Electric vehicle powertain: The 48V design challenge
Electric vehicles and the move to hybrid powertrains
|Figure 1: Increasing electric content delivers greater fuel savings
Electric vehicle (EV) technology is still at a very early stage and the so-called ‘range anxiety’ continues to slow the growth of the sector. If electric vehicles are to make longer journeys in similar times to internal combustion engine (ICE) vehicles, it will require an increase in battery capacity, improved vehicle efficiency, and more rapid charging stations.
The end goal is fully electrified battery electric vehicles (BEV) but, until the range issues are addressed, hybrid vehicles that mix ICE with electric propulsion form a useful interim step, allowing for dual fuel operation with greater efficiency than a simple ICE.
Hybrid vehicles can have different arrangements of the ICE and electric powertrains. The simplest arrangement is the series hybrid where the final drive is permanently electric and the ICE is solely used to power a generator that drives the electric motor or charges the battery – or both. In a parallel hybrid arrangement, the ICE and electric powertrains are essentially separate, although they can both drive the transmission through a mechanical coupler, either independently or together.
Electrification and the need for 48V systems
As might be expected, the changes to the powertrain are driving wholesale changes to the architecture of the mechanical, electromechanical and hydraulic systems in vehicles. Increasingly, mechanical pumps and cooling fans are being replaced by electrical equivalents and electrically-driven superchargers and turbochargers are becoming more popular.
These electrical alternatives to the traditional mechanical systems are generally lighter, more efficient and more reliable. While mechanical pumps present a load to the ICE the whole time it’s running, electric motors can be turned off when they’re not required, which saves energy. This is of particular value for the air conditioning compressor that’s only needed part of the time.
However, while these new electric pumps and motors are more efficient and reliable, they do consume significant amounts of power. This will vary depending upon the vehicle, but it is not unusual for an air conditioning compressor to consume 3-4kW and for a pump for electric power steering (EPS) to consume around 2kW.
With the existing 12V architecture in most passenger vehicles, it’s unrealistic and inefficient to try to drive loads of this magnitude, as the currents involved will be in the hundreds of Amperes. Losses in wiring and static losses in semiconductors are proportional to the square of the current so, with a 12V approach, overall vehicle efficiency would be significantly reduced.
The situation is further exacerbated as these large currents would need very substantial cabling, which is heavy, expensive and difficult to route within the tight spaces in modern vehicles. And the increased losses would necessitate additional thermal management such as fans, heatsinks and possibly water cooling. All of this reduces efficiency which increases CO2 and vehicle cost.
The move to 48V architectures in powertrains
Figure 2: Mild hybrids combine 12V and 48V electric system
In the same way that electricity grids transmit energy at higher voltages across long distances, automakers are increasing the voltage within vehicles to minimise resistive losses and increase efficiency. This, in turn, will reduce vehicle weight and cost while diminishing CO2 emissions.
There are many advantages to the 48V architecture. For example, it allows more power to be delivered through the existing cabling in vehicles. This is beneficial for powering devices that have made the transition from mechanical to electric power, such as AC compressors and EPS. The ability to deliver more power allows heaters (such as windshield heaters or heated seats) to warm up more quickly, improving comfort and convenience.
Moving to 48V also allows changes to be made to the powertrain. 48V-based ‘mild hybrid’ vehicles replace the starter motor with a belt-driven starter generator (BSG). This AC device is driven from the 48V rail via a bi-directional inverter that allows the battery to receive charge from regenerative braking. When the vehicle stops, the BSG re-starts the ICE and can also be used in conjunction with the ICE to provide additional torque for pulling away. The mild hybrid approach is significantly lower cost than full hybrids, and the auto industry estimates it will deliver up to 20% greater fuel economy, which will also reduce CO2 emissions.
While 48V may become the preferred voltage for future vehicles, for now automakers have too much invested in existing 12V systems to make a complete change. For this reason, vehicle electrical systems will include a mixture of 12V and 48V. Designers will partition this according to the devices being fitted to the vehicle with, in general, higher power devices being added to the 48V rail.
Design considerations and challenges with automotive 48V architectures
One reason that 48V was chosen as the future voltage for vehicles is that it’s the highest the voltage could go while still being considered ‘safe’, as it does not spike above 60V which is the SELV threshold.
Obviously, a new battery is required, although, as 48V is a multiple of 12V, four existing batteries can be used in parallel allowing proven technology to be used. The cabling is also able to change for the better as thinner cables can be used. This also allows for smaller bend radii which eases the task of designing for the small and cramped spaces in modern vehicles.
As the 12V and 48V systems will be linked and not isolated from each other, they’re able to transfer energy back-and-forth and also work together when peaks of energy are required. This does, however, require certain components to have higher voltage ratings than with a pure 12V system. For example, circuit protection components used in 12V systems would be designed to cope with spikes in the region of 40V which is obviously inadequate for a 48V system.
As always, there are further challenges with designing for vehicle applications. For electronic systems, the automotive arena is a very harsh environment. With the heavy electrical motors and switchgear, noise and transients are common and require protection to be added to any sensitive circuitry for both conducted and radiated noise.
Shock and vibration is also an issue, both from the road the vehicle is travelling over as well as small imbalances in the ICE. Vibration levels will vary depending upon whether the electronics are mounted on the ICE itself (ECU, electric supercharger, various fluid pumps etc) where vibration levels are higher, or on the chassis where vibration is generally less severe.
Due to the vibrations present, components must be rated to withstand long-term exposure without premature failure. Often, measures are taken to secure larger components (such as magnetics and capacitors) within assemblies including gluing and tie-down. If component values are able to be reduced, for example by increasing the switching frequency of a converter, then these measures may become less necessary, reducing manufacturing costs.
Passive components within 48V powertrains
Both capacitive and inductive components are widely used throughout the powertrain, especially for the storage of energy and filtering applications. Often seen as simple, these devices play an important role in ensuring modern power systems meet their performance goals and, consequently, represent a significant proportion of the value of electronic components deployed in a vehicle.
Examples of capacitive and inductive components for automotive applications
Inductive components range from single inductors that are used for filtering, as well as energy storage in power conversion through to common-mode chokes for filtering and pulse transformers that fulfil a variety of applications, including ensuring that semiconductor switches (MOSFETs, IGBTs) are properly controlled to avoid damage during high current operation.
Capacitors fulfil various roles from decoupling ICs where multi-layer ceramic capacitors are used, through to energy storage where other types with either a solid or wet electrolyte are used. As reliability is critically important in automotive applications, self-healing capacitors that can repair themselves after minor faults are often incorporated into circuitry.
With so much focus on the central processing of new automotive systems, engineers need to be careful not to overlook the importance of components such as connectors, capacitors, and magnetics in their powertrain designs. These components play a critical part in the reliability and functionality of automotive electronics, and the market is developing rapidly to meet increasing demands in this space. The choice on offer is substantial, and choosing appropriately is key.
Below we’ve highlighted our leading suppliers for components suited to powertrain applications.
If you require advice on selecting the right components for your design, our technical specialists are on hand to help.