Abstract:
A control circuit has an input terminal for receiving an input voltage, an energy accumulator for storing electric energy from the input terminal and supplying power to an electrical load, and an over-voltage protection unit for lowering the voltage at the input terminal when the voltage at an output terminal of the energy accumulator exceeds a predetermined threshold value. A motor device combines the control circuit with an electric motor as the load.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This non-provisional patent application claims priority under 35 U.S.C. §119(a) from Patent Application No. 201010138038.7 filed in The People&#39;s Republic of China on Mar. 26, 2010. 
       FIELD OF THE INVENTION 
       [0002]    This invention relates to an electrical power supply and in particular, to a power supply having over-voltage protection. 
       BACKGROUND OF THE INVENTION 
       [0003]    A known control circuit for a fan powered by an AC mains voltage and driven by a DC motor uses a capacitor to decrease the AC mains voltage, and a rectifier for rectifying the decreased AC voltage to a DC voltage. When the load is small or capacitance of the capacitor is large, the output voltage of the rectifier will increase, which could possibly damage electronic components powered by the rectifier. 
       SUMMARY OF THE INVENTION 
       [0004]    Hence there is a desire for a control circuit providing over-voltage protection for a power supply. 
         [0005]    Accordingly, in one aspect thereof, the present invention provides a control circuit comprising: an input terminal for receiving an input voltage; an energy accumulator for storing electric energy from the input terminal and supplying power to an electrical load; and an over-voltage protection unit for lowering the voltage at the input terminal when the voltage at an output terminal of the energy accumulator exceeds a predetermined threshold value. 
         [0006]    Preferably, a first switching unit is arranged between the input terminal and the energy accumulator and is configured to be conductive to electrically connect the input terminal to the energy accumulator when the voltage at the output terminal of the energy accumulator is lower than the threshold value and to be non-conductive to electrically disconnect the input terminal from the energy accumulator when the voltage at the output terminal of the energy accumulator exceeds the threshold value. 
         [0007]    Preferably, the first switching unit is a diode, the anode of which is electrically connected to the input terminal. 
         [0008]    Preferably, the over-voltage protecting unit comprises: a detecting unit which is configured to detect the voltage at the output terminal of the energy accumulator and produce a detecting signal indicating whether the detected voltage exceeds the threshold value; a second switching unit which is electrically connected between the input terminal and a voltage less than the threshold value and is configured to be conductive when the detecting signal indicates the detected voltage exceeds the threshold value and to be non-conductive when the detecting signal indicates the detected voltage is lower than the threshold value. 
         [0009]    Preferably, the second switching unit is electrically connected between the input terminal and a ground voltage. 
         [0010]    Preferably, the predetermined threshold value is essentially equal to the nominal voltage at the input terminal. 
         [0011]    Preferably, the control circuit includes a converter for converting an AC voltage to a DC voltage to be supplied to the input terminal. 
         [0012]    Preferably, a voltage decreasing unit is provided for decreasing a higher AC voltage to a lower AC voltage to be converted by the converter. Preferably, the voltage decreasing unit comprises an adjustable capacitor unit with adjustable capacitance. 
         [0013]    Preferably, the energy accumulator comprises a capacitor. 
         [0014]    According to a second aspect, the present invention provides a motor device comprising a motor and incorporating a control circuit as described above, to supply power to the motor. Preferably, the motor is a brushless direct current motor. 
         [0015]    Advantages of embodiments of the present invention include that the reliability of the circuit is high and the circuit cost is low. Besides, the speed of the motor can be easily adjusted. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    A preferred embodiment of the invention will now be described, by way of example only, with reference to figures of the accompanying drawings. In the figures, identical structures, elements or parts that appear in more than one figure are generally labelled with a same reference numeral in all the figures in which they appear. Dimensions of components and features shown in the figures are generally chosen for convenience and clarity of presentation and are not necessarily shown to scale. The figures are listed below. 
           [0017]      FIG. 1  is a block diagram of a control circuit in accordance with the preferred embodiment of the present invention; and 
           [0018]      FIG. 2  shows a voltage decreasing unit of the control circuit in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0019]    A control circuit  204 , in accordance with the preferred embodiment of the present invention, is used for controlling a single phase brushless DC motor having a permanent magnet rotor and a stator with stator windings  208 . The stator windings represent an electrical load. The control circuit  204  is powered by an AC power supply  206  and comprises a voltage decreasing unit  220 , an A-D converter  240 , an energy accumulator  210 , a driving unit  260 , a low-voltage generating unit  280 , an over-voltage protecting unit  230 , and a first switching unit  250 . 
         [0020]    The voltage decreasing unit  220  comprises an adjustable capacitor unit  222  ( FIG. 2 ) with adjustable capacitance for decreasing an AC voltage output from the AC power supply  206 . The adjustable capacitor unit  222  is electrically connected between the AC power supply  206  and the A-D converter  240 . The adjustable capacitor unit comprises at least two fixed capacitors  228  with fixed capacitance and a control unit  226  for respectively and selectively, electrically connecting the at least two fixed capacitors  228  between the AC power supply  206  and the A-D converter. The control unit  226  comprises at least two switches  221  which are respectively electrically connected in series with the at least two fixed capacitors  228 . Each switch  221  and a corresponding capacitor  228  form a switching capacitor and the at least two switching capacitors are electrically connected in parallel with each other. When a switch  221  is closed or conductive, its corresponding capacitor  228  is electrically connected between the AC power supply and the A-D converter. When a switch  221  is open or non-conductive, its corresponding capacitor  228  is electrically disconnected from the circuit with the AC power supply  206 . Therefore, by selectively controlling each switch  221 , the fixed capacitors  228  can be electrically connected or disconnected between the AC power supply  206  and the A-D converter  240  to adjust the capacitance of the adjustable capacitor unit  222 , thereby adjusting the output voltage of the voltage decreasing unit. The output voltage from the voltage decreasing unit is adjusted by adjusting the capacitance of the adjustable capacitor unit, thus the speed of the motor can be easily adjusted by a simple and low cost circuit. 
         [0021]    The A-D converter  240  is configured to convert the decreased AC voltage output of the voltage decreasing unit  220  to a DC voltage. Preferably, the A-D converter  240  comprises a bridge rectifier circuit. 
         [0022]    The energy accumulator  210  comprises a capacitor arranged to be charged by the output of the A-D converter  240  and to supply power directly or indirectly to the electrical load, i.e., the stator windings  208 . 
         [0023]    The low-voltage generating unit  280  is configured to generate a lower DC voltage from the output DC voltage of the energy accumulator  210 . The low-voltage generating unit  280  comprises a resistor and a Zener diode. One end of the resistor is electrically connected with the positive terminal of the capacitor  210 , the other end is electrically connected with the cathode of the Zener diode, and the anode of the Zener diode and the negative terminal of the capacitor  210  are electrically connected with ground. Further, a filtering capacitor electrically connected in parallel with the Zener diode may be arranged to make smooth or to stabilize the output of the low-voltage generating unit  280  at the cathode of the Zener diode. 
         [0024]    The driving unit  260  comprises a position detecting unit  262 , a controller  264  and an inverter  266 . The position detecting unit  262  may be a Hall sensor which is powered by the lower DC voltage and configured to detect the position of the rotor of the BLDC motor and output a corresponding position signal. The controller  264  is also powered by the lower DC voltage and configured to respond to the position signal to output a corresponding commutation signal. The controller  264  can be realized by a microcontroller or by a switching circuit constituted by electronic components such as resistor and switch. The inverter  266  is powered by the DC voltage output from the energy accumulator  210  and configured to respond to the commutation signal to control the power to the motor. 
         [0025]    The over-voltage protecting unit  230  is configured to carry out over-voltage protection for the DC voltage output from the energy accumulator  210  to prevent electronic components powered by the DC voltage from being damaged. The over-voltage protecting unit  230  comprises a detecting unit  232  and a second switching unit  234 . The detecting unit  232  is configured to detect the DC voltage at the output terminal of the energy accumulator  210  and to produce a detecting signal indicating whether the detected DC voltage exceeds a predetermined threshold value. 
         [0026]    The second switching unit  234  is configured to be conductive (closed) to lower the DC voltage at the output terminal of the A-D converter  240  to a voltage which is less than the threshold voltage when the detecting signal indicates that the detected DC voltage exceeds the threshold value, and to be non-conductive (open) to allow the A-D converter to function normally and output a DC voltage for the energy accumulator  210  when the detecting signal indicates that the detected DC voltage is lower than the threshold value. Preferably, the lower voltage is a ground voltage. This can be easily achieved by electrically connecting the second switching unit  234  between the output terminal of the A-D converter  240  and ground. 
         [0027]    The first switching unit  250  is electrically connected between the output terminal of the A-D converter  240  and the energy accumulator  210 . The first switching unit  250  is configured to be conductive to electrically connect the output of the A-D converter  240  to the input of the energy accumulator  210  when the DC voltage at the output terminal of the energy accumulator  210  is lower than the threshold value and to be non-conductive (open) to electrically disconnect the energy accumulator  210  from the A-D converter  240  when the DC voltage at the output terminal of the energy accumulator  210  is higher than the threshold value. Preferably, the first switching unit  250  is a diode, the anode of which is electrically connected with the output terminal of the A-D converter  240  and the cathode of which is electrically connected with the positive terminal of the energy accumulator  210 . 
         [0028]    The detecting unit  232  detects the output voltage of the energy accumulator  210 . If the output voltage of the energy accumulator  210  exceeds the predetermined threshold value, the second switching unit  234  is made conductive and the output voltage of the A-D converter  240  is reduced, preferably to 0 volts. As a result, the output voltage of the energy accumulator  210  is higher than the output voltage of the A-D converter  240 , the first switching unit  250  electrically disconnects the A-D converter  240  from the energy accumulator  210 , and the energy accumulator  210  supplies power to the motor from the stored energy. With the stored energy of the energy accumulator  210  being consumed, the output voltage of the energy accumulator  210  decreases. When the detecting unit  232  detects the output voltage of the energy accumulator  210  is lower than the threshold value, the second switching unit  234  is opened. As a result, the output voltage of the A-D converter  240  is higher than the output voltage of the energy accumulator  210 , the first switching unit  250  electrically connects the A-D converter  240  to the energy accumulator  210 , and the energy accumulator  210  is charged by the output of the A-D converter  240  and supplies power to the motor. Thus, the output voltage of the energy accumulator  210  can be kept basically stable relative to the threshold value, over-voltage protection can be realized and the reliability of the circuit can be improved. Besides, as electronic components with relatively lower voltage-resistant grade could be used in the circuit powered by the AC voltage output from the voltage decreasing unit, the circuit cost could also be lowered. 
         [0029]    In embodiments of the present invention, the A-D converter  240 , the energy accumulator  210 , the driving unit  260 , the over-voltage protecting unit  230  and the low-voltage generating unit  280  may be arranged on a single printed circuit board, and the adjustable capacitor unit  222  may be mechanically independent from the printed circuit board but electrically connected with the printed circuit board. Also, while the threshold voltage may be any desired value, it is preferred that the threshold voltage is substantially equal to the nominal voltage of the input terminal. The nominal voltage is the desired normal operating voltage, i.e., the design value of the output voltage of the A-D converter. 
         [0030]    In the description and claims of the present application, each of the verbs “comprise”, “include”, “contain” and “have”, and variations thereof, are used in an inclusive sense, to specify the presence of the stated item but not to exclude the presence of additional items. 
         [0031]    Although the invention is described with reference to one or more preferred embodiments, it should be appreciated by those skilled in the art that various modifications are possible. Therefore, the scope of the invention is to be determined by reference to the claims that follow.