Introduction

A power converter's reliability and EMC depend as much on the capacitors as on the switches, and the key parameters are the equivalent series resistance and the equivalent series inductance. This application note explains how ESR and ESL, ripple current, temperature and layout combine to determine the performance of a TDK EPCOS capacitor bank, and how to use an electrolytic and a film capacitor together.

ESR and Ripple Heating

The ripple current that a converter draws from its DC link flows through the equivalent series resistance of the capacitors and dissipates power, which raises the core temperature and ages the capacitor. An aluminum electrolytic has a higher ESR than a film capacitor, so at the same ripple it runs hotter; a film capacitor has a low ESR and loss, so it stays cool at high frequency. When you size a bank, use the ripple rating at your frequency and temperature, and remember that ESR rises as an electrolytic ages, which increases the heating over time.

ESL and High-Frequency Performance

The equivalent series inductance sets the high-frequency impedance of the capacitor and, in a DC link or snubber, the voltage overshoot across the switches. At the fast edges of a modern IGBT or wide-bandgap device, even a few nanohenries matter, which is why a low-ESL film capacitor is used for the high-frequency path. The layout is part of the ESL: a long busbar or a wide loop adds inductance that no capacitor can remove, so keep the commutation loop small.

Combining Film and Electrolytic

The practical approach is to use an aluminum electrolytic for the bulk energy and the low-frequency ripple, where its capacitance density and cost are best, and to add a film capacitor in parallel for the high-frequency ripple and the low-inductance path. The film part handles the fast content and the electrolytic handles the energy, and the two together give a low-impedance bus across the frequency range.

Temperature and Lifetime

An aluminum electrolytic ages with its core temperature, and lifetime roughly doubles for every 10 °C reduction, so keeping the capacitor cool is the most effective way to extend its life. A film capacitor does not dry out, so its life is much longer at the same temperature, which is why it suits long-life and high-frequency designs. Estimate the core temperature from the measured case temperature and the ripple heating, and keep the part well below its rated maximum.

Layout

Keep the commutation loop small for the film capacitors, place the electrolytics for airflow and away from heat sources, and keep the connections low-inductance. A tight layout reduces the voltage overshoot and the EMI, and a cooler layout extends the life of the electrolytics. Measure the bus ripple and the capacitor temperatures on the bench to confirm the design.

Getting Help

Our FAE team can help you choose between aluminum electrolytic and film capacitors, size the split, and review the layout. BeiLuo holds mainstream TDK EPCOS capacitors in regional stock and ships them with import declaration, certificate of origin and RoHS documents.