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AN2030 Datasheet(PDF) 2 Page - STMicroelectronics |
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AN2030 Datasheet(HTML) 2 Page - STMicroelectronics |
2 / 13 page 2/13 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 |
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