Part # Application Notes Motor Control ST7MC1K2 ST7MC1K4 ST7MC2M9 ST7MC2N6 ST7MC2R6 ST7MC2R7 ST7MC2S4 ST7MC2S6 ST7MC2S7 8-Bit Microcont datasheet

Part Manufacturer: ST Microelectronics

ST Microelectronics

Part Description: Enhanced sensorless startup control of BLDC motors using ST7FMC


Part Details:

AN2030 APPLICATION NOTE BACK EMF DETECTION DURING PWM ON TIME BY ST7MC INTRODUCTION The direct back EMF sensing scheme used by ST72141 synchronously samples the motor back EMF during PWM "off" time without the need to sense or re-construct the motor neutral in a sensorless BLDC motor drive system. Since this direct back EMF sensing scheme re- quires minimum PWM "off" time to sample the back EMF signal, the duty cycle can t reach 100%. Also in some applications, i.e. HVAC using high inductance motors, we see the zero crossing detection is unsymmetrical in the ST72141 sensorless drive system at high speed. It is found that the long settling time of a parasitic resonant between the motor inductance and the parasitic capacitance of power devices causes false zero crossing detection of back EMF. This application note provides an analysis of the resonant transient during PWM "off" time. As a result, the back EMF detection during PWM "on" time is used in ST7MC to solve the problem. AN2030 Rev 2 1/13 1 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: 3 v = - (1) a, b, c e 2 a, b, c Where Vx is the terminal voltage, ex is the back EMF of the floating phase. Figure 1. Direct Back EMF Sensing block diagram Back EMF Sensing R1 MCIC R2 MCIB motor R3 MCIA ST72141


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Application Notes Motor Control ST7MC1K2 ST7MC1K4 ST7MC2M9 ST7MC2N6 ST7MC2R6 ST7MC2R7 ST7MC2S4 ST7MC2S6 ST7MC2S7 8-Bit Microcont.pdf Datasheet