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Posted by Szymon Pasko

Electromagnetic interference: compact, power-compensated restrictors reduce interference signals at 80 – 400 amperes with just one magnet core

Vehicles with electric drives are becoming ever more popular. Based on recent figures from media company Bloomberg L.P., 1.1 million new e-vehicles were purchased worldwide in 2017 – by 2025, this figure is expected to rise perhaps as far as 11 million. However, and in particular, it is the electronic components of these vehicles that present many automotive manufacturers with new challenges. That is because excessively powerful electromagnetic interference (EMI) can cause electronic braking systems to malfunction. This is why suppression filters are used to reduce interference at these levels, in order to protect sensitive components. For this, existing filters need to be fitted with a bus bar filter with up to 21 ring cores. This increases the weight of EMI filters and they occupy valuable space within the restricted confines of a driveline. For this reason, Schaffner EMV AG has developed current-compensated restrictors for power levels of 80 – 400 A. With just one core, these units deliver the required level of EMI performance. They are therefore only half the weight of conventional solutions, and they only occupy about ¼ of the space previously needed. This in turn enables them to be installed in any hybrid or electric vehicle in a space-saving manner, where they provide reliable protection against EMI.

The electrical systems in modern electric vehicles generate electromagnetic interference when they are being operated at high performance levels. For example, the rapid charging of a battery in a short period of time entails a high level of energy transfer with a current strength of up to 400 A. In this way, powerful interference signals are created. These can cause a great many electrical components to malfunction – examples being BMS (battery management systems), electronic motor drive controllers, sensors and other safety functions. “In the past, bulky EMI filters were need to reduce interference to a non-hazardous level”, reports Dr. Szymon Pasko, R&D Lead Engineer at Schaffner EMV AG. “However, and in particular in modern electric vehicles, there is often no space, and the additional weight leads to higher levels of power consumption”. This is why Schaffner EMV AG developed current-compensated restrictors capable with just a single, ultra-compact restrictor of delivering the same level of EMI suppression as those bulky traditional EMI filter solutions. 
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Schaffner provides patented, current-compensated restrictors for the range of 80 – 400 Amperes. They filter electromagnetic interference convincingly with their high performance, compact design and good thermal characteristics.  

Efficient design through intelligent bus bar design

A normal restrictor consists of a magnetic core and two coils that are wound around one another. Firstly, this makes the level of EMI reduction dependent on the number of windings around the core. Secondly, it is dependent on the magnetic properties of the core. The biggest challenge for an EMI filter comes when a restrictor of this kind is used in high-current applications. In such cases, a current strength of 80 – 400 A is not uncommon, using conductors with a big cross section (32 mm2 – 160 mm2) which are difficult to wrap around the magnetic core. For that reason, in previous approaches involving applications in the high-current range (such as 125 A), two bus bars are directed through a ring core, so theoretically they constitute one single winding.


“To achieve the required reduction in EMI, this approach needs a large number of ring cores. We therefore looked for a way of increasing the amount of copper within a small area”, explained Pasko. “In the process, we recognised that, while is also possible to use the bus bars for the filter effect, in their previous form, they could not be arranged around the core in a space-saving manner”. For that reason, Schaffner modified the design of the bus bars to apply them better to the magnetic core.

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Up to 21 ring cores are needed for previous current-compensated restrictors to filter out electromagnetic interference factors at 125 Amperes. The compact, current-compensated restrictors made by Schaffner only weigh half as much as conventional restrictors and only take up about ¼ of the previous space. This enables them to be installed in a space-saving manner in any hybrid vehicle or electric vehicle, reliably minimising EMI-related safety risks.

The new, current-compensated restrictor from Schaffner with maximum dimensions of 95 x 56 mm only weights 1.2 kg. It is designed for a temperature range of -55°C to +150°C and the efficiency of this design permits operation up to 400 A. “Admittedly, we still used a magnet core with a bus bar here, but the outer bus cars now wind around this centre, which increases the filter function”, explained Pasko. “This specific arrangement achieves a similar effect to normal copper wire, but this design enables a great deal more metal to be used in a single current-compensated restrictor, which achieves better filter performance.” The previous solution with up to 21 ring cores is therefore no longer needed to reduce electromagnetic interference in a reliable manner.

Modular concept and Pre-Sales Service permit individual adjustments to be made

The housing on current-compensated restrictors is based on a modular design which makes it easy to adapt it to suit individual requirements. This enables Schaffner to assure the performance of coils across a bandwidth covering a vast array of different applications – from
10 kHz to 100 MHz. “We advise our customers in advance, and recommend appropriate materials for each field of application”, stated Pasko. “By way of example, a ring core made of the ferrite material MnZn or nano-crystals is suitable for applications in the kHz frequency range whereas a ferrite material consisting of NiZn performs better in the MHz frequency range”. It is also possible to use other metals such as aluminium or a compound of different magnetic materials. In this way, the experts are able to guarantee a bespoke solution for all electrical systems, regardless of their application within the electric vehicle sector.

This image shows the different forms of reduction of electromagnetic interference on vehicle drivelines. 
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