High Performance Busbars for e-Mobility

Sep 09, 2020

Reliability

The motor, battery and electrical control system are known as the tri-electric system of e-Mobility.

Rogers Corporation has many resources on interconnection and the linkage of batteries with our high performance ROLINX® busbars. We offer busbars that are used for cylindrical batteries, prismatic cells and those which are individually designed or jointly developed with major manufacturers in the e-Mobility field. Additionally, we have solutions for the interconnection and thermal management of pouch cells with PCM (Phase Change Material) which are offered by Rogers Elastomeric Materials Solutions business unit.

In this blog, high performance busbars used in electrical control systems are introduced. Generally, our ROLINX® busbars are widely used in high power inverters for pure electric vehicles (EVs) and plug-in hybrid cars, known as traction drivers, as well as in low power inverters of hybrid EVs, known as ISG (Integrated Starter Generator).

Nowadays, taking the compact integration into consideration, low power inverters with less than 60kW or even 75kW employ power PCBs (Printed Circuit Board) to make control units and power units, which are integrated on a single PCB. However, durability, vibration and thermal phenomena are essential in the field of e-Mobility.

With the development of SiC semiconductors and their reduced cost, more and more SiC MOSFET and IGBT are used in inverters which brings many new challenges.

Firstly, since the switching frequency of a SiC semiconductor is much higher, the stray inductance must be as low as possible in order to avoid a voltage spike. Unlike Si semiconductors, which have been developed many years ago and are limited by packaging technology and cost, there are a few modules of SiC semiconductors in the market. The most widely used in the market today are discrete and SiC MOSFETs. Semiconductors should be paralleled to reach the adequate rated power which demands more strict requirements for a low inductance. Meanwhile, the paralleled topology makes power expansion more flexible. In fact, the modularization makes the assembly easier while the discrete version allows a more compact structure.

Secondly, the temperature resistant of a SiC semiconductor is much higher. The maximum junction temperature of a SiC semiconductor is 175℃ and the extreme temperature may reach 600℃ while the same parameter of Si semiconductors equals to 150℃ which illustrates a big challenge for traditional PCBs. For the actual application, the SiC semiconductor´s case temperature can reach 125℃ whereas a Si semiconductor only reaches 85℃. The temperature resistant of the power unit and inverter also depends on the performance of the capacitors, inductors and several other components.

Consequently, a power unit with a low inductance and a high temperature resistant laminated busbar are still the best choice for electric vehicles´ inverters. Rogers has a wide range of laminated busbar solutions that meets the demand for a higher temperature resistant.

Low inductance is an advantage of laminated busbars and an area in which Rogers preforms. The stray inductance makes electric, magnetic and thermal phenomenon coupled in the inverter. It will lead to the deviation of the theoretical design or force the design margin too large that it increases overall cost. Besides, the power density is difficult to be improved. In addition, stray inductance affects the selection of power electronic components, power ratings, switching frequencies, the magnetic components´ performance, the loss and temperature rise. Therefore, the importance of low inductance busbars in inverter is obvious.

The skin effect, mutual inductance effect, commutating circuit´s length and interval distance, as well as busbars´ split structure have an influence on laminated busbars´ stray inductance. From our experience, the stray inductance increases by 0.3 times when the length of the converter circuit is doubled. Furthermore, the stray inductance increases by 0.8 times when the insulation thickness of the commutating circuit is doubled.

Due to the layouts´ asymmetry, components´placement and laminated busbars´ structure, the stray inductance of the commutating circuit is unbalanced. The stray inductance´s imbalance in the commutating circuit of laminated busbars leads to the asymmetry of stray inductance in the main circuit. Therefore, it is necessary to add an additional snubber circuit and a heat sink to suppress the unbalanced voltage and thermal stress. Thus, special attention should be paid to the balance of stray inductance in the commutating circuit.

Most of the power electronics solutions are a tradeoff between technology and cost. Due to different power and converter topologies, the laminated busbars´ physical structures are different. In order to obtain the optimal results, the laminated busbars´ design should follow the following principles:

  1. The insulation distance of commutating circuits of laminated busbars should be as small as possible on the premise of ensuring insulation.
  2. The layout of the laminated busbar should be as compact as possible on the premise of ensuring heat and temperature rise.
  3. The commutating circuit can be balanced by improving the components´ layout and busbars´ laminating sequence.

If you have any design questions or require assistance for the optimization of your design using busbars for your application, Rogers’ PES experts are available to help. Please contact us, if you have any questions.

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