9.1 Bulk Capacitance

Having appropriate local bulk capacitance is an important factor in motor drive system design. More bulk capacitance is generally beneficial but with the disadvantages of increased cost and physical size.

The amount of local capacitance needed depends on a variety of factors, including:

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

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

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

Figure 14. Example Setup of Motor Drive System With External Power Supply

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