Introduction

The DC link is the electrical reservoir of a voltage-source converter: it holds the bus voltage steady, supplies the switch current at each commutation and absorbs the energy that returns from the load. As switching frequencies and power densities rise, the electrolytic capacitor that once dominated the DC link has become the limiting component, because its ESR caps the ripple current it can carry and its electrolyte dries out over time. Faratronic metallized polypropylene film capacitors are built for this duty, with low ESR, low self-inductance and self-healing behaviour. This application note explains how to size and place them in a real inverter.

Capacitance and Bus Ripple

The capacitance is set by the allowable bus-voltage ripple. During one switching period the capacitor supplies the difference between the load current and the source current, and that current integrated over the period gives the charge that produces the ripple. A larger capacitance reduces the ripple, but beyond a point the ripple current and the thermal limit become the binding constraint, so it is rarely useful to oversize the capacitance alone.

Ripple Current

The RMS ripple current that flows through the capacitor is set by the modulation index, the load current and the switching frequency. That current multiplied by the ESR gives the internal loss, which raises the capacitor temperature above the ambient. The part must be chosen so the hot-spot temperature stays within its rating at the worst-case ambient, which is why the ripple-current figure, not the capacitance, usually decides the final value. At high modulation the ripple current is highest, so check the worst-case operating point, not the nominal one.

Placement and Loop Inductance

At high switching frequency the capacitor current is not simply a DC value; it is a series of fast pulses that follow the commutation loop. The inductance of that loop, not the capacitance, sets the voltage deviation the switch sees. Place the capacitor as close to the module as the layout allows and connect it with the shortest, widest conductors, so the loop area is small. A low-ESL film capacitor placed correctly is far more effective than a larger part on long leads.

Thermal Design

The capacitor rejects its heat to the ambient through its case and its mounting. In a hot enclosure, derate the ripple current or add airflow, and remember that a film capacitor is far more tolerant of high ambient temperature than an electrolytic, whose life halves for every ten degrees Celsius. That tolerance is a large part of the reliability gain in a photovoltaic or industrial installation.

Replacing an Electrolytic Bank

Moving from electrolytic to film is usually straightforward: match the total capacitance and the ripple-current rating, then choose a case and a connection that fit the space. The reward is a DC link with no dry-out mechanism, a long life and a self-healing dielectric. The BeiLuo FAE team helps with the sizing, the ripple estimate and the thermal check.