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A method and a circuit for controlling a motor and a brushless motor using the same are provided. The brushless motor includes three phase coils, wherein the first terminals of all the phase coils are coupled to a common node. The method includes following steps: among the above-mentioned three phas

A method and a circuit for controlling a motor and a brushless motor using the same are provided. The brushless motor includes three phase coils, wherein the first terminals of all the phase coils are coupled to a common node. The method includes following steps: among the above-mentioned three phase coils, when there is no current flowing through the first phase coil of the above-mentioned three phase coils and a current flows from the second terminal of the second coil to the second terminal of the third phase coil, detecting the voltage at the second terminal of the first phase coil to be a first specific voltage; detecting the voltage drop of a DC sensing resistor to be a second specific voltage; and utilizing the first specific voltage, the second specific voltage and the DC voltage supplied to the motor to estimate zero crossing points for controlling the motor.

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What is claimed is: 1. A method for controlling a motor, comprising: providing a motor, wherein the motor comprises at least three phase stator coils, each stator coil comprises a first terminal and a second terminal and the first terminals of the stator coils are coupled to a common node; providin

What is claimed is: 1. A method for controlling a motor, comprising: providing a motor, wherein the motor comprises at least three phase stator coils, each stator coil comprises a first terminal and a second terminal and the first terminals of the stator coils are coupled to a common node; providing a first power voltage; providing at least three upper-bridge switches, wherein each upper-bridge switch is respectively coupled between one of the second terminals of the above-mentioned three phase stator coils and the first power voltage; providing at least three lower-bridge switches, wherein each lower-bridge switch is respectively coupled between one of the second terminals of the above-mentioned three phase stator coils and the first end of an impedance element, wherein the second end of the impedance element is coupled to a second power voltage and the second power voltage is less than the first power voltage; among the stator coils, when there is no current flowing through a first stator coil, the upper-bridge switch coupled by the second terminal of the second stator coil is turned on and the lower-bridge switch coupled by the second terminal of the third stator coil is turned on: detecting the voltage at the second terminal of the first stator coil and defining the detected voltage as a first specific voltage; detecting the voltage drop across both ends of the impedance element and defining the detected voltage as a second specific voltage; judging whether a zero-crossing occurs by using the first power voltage, the first specific voltage and the second specific voltage; among the stator coils, when there is no current flowing through the first stator coil, the upper-bridge switch coupled by the second terminal of the second stator coil is turned off and the lower-bridge switch coupled by the second terminal of the third stator coil is turned on; and performing a proportion operation on the first specific voltage to judge whether a second zero-crossing occurs. 2. The method for controlling a motor according to claim 1, wherein when the first power voltage is represented by Vdd, the first specific voltage is represented by Vopen and the second specific voltage is represented by Vidc, then, the step ‘judging whether a zero-crossing occurs by using the first power voltage, the first specific voltage and the second specific voltage’ comprises: respectively reducing Vopen, Vdd and Vidc in a specific proportion into three voltages Vopen, Vdd and Vidc; scaling Vopen, Vdd and Vidc in a proportion of 2:1:1 and mixing the scaled voltages into a back electromotive force signal; and deciding the first zero-crossing occurs when the back electromotive force signal is greater than a predetermined voltage. 3. The method for controlling a motor according to claim 1, wherein the step ‘performing a proportion operation on the first specific voltage to judge whether a second zero-crossing occurs’ comprises: reducing the first specific voltage in a specific proportion into a back electromotive force signal; and deciding the second zero-crossing occurs when the back electromotive force signal is greater than a predetermined voltage. 4. The method for controlling a motor according to claim 1, further comprising following steps: judging which of the first zero-crossing and the second zero-crossing occurs earlier; and among the first zero-crossing and the second zero-crossing, selecting the earlier occurred zero-crossing and using the selected zero-crossing to control the motor for commutation. 5. A circuit for controlling a motor, wherein the motor comprises at least three phase stator coils, each stator coil comprises a first terminal and a second terminal, the first terminals of all the stator coils are coupled to a common node, the second terminal of each of the stator coils is coupled to at least an upper-bridge switch and at least a lower-bridge switch, each of the upper-bridge switches respectively determines whether a power voltage is supplied to one of the stator coils and each lower-bridge switch is coupled to a common voltage via an impedance element; circuit for controlling a motor comprising: a selection circuit, coupled to the second terminals of the stator coils, wherein when the upper-bridge switches or lower-bridge switches coupled by the second terminals of the first phase coil and the second phase coil are switched according to a pulse-width-modulation (PWM) signal, the voltage at the second terminal of the third phase coil is taken as a first voltage and coupled to the output terminal of the selection circuit; a zero-crossing detection unit, coupled to the output terminal of the selection circuit, wherein when the upper-bridge switch coupled by the second terminal of the first phase coil is turned on and the lower-bridge switch thereof is turned off, the power voltage and the voltage across the impedance element in the first voltage are removed to obtain a first back electromotive force voltage, the first back electromotive force voltage is compared with a first reference voltage and a first zero-crossing judgement signal is output, the zero-crossing detection unit comprising: a first zero-crossing detection circuit, for removing the power voltage and the voltage across the impedance element in the first voltage to obtain the first back electromotive force voltage, comparing the back electromotive force voltage with the first reference voltage, and outputting the first zero-crossing judgement signal when the upper-bridge switch coupled by the second terminal of the first phase coil is turned on and the lower-bridge switch thereof is turned off; and a second zero-crossing detection circuit, for taking the first voltage as a second back electromotive force voltage, comparing the second back electromotive force voltage with a second reference voltage, and outputting a second zero-crossing judgement signal when the upper-bridge switch coupled by the second terminal of the first phase coil is turned off; and a control circuit, for judging when a zero-crossing occurs according to the first zero- crossing judgement signal so as to control the motor. 6. The circuit for controlling a motor according to claim 5, wherein the selection circuit comprises a first proportionally voltages-reducing circuit for reducing the voltage at the second terminal of the third stator coil in a predetermined proportion to obtain the first voltage, and the first zero-crossing detection circuit comprises: a second proportionally voltages-reducing circuit, for reducing the power voltage in the predetermined proportion, defining the reduced power voltage as a second voltage, reducing the voltage across the impedance element in the predetermined proportion and defining the reduced voltage as a third voltage; and an analog differential amplifier, for scaling the first voltage, the second voltage and the third voltage in a proportion of 2:1:1, subtracting the scaled second voltage and the scaled third voltage from the scaled first voltage, amplifying the scaled first voltage after the above- mentioned subtraction operations to obtain the first back electromotive force voltage and outputting the first back electromotive force voltage. 7. The circuit for controlling a motor according to claim 6, wherein the first zero-crossing detection circuit further comprises: a reference voltage generator, for generating the first reference voltage, wherein when the upper-bridge switch coupled by the second terminal of the first phase coil is turned on, the lower-bridge switch thereof is turned off, the lower-bridge switch of the second phase coil is turned on and there is a current flowing through the third phase coil from the first terminal to the second terminal thereof, then, the first reference voltage is set to be a higher voltage level; when the level of the first back electromotive force voltage is lower than the higher voltage level, the first reference voltage is set to be a middle voltage level; when the level of the first sampling circuit back electromotive force voltage is higher than the middle voltage level, the first reference voltage is set to be a lower voltage level; a comparator, comprising a positive input terminal, a negative input terminal and an output terminal, wherein the positive input terminal receives the first back electromotive force voltage, the negative input terminal receives the first reference voltage and the output terminal outputs a comparison voltage; and a sampling circuit, coupled to the output terminal of the comparator to receive the comparison voltage for sampling the comparison voltage to obtain the first zero-crossing judgement signal when the upper-bridge switch of the first stator coil is turned on. 8. The circuit for controlling a motor according to claim 6, wherein the first zero-crossing detection circuit further comprises: a reference voltage generator, for generating the first reference voltage, wherein when the upper-bridge switch coupled by the second terminal of the first phase coil is turned on, the lower-bridge switch thereof is turned off, the lower-bridge switch of the second phase coil is turned on and there is a current flowing through the third phase coil from the second terminal to the first terminal thereof, then, the first reference voltage is set to be a lower voltage level; when the level of the first back electromotive force voltage is higher than the higher voltage level, the first reference voltage is set to be a middle voltage level; when the level of the first sampling circuit back electromotive force voltage is lower than the middle voltage level, the first reference voltage is set to be a higher voltage level; a comparator, comprising a positive input terminal, a negative input terminal and an output terminal, wherein the positive input terminal receives the first back electromotive force voltage, the negative input terminal receives the first reference voltage and the output terminal outputs a comparison voltage; and a sampling circuit, coupled to the output terminal of the comparator to receive the comparison voltage for sampling the comparison voltage to obtain the first zero-crossing judgement signal when the upper-bridge switch of the first stator coil is turned on. 9. The circuit for controlling a motor according to claim 5, wherein the second zero-crossing detection circuit further comprises: a comparator, comprising a positive input terminal, a negative input terminal and an output terminal, wherein the positive input terminal receives the second back electromotive force voltage, the negative input terminal receives the second reference voltage and the output terminal outputs a comparison voltage; and a sampling circuit, coupled to the output terminal of the comparator to receive the comparison voltage for sampling the comparison voltage to obtain the second zero-crossing judgement signal when the upper-bridge switch of the first stator coil is turned on. 10. The circuit for controlling a motor according to claim 5, wherein the control circuit receives the first zero-crossing judgement signal and the second zero-crossing judgement signal to obtain a zero-crossing indication signal and outputs a commutation-indicating signal according to the zero-crossing indication signal, wherein the commutation-indicating signal transits the state thereof according to the following rule: calculating i pieces of zero-crossing time prior to a state transition of the commutation-indicating signal, wherein the definition of the plurality of zero-crossing time comprises the following (1) or (2): (1) a time interval between a state transition of the zero-crossing indication signal from a first state to a second state and a state transition of the zero-crossing indication signal from the second state to the first state; (2) a time interval between a state transition of the zero-crossing indication signal from the second state to the first state and a state transition of the zero-crossing indication signal from the first state to the second state, wherein the k-th zero-crossing time is represented by Tk, i and k are natural number and 0<k<i; assigning each zero-crossing time with a weight value and the weight value of the k-th zero-crossing time is represented by Gk; defining a delay time Td with Td=(G1×T1+ . . . +Gk×Tk+ . . . +Gi×Ti)/(2×(G1+ . . . +Gi)); and transiting the state of the commutation-indicating signal in the delay time after the first zero-crossing time. 11. A brushless motor system, comprising: a motor, comprising at least three phase stator coils, wherein each stator coil comprises a first terminal and a second terminal, and the first terminals of all the stator coils are coupled to a common node; a plurality of upper-bridge switches, wherein at least one of the upper-bridge switches is coupled between the second terminal of each stator coil and a power voltage and each upper-bridge switch respectively decides whether a power voltage is supplied to the stator coils; an impedance element, wherein the first terminal of the impedance element is coupled to a common node voltage; a plurality of lower-bridge switches, wherein at least one of the lower-bridge switches is coupled between the second terminal of each stator coil and the second terminal of the impedance element; a selection circuit, coupled to the second terminals of the stator coils, wherein when the upper-bridge switches or lower-bridge switches coupled by the second terminals of the first phase coil and the second phase coil are switched according to a PWM signal, the voltage at the second terminal of the third phase coil is taken as a first voltage and coupled to the output terminal of the selection circuit; a zero-crossing detection unit, coupled to the output terminal of the selection circuit, wherein when the upper-bridge switch coupled by the second terminal of the first phase coil is turned on and the lower-bridge switch thereof is turned off, the power voltage and the voltage across the impedance element in the first voltage are removed to obtain a first back electromotive force voltage, the first back electromotive force voltage is compared with a first reference voltage and a first zero-crossing judgement signal is output, the zero-crossing detection unit comprising: a first zero-crossing detection circuit, for removing the power voltage and the voltage across the impedance element in the first voltage to obtain the first back electromotive force voltage, comparing the back electromotive force voltage with the first reference voltage, and outputting the first zero-crossing judgement signal when the upper-bridge switch coupled by the second terminal of the first phase coil is turned on and the lower-bridge switch thereof is turned off; and a second zero-crossing detection circuit, for taking the first voltage as a second back electromotive force voltage, comparing the second back electromotive force voltage with a second reference voltage, and outputting a second zero-crossing judgement signal when the upper-bridge switch coupled by the second terminal of the first phase coil is turned off; and a control circuit, for judging when a zero-crossing occurs according to the first zero-crossing judgement signal so as to control the motor. 12. The brushless motor system according to claim 11, wherein the selection circuit comprises a first proportionally voltages-reducing circuit for reducing the voltage at the second terminal of the third stator coil in a predetermined proportion to obtain the first voltage, and the first zero-crossing detection circuit comprises: a second proportionally voltages-reducing circuit, for reducing the power voltage in the predetermined proportion, defining the reduced power voltage as a second voltage, reducing the voltage across the impedance element in the predetermined proportion and defining the reduced voltage as a third voltage; and an analog differential amplifier, for scaling the first voltage, the second voltage and the third voltage in a proportion of 2:1:1, subtracting the scaled second voltage and the scaled third voltage from the scaled first voltage, amplifying the scaled first voltage after the above-mentioned subtraction operations to obtain the first back electromotive force voltage and outputting the first back electromotive force voltage. 13. The brushless motor system according to claim 12, wherein the first zero-crossing detection circuit further comprises: a reference voltage generator, for generating the first reference voltage, wherein when the upper-bridge switch coupled by the second terminal of the first phase coil is turned on, the lower-bridge switch thereof is turned off, the lower-bridge switch of the second phase coil is turned on and there is a current flowing through the third phase coil from the first terminal to the second terminal thereof, then, the first reference voltage is set to be a higher voltage level; when the level of the first back electromotive force voltage is lower than the higher voltage level, the first reference voltage is set to be a middle voltage level; when the level of the first sampling circuit back electromotive force voltage is higher than the middle voltage level, the first reference voltage is set to be a lower voltage level; a comparator, comprising a positive input terminal, a negative input terminal and an output terminal, wherein the positive input terminal receives the first back electromotive force voltage, the negative input terminal receives the first reference voltage and the output terminal outputs a comparison voltage; and a sampling circuit, coupled to the output terminal of the comparator to receive the comparison voltage for sampling the comparison voltage to obtain the first zero-crossing judgement signal when the upper-bridge switch of the first stator coil is turned on. 14. The brushless motor system according to claim 12, wherein the first zero-crossing detection circuit further comprises: a reference voltage generator, for generating the first reference voltage, wherein when the upper-bridge switch coupled by the second terminal of the first phase coil is turned on, the lower-bridge switch thereof is turned off, the lower-bridge switch of the second phase coil is turned on and there is a current flowing through the third phase coil from the second terminal to the first terminal thereof, then, the first reference voltage is set to be a lower voltage level; when the level of the first back electromotive force voltage is higher than the higher voltage level, the first reference voltage is set to be a middle voltage level; when the level of the first sampling circuit back electromotive force voltage is lower than the middle voltage level, the first reference voltage is set to be a higher voltage level; a comparator, comprising a positive input terminal, a negative input terminal and an output terminal, wherein the positive input terminal receives the first back electromotive force voltage, the negative input terminal receives the first reference voltage and the output terminal outputs a comparison voltage; and a sampling circuit, coupled to the output terminal of the comparator to receive the comparison voltage for sampling the comparison voltage to obtain the first zero-crossing judgement signal when the upper-bridge switch of the first stator coil is turned on. 15. The brushless motor system according to claim 11, wherein the second zero-crossing detection circuit further comprises: a comparator, comprising a positive input terminal, a negative input terminal and an output terminal, wherein the positive input terminal receives the second back electromotive force voltage, the negative input terminal receives the second reference voltage and the output terminal outputs a comparison voltage; and a sampling circuit, coupled to the output terminal of the comparator to receive the comparison voltage for sampling the comparison voltage to obtain the second zero-crossing judgement signal when the upper-bridge switch of the first stator coil is turned on. 16. The brushless motor system according to claim 11, wherein the control circuit receives the first zero-crossing judgement signal and the second zero-crossing judgement signal to obtain a zero-crossing indication signal and outputs a commutation-indicating signal according to the zero-crossing indication signal, wherein the commutation-indicating signal transits the state thereof according to the following rule: calculating i pieces of zero-crossing time prior to a state transition of the commutation-indicating signal, wherein the definition of the plurality of zero-crossing time comprises the following (1) or (2): (1) a time interval between a state transition of the zero-crossing indication signal from a first state to a second state and a state transition of the zero-crossing indication signal from the second state to the first state; (2) a time interval between a state transition of the zero-crossing indication signal from the second state to the first state and a state transition of the zero-crossing indication signal from the first state to the second state, wherein the k-th zero-crossing time is represented by Tk, i and k are natural number and 0<k<i; assigning each zero-crossing time with a weight value and the weight value of the k-th zero-crossing time is represented by Gk; defining a delay time Td with Td=(G1×T1+ . . . +Gk×Tk+ . . . +Gi×Ti)/(2×(G1+ . . . +Gi)); and transiting the state of the commutation-indicating signal in the delay time after the first zero-crossing time. 17. A method for controlling a motor, comprising: providing a zero-crossing indication signal and a commutation-indicating signal; calculating i pieces of zero-crossing time prior to a state transition of the commutation-indicating signal, wherein the definition of the plurality of zero-crossing time comprises the following (1) or (2): (1) a time interval between a state transition of the zero-crossing indication signal from a first state to a second state and a state transition of the zero-crossing indication signal from the second state to the first state; (2) a time interval between a state transition of the zero-crossing indication signal from the second state to the first state and a state transition of the zero-crossing indication signal from the first state to the second state, wherein the k-th zero-crossing time is represented by Tk, i and k are natural number and 0<k<i; assigning each zero-crossing time with a weight value and the weight value of the k-th zero-crossing time is represented by Gk; defining a delay time Td with Td=(G1×T1+ . . . +Gk×Tk+ . . . +Gi×Ti)/(2×(G1+ . . . +Gi)); and transiting the state of the commutation-indicating signal in the delay time after the first zero-crossing time.