Overview
The DC link is the heart of any voltage-source converter. It holds the bus voltage steady, supplies the switch current during each commutation and absorbs the energy that returns from the load. As switching frequency and power density rise, the electrolytic capacitor that once dominated the DC link has become the weakest component in the chain, because its ESR limits the ripple current it can carry and its electrolyte dries out over time. Faratronic film capacitors solve that problem: a metallized polypropylene film capacitor has far lower ESR, tolerates a much higher ripple current, self-heals a local breakdown and does not dry out, so its working life is measured in decades rather than a few thousand hours.
Choosing the DC-Link Capacitor
The first decision is the value and the voltage class. The bus voltage plus the switching overshoot sets the DC rating, so a 600 Vdc to 1200 Vdc part covers most industrial and photovoltaic inverters. The capacitance is then set by the allowable bus ripple, but the decisive check is the ripple current: the capacitor must carry the RMS current at the switching frequency without exceeding its hot-spot temperature. A film capacitor with low ESR carries that current far more easily than an electrolytic of the same value, which is why a single film part can replace several electrolytics and remove the dry-out failure mode at the same time.
PCB or Aluminum Case
The C3D series is a PCB-mount capacitor with a plastic case for compact boards, while the C3B series is a dry-type aluminum-case capacitor with screw terminals for higher current, where it bolts directly to the laminated busbar. The screw terminals matter because they make the shortest, lowest-inductance connection to the switch, which keeps the bus voltage clean at high current. Placement is as important as value: the capacitor should sit as close to the module as the layout allows, so its loop with the switch is small.
Protection and Layout
A DC link alone does not protect the switch. The commutation-loop inductance still stores energy that appears as a turn-off spike, so a snubber capacitor across each module clamps that spike and damps the ringing. The snubber must be mounted at the module terminals, because a snubber on a long lead adds its own inductance and clamps far less effectively. Together, the DC-link capacitor and the snubber define the electrical environment the module sees, and getting both right is what turns a working prototype into a reliable product.
Validation with BeiLuo
The BeiLuo FAE team helps size the bus capacitance, check the ripple current and thermal design and select the matching snubber. Sending the bus voltage, the ripple spectrum and the available space is enough to receive a recommendation with stock and lead time.