10.2 Bulk Capacitance Sizing

Having appropriate local bulk capacitance is an important factor in motor drive system design. It is generally beneficial to have more bulk capacitance, while the disadvantages are increased cost and physical size. The amount of local capacitance depends on a variety of factors including:

• The highest current required by the motor system
• The power supply's type, capacitance, and ability to source current
• The amount of parasitic inductance between the power supply and motor system
• The acceptable supply voltage ripple
• Type of motor (brushed DC, brushless DC, stepper)
• The motor startup and braking methods

The inductance between the power supply and motor drive system will limit the rate current can change from the power supply. If the local bulk capacitance is too small, the system will respond to excessive current demands or dumps from the motor with a change in voltage. When adequate bulk capacitance is used, the motor voltage stays stable and high current can be quickly supplied.

The data sheet provides a recommended minimum value, but system level testing is required to determine the appropriate sized bulk capacitor.

Figure 68. Motor Drive Supply Parasitics Example