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

5.2 PWM ON THE LOW SIDE

When the related switches are ON (example of Figure 12) the current passes through the two

switches and two motor windings.

When the low side is switched OFF, the current inside the motor continues to flow in the same

direction (Figure 14). While the switch is OFF, the current can only use the diode (D0 in our ex-

ample).

In this case the potential at A is (HV+Vf), the potential at B is (HV-Von) and the potential at M

is (2xHV+Von-Vf)/2. We can consider it to be HV (because in most cases Von = Vf).

In this case the microcontroller can read the BEMF voltage only if the Classic method is used

during ON time of the PWM signal because the signal in C is divided and filtered before the mi-

crocontroller input pin. Considering the ST method, the sampling of the BEMF is done only

during the OFF time of the PWM and as with the ST method, the signal is not divided and fil-

tered in C before the microcontroller pin, this method can not be used as the voltage in C is

close to HV during the PWM off time.

– Von is the voltage of the switch when it is turned ON

– Vf is the forward voltage of a diode

Figure 14. PWM on low side: Current during OFF time

C

A

B

HV

T4

T1

T0

T3

T2

T5

PWM

HV

PWM on T3; Free-wheeling diode D0

D0

D2

D4

D3

D5

D1

M