Powering the next wave of Electric Vehicles
Making the move to 48V
With the rise of 48V architectures in electric vehicles (EVs), the automotive industry is at a tipping point.
Since the 1950s, 12V architectures have been standard for cars’ electrical and electronic (E/E) systems and batteries. Over the years, there have been proposals to move to higher voltages, including using 48V for Mild Hybrid Electric Vehicles (MHEVs) equipped with high-voltage starter generators, but 12V has remained dominant.
Now, the time has come for 48V as an in-car standard. Battery Electric Vehicles (BEVs), full hybrids, and plug-in hybrids have high-voltage batteries as their main power source, which can provide 48V3. Moving from existing 12V systems to 48V promises reductions in wiring harness size and cost, with improved efficiency.
Benefits of 48V Architectures
Compared to 12V, the main benefit of a 48V system consists of lower power losses, and hence higher efficiency. 48V architecture also enables improved thermal management, with less heat dissipated from resistive power losses. The reason for this goes back to the fundamentals: Ohm’s Law dictates that V=IR, while power losses in a conductor are given by P=VI. Combining the two, we see that resistive power loss is given by I2R – and is proportional to the square of the current.
For example, to deliver 1200W at 12V requires 100A, while at 48V this needs 25A. With current reduced by a factor of four, power losses are reduced by a factor of sixteen. A 48V rail can also reduce PCB size, because lower current can deliver the same amount of power.
Alternatively, we can use thinner wiring with 48V than with 12V. This thinner wiring has higher resistance than cables we would use with 12V, but keeps power losses down to an acceptable level due to the lower current. With thinner wiring, manufacturers can substantially reduce weight and cost of wiring harnesses. Reduced weight improves efficiency, enabling longer range – a major competitive advantage – and reduced emissions.
While the benefits achieved vary widely, we can say that shifting from 12V to 48V in a typical EV could reduce the car’s weight by around 45 to 70kg, and enable a longer range by circa 10km. Additionally, 48V systems can deliver more power than 12V alternatives – essential for the power-hungry features and gadgets in today’s vehicles.
Types of Vehicle Architectures
As well as shifting to 48V, cars’ electrical architectures are changing. In a traditional ‘flat’ architecture, there are repeated connections from the power source to each device, system and electronic control unit (ECU). As cars become more complex, with more ECUs, this has led to redundant cabling, and cost, weight and space issues.
To address this, many cars now use a ‘domain’ architecture, with cabling organised into functional domains. While this improves scalability and simplifies wiring, it still leads to inefficient cabling.
Taking this further, a ‘zonal’ architecture organises wiring into several modular zones, with hardware gateways throughout the car, and ECUs linking to the nearest gateway, thus reducing the length of wiring required, and simplifying the system.
How to Overcome 48V Design Challenges
Migrating from 12V to 48V does create challenges for design engineers. This is partly because standards for 48V are less well-established, thus potentially requiring more engineering work to define a 48V system.
Engineers must consider the higher transient voltages with 48V, and how these can be contained. They must appraise the greater creepage and clearance distances needed for higher voltages, to prevent arcing between PCB traces. Additionally, 48V could lead to greater switching losses than in 12V systems.
With higher voltages and currents in a 48V system, there will be a more electrically noisy environment, requiring the right capacitors and other components to filter currents and cut RF emissions.
Key components of the 48V system
For a 48V system, several key components are required. Firstly, a battery: today’s BEVs already have a high-capacity 400V or 800V battery, which powers the 48V systems and the main powertrain. Lithium-ion batteries have dominated, providing high power density and, therefore, greater range and lighter weight.
Onboard Chargers
Figure 1: OBC Topology
The onboard charger (OBC) converts AC power from the grid into DC to charge the battery, and may have a bidirectional capability for sending power back to the grid. The OBC is used when charging from a standard or low-power AC outlet, such as a socket at your home.
An example of a high-efficiency OBC design is shown in Figure 1. This consists of a 'totem-pole' PFC stage, followed by an isolated CLLC DC-DC conversion stage. This includes both GaN and SiC MOSFET devices, with GaN devices used in the PFC stage since they have no reverse recovery losses.
SiC MOSFETs are used in the CLLC stage and output synchronous rectifiers, taking advantage of their similar efficiency to GaN in these stages, at a lower cost and with the ability to handle higher voltages.
DC/DC Converters
For an electric vehicle’s onboard DC/DC converter, the efficiency and high power density of GaN and SiC can make both a good choice, depending on specific application requirements.
A battery electric car will typically have a 400V or 800V high voltage battery, and a 12V or 48V low voltage system. To convert efficiently between high and low voltages, a bidirectional converter is usually required. A car using 48V as its main low-voltage system may also use multiple 48V-to-12V DC/DC converters, enabling a 12V supply to be obtained locally where needed for specific components, and therefore avoiding long, redundant cabling runs to carry 12V power.
BLDC Motors
Another key component is a 48V brushless DC (BLDC) motor, used for pumps, motors, and the internal combustion engine (ICE) starter in a hybrid vehicle. For smaller vehicles such as e-bikes, a BLDC motor that is small enough to be built directly in the wheel hub is often used, typically rated below 750W. To control the motor, an integrated chipset including a microcontroller (MCU), gate driver, and multiple power MOSFETs can be used.
Traction Inverters
Traction inverters convert DC power from the battery to AC to drive the vehicle’s electric motors. Although it offers excellent performance, GaN is typically only suited for applications up to around 650 V. Where higher voltages are needed, such as in a typical EV traction inverter, then silicon or SiC is more suitable. SiC is often the preferred choice, due to its higher efficiency than silicon, as well as enabling smaller and lighter devices.
Moving from silicon to SIC does raise some design challenges that need addressing, including the extra engineering effort needed to adapt to a new technology. SiC devices’ high-frequency switching can create issues with noise and electromagnetic interference (EMI), which require careful PCB layout, as well as parasitic inductances. Thermal management can also be difficult, with SiC’s high power density making heat dissipation tricky, particularly in compact spaces.
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SEE AUTOMOTIVETraction Motors and Regenerative Braking
Smaller vehicles such as e-bikes need light, compact and efficient power systems to provide the longest possible range from a battery, or to enable the use of a smaller battery while maintaining range at the same value. For example, e-bikes typically use a brushless DC (BLDC) motor, which is small enough to build directly into the wheel hub and often rated at less than 750W. To control the motor, an integrated chipset can be used, which includes a microcontroller, gate driver, and multiple power MOSFETs.
Power semiconductors in traction motors also play a vital role in regenerative braking systems. They need efficient, fast-switching semiconductors to capture and convert the vehicle's kinetic energy into electrical energy, which is then stored in the battery for re-use.
GaN-based devices can provide the excellent efficiency and compact size needed for these applications, but do create new design challenges. Similarly to SiC, as described earlier, they can raise issues around noise and EMC, parasitic inductances, and heat dissipation.
Cost considerations and the use of SiC and GaN
A BEV needs a variety of high-voltage power electronics, including DC/DC converters, power MOSFETs and IGBTs, and power management ICs. Avnet Silica’s power semiconductor portfolio supports both traditional 12V and 48V architectures, including all these devices.
Figure 2: Key technologies in an electric vehicle (STMicroelectronics)
Although the cost of these components in a 48V architecture is likely to be higher than that of a 12V system, the overall cost savings of moving to 48V will outweigh the higher bill of materials (BOM) cost.
While silicon remains the predominant choice in power electronics, another option is wide bandgap (WBG) semiconductors, such as Silicon Carbide (SiC) and Gallium Nitride (GaN). While these are typically more expensive than silicon, they offer important advantages.
For power conversion between the different voltages in the powertrain, as well as from AC to DC and vice versa, we need transistors that can switch rapidly and efficiently. SiC and GaN have become attractive due to their capabilities at higher temperatures, voltages, and efficiencies, enabling more compact and efficient power systems.
For instance, GaN devices are used in the PFC stage of OBCs because they have no reverse recovery losses. SiC MOSFETs, on the other hand, are used in the CLLC stage and output synchronous rectifiers, taking advantage of their similar efficiency to GaN in these stages, at a lower cost and with the ability to handle higher voltages.
For traction inverters, GaN is well-suited for applications up to around 650V. Where higher voltages are required, such as in a 800V traction inverter, then silicon or SiC is more suitable. SiC is, however, often preferred due to its higher efficiency compared to silicon. GaN-based devices can also provide the excellent efficiency and compact size needed for regenerative braking systems.
Moving from silicon to SiC and GaN raises design challenges, however, including extra engineering effort to adapt to new technologies. The high switching frequency of SiC devices can create issues with noise and electromagnetic interference (EMI), which necessitate careful PCB layout, as well as consideration of parasitic inductances. SiC’s and GaN's high power density can also make heat dissipation difficult, particularly in compact spaces.
Conclusions
The automotive/EV industry is already migrating from 12V systems to 48V. The two major advantages lie in the reduced weight and cost of wiring harnesses and the ability to provide higher power to in-car systems and devices.
Choosing the right components for 48V automotive applications depends on multiple considerations, including efficiency, power density, cost-effectiveness, and reliability.
Avnet Silica’s team possesses a wealth of expertise in power, automotive and EV sectors and are ready to help you. Engage with us to discover more about how we can support your specific needs.
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