The electronics systems of today’s electric and hybrid vehicles are crammed into extremely small spaces and are subject to vibration, impact and wide temperature fluctuations. A major assault to these electronic systems results from EMI/EMC. The evolution from mechanical to computerized automotive systems over the past 40 years has ushered in a myriad of features and benefits, as well as a greater need for electromagnetic compatibility (EMC) filtering, especially in today’s electric and hybrid vehicles.
Before the mid-1970s, auto engines were based on simple mechanical designs with circuitry too large to be practical. In the mid-1970s, electronics in vehicles typically involved the car radio and speakers. However, the gas crises in the 1970s prompted changes such as the addition of solid-state circuitry to control ignition timing and downsized cars and engines. By the 1980s, microchips were used for fuel injection and emission control applications. On-board computing became the norm in the 1990s, enabling greater control and functionality, while also making automobiles too complex for the average car owner to fix.
Since the early 2000s, the list of standard electronics grew to include GPS, entertainment systems and advanced climate control. As a result, electronics and on-board computing have become an essential performance-determining part of most vehicles on the road. It is now the age of fully computerized vehicles.
Hybrid and Electric Vehicles
Yesterday’s combustion-based vehicles have given way to dozens of electric and hybrid vehicles (e-vehicles). Electronics now control every aspect of these vehicles from basic operations to enhanced safety, comfort and convenience, as well as the powering of electrical drive systems, high-voltage batteries, inverters, and at least one electric motor. There are, however, potentially an unlimited number of problems that can occur with e-vehicles and their computing systems.
One major culprit, for example, is electromagnetic interference (EMI) and the physical damage it can cause. Attempts to mitigate such damage include the use of EMC components and filters to maintain the operation of the automotive communication networks and a variety of equipment found within the environment. EMC requirements for components and devices within the environment are regulated by international standards such as CISPR 25 or the EU Directive ECE-R10.
A priority when developing e-vehicles is to ensure that the individual systems, crammed into extremely restricted spaces, do not cause mutual interference. Given the unique design and punishing application environment of electric vehicles, design engineers must use electronic components that withstand vibration, impact, and wide temperature ranges, while reducing electromagnetic interference, voltage spikes and ground currents. It is also critical that interference does not affect systems outside of the vehicle.
When things go wrong
There are several places to search for the source of EMC problems when they do occur. Batteries and inverters are one of the first places to look. Inverters operate with pulse-width-modulation (PWM) control of the motor. The sharp edges of these pulses may cause EMC issues for both the input and output sides of inverters. This most often manifests as conducted and radiated emissions, which can be minimized by using a complete encapsulation or by shielding.
With the lack of a combustible engine, and given the space constraints of batteries and computing systems, designers must distribute weight and drive components throughout the vehicle. Most often the battery is situated in the rear of the vehicle, while the inverter is in the front. Motors are typically installed as close to the wheels as possible, either on the axles or in many cases directly on the wheels. As a result, the connection of the inverter and battery is made with a long shielded cable. This creates EMC risk for three reasons:
- High shield currents can be created that contribute to emissions in the high-frequency range.
- Voltage spikes that occur in longer cables can be large enough to damage the inverter and/or battery.
- There is a possibility of interference within the low-voltage system of the vehicle due to cable length.
The actual connection of the cable shielding to the battery shielding can also cause issues. The impedance of the connection, for example, must be tremendously low to provide the shielding required. This is further exasperated by vibration, impact and temperature fluctuations common to all vehicles. Finally, oxidation and corrosion may occur as the system ages, weakening the shielding connection and causing impedance to rise over time.
The best solution for dealing with EMC issues is to design systems using high-voltage DC filters. For example, 2-line high-voltage DC EMC filters for electric vehicles are designed for a maximum voltage of 600 V DC, which correlates to the standard voltages of high-voltage batteries.
To avoid significant loss, EMC filters must offer a very low DC resistance and should filter drive systems with power ratings in excess of 100 kW, with current capabilities in the range of 150 A DC to 350 A DC. In this way, conducted interference can be reduced by up to 70 dB, or a factor of 3,000, even when using an unshielded cable. EMC filter use can eliminate the need for shielded cables, ultimately saving weight and handling weight distribution and space challenges. EMC performance should be tested and confirmed in a test setup with the EMC filter in place to ensure results.
The use of 2-line high-voltage DC EMC filters, such as those offered by EPCOS, enables a considerable reduction in the number of conventional EMC measures in individual system components. Moreover, by using such filters, engineers can retain much-needed space and eliminate weight for the even more advanced on-board computing systems that are sure to arrive on the scene.
For additional information, please contact Joseph Pulomena, at firstname.lastname@example.org or 732-906-4300