Have on-board chargers (OBC) reached a limit? For a while, it seemed that a new OBC with faster charging capabilities and new functionalities was coming out every other year. Today, on-board chargers are garnering less attention from the press, but it doesn’t mean that they are less critical, less challenging to conceive, or less innovative. In fact, the current perception lull is a strong signal that markets are waiting for companies to democratize fast on-board chargers and bring long-awaited features like V2H (vehicle to the home) or V2G (vehicle to the grid) to mainstream consumers. That’s precisely why ST released a white paper to help guide engineers working on this problem and others.
What are the challenges behind the design of a modern OBC?
Complexity
In very simplistic terms, an OBC is the electronic module of a vehicle responsible for converting alternating current into direct current to charge the battery by communicating with an electric vehicle supply equipment (EVSE), such as a wall charger, which determines the available current. This definition highlights the complexity behind on-board chargers. They must include a power factor correction and rectification to deal with the AC power. They must be as efficient and powerful as possible to help charge the battery faster. They must support the communication protocols used by the EVSE. They must deal with high electromagnetic interference. And that’s only naming the most common issues.
Moreover, OBC manufacturers have had to face a new set of challenges as they design multiple car platforms while reusing modules to save costs and shorten their time to market. Indeed, until today, car makers have only had a few EVs in their catalog. However, as demand increases, consumers ask for greater variety, leading manufacturers to create new and often more cost-effective solutions. A flexible and scalable OBC is critical to meet market demands, as creating a new OBC from scratch for each model would be too cost-prohibitive. However, designing a flexible and scalable OBC is more complex, which means engineers have to find innovative solutions.
Diversity
Additionally, markets are increasingly asking OBCs to do more, such as feeding power back to the grid or a home, which further complicates designs, but is critical to an EV’s popularity. For instance, in North America, pick-up truck makers are already advertising how their electric models can power a whole house or run an outdoor home theater setup. Others are touting the high-powered capabilities of their on-board chargers, with some offering 22 kW to hasten charging times, while cities and other commercial projects are already talking about 50 kW. Moreover, “smart” models provide remote monitoring and management features through mobile apps to stand out from the competition.
Reality
The problem is that engineers working on these new designs are also fighting another war behind the scenes: democratization. Based on the most abstract theoretical models, it’s possible to imagine building the ultimate on-board charger capable of “filling up” an EV battery in seconds. It would cost millions, be larger than the car itself, require a grid that would exceed most international standards, and be so difficult to manufacture that there would only be one in the world. In practice, engineers must find ways to make their OBC more cost-effective while offering the new features that markets are demanding. OBCs must support faster and faster charging times while getting smaller and lighter.
How can the ST on-board charger white paper help?
New architecture
The ST white paper explores some of the solutions that are already making a difference in new on-board charger designs. For instance, the industry is witnessing an architectural shift from a traditional two-stage OBC architecture to a single-stage architecture. Instead of having the PFC rectifier and the DC-DC converter on two different stages, engineers are finding ways to put them on a single stage, thus simplifying the overall design, which reduces costs and increases reliability. This is in part possible thanks to silicon carbide MOSFTETs, like the SCT025W120G3. By using wide bandgap devices, engineers can improve efficiency and provide fast charging while keeping their BOM down.
New integration
Another solution is to adopt what we commonly refer to as a “combo design” or an X-in-1 approach. In a nutshell, it means combining various elements of an electrical vehicle. For instance, bundling the OBC and the auxiliary DC-DC converter under one design can offer additional levels of efficiency and cost savings. When using a microcontroller like the Stellar E1 SR5E1E7, engineers have enough computational throughput to handle the additional logic and can drive wide-bandgap devices easily. It also makes it far easier to ship smart features like monitoring. Instead of using multiple DSPs, engineers can use one microcontroller, therefore vastly simplifying development.
New engineering approach
The X-in-1 approach also highlights the importance of looking at the car architecture as a whole rather than a patchwork of modules. The shift is already happening as manufacturers increasingly move to zonal architectures. And as engineers must now contend with electronic fuses, bundles that also contain the battery management systems, and a move to 1,000 V, a holistic view of the car can help them answer these new challenges to create a design that will scale from mainstream cars to luxury models. To get started, the ST white paper will point readers to design principles, component selection, and other considerations that will help teams ship their best OBC module faster.
