The shift to 48V in automotive systems: What you need to know

One of the biggest current trends in car design is the shift to hybrid and fully electric vehicles. Less obvious to the public, perhaps, is a fundamental change in the architecture of vehicles’ electrical systems, with the introduction of 48V DC systems to work alongside the 12V DC systems we have used for decades. But what is driving this change, and what will it mean for companies developing vehicle electronics, and the component suppliers that serve them?
 

Power play

It’s here that your high-school physics lessons prove their value. Remember ‘watts equals volts times amps’? This handy equation tells us that you can deliver the same power electrically using either high voltages and low currents, or high currents and low voltages. A second equation, ‘watts equals current squared times resistance’, tells us that it makes sense to deliver power at high voltages and low currents, to minimise resistive losses.

This is one of the key reasons for a shift to 48V: modern cars, their energy-hungry climate-control, powerful driver-assistance computers, and complex infotainment systems together mean that cars now consume much more electrical energy than they used to. It makes sense, therefore, to shift to 48V to minimise resistive losses. The shift to 48V also allows higher currents, and therefore greater power delivery, so features such as window demisters and heated seats can be more effective by working more quickly.

These may seem like trivial reasons for making such a substantial change to a system that has served us well for decades, but they sell cars – some US truck makers are promoting their use of 48V systems by pointing out that they will enable users to run power tools from their vehicles. Potentially much more important, adding 48V systems will enable hybridisation, bringing greater fuel economy and reduced emissions to vehicles at relatively low cost.
 

The shift to hybrids

The first step towards hybridisation is to use 48V systems to help run internal combustion engines more efficiently, by driving auxiliary functions such as power steering racks, brake vacuum pumps and water pumps with electric motors rather than using a power take-off from the engine. This can be more energy efficient than mechanical drive, since the operation of these functions can be more closely matched to the vehicle’s needs. For example, intermittent electrical drive is more efficient than continuous mechanical drive for running vehicle air conditioning, because an AC system’s compressor is off more often than it is on.

The next step is a ’mild hybrid’, which replaces a car’s traditional starter motor with a motor generator unit (also known as a belt-driven starter generator). This acts as generator under braking, recharging the vehicle’s battery. When the vehicle comes to a stop, the motor generator, driven by AC power provided though an inverter from the battery, can restart the internal combustion engine and set the vehicle moving again more quickly by providing additional torque. Industry estimates say this approach can improve fuel economy by 10 to 20% at a much lower cost than a full hybrid approach.

Using a slightly more powerful motor generator will enable a car to creep through a parking lot or a city’s low-emission zone on electric power, reducing fuel consumption and emissions. More complex hybrids use much more powerful motors to handle traction at normal road speeds. They also use the electric motor to smooth out torque delivery from the internal combustion engine, when changing gears or pulling away from a stop, and to provide additional power as necessary. These two strategies help keep the engine operating at its peak efficiency, reducing emissions and making the vehicle more responsive and fun to drive.
 

Making the transition

The industry has too much invested in 12V electrical systems to try to make an overnight transition to 48V. Instead, the shift to 48V will come gradually, with the introduction of 48V infrastructure to run alongside the ‘legacy’ 12V system. A generic 48V system is likely to include a 48V battery and battery controller, the motor generator unit and inverter, power bus and connection points, as well as a DC/DC converter to transfer power between the two systems as needed.

Shifting to 48V will demand the development of a wide range of new componentry that can operate at this higher voltage and meet the demanding standards of the automotive industry. For example, mild hybrids will need efficient inverter circuits to be able to contribute energy to, and draw energy from, the onboard 48V battery without excessive losses. Wiring looms may have to be upgraded to handle the higher voltages and currents, as will any devices that are switching significant amounts of power at 48V.

We’ve already talked about using electric power to fill gaps in an internal combustion engine’s power curve, or just to provide a boost. In most cases the energy for this will be drawn from a battery, but there is an alternative approach: a super-capacitor. Eaton offers an XLR 48V super-capacitor module (right) designed for high power, frequent charge/discharge systems in hybrid or electric vehicles, public transportation, and related applications. Eaton argues that adding such a model to a hybrid can reduce battery size and weight or even replace batteries altogether in some cases. The 166F, 5milliohm module is made up of 18 supercapacitor cells, and includes cell-voltage management circuitry and an over-voltage alarm. It’s sealed to IP65, making it suitable for use in dusty areas and where it will be jet washed.

You should also expect rapid development in power electronics to serve the opportunity created by the shift to 48V systems.

One key area will be charging control and battery management. Companies such as NXP are already producing relevant battery-management devices, such as the MC33771B Li-ion battery manager for up to 14 battery cells with built-in current balancing.

Efficient DC/DC conversion is also important. ON Semiconductor has developed highly integrated modules, such as its FTCO3V85A, an 80V, low Rds(on) automotive-qualified power module featuring a three-phase MOSFET module, for use in 48V DC/DC conversion.

Vendors are also exploring multiple approaches to partitioning 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 pre-driver channels independently, enabling a variety of motor 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 vehicles. It has three half-bridge drivers, an 11V DC/DC step-down converter, two linear regulators, and a 16bit microcontroller. The 11V output can power six gate drivers, internal linear regulators, and external loads such as external Hall sensors.

Lower down the integration chain, companies such as Infineon and ON Semiconductor are working on power transistors optimised for 48V automotive applications. Infineon is using its OptiMOS 80V/100V trench technology and leadless TOLL or TOLG packages to build basic devices for 48V applications. ON Semiconductor is introducing a family of very low resistance 80V N-channel PowerTrench MOSFETs in a compact TOLL package, which it says are a good fit for high-current 48V applications.

The increased currents and voltages used in 48V systems will inevitably lead to a more electrically noisy environment. Some manufacturers are already working on ways to counter this problem. For example, Panasonic’s EEH-ZE series hybrid aluminium electrolytic capacitors (left) have been designed for use in filtering the inputs and outputs of power converters and voltage regulators, for power and battery decoupling, and in a variety of automotive applications. The parts are surface-mountable and comply with AEC-Q200.

Yageo makes two ranges of multilayer ceramic capacitors for the automotive market. The AC series, NP0/X7R parts are available with working voltages ranging from 6.3V to 630V, and at capacitances ranging from 0.2pF to 2.2mF. They’re designed for a range of automotive applications, including entertainment, comfort, security and infotainment. Importantly, the parts are free of lead and halogens, comply with RoHS requirements, and meet the AEC-Q200 automotive quality standard.

The AS series parts are available in capacitances from 1nF to 4.7uF, and with operating voltages from 10V to 250V. They have similar applications, environmental and quality characteristics to the AC series parts, but have soft terminations made up of multiple layers of plating. The soft terminations act as a form of strain relief, making the parts less likely to crack if the board to which they are soldered is flexed.

The shift to e-mobility is also creating demand for new forms of passives. TDK’s CeraLink capacitors are designed to act as ripple-current suppressors, DC link capacitors, and snubbers in the own generation of fast-switching automotive power supplies and inverters made possible by the availability of new IGBTs and MOSFETs, where low equivalent series resistances and inductances are important.

Vishay has developed a range of surface-mountable power inductors for the automotive industry. The IHLP parts (right) are made by forming a copper coil onto a lead frame and then encapsulating it in powdered iron and epoxy. The inductors can withstand operating temperatures of 125°C, have low DC resistance and core loss, are self-shielding, and are smaller and lighter than alternatives.

The shift to 48V, especially at relatively high currents, also demands a shift in the way that a vehicle’s electrical systems are interconnected. Low resistance, sealing, durability, reliability and price are among the factors that should all play a role in informing your choice of connector.

Aptiv is responding to this market opportunity with its PowerPack 1000 line of high-current, highly sealed connectors. They are designed to be compact, easy to assemble, robust against vibrations, with a proven terminal design. The connectors can carry currents of up to 145A at 85C, and will work with cables of 8 to 10sq mm, or 19 to 25sq mm. The operating temperature range is –40°C to +125°C and sealing specs include IP67, IP6K9K and S3. The company suggest the PowerPack 1000 connectors are ideal for use in power distribution boxes, start-stop systems, electric turbos, battery connections, active suspension, and for motor generator units.

Adding a 48V subsystem to today’s vehicles will boost their efficiency and reduce their emissions. It will also the pave the way to future advanced hybrid and fully electric vehicles that bring exciting new features to the world of personal transport. 

Connectors, capacitors, and magnetics play a critical role in the reliability and functionality of these systems, and the market is developing rapidly to meet increasing demands in this space. The choice on offer is substantial, and choosing appropriately is key.Explore Avnet Abacus' automotive solutions from some of the world's leading suppliers, or if you need advice on your design, click the Ask an Expert button to get in touch with one of our technical specialists in your local language.

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