Control circuit and motor device

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.

CROSS REFERENCE TO RELATED APPLICATIONS

This non-provisional patent application claims priority under 35 U.S.C. §119(a) from Patent Application No. 201010138038.7 filed in The People's Republic of China on Mar. 26, 2010.

FIELD OF THE INVENTION

This invention relates to an electrical power supply and in particular, to a power supply having over-voltage protection.

BACKGROUND OF THE INVENTION

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

Hence there is a desire for a control circuit providing over-voltage protection for a power supply.

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.

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.

Preferably, the first switching unit is a diode, the anode of which is electrically connected to the input terminal.

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.

Preferably, the second switching unit is electrically connected between the input terminal and a ground voltage.

Preferably, the predetermined threshold value is essentially equal to the nominal voltage at the input terminal.

Preferably, the control circuit includes a converter for converting an AC voltage to a DC voltage to be supplied to the input terminal.

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.

Preferably, the energy accumulator comprises a capacitor.

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.

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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A control circuit204, 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 windings208. The stator windings represent an electrical load. The control circuit204is powered by an AC power supply206and comprises a voltage decreasing unit220, an A-D converter240, an energy accumulator210, a driving unit260, a low-voltage generating unit280, an over-voltage protecting unit230, and a first switching unit250.

The voltage decreasing unit220comprises an adjustable capacitor unit222(FIG. 2) with adjustable capacitance for decreasing an AC voltage output from the AC power supply206. The adjustable capacitor unit222is electrically connected between the AC power supply206and the A-D converter240. The adjustable capacitor unit comprises at least two fixed capacitors228with fixed capacitance and a control unit226for respectively and selectively, electrically connecting the at least two fixed capacitors228between the AC power supply206and the A-D converter. The control unit226comprises at least two switches221which are respectively electrically connected in series with the at least two fixed capacitors228. Each switch221and a corresponding capacitor228form a switching capacitor and the at least two switching capacitors are electrically connected in parallel with each other. When a switch221is closed or conductive, its corresponding capacitor228is electrically connected between the AC power supply and the A-D converter. When a switch221is open or non-conductive, its corresponding capacitor228is electrically disconnected from the circuit with the AC power supply206. Therefore, by selectively controlling each switch221, the fixed capacitors228can be electrically connected or disconnected between the AC power supply206and the A-D converter240to adjust the capacitance of the adjustable capacitor unit222, 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.

The A-D converter240is configured to convert the decreased AC voltage output of the voltage decreasing unit220to a DC voltage. Preferably, the A-D converter240comprises a bridge rectifier circuit.

The energy accumulator210comprises a capacitor arranged to be charged by the output of the A-D converter240and to supply power directly or indirectly to the electrical load, i.e., the stator windings208.

The low-voltage generating unit280is configured to generate a lower DC voltage from the output DC voltage of the energy accumulator210. The low-voltage generating unit280comprises a resistor and a Zener diode. One end of the resistor is electrically connected with the positive terminal of the capacitor210, 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 capacitor210are 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 unit280at the cathode of the Zener diode.

The driving unit260comprises a position detecting unit262, a controller264and an inverter266. The position detecting unit262may 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 controller264is also powered by the lower DC voltage and configured to respond to the position signal to output a corresponding commutation signal. The controller264can be realized by a microcontroller or by a switching circuit constituted by electronic components such as resistor and switch. The inverter266is powered by the DC voltage output from the energy accumulator210and configured to respond to the commutation signal to control the power to the motor.

The over-voltage protecting unit230is configured to carry out over-voltage protection for the DC voltage output from the energy accumulator210to prevent electronic components powered by the DC voltage from being damaged. The over-voltage protecting unit230comprises a detecting unit232and a second switching unit234. The detecting unit232is configured to detect the DC voltage at the output terminal of the energy accumulator210and to produce a detecting signal indicating whether the detected DC voltage exceeds a predetermined threshold value.

The second switching unit234is configured to be conductive (closed) to lower the DC voltage at the output terminal of the A-D converter240to 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 accumulator210when 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 unit234between the output terminal of the A-D converter240and ground.

The first switching unit250is electrically connected between the output terminal of the A-D converter240and the energy accumulator210. The first switching unit250is configured to be conductive to electrically connect the output of the A-D converter240to the input of the energy accumulator210when the DC voltage at the output terminal of the energy accumulator210is lower than the threshold value and to be non-conductive (open) to electrically disconnect the energy accumulator210from the A-D converter240when the DC voltage at the output terminal of the energy accumulator210is higher than the threshold value. Preferably, the first switching unit250is a diode, the anode of which is electrically connected with the output terminal of the A-D converter240and the cathode of which is electrically connected with the positive terminal of the energy accumulator210.

The detecting unit232detects the output voltage of the energy accumulator210. If the output voltage of the energy accumulator210exceeds the predetermined threshold value, the second switching unit234is made conductive and the output voltage of the A-D converter240is reduced, preferably to 0 volts. As a result, the output voltage of the energy accumulator210is higher than the output voltage of the A-D converter240, the first switching unit250electrically disconnects the A-D converter240from the energy accumulator210, and the energy accumulator210supplies power to the motor from the stored energy. With the stored energy of the energy accumulator210being consumed, the output voltage of the energy accumulator210decreases. When the detecting unit232detects the output voltage of the energy accumulator210is lower than the threshold value, the second switching unit234is opened. As a result, the output voltage of the A-D converter240is higher than the output voltage of the energy accumulator210, the first switching unit250electrically connects the A-D converter240to the energy accumulator210, and the energy accumulator210is charged by the output of the A-D converter240and supplies power to the motor. Thus, the output voltage of the energy accumulator210can 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.

In embodiments of the present invention, the A-D converter240, the energy accumulator210, the driving unit260, the over-voltage protecting unit230and the low-voltage generating unit280may be arranged on a single printed circuit board, and the adjustable capacitor unit222may 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.