Important aspects and solutions for more power-dense and efficient on-board charger designs 

Development and innovation in the automotive industry continue at a rapid pace in almost all aspects of vehicle design, including the chassis, the powertrain, infotainment, connectivity, and driving-assistance systems. A topic that challenges the fast and wide adoption of battery electric vehicles (BEVs), due to increased driver concerns and stress, is the time taken to recharge these vehicles — especially on long highway journeys. The design of the on-board charger (OBC) is an area that is coming under more scrutiny than most. 

In the quest for more power-dense and more efficient OBCs, designers are looking to advanced technology to make the next step in innovation by moving from current silicon-based solutions to power-semiconductor technologies that use wide-bandgap (WBG) materials, such as silicon carbide and gallium nitride. Compared with traditional topologies based on silicon devices, once the improved figures of merit (FoM) of WBG devices are well-understood, innovative ideas could help designers develop new topologies at higher switching frequencies with modulation schemes that were not possible or were too complex to be implemented before. In addition, an effective thermal design (i.e., innovative packaging that includes new cooling concepts) opens up the horizon for upcoming OBC designs in terms of power density and efficiency. 

This article identifies trends in OBC designs, compares the FoM among semiconductor technologies, and introduces new surface-mount device (SMD) packaging. The comprehensive solutions bring innovation in different topologies, offer more efficiency and power density, and enable bidirectionality that integrates EVs into the smart grid. 

OBC trends

The role of an OBC is to convert AC power from the power grid into a DC voltage that can be used to charge the traction battery. Because the OBC can perform this function only while the vehicle is stopped and the DC/DC only while the vehicle is moving, this concept adds extra weight that must be carried around but also be cooled. Thus, the size and weight of the OBC plus the DC/DC must be minimized to reduce its effect on the driving range as well as the volume occupied in the e-Powertrain compartment while still allowing for rapid and efficient charging. 

Similarly, under the influence of future power-grid regulations, which are moving toward smart grids, and the possibility of having an emergency power supply for blackouts or natural disasters, OBCs are also impacted by the fact that they need to allow a bidirectional power flow. 

These aspects are closely linked and interrelated when identifying the five key challenges for OBC designers: 

  1. Power classes are continuously increasing to accelerate the charging times. Current plug-in hybrid vehicles and BEVs have OBCs in the range of a 3.6- to 7.2-kW power class. Current OBC designs at original design manufacturers for the next generation of EVs in the next three to five years are moving up to the range of a 7.2- to 11-kW power class. In the case of luxury cars or high-profile cars with 800-V batteries, OBCs can be designed for up to 22 kW. 
  1. The increase in power density is significant, as it implies a reduction in size and weight, contributing to an extended EV driving range. Enhancing efficiency not only reduces the heat buildup within the OBC (which reduces size and increases power density due to reduced thermal management), but it also allows more energy from a finite grid source to be delivered to charge the traction battery, thereby reducing charging time. 
  1. The increase in efficiency is closely related to the first two trends. It is impossible to achieve higher power classes at higher power density without having higher-efficiency power converters. Similarly, extra regulations for OBC converter efficiency targets might be introduced, such as tank-to-wheel efficiency. 
  1. The requirement for bidirectional operation is another great challenge for OBC designers. As EVs become more and more common, the pressure on the power grid will increase significantly, especially as people recharge their vehicles in the evening after their daily commute. Electricity suppliers recognize that there is a significant amount of energy stored in EVs that may not be needed immediately. This could either be returned to the power grid or used to power a single house at peak times to reduce peak energy demands. However, to do so, the OBC needs to be able to transfer energy from the traction battery back to the grid instead of only charging the EV. 
  1. Battery voltages are increasing. Even though the 400-V batteries will remain mainstream for the next five-plus years, 800-V batteries are already getting more attractive. The main motivation is primarily to reduce currents and the associated I2R losses in cabling when charging and when delivering energy to the traction motors. 

Figure 1 looks at the most important trends with corresponding implications in the OBC design and the key solution enablers for each. 

Figure 1: Trends in OBC design that represent several challenges to power designers 

The topology chosen will be essential to address these challenges, as will be the technology used — especially for the switching components. In most cases, WBG solutions can contribute significantly to providing the performance benefits needed. 

Figures of merit and technical differences among Si, SiC, and GaN 

The first step when analyzing which WBG device is suitable for OBC applications would be to compare the different FoMs of each technology. These are summarized in Figure 2. 

FoM comparison among the different technologies.
Figure 2: FoM comparison among the different technologies offered by Infineon 

Each of these FoMs has a distinct meaning and quantifies the implications that exist in the different topologies. It is worth mentioning that the lower the value of the FoM, including the RDS(on) temperature coefficient, the better the positioning of that technology in the application. The proper selection of the technology is not made by choosing only one FoM but a combination of them. 

For example, the combination of RDS(on) × Qrr + RDS(on) × Eoss + RDS(on) temperature coefficient indicates that either SiC or GaN is the best candidate for a hard-switching application, such as continuous-conduction–mode totem-pole power-factor correction (PFC), and has the best price-performance ratio. On the other hand, the combination of RDS(on) × Qoss + RDS(on) × Qg + VF + RDS(on) temperature coefficient indicates that GaN is the most suitable technology to be used in soft switching applications, working at very high switching frequencies (>400 kHz). As a result, a system-level cost reduction is achieved by reducing the size of the magnetics and other passives as well as reducing the cooling efforts for a very high-power–density design at an efficiency level that would be difficult for silicon-based solutions to match. 

New thermal design through innovative package and cooling approach

While acknowledging the benefits of WBG technology, designers must also be aware that improved thermal performance plays a crucial role in achieving these important goals. 

In current SMD designs, the conduction path for the heat is downward, through the legs of the component into the PCB, which is bonded to a heatsink. In challenging applications, the power components may be mounted on an insulated metal substrate (IMS), which improves thermal performance, as it is a better conductor of heat than standard FR4 materials. However, the natural inclination of heat is to rise, making bottom-side cooling (BSC) seem somewhat counterintuitive. 

Through innovative packaging, Infineon has developed top-side–cooled (TSC) discrete semiconductors and ICs. This concept not only takes advantage of the natural upward flow of heat but delivers many additional benefits that are advantageous to the OBC design and other similar applications. 

In BSC, a cold plate/heatsink is typically attached to the bottom side of the PCB/IMS to dissipate heat. This precludes the placement of components on one side, reducing power density by a factor of 2. Also, the semiconductor devices are thermally bonded to the PCB, which means they will operate at the same temperature. As the glass transition temperature of the FR4 PCB is lower than the operating temperature of many WBG devices, they cannot be used to their full potential. 

TSC permits double-sided component placement.
Figure 3: TSC permits double-sided component placement, allowing for a doubling of power density. 

With the cold plate bonded to the top side of the power components, these issues are easily addressed, allowing for components to be placed on both sides, and WBG devices can be used over their entire operating temperature range. 

As IMS typically involves using a separate FR4 PCB for the driver and passive components, there can be a significant distance between the gate driver and the transistor, which inevitably increases the parasitic effects that cause ringing, which is a very delicate topic when using WBG devices. 

Double-sided components benefits.
Figure 4: Double-sided components allow for short gate traces, eliminating the parasitics.

As TSC allows all components to be placed on the same double-sided PCB, the driver can be placed directly below the corresponding transistor, eliminating parasitic effects due to the PCB. This enhances system performance and prolongs the lifetime of the MOSFET. 

Multiple options exist for thermally bonding the transistor packages to the heatsink/cold plate. In general, the most straightforward approach is to place a single, thermally conductive gap-filler pad between the MOSFETs and heatsink. With a thickness of about 0.5 mm, this gives the best thermal performance, provided that any voids in the gap filler are addressed. 

Gap Filler.
Figure 5: A gap filler is the preferred thermal bonding approach, with the addition of an insulator for HV applications.

In higher-voltage applications, the gap filler isn’t reliable in providing sufficient electrical isolation between the transistors and the conductive heatsink. In this instance, a thermally insulating material of about 0.1 mm is placed between the gap filler and the heatsink to provide the appropriate level of electrical isolation while maintaining excellent thermal performance. 

Infineon’s QDPAK (PG-HDSOP-22-1) devices are specifically designed to take advantage of the benefits of TSC. A variety of features is offered to suit different applications. A Kelvin source pin is provided for high levels of controllability and full-load efficiency. The symmetrical parallel lead layout ensures mechanical stability on the PCB as well as easy assembly and test. 

TSC-capable QDPAK.
Figure 6: The TSC-capable QDPAK offers several manufacturability/assembly benefits. 

Topologies with bidirectional power flow enabled by WBG devices

Let’s start with one of the most widely used configurations in a single AC phase for bidirectionality, allowing the vehicle-to-grid (V2G)/vehicle-to-load (V2L)-AC load functions. For a 3.6-kW power-class solution, a single phase for the PFC topology is enough. However, for the 7.2-kW power class, interleaving phases with the totem pole are recommended to keep the proper efficiency and thermal management. The HV/HV DC/DC converter can be realized with either CLLC or dual-active–bridge topologies. The selection depends on the desired peak/overall efficiency throughout the load as well as the preferred controllability. 

In terms of power semiconductors, the recommendation for each topology is depicted in Figure 7. The proper selection of the WBG technology depends on each customer’s value drivers, like efficiency, power density, cost, system requirements, and the selected topology. It is worth mentioning that if the OBC in discharging mode (i.e., vehicle-to-everything, or V2X) works as a voltage source with a power factor equal to 1, then the slow leg (Q5 and Q6) can be populated with silicon superjunction (SJ) transistors. But if the OBC needs to handle reactive power (power factor does not equal 1), then the slow leg must be populated with WBG transistors, as hard-commutation events are bound to happen. 

Based on the topology shown in Figure 7, keeping Q5 fully off and Q6 fully on makes it possible to enable the vehicle-to-vehicle (V2V), V2L-DC load, and vehicle-to-DC (V2DC)-microgrid options. In this case, the front-end converter works as an interleaved buck converter. 

By sharing the same power circuit, both bidirectional AC/DC and bidirectional DC/DC power transmission can be realized efficiently and conveniently without additional devices and costs. 

Figure 7: Using SiC MOSFETs and/or GaN HEMT transistors instead of IGBTs and SJ devices enables the possibility of doubling switching frequencies both in PFC and CLLC in order to achieve power densities >4 kW/l. 

The configuration shown in Figure 7 can be the building block for a three-phase AC system. In other words, each building block is connected to each phase of the AC grid and has the secondary sides of the HV/HV DC/DC converters tied together. This approach makes it possible to get 11-kW (3× 3.6 kW) and 22-kW (3× 7.2 kW) OBC designs. 

Another attractive and simpler approach could be made by combining WBG devices for three-phase AC systems, as shown in Figure 8. In this configuration, there are two possible scenarios: 

  1. Depending on the AC grid supply configuration and consequently the DC bus voltage at the output of PFC (in this base, the B6/voltage-source converter), the HV/HV DC/DC converters can be connected in series (for a three-phase input) or in parallel (for single-phase input). The purpose of this is to keep the same input-to-output voltage-conversion ratio of the transformers. 
  1. Depending on the power density, thermal management, and efficiency requirements of the OBC system, the HV/HV DC/DC converter can be either in series or parallel on the primary side. 
A three-phase/single-phase bidirectional solution.
Figure 8: A three-phase/single-phase bidirectional solution by using different-voltage–class SiC MOSFETs and GaN HEMT, as well as a series/parallel arrangement in the HV/HV DC/DC converter for different battery voltage classes. 

According to the output voltage/battery voltage range, the proper voltage-class selection must be made, which is either a 650-V GaN HEMT/750-V SiC MOSFETs or 1,200-V SiC MOSFETs. Selecting the targeted efficiency, power density, and controllability will determine the topology and the power-semiconductor technology. 

For a less complex approach and keeping the number of components to a minimum, the HV/HV DC/DC converter can consist of a single converter using 1,200-V SiC transistors. 

Summary

The megatrends of electrification and digitalization are significantly impacting the design, manufacturing, and commercialization of EVs as well as electromobility technology. OBCs are under the spotlight as one of the key role players in the faster adoption of BEVs and plug-in hybrid vehicles, competing with internal-combustion–engine cars in terms of charging times and driving coverage. 

Five major trends have been identified that influence OBC design. In all of them, the usage of WBG technology enables better power density and efficiency and extends the power classes. Designers must fully understand the FoMs of each technology to accurately choose the most suitable solution for the different topologies. Furthermore, the technology itself is insufficient to improve the different aspects of future OBC designs. Innovative packaging with a new TSC approach is essential to further exploit the benefit of WBG transistors. Finally, WBG devices enable innovative topologies with advanced control algorithms that allow bidirectional power flow and, consequently, the integration of BEVs into the AC grid. 

Infineon’s semiconductor solutions can realize core functionalities in electrification, extend driving range, reduce charging times, and enable the bidirectional energy flow between the high-voltage systems with the traditional 12-V domain. Learn more about the company’s offering for hybrid-vehicle on-board battery chargers here. Full details about its WBG technologies can be found here

References 

May 10–12, 2022: Visit us at PCIM Europe in Nuremberg 

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