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BACK EMF DETECTION DURING PWM ON TIME BY ST7MC

1 TRANSIENT ANALYSIS DURING PWM OFF TIME

Generally, a brushless dc motor is driven by a three-phase inverter with what is called six-step

commutation. The conducting interval for each phase is 120° by electrical angle. Therefore,

only two phases conduct current at any time, leaving the third phase floating. This opens a

window to detect the back EMF in the floating winding.

For the direct back EMF sensing scheme, the PWM signal is applied on high side switches

only, and the back EMF signal is synchronously sampled during the PWM off time. The low

side switches are only switched to commutate the phases of the motor. The true back EMF

can be detected during off time of PWM because the terminal voltage of the motor is directly

proportional to the phase back EMF during this interval. Also, the back EMF information is ref-

erenced to ground, which eliminates the common mode noise; and the synchronous sampling

rejects the high-frequency switching noise. Only three resistors are required to detect the back

EMF, as shown in Figure 1.

Ideally, the terminal voltage for the floating phase is directly proportional to the back EMF

signal in steady state during PWM off time [1]. The equation is as following:

Where Vx is the terminal voltage, ex is the back EMF of the floating phase.

Figure 1. Direct Back EMF Sensing block diagram

The equation (1) is valid only in steady state.

v

ab c

,,

3

2

---e

ab c

,,

=

(1)

MC00

MC01

MC02

MC03

MC04

MC05

MCIC

MCIB

MCIA

ST72141

motor

Microcontroller

Power stage

Back EMF Sensing

R1

R2

R3

2