Plastic optical fibre for battery management systems in 48 V powertrain architectures

By KDPOF
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Electromagnetic noise is a major topic in any electrical power train, either full electrical or hybrid architectures. Using plastic optical fibre for battery management systems provides a new way to implement 48 V powertrain systems Read More


Electromagnetic noise is a major topic in any electrical power train, either full electrical or hybrid architectures. The primary source of electrical noise generation is the continual switching of the IGBTs and MOSFETs embedded within the power electronic subsystems.

The DC-AC inverter produces an AC waveform from the battery current that drives the electric motor or, alternatively, rectifies the current from the motor to store it in the battery. On the DC-AC inverter board, AC waveforms in the order of 8-10 KHz are typically generated.  The DC-DC bidirectional converter steps up the voltage of the battery pack to that employed on the high voltage bus and generates harmonics in the 50 KHz band. Another source of high frequency AC ripple on the DC link comes from the electrical motor itself. The electrical machine produces odd harmonics arising from the non-sinusoidal back-EMF waveform.

The control of all the subsystems involved in the electrical powertrain requires a communication bus that transports the control, actuation and sensor signals among the different components. The communication bus has to be immune to the above-mentioned electromagnetic noise and, at the same time, comply with the mechanical, temperature, and weight constraints of the overall vehicle.

Figure 1: A hybrid powertrain architecture

An optical-based communication technology like 1000BASE-RH complies with these requirements. Moreover, it can operate at 100 Mbit/s for most current needs while also supporting future needs at 1 Gbit/s.

Isolation risks in battery management systems

The batteries used in electrical powertrains consist of physical clusters of cells, assembled in enclosures called packs. Packs typically contain between six and twenty-four cells in series. Altogether, a commercial battery involves one hundred or more cells providing hundreds of volts. A typical li-Ion battery consists of approximately 96 stacks of cells, developing a total voltage in excess of 400 V.

It is generally accepted that more than 60 volts may be lethal to human beings, so safety is a key concern for more than the surrounding electronic equipment. Although inherently dangerous, stacks need to be monitored and managed. For this purpose, a safe and reliable communication system between the cell monitoring devices in the packs and the central battery management system (BMS) is needed. For lithium-ion chemistry, for example, it is necessary to monitor the voltage of each cell. Moreover, although individual cell temperature monitoring is not mandatory, the facility to do so should be available. These measurements are taken by specific standard product ICs (ASSPs), which typically handle from six to twelve cells.

Figure 2: Battery packs grouped into a series of electrically separate clusters

A common approach to organizing cells in a battery stack is to group the battery packs into a series of electrically separate clusters, as shown in figure 2.

Each cell pack communicates with a control module that in turn sends and receives control information from the BMS. The chain of cells is connected in series with a switch that is normally closed when the vehicle is in normal operation. In emergency situations, the switch is opened so the stack voltage disappears at the terminals. In order not to compromise the isolation provided by the open switch, it is necessary to ensure that there is no alternative electronic path bridging the switch terminals. Thus, the top half of the stack should be electrically isolated from the bottom half when the switch is in an open position. However, the need for a communication bus between each pack of cells and the control module may create an undesired parallel path to the switch.

It is thus required that each pack communicates with the control module across an isolation barrier. In other words, the communication bus should provide galvanic isolation. Moreover, the required cabling for the communication bus degrades the electromagnetic compatibility (EMC) performance of the system.

All in all, once again, a communication system that is not based on copper wires is the best choice to ensure a safe, robust and long-term galvanic isolation between the cell monitoring structures and the BMS.

48 Volts bridging in new electrical architectures

With regulations driving car companies to reduce Green House Gases (GHG) emissions further by 2021, a new hybrid architecture concept based on a two voltage power line (12/48 volt) is already on the advanced marketing announcements of OEMs and Tier-1. A 48 volt dual voltage electric system converts high power consumers to 48 volt, thus reducing electrical losses and wire weight. The first 48 volt hybrid models will enter the market in 2020 and will incorporate e-chargers (electrical turbos) with a 10%-15% gain in fuel economy.

The new 48 volt electrical architecture pushes the envelope in terms of EMC and safety requirements. As an example of this industry-wide new technological wave, the German VDA published recommendation 320, covering electric and electronic components in motor vehicles for the development of a 48 volt power supply. This document defines requirements, test conditions and tests performed on electric, electronic and mechatronic components and systems for use in motor vehicles with a 48 volt on-board power supply.

The first requirement for components with a 48 volt connection, as stated on recommendation 320, reads:

A single error must not cause a short circuit between the 48 volt supply and the 12/24-volt supply…. ground connections on the 48 and 12/24-volt lines must be physically separated from one another.

Figure 3: Failure modes

The challenge for automotive electronics designers is to implement a battery management system (BMS) that provides for a combination of safe operation, long battery life and the separation of the low- and high-voltage domains without requiring the use of numerous components in a complex circuit design. Since the development of 48 volt automotive power systems is in its infancy, there is as yet no single, preferred architecture or approach to achieving these goals.

New safety precautions are needed while designing the 48 volt electrical system. Even a single malfunction between 48 volt and the12-volt electrical system will lead to short circuit, and this can damage the complete 12 volt system due to overvoltage. Moreover, since the chassis is a common ground potential for both 12 volt and 48 volt, loss of this common ground may lead to a damage of the 12 volt system due to the reverse voltage on the 12 volt system.The need for a ubiquitous communication network within the vehicle, and in particular between ECUs belonging to different voltage domains represents a source of potential hazards. It imposes the additional requirement of galvanic isolation between the communicating nodes. Any event like the ones mentioned above that imply the 48 volt crossing into the 12 volt might destroy communicating ECUs with line transceivers that do not provide sufficient galvanic isolation.

Plastic Optical fibre 1000BASE-RH, being based on a dielectric media, will perform as a perfect galvanic barrier between domains, and provide up to 1 Gbit/s communication speed at the same time.

Figure 4: Galvanic Isolation

Summary

Upcoming regulations on GHG emissions and safety are inducing a technological leap in the automotive industry. Electrical powertrains, with their complement of 48 volt architecture along with all the functions required for different levels of autonomous driving, are the containers hiding a plethora of innovations aimed towards the new stringent requirements.

Further electrification of the powertrain and surrounding systems requires a communication architecture to control the actuation and sensing. Unless the technology supporting the communication buses guarantees a reliable isolation between modules and a negligible contribution to the electromagnetic radiation, the technical constraints will increase the efforts, costs and timing of bringing these new innovations to the mass market.

1000BASE-RH, the Ethernet specification for a Gigabit capable, Plastic Optical Fibre-based communication protocol is suited to the new architectures as it provides a natural galvanic isolation between communicating modules, a radiation free harness and the ability to run at 100 Mbps for the early requirements of this technology.

 


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