1.  Field of the Invention
The instant disclosure relates to a shutdown method for motor; in particular, to a shutdown method for motor that can prevent the motor from the damage caused by the back current and can avoid generating the over current and a motor driving circuit using the same. 
2.  Description of Related Art
With the development of technology, the motor has become an essential electric device.  The common motors, such as the DC motor, the AC motor, the stepping motor and the like, have been widely used to drive fans. 
The motor comprises the higher gate switch, the lower gate switch and the driving circuit.  The driving circuit controls the tuning on and off of the higher gate switch and the lower gate switch.  The higher gate switch and the lower gate switch are related to the working status of the motor.  Precisely, the driving circuit controls the turning on and of the higher gate switch and the lower gate switch, to output the voltage for changing the polarity of the coil of the motor rotator.  Via the mutual effect between the polarity of the coil and the polarity of the motor rotator, the motor rotator would rotate because of the polarity of the coil.  The change of the magnetic flux results in the reverse voltage, which is the Back Electromotive Force (BEMF). 
When the motor is shut down, the power supply stops providing the supply voltage to the motor, such that the supply voltage starts to decrease.  When the supply voltage is lower than the BEMF, the back current would be generated inside the motor because of the BEMF.  The back current flows back to the higher gate switch and the lower gate switch, which damages the higher gate switch, the lower gate switch and the power supply. 
In practice, a protection diode would be configured between the power supply and the motor driver to prevent the back current inside the motor from flowing back to the power supply.  Thus, when shutting down the motor, the back current generated by the BEMF can just flow to the storage capacitor of the motor driver, which results in the increase of the BEMF.  However, if the BEMF is too large, it is possible to damage the motor drover.  In order to prevent the motor driver from being damaged by the back current, in a traditional way, after the power supply stops providing the supply voltage, the higher gate switch is turned off and the lower gate switch is turned on.  Thereby, the back current flows to the lower gate switch to prevent the power supply and the higher gate switch from damaging by the BEMF. 
However, there is a disadvantage in this traditional way.  The value of the back current is directly proportional to two factors.  One is the voltage value of the BEMF, and the other one is the voltage of the motor coil, which is the voltage of the node between the higher gate switch and the lower gate switch.  If the BEMF is larger, the back current would also be larger.  If the voltage of the motor coil is smaller, the voltage to resist the BEMF would be smaller.  Thus, when the lower gate switch is turned on, the back current would be so large as to damage the lower gate switch.  In other words, the traditional way cannot protect the higher gate switch, the lower gate switch and the power supply simultaneously.