EMC Technology Report: The effect of split-voltage 12V/48V in modern cars

In the first 100 years of automotive history the motor car required no electronics, but in the 1980s microprocessors made an appearance, primarily for engine management, and by the 1990s these were almost standard fitment in on-board diagnostics. Today, a typical vehicle has dozens of embedded processors managing everything from passenger comfort and entertainment to critical vehicle safety systems. In fact it’s no longer possible to build vehicles that meet safety requirements without microprocessors that control systems such as the anti-lock brakes and advanced driver assistance systems. Author: Peter Els Furthermore today’s tightening emissions regulations have forced OEM’s to turn to electrification, spawning micro and plugin hybrids using 48V traction motors. With many existing systems designed around 12Volts, split-voltage12V/ 48V architecture has to be used: 12 V for infotainment, lighting and many driver assistant systems and 48V for traction, power steering, kinetic energy recuperation, and HVAC. These complex split-voltage systems call for DC/DC convertors. Fitting all these electrical and electronic systems into a very confined space exacerbates the problem of controlling electromagnetic interference (EMI) created through radiated and conducted emissions, sometimes referred to as crosstalk. If not properly controlled, the interference can cause a system malfunction, or even failure. With most systems now fully electronic the need to contain EMI is more important than ever. While radio interference might be a nuisance, it is infinitely more serious when an ABS or airbag suffers a malfunction because a vehicle passes close to a TV tower. Thus, with 12/ 48 Volt systems and the plethora of electronics and electro-mechanical devices fitted to modern vehicles, ensuring electromagneticcompatibility (EMC) has become an important part of the automotive electronic designer’s function. [eventPDF] Several suppliers are already involved in developing EMC components for dual voltage systems: TDK Corporation recently announced a new BCEM series of EPCOS high-current chokes designed specifically for buck-boost converters in 48-V automotive power systems. These innovative SMD inductors have very compact dimensions of only 27.5 mm x 29.5 mm x 14.5 mm thanks to their ERU27 ferrite cores with flat windings. The new chokes feature inductance values of 3 µH to 11 µH and are designed for saturation currents of up to 50 A and operating temperatures of between -40 °C and +150 °C. The components are suited for switching frequencies from 100 kHz to 200 kHz, are RoHS-compatible and are qualified to AEC-Q200. The new components are used as storage chokes in buck-boost converters of vehicles linking the 12-V and 48-V levels. Six or eight-phase interleaved converters are used in order to keep the ripple currents as low as possible and transfer high power levels. Since each of the converters needs a storage choke, the compact dimensions of the new high-current chokes along with their current capability are especially beneficial. Common EMI sources effecting modern vehicles In a digital system, clock pulses that are generated to operate the logic are often the root cause of EMI noise. As these clock pulses are developed (typically a square waveform), they have a given rise time. The rise time, as it gets shorter, has a tendency to create a broadband energy pulse on the leading edge. This is commonly known as overshoot and ringing. The energy present in the overshoot and ringing is the basis for generating other higher frequencies called harmonics. These higher frequencies are multiples of the clock frequency. Both odd and even multiples (harmonics) exist. In most cases, the odd harmonics (observed at 3, 5, 7, and 9 etc. times the desired or fundamental clock frequency) create most of the EMI noise problems. However, even harmonics do exist and must not be ignored. Another EMI issue involves intentional transmitters such as mobile phones, Bluetooth devices, and even commercial broadcast signals. The EMI of these external sources will interfere with the vehicle's electronic system when the vehicle is in close proximity to the transmitting antenna. In short, the new automotive electromagnetic environment is a very complex one involving many internal and external factors. Dennis Bogden, retired director of Powertrain Electronics Engineering at General Motors, points out that there are five different EMC domains: radiated immunity, radiated emissions, conducted immunity, conducted emissions and electrostatic discharge (ESD). Under these conditions engineers, both at a component and system level face the following problems when designing and qualifying systems for EMC: Wiring design impacts both conducted and radiated emissions, because of this EMC testing done at a component level will not provide a complete picture. It’s possible for a component to perform perfectly during component testing but fail once integrated into a system. In extreme cases, shielded wiring may be required, but twisted pairs or just keeping certain wires away from each other may be the solution. Software is an important consideration for some EMC problems. From an immunity point of view, the software is not impacted, but the right software can reduce EMI. A switch provides a simple example. With a switch, switch bounce and arcing can occur. A software routine that monitors the switch bounce and provides adequate settling time before making a reading is an effective solution. Microprocessor clock frequencies and software loops can pose unique problems that are normally easy to detect. It’s important to select the microprocessor crystal frequency so that it doesn’t overlay other radio frequency bands, while software executing periodic timing loops can also create problems. Certain control techniques also produce problems. For example, pulse width modulation (PWM) control for motors and solenoids tend to generate both conducted and radiated waveforms. If run at the right frequency, a conducted path coming back through the wiring can affect other components in the system. With 48V architecture gaining in popularity the number of motors used in cars is expected to surpass 200 per car this year. Controlling EMI to improve electromagnetic compatibility Optimal EMC can be achieved through: Ensuring all components in the system are within the EMI specifications Components that emit high levels of EMI are adequately shielded Selecting the correct component design to minimize EMI By way of example: In brushless DC (BLDC) motor controls, advanced power semiconductors and peripheral components are integrated to reduce size and weight. This integration comes in many forms; from simple power modules consisting of multiple power devices (such as power MOSFETs and IGBTs), to intelligent power modules (IPMs) that incorporate power devices, their drive circuits (pre-drivers), protection circuits and so on. By applying metal-based Insulated Metal Substrate Technology (IMST) EMI emanating from the motor control can be significantly reduced. With an IMST-based IPM, there is a distributed capacitance through an insulating resin between the aluminium sheet portions and copper foil portions of the metal substrate. This works to effectively reduce switching noise of the power device when compared to a discrete configuration: Image Credit: EETimes Automotive Further, as the parasitic inductance of the wiring layout and wire bonding on the IMST substrate can be reduced, noise can be lowered by inhibiting the high voltage surges that occur during switch state transition when a high-voltage, high-current device is hard-switched. For radiated EMI; once proper circuit layout is developed (minimize frequency, current and loop area), if the circuit still does not meet its EMI requirements, the only thing left to do is provide some sort of shielding. However, system designers have recently been evaluating the effects of layout of the hardware within the system, to improve EMC and reduce costs. Depending on the vehicle design and geometry and layout, hotspots of EMI can be created.By correctly positioning EMI sources and receptors designers are able to reduce the amount of shielding required, thereby driving down EMC costs. EMC testing ensures compliance in dual voltage systems and connected vehicles ISO 26262 is designed to ensure that vehicle electronics meet the criteria for reliability and safe operation. The standard employs a risk-based approach for assigning components to various risk classes and defines procedures for the development, operation and service of components depending on their class. Design and testing to ensure electromagneticcompatibility (EMC) plays a key role in these procedures. Nowadays all car manufacturers are highly aware of the fact that EMC testing is an important part of car electronics development and understand that EMC issues become costlier the later they are discovered. For this reason they do not just rely on a final test inside the car but insist on tests of the electronic control unit (ECU) and even on test results of the integrated circuits used in the design before deployment in vehicles. However, The introduction of hybrid vehicles has brought with it major new components that were previously absent in conventional vehicles: Electrical Energy Storage Systems (EESS) such as a battery or Super (Ultra) capacitors, PECs, electric generators, electric motors, high voltage bus (cabling) and more ECUs. All require new forms of EMC testing as some issues in the standards and test setups must be updated: cars cannot be measured in the idle state and new LISNs must be used in the high-power network, whilst magnetic field evaluation for safety must be mandatory. Many OEMs realize that EMC is more than meeting regulations. Fiat Chrysler Automobiles with its test facility in Michigan claims that its electromagnetic compatibility requirements are more stringent than any worldwide standard. Chrysler chooses to conduct the majority of component tests in-house: Robert Kado, manager at the EMC lab, claims: "Suppliers will come and work with us in the lab and that testing is done well in advance [of a vehicle launch]." Full vehicle tests are grouped into subsystems and consider every function in the car including the instrument cluster, infotainment system, HVAC, and collision warning, to name but a few. To ensure compatibility, the radio frequency emission and immunity characteristics of all components and systems in the vehicle must be analysed. This includes fully characterizing the systems with regard to immunity to electrostatic discharge (ESD) and other transient voltages. Furthermore, inductive components, such as motors and solenoids, must be evaluated to determine their potential to generate transient voltages within the vehicle’s electrical system. One particular component test, for example, involves radiating a component using an ESD simulator and monitoring its response. The FCA team believes that simulation can be used as an up-front design tool; however, its suitability for evaluating EMC remains limited. Things like field strength and the geometry of the vehicle and layout of the machine metal can be measured to identify where the hotspots are, based on waves bouncing and resonating in a certain way. There are also certain ‘rule checkers’ that can be applied when simulating the design layout of a circuit board: "For example, we might know that it is dangerous having one particular type of chip next to another. That level of work is often performed by suppliers and qualified during design reviews," explains Kado. Despite individual components being qualified for EMC the complete vehicle has to comply as well. Full vehicle tests take place in a semi-anechoic chamber. The walls inside the chamber are lined with carbon impregnated foam that absorbs radio waves and dissipates them into a form of very low heat that doesn’t affect the temperature of the chamber. To simulate real world conditions a turntable enables all sides of the vehicle to be tested, while a rolling road is used for analysing any EMI generated by the powertrain and control systems. This test is typically performed at 19, 37 and 56km/h. Image Credit: Automotive testing international The current internal combustion engine powered vehicles usually produce two main types of electromagnetic interference (EMI): the ignition system, the electrical motors and the switches producing broadband EMI and the electronic devices generating narrowband interferences. Electric vehicles use high-power electronics to drive the electrical engine which produces high-level low-frequency EMI. In hybrid cars this new emission is added to the ignition systems one. Plug-in electric vehicles introduce a new device in the mains network and, because their characteristics and the power involved, special considerations must be taken into account. In order to achieve effective EMC proper EMI design procedures need to be followed during system as well as component development. There are a myriad of EMI sources and receptors in today's motor vehicles and in the environment in which they operate. Today's cars are packed with electronic, electro-mechanical and electrical systems, and will only get more sophisticated and complex as time goes on. The control of electromagnetic interference is essential. Not only because there are mandatory requirements, but also for the proper functionality of the entire system. Devices installed into automobiles and devices brought on board by passengers must not interfere with each other. This is especially true for the safe operation of the vehicle. The achievement of complete electromagnetic compatibility is essential. Sources: Avoiding EMC Problems in Automotive Systems - Electronic Design (Randy Frank) Compact high-current chokes for 48-V automotive power systems - http://www.epcos.com Controlling automotive electronic emissions – EETimes (John Noto, Gary Fenical, and Colin Tong) In-Vehicle Electric Control Systems Using Intelligent Power Modules – EETimes Automotive (Matt Tyler) Automotive testing international – EMC testing at Fiat Chrysler Automobiles About the Author After qualifying as an Automotive Engineer, Peter has spent 40 years in the South African Automotive industry. Having started his career with Nissan’s product development division, automotive technology and engineering remain his passion.

EMC Technology Report: The effect of split-voltage 12V/48V in modern cars

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