Figure 8-32 shows the operation of active demagnetization during the BLDC motor commutation. As shown in Figure 8-32 (a), the current is flowing from HA to LC in one commutation state. During the commutation changeover as shown in Figure 8-32 (b), the HC switch is turned on, whereas the commutation current (due to motor inductance) in OUTA flows through the body diode of LA. This incorporates a higher diode loss depending on the commutation current. This commutation loss is reduced by turning on the LA for the commutation time as shown in Figure 8-32 (c).

Similarly the operation of high-side FET is realized in Figure 8-32 (d), (e) and (f).

Figure 8-32 ASR in BLDC Motor Commutation

Figure 8-33 (a) shows the BLDC motor phase current waveforms for automatic synchronous rectification mode in BLDC motor operating with trapezoidal commutation. This figure shows the operation of various switches in a single commutation cycle.

Figure 8-33 (b) shows the zoomed waveform of commutation cycle with details on the ASR mode start with margin time (t_margin) and ASR mode early stop due to active demag. comparator threshold and delays.

Figure 8-33 Current Waveforms for ASR in BLDC Motor Commutation