Appropriate local bulk capacitance is an important factor in motor drive system design. Having more bulk capacitance is generally beneficial, although the disadvantages include increased cost and physical size. Bulk capacitors near the motor driver act as a local reservoir of electrical charge to smooth out the motor current variation.

Experienced engineers often use general guidelines about bulk capacitance to select the capacitor values. One such guideline says to use at least 1 to 4μF of capacitance for each Watt of motor power. For example, a motor which draws 2 Amps from a 24V supply has a power of 48 Watts, leading to bulk capacitance of 48 to 192μF, using this general guideline.

The voltage rating for bulk capacitors must be higher than the operating voltage, to provide margin for cases when the motor transfers energy to the supply.

A large value of bulk capacitance is desired to provide a constant motor supply voltage during current transitions, such as motor start-up, changes in load torque, or PWM operation. A working estimate of the required capacitance for consistent supply is essential to reduce complexity, cost and size of board electronics. We can use a general guideline method to find an appropriate capacitor size based on the expected load current variation and allowable motor supply voltage variation:

Where:

C_BULK is the bulk capacitance

$k$ is a scale factor to account for the ESR for typical capacitors in this type of application; based on the lab measurements with DRV8718-Q1EVM, k ≈ 3 is practical for these cases.

ΔI_MOTOR is the expected variation in motor current, i_max – i_min

T_PWM is the PWM period which is the reciprocal of the PWM frequency

ΔV_SUPPLY is the allowable variation in the motor supply voltage

Figure 7-7 plots several data points and applies this general guideline, showing relatively good agreement.

For more information please see the Application Note Bulk Capacitor Sizing for DC Motor Drive Applications .