Decelerating the motor quickly requires the motor mechanical energy to be extracted from the rotor in a fast and controlled manner. However, the supply voltage (VM) increases if the motor mechanical energy is returned to the power supply during the deceleration process. MCF8315C is capable of decelerating the motor quickly without pumping energy back into the supply voltage by using a novel technique called active braking. ACTIVE_BRAKE_EN should be set to 1b to enable active braking and prevent DC bus voltage (VM) spike during fast motor deceleration. Active braking can also be used during reverse drive (see Section 6.3.10.3) or motor stop (see Section 6.3.20.3) to reduce the motor speed quickly without DC bus voltage (VM) spike.

The maximum limit on the current sourced from the DC bus (i_{dc_ref}) during active braking can be configured using ACTIVE_BRAKE_CURRENT_LIMIT. The D-axis current reference (i_{d_ref}) is generated from the error between DC bus current limit (i_{dc_ref}) and the estimated DC bus current (i_dc) using a PI controller as shown in Figure 6-42. The gain constants of PI controller can be configured using ACTIVE_BRAKE_KP and ACTIVE_BRAKE_KI. During active braking, the DC bus current limit (i_{dc_ref}) starts from zero and linearly increases to ACTIVE_BRAKE_CURRENT_LIMIT with current slew rate as defined by ACTIVE_BRAKE_BUS_CURRENT_SLEW_RATE.

Figure 6-42 Active Braking Current Control Loop for i_{d_ref}

ACTIVE_BRAKE_SPEED_DELTA_LIMIT_ENTRY sets the minimum difference between the initial and target speed above which active braking will be entered. For example, consider ACTIVE_BRAKE_SPEED_DELTA_LIMIT_ENTRY is set to 10%; if the initial speed is 100% and target speed is set to 95%, MCF8315C uses AVS instead of active braking to reach 95% speed since the difference in commanded speed change (5%) is less than ACTIVE_BRAKE_SPEED_DELTA_LIMIT_ENTRY (10%).

ACTIVE_BRAKE_SPEED_DELTA_LIMIT_EXIT sets the difference between the current and target speed below which active braking will be exited. For example, consider ACTIVE_BRAKE_SPEED_DELTA_LIMIT_EXIT is set to 5%; if the initial motor speed is 100% and target speed is set to 10%, MCF8315C uses active braking to reduce the motor speed to 15%; upon reaching 15% speed, MCF8315C exits active braking and uses AVS to decelerate the motor speed to 10%.

ACTIVE_BRAKE_MOD_INDEX_LIMIT sets the modulation index below which active braking will be used. For example, consider ACTIVE_BRAKE_MOD_INDEX_LIMIT is set to 50%, ACTIVE_BRAKE_SPEED_DELTA_LIMIT_ENTRY is set to 5%, ACTIVE_BRAKE_SPEED_DELTA_LIMIT_EXIT is set to 2.5%. If the initial motor speed is at 70% (corresponding modulation index is 90%) and target speed is 40% (corresponding modulation index is 60%), MCF8315C uses AVS to decelerate the motor till target speed of 40% since the modulation index (60%) corresponding to final speed is higher than ACTIVE_BRAKE_MOD_INDEX_LIMIT of 50%. In the same case, if final speed command is 10% (corresponding modulation index is 30%), MCF8315C uses AVS till 30% speed (corresponding modulation index is 50%), switches to active braking from 30% to 15% speed (final speed of 10% + ACTIVE_BRAKE_SPEED_DELTA_LIMIT_EXIT of 5%) and uses AVS again from 15% to 10% speed to complete the active braking. TI recommends starting active braking tuning with ACTIVE_BRAKE_MOD_INDEX_LIMIT set to 100%; if there is a DC bus voltage (VM) spike observed during active braking, reduce ACTIVE_BRAKE_MOD_INDEX_LIMIT in steps so as to eliminate this voltage spike. If ACTIVE_BRAKE_MOD_INDEX_LIMIT is set to 0%, MCF8315C decelerates in AVS (even when ACTIVE_BRAKE_EN is set to 1b) in the forward direction; in reverse direction (during direction change), ACTIVE_BRAKE_MOD_INDEX_LIMIT is not applicable and therefore MCF8315C decelerates in active braking.