On the path to 48V vehicle electronics
Back in 1879, Karl Benz was granted a patent on an internal combustion engine (ICE). In the 140 years since, what began as a horseless carriage powered by Benz’s invention has steadily evolved into today’s modern car. In the past 20 years, though, the pace of this evolution has picked up, as the ICE has been challenged by the introduction of electric motors, either in hybrid or, increasingly, battery electric vehicles (EV). Over the next ten years, things will quicken once more, as personal mobility undergoes a radical rethink. The result of this will be a complete re-imagining of vehicle architectures, the subsystems of which they are built, the algorithms that control them, and even fundamentals such as the operating voltage of their electrical systems.
Drivers for change
Why the pressure for change? One key factor is the need to reduce mobility’s impact on the climate. Legislators are introducing stringent new regulations for vehicle emissions. Norway, for example, says that, by 2025, all new passenger cars should have zero emissions. The Netherlands wants half of all cars sold in 2025 to be EVs. Denmark wants to end sales of ICE cars by 2030, India wants all vehicles to use battery power by the same date, and China plans to end sales of ICE cars. Meanwhile, both the EU and US are strengthening their emissions standards.
The second driver for change is that we are demanding much more of our vehicles, and it makes sense to service that desire using electrical energy. For example, what used to be a dashboard with a mechanical speedo and rev counter is now an Internet-connected, app-enabled multimedia infotainment system. What once was a mechanical distributor and carburetor is now a sophisticated, multi-sensor, multi-actuator engine control unit. And where a parking sensor was once reserved for the high-spec versions of a particular model, today’s cars increasingly include advanced driver assistance systems such as adaptive cruise control, lane keeping, emergency braking, blind spot monitoring and more.
Alongside these comfort features, car owners also want their vehicles to be more fun to drive, and this is leading to the introduction of electric power steering, electronically controlled automatic transmissions, active roll control, electric turbocharging and even electric traction assistance. It all adds up to a lot of electrical energy flowing around a vehicle.
Pure EV technology, although offering lower-emission mobility, still falls short of delivering the range and convenience of ICE vehicles. The short-term fix for traditional car makers, therefore, is to start their vehicles along a path of steadily increasing hybridization. This will deliver considerable emissions reductions and fuel economy improvements without vendors having to make the leap to battery-only EVs.
This process begins with simple measures such as shifting traditionally engine-driven subsystems to electric motor drive, and ends with an intimately interconnected, co-dependent marriage of electric and ICE systems to deliver both traction power for the vehicle and electrical power for its subsystems. The greater flow of electrical energy needed to enable this strategy is forcing a shift from today’s 12V and 24V DC vehicle electrical systems to 48V DC. The approach is being promoted by Germany’s Big Five auto makers Audi, BMW, Ford, Mercedes, Porsche, and VW – and standardised through organisations such as ISO.
Why the choice of 48V? First, the higher operating voltage reduces resistive losses in energy transmission compared to 12V systems. Second, it allows for more powerful electric motors and generators. Third, 48V is below the 60V DC safety threshold that standards organisations have set as the point at which electrical systems have to be more heavily shielded any physically protected.
The hybridization journey
Toyota’s Prius, introduced back in 1997, demonstrated what may have seemed surprising at the time. Despite the additional cost, weight and complexity of fitting additional batteries, a motor/generator, complex wiring harnesses and more sophisticated control systems, it is worth it for the resultant reduction in fuel consumption and emissions.
Since then, an entire taxonomy of hybrids has emerged.
Starting with the simplest, a micro-hybrid uses a sub-5kW, 12V motor/generator to help vehicles start and stop. This form of hybrid can turn the ICE off when it is idle and restart it when needed. When drivers take their feet off the accelerator, the motor/generator helps slow the vehicle by turning some of its kinetic energy into 12V power to recharge the onboard battery. Industry estimates suggest this approach can reduce CO2 emissions by up to 4%.
A ‘mild hybrid’ goes a step further, by adding a 48V power bus and offering an additional feature: its motor/generator is powerful enough (5 – 13kW) to provide additional torque to the drivetrain, for example when a vehicle is pulling away from stationary, or a torque boost, for example when accelerating hard. There are two advantages of this ‘fill’ and ‘boost’ strategy. The first is that the electric motor’s torque is available instantaneously, improving responsiveness. The second is that it can help keep the ICE close to its most efficient operating point. Industry estimates suggest this approach can reduce emissions by up to 21%.
The familiar full hybrid, whose motor generator has enough power (20 – 40kW) to drive the vehicle on its own, needs a larger, heavier battery to store the required energy. This approach can reduce emissions by up to 30%. A plug-in hybrid, which uses energy from the grid to charge its traction batteries, can cut emissions by up to 75%, while a full EV doesn’t have any emissions (at least from the vehicle itself).
The automotive industry has built up a vast ecosystem of parts and experience around the 12V DC power bus, and this isn’t going to be discarded in the shift to 48V systems. This means that almost all hybrid vehicles will have to run dual power buses and battery systems to serve the needs of the 12V ecosystem and the emerging 48V equipment types.
Each form of hybrid will also need additional electronics. A 12V micro-hybrid is likely to need a voltage stabilisation system, as well as a dual battery manager. Mild hybrids will need 48V DC/DC converters to manage the flow of energy between the 48V and 12V systems, as well as 48V belt-driven starter/generators. Full hybrids, plug-in hybrids and battery EVs will run their traction batteries at much higher voltages, and so will need a whole family of high-voltage power electronics systems, as well as high-voltage axle-drive motor/generators to actually power the vehicles.
The challenge or opportunity here for carmakers, subsystem builders and component suppliers is that the shift to 48V is creating vast scope for innovation in the industry.
For example, adding a parallel electric subsystem to an existing ICE vehicle demands space, and so component and subsystem makers need to increase the integration of their offerings. Wiring looms and connectors are expensive, complex and costly, so innovations in this area would be welcomed. And there’s plenty of opportunity to offer more efficient, compact and robust power-electronics components for in-vehicle use.
Some car companies are developing 48V battery EVs with ranges of up to 150km for use in urban areas. These are going to need onboard chargers and battery managers, which can be controlled by emerging subsystems, such as NXP’s battery management devices. The MC33771B is a Li-ion battery manager for up to 14 battery cells. It has features such as current balancing between cells. The parts also meet the demands of the ISO 26262 vehicle functional safety standard.
Another critical enabler of the shift to 48V is compact, efficient DC/DC conversion. Companies such as ON Semiconductor have highly integrated modules, such as its FTCO3V85A, an 80V, low Rds(on) automotive-qualified power module featuring a three-phase MOSFET module for 48V DC/DC conversion. It includes a precision shunt resistor for current sensing, an NTC for temperature sensing, and an RC snubber circuit. This device is part of full family of dedicated power modules based on proprietary transfer-molded technology to address specific 48V applications like 48V Belt Start/Generator, Battery Disconnect Unit. Turbo Charger and other 3 phase motor unit for auxiliary functions.
Automotive systems will also need more efficient, rugged power transistors. Some vendors are already responding, with companies such as Infineon using its OptiMOS 80V/100V trench technology and leadless TOLL or TOLG packages to build basic devices for 48V applications. Similarly, ON Semiconductor is introducing a family of very low resistance 80V/100V N-channel PowerTrench MOSFETs in a compact TOLL package, like the FDBL8636x, which it says are a good fit for high-current 48V applications.
The fluid nature of the hybrid and EV market also provides scope for vendors to experiment with different ways to partition key components of the drivetrain.
For example, STMicroelectronics offers the L9907, a smart-power FET driver for three-phase brushless DC motors. It is built in the company’s BCD-6s process and can control six external FETs independently, enabling a variety of control strategies for three-phase brushless DC motors.
Elmos has taken a different approach with its E523.52, a programmable, high-voltage brushless motor controller for 24V and 48V cars and commercial vehicles. It has three half-bridge drivers, an 11V DC/DC step-down converter, two linear regulators, and a 16bit RISC microcontroller with 32Kbyte of flash memory. The 11V output can power six gate drivers, internal linear regulators, and external loads such as external Hall sensors.
Each approach has its merits. The exciting thing in the EV market at the moment is that there is still room to experiment with different approaches to motor control and energy management, and suppliers willing to provide the parts to do it.
Getting started with 48V systems
One of the most exciting opportunities for vehicle makers in the shift to hybrid vehicles is to increase the software content of their vehicles, so that they can be brought to market more quickly, adapted when in the field, and even become platforms for new revenue streams through the provision of onboard services.
The challenge here is twofold: finding people with the right level of experience of 48V power systems; and finding the software engineers who can develop code that meets the rigorous requirements of automotive standards such as ISO 26262, and make it secure against attack.
Distributors such as Avnet Silica are already seeing an uptick in demand for help with implementing secure subsystems and interfaces to key 48V applications such as inverters, DC/DC converters, and battery management systems.
It’s a long way from replacing a horse with an engine, as Benz did back in 1879, but getting effective help to manage the safety and reputational risks of shifting to 48V hybrid and battery electric vehicles is another important step in the evolution of cars.
By Gregor Knappik (Avnet Silica)
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