Reliable DC Link Systems for Next Generation High Power Applications by Dominik Pawlik

Jun 29, 2018

Reliability

Nowadays the requirements for high power density, increased reliability and low inductance are not only important for busbars but also for complete inverter design. In higher-power applications such as traction, solar and wind inverters and the powertrains of electric vehicles (EVs) and hybrid electric vehicles (HEVs), energy must be channeled with minimal combining and distribution loss.

An ongoing challenge for these inverter systems is to develop the DC link system including distribution components such as laminated busbars with integrated capacitors that are rugged enough to handle high voltage and current but also offer very low inductance to minimalize switching losses in the converter.

A solution lies in a different approach to laminated busbar design, using an assembly configuration of a laminated busbar and typically film capacitors. With this approach, it has been possible to reduce significant equivalent series inductance (ESL) and keep the compact design and increased power density.

Not only the busbar

An important step in making these capacitor busbar assemblies is the method of attaching the capacitor to the busbar. The combination of materials in each component exhibits a complex coefficient of thermal expansion (CTE) not only between each layer of material in the laminated busbar and the capacitor. In addition, the interface between the busbar and the capacitor is subject to stress caused by vibration and changes in temperature (from both environmental temperature and heating induced by current.)

With low ohmic resistance and good heat transfer characteristics, the capacitor contributes a stable thermal interface to the busbar assembly. The capacitors are attached to the laminated busbar assembly by specific soldering technics to earlier patented by Rogers “connection island” on the busbar structure. The interconnection method contributes low resistance and inductance for low ESL of the combined assembly. These integrated busbar-capacitor assemblies can switch voltages from 300 to 1500V and current up to 1000A. The capacitance ranges from 10 to 2000μF, with capacitance values maintained to a tolerance of ±5%(J). These enhanced assemblies are rated for minimum operating temperatures from -40 to +105°C for use at the maximum voltage rating. The attachment method used to combine the busbar and capacitor is critical in maintaining low resistance at high power levels since excessive contact resistance of all connection points along a busbar leads to thermal junctions which can jeopardize reliability at high power levels.

Selection of materials

The choice of materials is also critical in determining the ultimate high-power performance from the capacitor busbar component. For the laminated busbar, for example, the cross-section size as well as the choice of conductor material will determine the busbar’s current-carrying capacity. Laminated busbars usually employ copper or aluminum conductors, which may be plated with an additional metal such as silver, tin, or nickel to avoid an oxidation effect. The choice of busbar materials, such as conductors and insulators, will also limit process manufacturing temperatures for interconnecting circuits to a busbar. Solder-free attachment methods, for example, require high processing temperatures and busbar materials capable of withstanding those temperatures during manufacturing. Insulation materials separating busbar conductors should exhibit stable dielectric constant with temperature, to minimize variations in capacitance and voltage.

Not only ESL

Paralleling discrete capacitors dramatically improves ESL and also achieves the desired capacitance and ESR ratings of the capacitor/busbar combination components, but, like the busbar itself, materials must be carefully chosen in consideration of potential thermal effects at high power levels. One goal is low inductance in combination with the busbar, and the capacitor’s use of polyester and polypropylene dielectric materials contribute to low ESR and ESL.

Polypropylene film metallized with zinc alloy, for example, is used for capacitors in EV and HEV drivetrain applications where it is important to maintain consistent performance even under the severe temperature conditions found in vehicular electronic applications.

With evermore compact inverter designs now leveraging new WBG semiconductors such as SiC, the operating temperature range is increasing to above 125°C. The material used for laminated busbars as well as the capacitors need to withstand these requirements. The new innovative assemblies use dielectric films with temperature up to 150°C and high temperature films for the capacitors that have comparable characteristics to polypropylene films but with much higher energy density.

The excellent thermal characteristics of materials used in the capacitors, soldered to busbars which are formed to a multilayer construction of conductive metals and dielectric materials, provides a nearly continuous thermal path that is essential for avoiding hot spots at high power levels. In this choice of materials for both components, this combination provides a busbar with an integrated capacitor that is well equipped for the higher voltages and currents found in many modern applications, such as EVs, HEVs, as well as solar and wind-turbine inverters.

If you have any design questions or require assistance for the optimization of a laminated busbar or capacitor-busbar assembly for your application, Rogers’ PES experts are available to help. Please Contact Us if you have any questions.

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