SiC Technology Advancement Fuels the Shift to 800V Automotive Charging Systems
Abstract: As electric vehicles (EVs) gain traction in response to the perpetual range anxiety, automakers tirelessly seek to extend driving range and reduce charging times. The usability of EVs significantly hinges on charging methodologies, where the role of on-board chargers (OBC) remains critical due to the scarcity of high-power charging stations. To optimize the performance of on-board chargers, manufacturers are exploring novel technologies like Silicon Carbide (SiC). This technical article delves into the importance of OBCs and how semiconductor switch advancements are propelling their performance to unprecedented levels.
Introduction: The automotive landscape is peppered with vehicles employing various propulsion systems from the traditional internal combustion engine (ICE) vehicles, the synergy of ICE and electrical systems in hybrid electric vehicles (xHEVs), to pure electric vehicles (xEVs). xHEVs comprise of both mild hybrid electric vehicles (MHEVs) and full hybrid electric vehicles (FHEVs).
MHEVs rely primarily on the ICE integrated with a small battery (typically 48V). Yet, MHEVs aren't capable of operating solely on electric power, and the electrical motor serves to moderately reduce fuel consumption.
In contrast, FHEVs offer greater flexibility as they seamlessly alternate between ICE and battery-powered electric motor operation (typically operating within a 100-300V voltage range). FHEVs additionally capitalize on regenerative braking technology, harnessing energy during deceleration to enhance efficiency.
All xEVs, inclusive of plug-in hybrid electric vehicles and battery electric vehicles (BEVs), are equipped with regenerative braking systems. However, with larger battery packs, these vehicles largely rely on OBCs for charging (Figure 1).
The simplest charging method is through a cable that connects the EV's OBC to a wall socket (usually necessitating ground fault protection). While convenient, the residential level 1 system (or SAE AC level 1 as per J1772 standard) operates around 1.2kW, which equates to adding about 5 miles of range per hour[1]. Level 2 systems (or SAE AC level 2) usually tap into a multi-phase AC supply from the grid, commonly found in public buildings and commercial facilities, delivering up to 22kW power and potentially adding 90 miles of range per hour.
Both level 1 and 2 chargers provide AC to EVs, hence the OBC plays a crucial role in converting AC input to DC output for battery charging. Most deployed chargers currently are level 2.
High-power DC chargers, commonly referred to as level 3, SAE level 1 and 2 direct current (DC) or IEC mode 4 chargers, output DC voltage capable of directly charging batteries, circumventing the need for an OBC. These chargers range from 50kW to upwards of 350kW and can charge batteries to 80% capacity in roughly 15-20 minutes. Given the high-power levels and the requisite electrical grid infrastructure modifications, rapid charging stations, although swiftly proliferating, remain relatively scarce.
Many automakers are gradually transitioning from 400V to 800V batteries, aiming to extend EV driving range by enhancing system efficiency, improving performance, accelerating charging speeds, and lessening the weight of cables and the battery itself.
OBC Analysis
An OBC is typically a two-stage power converter consisting of a Power Factor Correction (PFC) stage and an isolated DC-DC converter stage. It's noteworthy that while non-isolated configurations are feasible, they aren't commonly used. The PFC stage rectifies AC supply, maintains a power factor over 0.9, and generates a regulated bus voltage for the DC-DC stage.The demand for bidirectional systems has significantly spiked in recent years. Bidirectional systems allow EVs to supply reverse power flow from battery to source, offering dynamic grid load balancing (V2G: Vehicle-to-Grid) or managing power outages (V2L: Vehicle-to-Load).
Traditional PFC approaches involve a diode rectification bridge combined with a boost converter where the bridge turns AC voltage into DC, with the boost converter elevating the voltage. Enhanced versions of this basic circuit utilize interleaved boost topologies, paralleling multiple converter stages to reduce ripple current and heighten efficiency. These PFC topologies typically leverage silicon-based technologies like superjunction MOSFETs and low Vf diodes.
The emergence of Wide BandGap (WBG) power switches, especially SiC power switches, has paved the way for novel design approaches. These switches offer reduced switching losses, lower RDS(on), and the advantage of low reverse recovery body diodes.
For mid to high-power PFC applications (typically 6.6kW and above), bridgeless totem-pole topologies are gaining popularity (Figure 2). In this topology, the slow leg (Q5-Q6) switches at grid frequency (50-60Hz), while the fast leg (Q1-Q4) performs current shaping and voltage boosting, operating in hard-switching mode at higher frequencies (typically 65-110kHz). While bridgeless totem-pole topology substantially improves efficiency and reduces the number of power components, it introduces increased complexity in control.
The DC-DC stage generally employs an isolated topology with a transformer providing isolation, primarily aiming to adjust output voltage in accordance with battery charging state. While half-bridge topologies are plausible, dual active bridge (DAB) converter schemes like resonant converters (e.g., LLC, CLLC) or phase-shifted full-bridge (PSFB) converters are primarily used. Resonant converters, particularly LLC and CLLC, have garnered attention due to their broad benefits, including wide soft-switching range, bidirectional operation, and the convenience of combining the resonant inductor and transformer into a single power transformer.
SiC Application in OBC
For 400V battery packs, 650V-rated SiC devices are often the go-to choice. However, for 800V configurations, devices with a rating of 1200V are required due to higher voltage demands.The adoption of SiC in OBCs stems from its impressive Figure of Merits (FOM). SiC devices offer lower specific RDS(on), switch losses, reverse recovery diode, and breakdown voltage per unit area. These advantages enable SiC-based solutions to operate reliably at higher temperatures. Leveraging these stellar attributes allows for the design of more efficient, lighter implementations. As such, systems can achieve higher power levels (up to 22kW), a feat tough to realize with traditional silicon-based solutions (e.g., IGBTs or superjunctions).
While a higher power OBC in EVs may not directly affect vehicle range, it significantly reduces charging time, aiding in the mitigation of range anxiety. To achieve faster charging speeds, the power of OBCs is continually being ramped up. SiC technology plays a pivotal role, making these systems more efficient, ensuring the energy from the grid is converted effectively, minimizing energy wastage. The technology allows for the creation of more compact, lightweight, and reliable OBC systems.
In conclusion, CISSOID and Silicon Mobility have collaborated to develop a comprehensive SiC inverter reference design, offering customers an innovative and customizable solution for motor drives up to 350kW/850V. This reference design includes SiC-based high voltage power modules, integrated gate driver boards, control boards, DC and phase current sensors, DC link capacitors, and integrated liquid cooling for EMI filtering, as well as Silicon Mobility's ultra-fast and secure OLEA T222 FPCU. BONCHIP, as an authorized CISSOID agent, has now started offering the complete SiC inverter development kit, welcoming customers to explore and integrate these advanced solutions into their applications. Dave Hutton, CEO of CISSOID, emphasizes that customers now can access a turnkey SiC inverter reference design, licensed to use Silicon Mobility's OLEA APP INVERTER control software, and design their software applications. This approach provides flexibility and customization options, allowing customers to purchase either the complete inverter Bill of Materials (BOM) or just the SiC Intelligent Power Modules (IPM) and control...
