One of the most significant changes that drivers experience when switching to electric vehicles (EVs) is the change in refueling methods. Specifically, they no longer need to drive to a gas station; instead, they must find available charging points.

Although the number of public charging stations is rapidly increasing, many people still prefer to charge at home. Many high-power public charging stations provide direct current (DC) to charge batteries directly, but home charging stations provide alternating current (AC), requiring an on board chargers (OBC) to convert it to DC for charging the vehicle.

Figure 1: Typical electric vehicle drivetrain analysis

The technology for electric vehicles is advancing rapidly, with automakers moving from 400V to 800V battery architectures. Meanwhile, consumer demand continues to grow, and battery capacities (kWh) are increasing, all of which necessitate continuous advancements in OBCs. Additionally, many users wish to improve charging speed; therefore, the power of OBCs has increased from the early designs of 3.6 kW to 7.2 kW or 11 kW without exceeding the capacity of the power grid.

Figure 2: Classification of pure electric vehicle (BEV) charging piles

Key Design Considerations for OBC

Before embarking on the comprehensive design of an OBC, designers must understand the key design parameters that will influence the selection of components and topologies. Power levels directly affect user experience, making the determination of power level a crucial first step. Simply put, the higher the power of the OBC, the shorter the time required to charge the battery. In many cases, users charge their cars at home, where they are often busy with other tasks or resting, so charging time may not be a major issue. However, for charging needs during trips, charging time becomes critical. Level 2 charging stations typically have rated powers of about 7.2 kW or 11 kW. The power level design of the OBC should match the grid capacity and circuit breaker limitations (e.g., maximum current). For a 230V grid, for example, a single-phase 7.2 kW Level 2 charging station can draw up to 32A. The 11 kW Level 2 charging station is optimized for three-phase AC input, drawing up to 16A per phase.

As electric vehicles accelerate their global market penetration, the differences in grid voltage across different countries/regions pose challenges for vehicle charging. North America widely uses 110V AC, while Europe and China commonly use 230V AC. The power industry typically adopts a “universal input” design with 86-264V AC, allowing the same OBC to be used regardless of where the vehicle is delivered.

By using the same charging port, electric vehicles can charge via fast charging stations providing DC without needing to perform AC-DC conversion inside the OBC; thus, a bypass function is often designed to allow DC to flow directly into the high-voltage battery.

Energy efficiency is a crucial parameter for OBCs. The higher the efficiency, the more energy can be delivered to the battery in a given time, thereby reducing charging time, especially when the power per phase of the grid is close to its limit.

Lower efficiency in OBCs results in more heat generation within the device. This not only leads to energy waste but also requires additional cooling measures, which can be challenging due to the limited space in modern electric vehicles. Increased size and weight of the OBC add to the vehicle's overall weight, increasing energy consumption during driving and ultimately reducing the vehicle's overall range.

Improving efficiency is the primary task for power supply designers, a complex challenge that requires a multifaceted approach. While the choice of conversion topology and control schemes significantly impacts efficiency, the selection of components (especially MOSFETs) is also crucial in achieving optimal energy efficiency.

Power Stages in OBC Design

Figure 3: Block diagram of the main power stages in a typical OBC

Typically, an OBC consists of three main modules: an EMI filter, a Power Factor Correction (PFC) stage, and an isolated DC-DC converter with separate primary and secondary sections. The PFC stage is located at the front end of the OBC and performs several important functions. First, it rectifies the input AC grid voltage into a DC voltage, commonly referred to as the “bus voltage.” It also regulates this voltage, typically maintaining it around 400V, depending on the input AC grid voltage.

Another important function of the PFC stage is to improve the power factor. Without the PFC enhancing the power factor, a low power factor acts as a pollutant to the grid and increases energy consumption. To this end, the PFC stage works to keep the voltage and current waveforms in phase and shapes the current waveform to be as close to a pure sine wave as possible, thereby reducing Total Harmonic Distortion (THD). A good PFC stage will bring the circuit's power factor close to 1.

The DC-DC converter serves two roles: one is to isolate the voltage from the grid; the other is to convert the bus voltage from the PFC stage to levels suitable for charging the electric vehicle, i.e., 400V or 800V.

The primary of the DC-DC converter will “chop” the DC bus voltage, adjusting its amplitude so that it can pass through the transformer between the primary and secondary, while the secondary will rectify the output voltage and adjust it to levels suitable for battery charging.

Conclusion

Designing an efficient OBC is not an easy task, and its size and performance significantly impact the operation of electric vehicles and overall customer experience. Relevant designs must be capable of handling various input voltages while efficiently converting kilowatt-level power in a lightweight, compact structure.

There are many topologies and control schemes to consider, and a wide variety of components available, all of which will collectively determine the final performance of the design.

To simplify design tasks, many designers prefer to source components from a limited number of suppliers, ideally establishing a long-term partnership with a single supplier.