When it comes to connecting circuits while minimizing inductance, a bus bar is essential. Similar to a wire or cable, a bus bar’s purpose is to connect two or more points of a circuit. However, bus bars offer many advantages over conventional cables.
One such benefit is that a bus bar assembly can be manufactured so that it minimizes stray circuit inductance while making all required component connections. When one analyzes the common causes of reduced efficiency and unexpected component failure, it becomes easy to see the importance of using a low-inductance, laminated multi-layer bus bar to eradicate these problems. In short, bus bars lead the way for design advances.
A designer’s best component option
The growing worldwide demand for power conversion and distribution technology – particularly in the military, industrial and renewable energy fields – fueled the rise of bus bars. These components have enabled designers to mitigate much of the “stray” inductance in numerous applications. In a motor drive, converter or other typical switching application, for example, energy stored in a circuit’s stray inductance will be realized in the form of an overshooting voltage spike added to the dc bus voltage and felt across the IGBT at the time of turn-off.
In this scenario, the circuit designer must weigh several options. A diode in the same circuit will experience a similar voltage increase when it is reverse biased as a result of turning an IGBT on elsewhere in the circuit. Alternatively, the designer could increase the IGBT gate resistance (RG) and reduce the turn-off di/dt. This would reduce the overshoot voltage but also increase the turn-off loss and not take full advantage of the new semiconductor technology.
The designer could also choose a higher voltage IGBT, but higher voltage IGBTs have greater on-state voltage (VCE(SAT) and therefore, increased conduction loss.
The illustration above shows the designer’s best option: a laminated multi-layer bus bar, which is an effective choice for minimizing circuit stray inductance. With this solution, the capacitors and IGBT modules are closely connected to the conductors of the bus bar. These are usually made of copper and plated with Sn, Ni or another metal to improve connection integrity and slow oxidation. Component terminal offsets are compensated for with bushings or formed areas of the bus bar conductors themselves. The formed and plated conductors are then laminated together, separated by a thin layer or multiple layers of material having a high dielectric strength such as Mylar or Nomex.
Life testing and design are critical to high-performance outcomes
When designing to get the maximum benefits from any capacitor, one should do so with the intended application in mind. The first part of the life testing process should focus on determining what stresses the component will see and how long it must survive. For example, temperature and voltage stress levels are two common factors in determining capacitor life that almost all designs share. Care should also be taken to ensure that the shape parameter obtained in the accelerated testing using the Wiebull distribution function is a close fit to the shape parameter one would obtain when using the operating stress level.
The material selection is also a critical element of designing components for high performance. When conductors are properly shaped and routed so that current flows equally and in the opposite direction through each, their opposing magnetic fields will effectively cancel each other. The closer the conductors are together, the greater the cancellation effect. Therefore, the dielectric material selected should be as thin as possible while still having a dielectric strength appropriately in excess of the application voltage, resulting in little added circuit inductance.
The IGBT packaging is also important, as some inductance can be attributed to that material, as well. Many modern semiconductor packages employ miniature versions of multi-layer bus bars internally, as most semiconductor designers are keenly aware of the detrimental effects of stray inductance.
The bus bar’s low profile belies its importance
There aren’t many who point to the bus bar as a cutting-edge technology. In fact, this component’s contribution to the improvement in circuit performance often goes unnoticed. That’s a shame, since a properly designed multi-layer bus bar enables circuit designers to fully realize the performance benefits of today’s modern power semiconductors. Wires and cables can’t match the bus bar when it comes to low-inductance circuit construction. The bus bar keeps a low profile, but it has stealthily become a critical ingredient in the creation of high-performance switching applications.
Ken Brandmier has more than 20 years of experience in electronics related engineering research, product development and manufacturing. At Custom Electronics Inc., he supervises the development of new product lines including: single and multi-layer busbars, high-voltage semiconductor packages and several application specific capacitor charging and discharge circuits.