Ansemy (ON): Key considerations for designing OBC for car chargers

The biggest change felt by drivers after switching to electric vehicles (EVs) may be a different way of replenishing energy. Specifically, they no longer need to drive to gas stations, but 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 to charge the battery, but household charging stations use alternating current, so they must be converted to direct current using an in car charger (OBC) to charge the car.

Electric vehicle technology is developing rapidly, and car manufacturers are migrating from 400V to 800V battery architectures. At the same time, consumer demand continues to grow and battery capacity (kWh) continues to increase, all of which require continuous improvement in OBC. In addition, many people hope to improve the charging speed of electric vehicles, so the power of OBC has been increased from the early design of 3.6 kW to 7.2 kW or 11 kW without exceeding the power supply capacity of the grid.

　　Key design considerations for OBC

Before embarking on a comprehensive design of OBC, designers must understand the key design parameters that will affect device and topology selection.

The power level directly affects the user experience, so determining the power level is a crucial first step. Simply put, the higher the power of the OBC, the shorter the time required for battery charging. In many cases, users will charge their cars at home while they are usually busy with other tasks or resting, so charging time is not a big problem.

However, for the charging needs during travel, charging time is crucial. The rated power of a Level 2 charging station is generally about 7.2 kW or 11 kW. The power level design of OBC should match the grid capacity and circuit breaker limitations (such as maximum current). Taking the 230V power grid as an example. In single-phase design, a 7.2 kW level 2 charging station will consume up to 32A of current. The 11 kW level 2 charging station has been optimized for three-phase AC input, consuming up to 16A of current per phase.

Electric vehicles are accelerating their popularity in the global market, but the voltage differences in the power grids of different countries/regions pose challenges to car charging. 110V AC power is widely used in North America, while 230V AC power is more common in Europe and China. The power industry usually adopts a "universal input" design of 86-264V AC, so that the same OBC can be used no matter where the vehicle is transported.

By using the same charging port, electric vehicles can be charged using a fast charging station that provides direct current on the roadside. At this time, there is no need for AC-DC conversion inside the OBC. Therefore, a bypass function is usually designed to allow direct direct DC power to flow into the high-voltage battery.

Energy efficiency is a key parameter of OBC. The higher the energy efficiency, the more electricity is delivered to the battery within a given time, which can shorten the charging time. This is particularly effective when the power of each phase of the grid is close to the limit.

The lower the energy efficiency of OBC, the more heat is generated inside the equipment. This not only causes waste, but also requires additional cooling measures, which is quite challenging for modern electric vehicles due to their limited space. The increase in size and weight of OBC will increase the weight of the vehicle, increase the power consumption during driving, and ultimately lead to a reduction in the overall range of the vehicle.

Improving energy efficiency is the top priority for power designers, and this is a complex challenge that requires multiple approaches. Although the conversion topology and control scheme also have a significant impact, the selection of devices (especially MOSFETs) plays a significant role in achieving better energy efficiency.

Power stage in OBC design

Usually, OBC mainly consists of three modules: EMI filter, power factor correction (PFC) stage, and isolated DC-DC converter with independent primary and secondary parts.

　　The PFC level is located at the front end of the OBC and is responsible for executing many important functions. Firstly, it rectifies the input AC grid voltage into DC voltage, commonly referred to as "bus voltage". In addition, this voltage will be adjusted, usually to maintain around 400 V, depending on the input AC voltage of the power grid.

Another important function of PFC level is to improve power factor. If PFC is not used to improve power factor, then low power factor will be more like a pollution source to the power grid, and power consumption will also increase.. For this purpose, the PFC stage will strive to maintain the voltage and current waveforms in phase and shape the current waveform as close to a pure sine wave as possible, thereby reducing total harmonic distortion (THD). A good PFC level will bring the power factor of the circuit close to 1.

The DC-DC converter has two functions: one is to isolate the voltage from the power grid; Another is to convert the bus voltage from the PFC level into a voltage level suitable for charging electric vehicles, which is 400 V or 800 V.

The primary of the DC-DC converter will "cut" the DC bus voltage, adjust its amplitude so that it can pass through the transformer between the primary and secondary, and the secondary will rectify the output voltage and adjust it to a level suitable for charging the battery.

Conclusion

Designing an efficient OBC is not an easy task, as its size and performance have a significant impact on the operation and overall customer experience of electric vehicles. The relevant design must be able to handle various input voltages and achieve kilowatt level power conversion as efficiently as possible in a lightweight and compact structure.

There are many topologies and control schemes to consider, and a wide range of devices to choose from, which together determine the performance of the final design.

To simplify design tasks, many designers tend to purchase components from a limited number of suppliers, ideally establishing a long-term partnership with only one supplier.

Ansemi offers a wide range of discrete devices and power modules that can meet the design requirements of a complete OBC power system in one stop.