How to Build a Faster In-Vehicle Charger Using 1200V EliteSiC MOSFET Modules?

Early electric vehicles (EVs) had limited driving range due to the difficulty in storing sufficient energy to power a powerful main drive motor. To extend the driving range, EV manufacturers increased the energy capacity of the vehicle batteries. However, larger batteries resulted in longer charging times.

To enable fast and efficient charging of larger EV batteries, in order for electric vehicles to become more widely adopted and developed, in 2021, the average battery capacity of the top 12 EVs in the market was 80 kWh. Consumers primarily use the onboard charger (OBC) of the vehicle for charging at home. To ensure reasonable vehicle charging times, OEMs have increased the power capacity of OBCs from 6.6 kW to 11 kW, and even up to 22 kW. With a 6.6 kW OBC, these electric vehicles require 12.1 hours for a full charge. Increasing the OBC power to 11 kW reduces the charging time to 7.3 hours, and with a 22 kW OBC, it only takes 3.6 hours to fully charge the vehicle.

It is worth noting that DC fast charging stations can provide approximately 250 kW of power, allowing for a full charge of the mentioned battery capacity in just 20 minutes, and these charging stations do not utilize the vehicle’s OBC. However, according to data from the California Energy Commission, the average cost of purchasing and installing commercial DC fast charging stations exceeds $100,000. At this price point, DC fast charging stations are only viable for industrial and commercial applications, where the same charging station can be used by multiple vehicles. Currently, consumers rely on OBCs for home charging, and reducing charging time is the main driving factor behind increasing OBC power to above 6.6 kW.

The two key factors influencing OBC design are voltage and switching frequency.

Battery voltage is increasing from 400 V to 800 V or even higher. Higher battery voltage increases the energy capacity of the battery (energy capacity = voltage x ampere-hour capacity). For example, doubling the voltage would double the battery capacity (in kilowatt-hours) and the driving range of the vehicle. Operating at higher voltage also reduces the current required for the entire vehicle, thereby lowering the cable costs between the power system, battery, and OBC.

Switching frequency determines the size and weight of magnetic components, such as inductors, required by the vehicle. By increasing the switching frequency, smaller and lighter magnetic components can be used, which are less expensive than larger components. Their reduced weight also decreases the mass of the onboard charger, allowing engineers to allocate more weight to other areas of the electric vehicle without changing the overall vehicle weight. The compact size also facilitates sleek vehicle designs. A smaller package size reduces the likelihood of the OBC housing becoming dangerous projectiles in the event of a collision, thus increasing safety. In summary, higher switching frequencies enable designers to achieve higher power densities within smaller physical dimensions.

In conclusion, higher voltage and higher switching frequency can significantly enhance the capacity of the OBC. The challenge for developers is to ensure that the components they use can withstand the higher voltage and switching frequency. It is worth noting that even for lower voltage designs (such as 400 V), higher switching frequencies can still provide benefits by reducing the size and weight of magnetic components.

Silicon carbide (SiC) enables higher switching frequencies.

Current generations of OBC architecture utilize Superjunction MOSFETs and IGBT components. However, these technologies are suitable for low-voltage applications that operate at lower switching frequencies. Specifically, the efficiency of silicon-based Superjunction MOSFETs decreases as the voltage increases. While IGBT-based devices can be used for higher voltage applications, their performance at higher frequencies is not optimal.

To deliver faster charging speeds, onboard chargers require a new topology specifically designed for higher voltage and higher switching frequencies. Additionally, this new topology needs to simplify the overall power system design while providing higher power capabilities. With the help of silicon carbide (SiC) technology, such new topologies become possible.

Compared to traditional Superjunction MOSFETs and silicon-based IGBTs, SiC-based devices and modules offer several advantages. For example, in most cases, as power increases, the overall system losses also increase. However, SiC-based MOSFETs allow OEMs to create better power conversion circuits within the OBC system. The result is that OEMs can improve the overall efficiency from generation to drive, and importantly, maintain such efficiency at higher voltage levels.

Maximizing the efficiency of the charging system, in alignment with the electric propulsion system, not only extends the driving range of electric vehicles but also reduces the cost of charging the vehicle. Therefore, adopting SiC technology to enhance OBC efficiency not only meets consumer demands and competition pressures but also lowers the operational cost of electric vehicles and improves their overall sustainability. With the introduction of 11 kW and 22 kW electric vehicles, SiC technology will continue to contribute to efficiency improvements and cost savings.

SiC-based power systems can improve system efficiency and power density, partly due to smaller passive components with lower resistance and lower conduction losses. As a result, compared to Superjunction MOSFETs and IGBTs, SiC offers excellent thermal performance, minimizing power dissipation and requiring relatively less heat dissipation for the system.

For example, let’s consider a 3.6 kW IGBT charger with 94% efficiency, resulting in 200 W of losses. However, as the OBC rated power increases to 11 kW, the 94% efficiency translates to 660 W of losses. Generating more than three times the losses adversely impacts the thermal system design, imposes higher loads on the power supply, and further reduces efficiency.

A SiC-based OBC can achieve approximately 97% efficiency, depending on the design. For an 11 kW system, this would result in around 230 W of losses, equivalent to what the existing 3.6 kW system handles. Therefore, the existing heat dissipation system used for the 3.6 kW IGBT system can support the SiC-based 11 kW system. In other words, the heat dissipation device for an 11 kW system based on IGBTs would need to operate more frequently, consuming additional power and reducing overall efficiency, leading to increased operational costs.

SiC-based OBC design

The functionality of an onboard charger (OBC) can be divided into two main stages.

The first stage is Power Factor Correction (PFC), which is the initial phase of the AC/DC converter. It serves three purposes: converting AC power to DC power, boosting the input voltage to the correct DC voltage, and achieving unity power factor. The role of achieving unity power factor ensures that the current and voltage are in phase. Systems without effective unity power factor can cause disturbances to the electrical grid.

The second major stage is the regulated DC/DC converter for charging. The charging voltage is not constant but varies based on specific battery configuration profiles. These profiles allow engineers to achieve the best possible charging experience in terms of efficiency, charging time, and extending battery life.

Traditionally, in a 3.6 kW system, the PFC stage uses a 4-diode bridge to convert AC power to DC power, followed by one or multiple phases of boost converters. Typically, this requires one MOSFET and rectifier per phase or two MOSFETs.

To scale up from 3.6 kW to 11 kW, three 3.6 kW circuits are paralleled (see Figure 1). To achieve 22 kW, six 3.6 kW circuits are paralleled. When using SiC, fewer power devices are required to achieve 11 kW or 22 kW, simplifying the overall design and achieving higher efficiency.

ON Semiconductor offers NVXK2KR80WDT, NVXK2TR80WDT, and NVXK2TR40WXT 1200V EliteSiC MOSFET modules that can be utilized in OBC applications for electric vehicles to leverage the advantages of SiC. These EliteSiC modules can enhance OBC design. NVXK2KR80WDT is a Vienna rectifier module that integrates a 1200V 80 mΩ EliteSiC MOSFET, with both SiC and Si diodes mounted on an Al2O3 ceramic substrate. NVXK2TR80WDT is a dual half-bridge module featuring 1200V 80 mΩ EliteSiC MOSFETs mounted on an Al2O3 ceramic substrate. NVXK2TR40WXT is a dual half-bridge module equipped with 1200V 40 mΩ EliteSiC MOSFETs mounted on an AlN ceramic substrate for improved current handling capabilities.

Figure 2 demonstrates how these SiC-based modules can provide all three phases of the 11 kW PFC stage through a single circuit without the need for three parallel circuits. Alternatively, three NVXK2KR80WDT modules can be employed to achieve a three-phase Vienna rectifier, with each module handling one phase. For the second stage, the DC/DC converter (two NVXK2TR80WDT modules or two NVXK2TR40WXT modules) constitutes the primary and secondary side bridges of a CLLC resonant converter. This topology reduces the overall component count and improves efficiency, with approximately a 50% reduction in components. This topology can also be applied to a 22 kW system.

Engineers can simplify their designs by using a range of modules instead of discrete components while ensuring a compact design with high power density. The modules integrate the design of discrete components, reducing complexity and minimizing the design and assembly work for OBC manufacturers, while providing enhanced reliability.

ON Semiconductor offers a wide range of power device combinations that simplify engineering and provide different trade-off options, offering engineers greater flexibility. For example, compared to the RqJC (thermal resistance from junction to case) of 0.47°C/W for NVXK2xx40WXT, the NVXK2xx80WDT has a higher RqJC of 1.84°C/W (per watt rise in temperature). While the xx80WDT generates more heat, it is smaller and cheaper than the xx40WXT, which has better thermal performance. This allows developers to choose the appropriate device to match the rated power of a specific application and make trade-offs between size/cost and heat dissipation.

It’s important to note that comparing the RqJC of modules with discrete devices is not a one-to-one comparison. The module already has an embedded electrical insulation layer, which would need to be added to a discrete solution. Additionally, comparable devices in discrete packages have significantly higher thermal resistance compared to individual discrete components, both at the external and internal thermal interfaces.

Another factor to consider is the profile. Due to the potential for integration, the module has much better clearances than a discrete solution. For example, the IEC-60664-1 standard requires a minimum gap of 5.0 mm for packaging. Choosing a module ensures compliance with clearance requirements while simplifying engineering design.

Load Balancing

A typical charging scenario is when drivers charge their electric vehicles overnight after returning home from work. With an increasing number of electric vehicles on the road, a major challenge for power companies will be the demand for load balancing. Currently, stakeholders are conducting research to create coordinated smart grids, both at the national and global levels. One potential strategy is for power companies to utilize electric vehicle batteries at different times and locations to help maintain grid stability and meet the charging demands during peak periods.

One of the advantages of these new SiC-based topologies is their bidirectional capability, which is likely to support various coordinated smart grid strategies as they are implemented. Given the evolving regulations that may introduce new functionalities that existing EV architectures may struggle to accommodate, this capability helps facilitate future-oriented designs.

Bidirectional OBCs also enable electric vehicles to act as residential emergency generators. For example, during power outages caused by heavy snowfall, households with electric vehicles can use them to power essential devices such as heaters and lighting, with power output reaching up to 60 kilowatt-hours, depending on the battery capacity. With advancements in technology, electric vehicles can serve as generators in various professional settings, such as providing power at remote construction sites.

ON Semiconductor is at the forefront of introducing SiC-based power modules that comply with automotive standards for use in onboard chargers. With 15 years of SiC module production experience, ON Semiconductor has a strong track record and a long history of delivering value and quality to customers.

ON Semiconductor is also one of the few SiC manufacturers with a fully integrated supply chain. From SiC crystal growth to wafer manufacturing, and from modules to discrete devices, ON Semiconductor has its own in-house SiC manufacturing and assembly processes to ensure that power devices meet high-quality standards. ON Semiconductor not only provides end-to-end SiC solutions but also possesses excellent operational capabilities and fast response times.

Next-generation onboard chargers need to handle high voltages and increasing switching frequencies to deliver the efficiency and power density required by automotive manufacturers. Silicon carbide technology supports new topologies that enable power engineers to meet these emerging requirements while reducing the size, weight, cost, and complexity of OBCs. With a comprehensive portfolio of power products, ON Semiconductor can help accelerate OBC designs, provide application flexibility for developers, and create future-oriented designs to adapt to evolving regulations and support new applications.