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

When the PWM signal is ON when using the Classic method, the voltage at the virtual ground

will be HV/2 divided and filtered and the voltage at point C will be (HV/2 + BEMF) divided and

filtered so the comparison can be done by the comparator in the microcontroller as it will stay

within the microcontroller supply voltage range (0 - Vdd) after division and filtering.

When the high side switch PWM is OFF, the current inside the motor continues to flow in the

same direction (Figure 13). While the switch is off, the current can only use the diode (D5 in

our example).

In this case the potential in A is the Von of the T3 switch, the potential in B is -Vf of D5 and the

potential at M is (Von-Vf)/2. It is close to zero because in most cases Von = Vf.

In this case the microcontroller can read the complete BEMF voltage referred to the ground

terminal on the phase C.

This is the configuration used for the ST method where the sampling of the BEMF signal is

only done during the Off time of the PWM signal. No dividers nor filters are needed externally

as we are looking at a signal that is close to the MCU ground level.

Note: This sampling method of the BEMF signal is also appropriate for the classic method. In

fact the sampling of the BEMF can be done either during ON or OFF time of the PWM for the

classic method.

Figure 13. PWM on high side: Current during OFF time

PWM

PWM on T2

Free-wheeling

current during OFF TIME

diode: D5

AN2009 데이터시트(PDF) 20 Page - STMicroelectronics