Sensorless start-up method for driving a brushless DC motor

1. A sensorless start-up method for driving a brushless DC motor, comprising:

performing a plurality of detection modes in sequence, such that with respect to each of the detection modes a composite magnetic field generated by a motor current flowing through motor coils points in a different direction, wherein each mode is performed to last for a corresponding detection time such that the motor current gradually increases to a peak value at an end of the corresponding detection time;

comparing each of the peak values of the motor currents generated by the detection modes with respect to one another so as to select, from the plurality of detection modes, one detection mode which generates a largest one of the peak values of the motor currents;

determining an initial position of a rotor on a basis of a direction of a composite magnetic field generated by the detection mode which generates the largest one of the peak values of the motor currents; and

driving the brushless DC motor to start rotating based on the initial position of the rotor,

wherein:

the corresponding detection time of each of the odd-numbered detection modes is implemented by a first group detection time (T G1 );

the corresponding detection time of each of the even-numbered detection modes is implemented by a second group detection time (T G2 ); and

the first group detection time (T G1 ) and the second group detection time (T G2 ) satisfy an equation as follows:

T

G

⁢

⁢

2

=

-

(

L

R

)

⁢

ln

⁡

(

4

⁢

⁢

e

(

-

(

R

L

)

⁢

T

G

⁢

⁢

1

)

-

1

3

)

wherein L represents an equivalent inductance of any of the motor coils and R represents an equivalent resistance of any of the motor coils.

2. The sensorless start-up method according to claim 1 , wherein:

the plurality of detection modes include twelve detection modes for generating twelve composite magnetic fields, respectively, any adjacent two of which have a difference of 30 degrees in direction.

3. The sensorless start-up method according to claim 2 , wherein:

the motor coils include a first phase coil, a second phase coil, and a third phase coil, coupled together to form a Y-shaped structure.

4. The sensorless start-up method according to claim 3 , wherein:

at a first detection mode of the twelve detection modes, an input terminal of the first phase coil is coupled to a ground potential, an input terminal of the second phase coil is coupled to a supply voltage, and an input terminal of the third phase coil is coupled to the supply voltage;

at a second detection mode of the twelve detection modes, the input terminal of the first phase coil is coupled to the ground potential, the input terminal of the second phase coil is coupled to the supply voltage, and the input terminal of the third phase coil is floated;

at a third detection mode of the twelve detection modes, the input terminal of the first phase coil is coupled to the ground potential, the input terminal of the second phase coil is coupled to the supply voltage, and the input terminal of the third phase coil is coupled to the ground potential;

at a fourth detection mode of the twelve detection modes, the input terminal of the first phase coil is floated, the input terminal of the second phase coil is coupled to the supply voltage, and the input terminal of the third phase coil is coupled to the ground potential;

at a fifth detection mode of the twelve detection modes, the input terminal of the first phase coil is coupled to the supply voltage, the input terminal of the second phase coil is coupled to the supply voltage, and the input terminal of the third phase coil is coupled to the ground potential;

at a sixth detection mode of the twelve detection modes, the input terminal of the first phase coil is coupled to the supply voltage, the input terminal of the second phase coil is floated, and the input terminal of the third phase coil is coupled to the ground potential;

at a seventh detection mode of the twelve detection modes, the input terminal of the first phase coil is coupled to the supply voltage, the input terminal of the second phase coil is coupled to the ground potential, and the input terminal of the third phase coil is coupled to the ground potential;

at an eighth detection mode of the twelve detection modes, the input terminal of the first phase coil is coupled to the supply voltage, the input terminal of the second phase coil is coupled to the ground potential, and the input terminal of the third phase coil is floated;

at a ninth detection mode of the twelve detection modes, the input terminal of the first phase coil is coupled to the supply voltage, the input terminal of the second phase coil is coupled to the ground potential, and the input terminal of the third phase coil is coupled to the supply voltage;

at a tenth detection mode of the twelve detection modes, the input terminal of the first phase coil is floated, the input terminal of the second phase coil is coupled to the ground potential, and the input terminal of the third phase coil is coupled to the supply voltage;

at an eleventh detection mode of the twelve detection modes, the input terminal of the first phase coil is floated, the input terminal of the second phase coil is coupled to the ground potential, and the input terminal of the third phase coil is coupled to the supply voltage; and

at a twelfth detection mode of the twelve detection modes, the input terminal of the first phase coil is coupled to the ground potential, the input terminal of the second phase coil is floated, and the input terminal of the third phase coil is coupled to the supply voltage.