Overview
Inverters and motor drives are among the highest-volume applications for power capacitors, because the DC link that feeds the switching bridge must store energy, smooth the ripple and provide a low-inductance path for the high-frequency current that a fast switch demands. The capacitor bank and the snubber set the bus ripple, the hold-up time and the voltage overshoot, and therefore the reliability of the drive. TDK EPCOS aluminum electrolytic, film and snubber capacitors serve this role, and BeiLuo supplies them with genuine traceability and FAE support.
The DC Link and Its Ripple
A motor drive converts a DC bus into three-phase AC, and the switching bridge draws a pulsed current from the bus. The DC-link capacitor supplies that pulse, so it sees a large ripple current at the switching frequency in addition to the DC voltage. An aluminum electrolytic screw-terminal capacitor is used for the bulk energy because its busbar connection handles the high current and its large can dissipates the ripple heating; the B43713 series offers a high ripple rating and a 12,000 to 15,000-hour life. A DC-link film capacitor, such as the low-inductance B25620, is often added in parallel to provide a low-inductance, high-frequency path and to handle the fast dV/dt of a modern power stage.
Sizing the Bank
The capacitance sets the bus ripple and the hold-up, and the total ripple rating sets how much pulse current the bank can absorb. Because the capacitance is large, several capacitors are often paralleled on a busbar; each carries a share of the ripple, so the bank rating is the sum of the individual ratings. Keep the busbar impedance balanced so the current shares evenly, and verify the ripple on each capacitor under load.
IGBT Snubbering
At the switching edge, the energy stored in the commutation-loop inductance produces a voltage spike across the IGBT. A snubber capacitor close to the module absorbs that energy and limits the overshoot, keeping the device within its safe operating area. Because the snubber must respond within a fast edge, its equivalent series inductance is the key parameter, and TDK EPCOS offers parts with ESL as low as about 6 nH. The snubber is mounted as close to the module as possible with a short loop so that the low ESL actually helps.
Life and Thermal Design
An aluminum electrolytic capacitor ages with its core temperature, and lifetime roughly doubles for every 10 °C reduction, so keeping the capacitors cool is the most effective way to make the drive last. A film capacitor does not dry out, so it suits long-life and high-frequency designs. Space the capacitors for airflow, keep them away from the hottest components, and check the ripple margin at the worst-case case temperature.
Selection and Supply
Selection starts with the bus voltage and the required capacitance, ripple and inductance, then narrows by case size, terminal type and life, choosing a screw-terminal electrolytic for the bulk energy, a film capacitor for the low-inductance high-frequency path and a low-ESL snubber for the IGBTs. BeiLuo keeps mainstream TDK EPCOS capacitors in regional stock, ships them with import declaration, certificate of origin and RoHS documents, and supports ripple, lifetime and layout design with an in-house FAE team. From a compact servo drive to a large industrial inverter, our engineers help you choose the capacitors that keep the drive reliable and the bus stable.