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PWM MANAGEMENT FOR 3-PHASE BLDC MOTOR DRIVES USING THE ST7MC

2 SIX-STEP, 120° DRIVE AND PWM POWER CONTROL

To control a BLDC motor with the best efficiency, we have to know the rotor position at all

times. To achieve this there are two modes. One is called the sensor mode, where the infor-

mation read back from the motor is the one coming from Hall Effect sensors (1 per phase).

The other one is the sensorless mode, where the Back Electromotive Force (BEMF) signal in-

formation is the one read back from the motor. The strength of the ST7FMC is that it is com-

patible with all the modes to control a BLDC motor, whether it is sensorless or not and it is

even suitable with different variations within the sensorless or sensor modes which will be de-

tailed later on in this application note.

In sensorless mode, in order to be able to read the BEMF information, the phase switching has

to include a dead time during which no current flows in one of the motor windings.

As shown in Figure 4, in six-step, 120° drive, power is removed from each winding every three

steps. During this dead-time phase, it is possible to detect the BEMF zero-crossing event on

this non powered winding (see Figure 3 for an example).

In order to control the speed, the torque or the power applied to the motor, a PWM signal is

usually logically ANDed with the switch control signals. This control is implemented by modi-

fying the duty cycle of this logically ANDed PWM signal.

With this method, the BEMF voltage is referred to point M of the motor and not to ground. Be-

cause this point is at high voltage, the microcontroller cannot read its value directly.

Note: In sensor mode the six-step drive or the sinusoidal drive can be implemented as the

feedback signal comes independently from the Hall Effect sensor in the dedicated input of the

microcontroller. However, in six-step drive, the method will be exactly the same as the one de-

tailed for the sensorless mode. For the sensor sinusoidal mode, the PWM management is the

same as for the AC induction motor drive.

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