Bus Bars: The Art of Optimized Power and Signal Interconnectivity


Dr. Philippe Roussel, VP Global Strategic Marketing at Mersen

Today’s inverter manufacturers, power electronics system integrators and engineers throughout various industrial markets look for solutions to reduce assembly footprint and to optimize connectivity in their designs. Bus bars solutions from Mersen offer an attractive alternative interconnectivity method for power and signal circuits.

Introduction

Mersen is a global expert in electrical power and advanced materials. Mersen designs innovative solutions to address its clients’ specific needs to enable them to optimize their manufacturing process across numerous industrial sectors.

The Challenge

With the growth of power conversion systems, comes the need for designers to find ways to optimize various aspects of their systems such as reduced foot print and enhanced performance. Traditionally industrial power conversion systems such as inverters, converters, rectifiers and drives to name a few, utilize standard wiring and cables to interconnect the power and signal circuitry. Though wires and cables can serve the connectivity needs, considering a custom designed bus bar platform can bring in several added benefits towards a well optimized solution.

Bus Bars and Their Benefits

Laminated bus bar is an engineered component consisting of layers of fabricated copper separated by thin dielectric materials, laminated into a unified structure. Bus bars reduce system costs, improve reliability, increase capacitance, and eliminate wiring errors. They also lower inductance and lower impedance. Plus, the physical structure of bus bars offers unique features in mechanical design. Multilayer bus bars offer a structural integrity that wiring methods just can’t match.

Straight to the Optimum Design

With over 100 cumulative years of experience designing, manufacturing and testing bus bars, Mersen engineers have built a strong knowledge base of helping customers towards the optimized system.

Mersen engineers can work with inverter manufacturers at very early stages of design to efficiently integrate the passive components such as cooling, bus bars and fuses within the inverter. Using a variety of Multiphysics simulation toolsets, Mersen engineers can evaluate the thermal and electrical performance of the heat sinks and bus bars within the overall inverter design. This approach is called the “Integrated Architecture” approach and allows for the most optimum design footprint of the overall inverter assembly by sourcing the cooling and bus bar solutions along with semiconductor protection fuses all from one source.

Figure 1: Mersen Integrated Architecture approach

Optimization Starts with Customer Needs in Mind

As each bus bar design can be unique, a “prototype” is usually produced before final production run. To ensure the prototypes are as close as possible to the optimum design, multi-physics simulations are used in the conception phase, so that any design flaws can be spotted and eliminated before going through the prototype manufacturing process.

Mersen’s multi-physics simulation tools include mechanical, electrical and thermal simulations. During the simulation stage, two main design aspects are considered. The first aspect touches upon the need for space optimization in the final assembly, and second aspect addresses the constraints associated with the use of different heterogeneous materials.

In the bus bar design the major characteristics are layer dimensions, layer assembly, interconnection configuration and insulation material and thickness. Thermal performance is

Key in laminated bus bars. Mersen application engineers can conduct temperature rise simulation on the bus bar prototypes. Engineers simulate the Joule effect heating the conductors when the current is flowing and also heat concentration at the interfaces with power modules and capacitors. The simulation process allows a significant reduction of product development time.

When it comes to electrical simulation, engineers and technicians simulate the routing of contacts to meet all clearance and creepage requirements. The current distribution is then simulated to check compliance with admissible current density in order to limit self-heating of the bus bar.

Figure 2: Bus bar thermal and electrical simulations

Rigorous testing is completed on each part prior to shipment to ensure long term reliability. In addition to dielectric withstanding, or insulation breakdown testing (aka “HiPot”), Mersen performs Partial Discharge (PD) testing using the state of the art PD test station. Mersen’s engineering and quality team uses DFM (Design For Manufacturability) techniques such as FMEA (Failure Mode Analysis) to evaluate each order to assure smooth transition without failures throughout the process.

Innovations to Better Serve The Future

Mersen has developed several recent innovative technologies to bring additional features to bus bar products to better serve certain requirements of specific applications and industries.

Low Inductance / High T° Bus Bar

Today’s high frequency Wide Band Gap Semiconductor manufactures are pushing the envelopes of power densities and high frequency switching capabilities within their power modules. As a result of this, temperature and inductance criteria within the power modules such as SiC or GaN modules now require superior connectivity methods within the modules themselves.

Mersen has developed the manufacturing process to manage high temperatures and low inductance dielectric and associated adhesives to comply with new constraints imposed by wide band gap semiconductors and latest IGBT generations. As a result, our bus bars can now handle up to 220°C operation T° with inductance as low as 35nH and a lifetime operation of 25 years!

Figure 3: Internal high T° / Low L bus bar for Power Modules

Monitoring Bus Bar for Battery Applications

To cope with the growing demand of constant voltage and temperature monitoring in power electronics applications such as lithium-ion and ultra-capacitors packs, Mersen has engineered an innovative concept that combines in a single customized device a laminated bus bar, a flexible circuit, thermal sensors and other custom electronic components. It allows, with only one part, to make both the power connections and the signal collection from each cell independently, so the status of each cell is delivered to the Battery Management System via a custom connector.

Figure 4:  Smart monitoring bus bar for battery cells connection

Low Partial Discharge Design to Handle

High Voltage 5KV+ Breakdown

Partial discharge (PD) is a localized dielectric breakdown of a small portion of a solid or fluid electrical insulation system under high voltage stress. Mersen is working on increasing our capabilities to be able to design and manufacture bus bars featuring low partial discharge for voltage applications up to 5kV.

Increasing the working voltage application and a passing a positive partial discharge test can greatly contribute to a long life cycle of bus bar under high voltage applications. This testing is critical to inverter and drive systems manufacturers.

Figure 5: Low Partial Discharge Optimized Bus bars

Bus bars with flexible floating connectors

Temperature cycles and mechanical environmental constraints (shock & vibration) can damage devices (capacitors, modules…) at fixture points. To avoid this, Mersen offers various floating connector designs that allow ±1mm mechanical deviation with minimal impact on electrical resistance and inductance

Figure 6: Bus bars with flexible floating connectors

Serving Customer Globally

Mersen has a global industrial footprint for laminated bus bars in the three dedicated ISO 9001 certified facilities cover the major economic regions in the world.

  • Rochester – New York in USA is the center-of-excellence facility for the North American market. This site is AS9100C registered and ITAR Certified.
  • Angers – France is the center-of-excellence facility for European market. Angers is IRIS Certified to better serve the rail industry.
  • Shanghai – China is the manufacturing facility to serve the Chinese market.

A fourth manufacturing bus bar facility is currently being commissioned in India & will be producing in the first months of 2018.

State-of-the-art metal fabrication

In-house capabilities include: CNC stations, punch presses, CNC press brakes, CNC Bending press. Our metal assembly processes include ultrasonic welding, induction brazing, torch brazing and soldering.

Plating

Many available plating finishes are available: tin, tin-lead, nickel, copper, silver and gold.

Key parameters of plating processes are monitored throughout the process, and end of line testing includes thickness measurements.

Properly selected insulation

Insulation is of course at the core of laminated bus bars, and impacts directly the electrical integrity of the product. Various types of insulation materials are used: Polyimide, PVF, PET, rigid insulation, epoxy glass, GPO and Phenolic.

Assembly and lamination

Dedicated production tooling is manufactured for the lamination of each of our bus bars. Additional hardware and interconnection devices and electrical components can be mounted either before or after lamination.

Sub Assembly and Full Power Stack Assembly Services

Drawing on our expertise in bus bar, cooling and fuses, and based upon our multi-physics simulation tools, Mersen is positioned at the center of the inverter design cycle. We are now helping our customers to benefit from the best of these passive devices through a design optimization and assembly service that encompasses:

  • Conception optimization and design cycle-time reduction, with testing and design validation in our labs.
  • Preferential access to main components (Cooling, bus bar, capacitors, fuses and gate drivers)
  • Overall price optimization / Scale effect
  • A total customization approach: no pre-defined technical bricks.

From simple [bus bar + capacitor] sub-assembly to full inverter assembly, Mersen is the preferred partner for inverter / stack design-houses with limited or no production capability.

Figure 7: Power Stack Assembly Service

In conclusion, bus bars have evolved into an integral component and embedded platform for many power conversion systems, providing not only an optimized interconnection but also helping power electronics manufacturers to push the performance thresholds of their products

More information: Mersen    Source: Bodo's Power Systems, March 2018