Designing DC-DC converters for next generation electric vehicles
It will be obvious to anyone who’s looked at buying a car in the past few years that things are changing significantly. A decade ago, the only real fuel choices were fossil fuels, diesel or petrol. But now there’s also mild hybrid electric vehicles (MHEV), hybrid electric vehicles (HEV) and battery electric vehicles (BEV) to consider.
From the consumer’s point of view these vehicles are different; they don’t travel as far as conventional vehicles, they can take hours to recharge and they contain a lot more technology. However, there are more changes in vehicles, many of which won’t be visible to the casual observer – but, nonetheless, have a significant impact on the vehicle design.
Changing architectures drive different power needs
The vast majority of internal combustion engine (ICE) vehicles have had a relatively simple architecture with a 12V battery powering the electrical components, replenished by an alternator. Several key components, such as water pumps, power steering pumps and fans have been belt-driven and therefore required no electrical power.
However, with the move to hybrid and electric vehicles (xEV), there’s less opportunity to belt-drive these ancillaries. This, coupled with the need for efficiency means modern vehicles (xEV and even some ICE vehicles) are replacing these electro-mechanical devices with electrically-driven alternatives. These new versions are smaller, lighter and more reliable – they also facilitate more efficient operation. For example, the air-conditioning pump doesn’t need to run constantly, but a belt-driven pump presents a constant load to the ICE, while one driven by an electric motor is only powered as needed.
However, many of the pumps and motors in a vehicle require relatively high levels of power – in the kilowatt region (see Figure 1 below). As a result, along with devices such as heated seats, which are similarly power-hungry, these applications aren’t suited to being driven from the 12V battery common in most vehicles.
The relatively low voltage in a 12V system results in high currents that require substantial cabling, which is expensive, heavy and difficult to route through the small spaces in modern vehicles. The weight of the cabling, and the losses associated with the high currents, have a negative impact on the efficiency of the vehicle.
In order to address this, automakers are gradually introducing 48V systems to drive the higher power fans, motors and heaters. As they require one-quarter of the current, the cable size can be reduced, which has a positive impact on the cost of the wiring and the efficiency of the vehicle.
However, as there has been so much invested in 12V systems over the years, the 48V systems will run alongside the existing 12V systems, so that existing low-power systems (such as infotainment, driver assistance, ECUs and others) can remain unchanged.
Bi-directional DC-DC converters for xEV – design considerations and challenges
In a typical vehicle architecture, the 12V and 48V systems remain relatively separate and are generally segmented on the basis that the lower power applications are connected on the 12V side, while the higher power applications (generally those requiring motors and / or heating elements) are connected to the 48V.
Many of the modern systems being implemented in vehicles such as active chassis management, adjustable suspension and electric superchargers / turbochargers are being designed for the 48V bus. Additionally, the 48V bus can support engine-starting in ICE vehicles, making start-stop operation smoother.
At the heart of these mixed voltage systems, and providing the bridge between the two voltages, is a bi-directional DC-DC converter. This important sub-system is both a step-down (‘buck’) and step-up (‘boost’) converter that allows either battery to be charged from the other. In the early stages of this technology, two separate DC-DC converters were used, one for each direction. However, the bi-directional approach allows the same external components (including passive devices such as inductors and capacitors) to be used for both the step-up and step-down conversions. As a result, size and weight are reduced which improve the efficiency / range of the vehicle and lower the manufacturing cost.
Figure 2: Mixed 48V / 12V systems are generally segmented on the basis of power requirements
The bi-directional DC-DC converter is also able to combine energy from both systems to provide as much power as possible when current draw is at its greatest – for example, when starting the vehicle.
There is little need for galvanic isolation in automotive systems, as all voltages are separated extra low voltage (SELV), part of the reason that 48V was chosen. So bi-directional DC-DC tend to be non-isolated to avoid the weight and cost of a transformer. Therefore, a non-isolated multidevice interleaved bi-directional DC-DC converter (MDIBC) is a common solution.
Figure 3: Schematic of a typical MDIBC
The 12V power is derived from a sealed lead acid (SLA) battery while the 48V source can be either a battery or a supercapacitor or, quite often, a combination of both, which gives the ability to deliver peaks of current when needed.
The multi-phase approach of the MDIBC shown, which relies upon the gate drive signals to be interleaved, reduces the input ripple. In fact, acceptable levels of input and output ripple can be achieved without increasing the value (and therefore, size and cost) of the passive components. The trend towards using wide bandgap (WBG) semiconductors is allowing operating frequencies to increase, which reduces the size of the passive components.
Unlike many conventional topologies, the MDIBC has commonality with the control circuit, thermal management and also the DC link capacitor, all of which add to the overall reliability. The ability to allow power to flow bi-directionally means that it can accommodate systems such as regenerative braking that return power to the battery and increase overall efficiency and range of the vehicle.
Standards applicable to 48V systems
While there are many standards and regulations applicable to the design of electronic systems for vehicles, as the technology evolves, the standards will need to be reviewed and updated. While many of the existing standards (such as ISO 7637) cover the ‘traditional’ vehicle voltages of 12V and 24V, as well as much higher voltages, there is something of a gap around 48V.
Figure 4: 48V will improve existing systems and enable new comfort and safety features
There will be wholesale changes in the electrical architecture as a result of the introduction of 48V to vehicles. The voltage will be four times higher and currents will not necessarily reduce, as 48V will be used to power systems requiring more energy. This could mean additional loads will be placed on system components including fuses and relays, leading to increased risk of arcing. The move to 48V will enable further system changes, including the replacement of mechanical relays with solid state devices, introduction of smart fuses, optimised wiring harnesses and fault tolerant features – all leading towards fully autonomous vehicles.
The new ISO 21780 now provides a single global standard for 48V automotive systems, which simplifies design and lowers costs for automakers in international markets. The standard was recently finalised, and the first ISO 21780 compliant DC-DC converters were released in late 2019.
Mission-critical passive components for automotive DC-DC converters
While much is written about the switching semiconductors such as IGBTs and MOSFETs, as well as the advanced controllers available, passive components have an equally important role to play in automotive DC-DC conversion.
Both inductors and capacitors are used extensively for applications such as energy storage, filtering, decoupling and noise reduction. In fact, it’s common for the cost of passive components to far outweigh the cost of semiconductors within a DC-DC converter bill-of-materials (BoM).
The automotive environment is harsh, with high temperatures, transients, noise, and other hazards present. In selecting the appropriate components for an application, both electrical and mechanical parameters have to be considered.
A variety of capacitor types will be needed, including the simple multi-layer chip capacitors (MLCC) that are used for applications such as decoupling, smoothing and snubbing. Film capacitors, such as polypropylene film, offer high voltage capability and are often used for filtering unwanted EMI. And some types offer the ability to self-heal, which adds robustness for automotive applications.
Magnetic components such as inductors are also used in a variety of power applications, from filtering to providing energy storage in buck converters. Current handling capability is an important factor, from the perspective of saturation as well as the amount of current the winding is able to carry. Many types of inductor are available including some with thick strip-wound copper wire that minimises DC resistance and the associated losses. Shielding is another important consideration, especially in switching applications where the generation of, and susceptibility to, electro-magnetic interference (EMI) must be considered.
Other magnetic components include common-mode chokes, current sense transformers for system monitoring and control, and small PCB-mount transformers that are used in the drive circuitry for switching semiconductors such as IGBTs and MOSFETs.
As automotive systems are mostly modular in nature, allowing for servicing and repair, connectivity forms an important part of the system. As currents and voltages increase, so the specification of connectors must be considered carefully so they don’t introduce losses into the system. The mechanical properties of connectors and their resistance to heat are also important, as they’re often used in under-the-hood applications such as start-stop systems and electric turbos / superchargers.
While it’s not something that will be immediately apparent to many buyers of vehicles, the fundamental power architecture is changing, even in ICE vehicles. As more hydraulic and mechanical components transition to electrical / electronic alternatives, the existing 12 V architecture is being augmented (and eventually replaced) by 48 V systems. This approach brings greater efficiency and also saves cost as cabling can be thinner and lighter.
The 48 V is generated from a battery or supercapacitor (sometimes both) by a DC-DC converter that is required to work bidirectionally, meaning that energy recouped during braking can be returned to storage, thereby increasing the electric range of the vehicle.
As the voltage increases, components that can withstand the additional stresses of 48 V operation (as well as tolerate the harsh automotive environment) must be selected by designers of these DC-DC converters. Given the high level of electrical / electronic content in modern vehicles, many component manufacturers are now offering devices specifically for these new applications, including inductors, capacitors, filters, chokes, current sensors and connectors.
Below we’ve highlighted our leading suppliers for components suited to DC-DC conversion applications.
If you require advice on selecting the right components for your design, our technical specialists are on hand to help.