Innovative filter solutions for e-mobility

As cars are equipped with more and more electronic systems, so the demands on EMC increase. This applies in particular to electric vehicles (xEVs) with their additional power electronics for the motor inverters. An elegant and weight-saving solution to the resulting EMC problems is offered by new filter solutions from TDK.

Regardless of whether they have a hybrid or fully electric drive, electric vehicles, or xEVs, are packed full of electronics which far exceed the values familiar from combustion-engined vehicles in terms of both quantity and technical requirements. For, in addition to the increasingly complex systems for safety, comfort and communication, it is the electrical drive systems – comprising high-voltage battery, inverter and at least one electric motor – that require the use of electronic systems. Consequently, the number one priority when developing such vehicles is to ensure that the individual systems, crammed into very restricted spaces, do not cause mutual interference. It is also important that no interference affects systems outside the vehicle. These EMC requirements are regulated by international standards such as CISPR 25 or the EU Directive ECE-R10.

EMC problems caused by shielded cables

The inverters operate with pulse width modulation in order to control the motor with the required power and speed. The steep edges of the pulses cause considerable EMC problems on both the input and output sides of the inverters, which manifest themselves as both radiated and conducted emissions. In order to keep these to a minimum, most designs are based on a complete encapsulation or shielding of the entire system.

To save space and improve the weight distribution, the individual drive components are distributed throughout the vehicle. The battery is usually accommodated at the rear and the inverter at the front. The motor or motors are installed on the axles or – where designed as wheel-hub motors – directly on the wheels. Connection of the inverter to the battery thus necessitates a correspondingly long, shielded cable. This, however, entails significant potential risk for the EMC because, on the one hand, high shield currents can be generated that contribute to high emissions in the high-frequency range. On the other hand, voltage spikes of sufficient magnitude can even damage the inverter or battery. Moreover, the possibility of interference being coupled into the low-voltage system of the vehicle cannot be ruled out.

The electrical and mechanical connection of the cable shielding to the shielding of the battery and inverter poses a further problem. The impedance of this connection must be extremely low in order to guarantee adequate shielding. But the vibrations and impacts inherent in vehicles create forces that weaken the shielding connection, causing a gradual long-term rise in impedance. Aging processes caused by oxidation, or even corrosion, are not insignificant either. Figure 1 shows the setup for the measurement of electromagnetic emissions for power electronic devices according to CISPR 25.

Figure 1:

Measurement setup according to CISPR 25

The radiated and conducted emissions of a system with shielded cable between the battery and inverter are depicted in Figure 2.

Figure 2:

Emissions with shielded cable:
Although the use of a shielded cable between battery and inverter reduces the radiated emissions (left), it does not reduce the conducted emissions (right).

Clear improvement in EMC through use of new filters

In order to cope with the growing demands made on EMC, TDK has developed the P100316* series of EPCOS two-wire high-voltage DC filters specially tailored to the needs of electric vehicle drive systems. These filters are designed for a maximum voltage of 600 V DC and correspond to the typical voltages provided by high-voltage batteries. Their current capabilities are around 150 A DC or 350 A DC, which means that even drive systems with power ratings considerably in excess of 100 kW can be filtered. The DC resistance is only 0.05 mΩ for all types, which means that there are no significant losses even in the case of high currents.

As shown in Figure 3, it was possible to confirm the excellent filtering effect of the new filters in a test setup.

Figure 3:

Emissions when using the EPCOS high-voltage DC filter:
Plain to see: By using the new EPCOS EMC filter between battery and inverter, it was possible – despite using an unshielded cable – to dramatically reduce the conducted interference in particular.

Despite using an unshielded cable, the conducted interference in particular could be reduced by up to 70 dB, or a factor of 3000. There is a further advantage to the use of the new filters: They enable a considerable reduction to be made in the number of conventional EMC measures in the individual system components.

Apart from outstanding electrical values, the filters are also impressive for their low weight and compact dimensions – factors that help make them suitable for use in vehicles. Depending on the individual type, dimensions vary between 121 mm x 52 mm x 52 mm and 186 mm x 65 mm x 65 mm.

The new series comprises 12 types that not only have two different current capabilities of 150 A DC and 350 A DC, but also feature different filtering characteristics. This means it is possible for development engineers to obtain a filter precisely tuned to their specific EMC problem. In addition to the versions with a general common-mode rejection ratio, types are available that exhibit a particularly high filtering effect in the long-wave spectrum between 150 kHz and 300 kHz.

Extending the life of motors

High-frequency shielding currents represent only a part of the challenge in ensuring electromagnetic compatibility. At the output of the inverter, for example, the steep pulse edges cause voltage spikes that are further increased by cable inductances. Under unfavorable conditions these voltage spikes can result in arcing that could destroy the motor windings. At the same time leakage currents occur that flow through the motor bearings causing sparks. This in turn can damage the bearing balls or rollers, causing them to fail prematurely.

One remedy for this is the use of ferrite ring cores through which the motor cables are routed. This significantly reduces common-mode interference and the leakage currents to a non-critical level. Here too, TDK offers a wide range of solutions: Ring-cores with different geometries and in different ferritic materials that are optimized for certain frequency ranges and can thus be tailored to every drive system.

Table: Technical data of EPCOS two-wire HV DC filters

* Based on passive cooling. Higher operating temperatures are possible with active cooling.