5  and the Zener diode Vz generate a voltage driving the switching transistor SW using a voltage applied to the over-voltage protection circuit  115 . The capacitor C 7  removes noises in the over-voltage protection circuit  115 . 
         [0072]    The resistor R 6  is used for current limitation, and the resistor R 7  stabilizes a voltage supplied to the gate of the switching transistor SW. 
         [0073]      FIG. 7  is a circuit of a voltage detector IC included in the over-voltage protection circuit  115  of  FIG. 4 , which is implemented with a single IC chip. 
         [0074]    As shown in  FIG. 7 , the voltage detector IC includes a reference voltage generator generating a reference voltage Vref, a comparator OP and a transistor TR. The comparator OP compares a voltage input through a reference lead R thereof with the reference voltage Vref, in which the input voltage is previously generated through voltage division. When the input voltage is greater than the reference voltage Vref, the comparator OP outputs a high level signal such that the transistor TR can be turned on. On the other hand, if the input voltage is less than the reference voltage Vref, the comparator OP outputs a low level signal such that the transistor TR can be turned off. 
         [0075]    With reference to  FIG. 6  and  FIG. 7 , operations of the over-voltage protection circuit  115  according to the embodiment of the present invention are described in detail below. 
         [0076]    First, when the full wave rectified voltage of the second rectifier  114  is divided by the resistors R 3  and R 4  and then applied to the reference lead R of the voltage detector IC, the comparator OP of the voltage detector IC compare the input voltage with the reference voltage Vref. 
         [0077]    When the input voltage is greater than the reference voltage Vref, the comparator OP outputs a high level signal such that the transistor TR can be turned on. When the transistor TR is turned on, since the voltage between the cathode lead K and the anode lead A of the voltage detector IC is close to zero, the switching transistor SW is turned off. In that case, the leads a and b of the over-voltage protection circuit  115  are electrically broken from one another. Therefore, the power is not transmitted from the second rectifier  114  to the second smoothing unit. 
         [0078]    On the other hand, when the input voltage is less than the reference voltage Vref, the comparator OP outputs a low level signal such that the transistor TR can be turned off. In that case, the driving voltage, which is generated by the resistor R 5  and the Zener diode Vz of the over-voltage protection circuit  115 , is applied to the gate of the switching transistor SW, such that the switching transistor SW can be turned on. Therefore, the power is transmitted from the second rectifier  114  to the second smoothing unit  116 . 
         [0079]      FIG. 8  illustrates simulation waveforms in a case that an over-voltage is applied to the over-voltage protection circuit  115 . 
         [0080]    More specifically, waveform D denotes current flowing between the leads a and b of the over-voltage protection circuit  115 , in which the current does not flow therebetween before a voltage is applied thereto and after an over-voltage is applied thereto, except for a short period d where a current briefly flows when a voltage is applied thereto. Here, such a period d is a period from a time point when the switching transistor SW is turned on as a voltage is applied thereto to a time point when the switching transistor is turned off again as an over-voltage is detected. 
         [0081]    Waveform E denotes a voltage between the gate and the emitter of the switching transistor, in which the voltage is maintained below the threshold voltage when an over-voltage has been detected. 
         [0082]    As shown in  FIG. 8 , the over-voltage protection circuit  115  detects the over-voltage and maintains the voltage between the gate and the emitter of the switching transistor SW below the threshold voltage, thereby turning off the switching transistor SW. Therefore, the over-voltage cannot be applied to the SMPS  118 . 
         [0083]    Although the second over-voltage protection circuit  115  of  FIG. 6  is illustrated in detail to describe the over-voltage protection circuit of  FIG. 4 , the structure and function thereof are the same as the first and third over-voltage protection circuits  105  and  27 , only except that values of the respective elements in the respective protection circuits are different from each other according to magnitudes of voltages applied to the protection circuits. 
         [0084]    As mentioned above, the power supply device according to the embodiment of the present invention can compare a voltage from the external power source with the predetermined reference voltage to detect over-voltages, and break the power supplied to the system when the over-voltages are applied to the system, thereby protecting the system against the over-voltages. 
         [0085]    Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.