SiC Drives Onboard Charging as Voltage Levels Increase
11 November 2023
Figure 1: Multiple types of EVs are in use today including MHEVs, FHEVs, PHEVs and BEVs
Electric vehicles (EVs) are gaining wider acceptance in various forms (hybrid, pure electric, etc.) despite the persisting concerns about ‘range anxiety’. In order to increase adoption levels, automakers continue to work on extending driving distances and reducing charging times to overcome this significant hurdle. The way that EVs are charged significantly impacts upon vehicle usability and convenience.
With a limited number of high-power charging stations in place, a considerable portion of EV owners still rely on their onboard chargers (OBCs) to charge their vehicles. To improve OBC performance, automakers are looking toward new technologies such as silicon carbide (SiC). This article underlines the importance of OBCs and how advancements in semiconductor switches will take their performance to the next level.
There are a wide variety of vehicle propulsion systems currently on the market - from vehicles solely powered by internal combustion engines (ICEs) through to hybrid models (xHEVs) that use ICEs and electric power in combination, plus completely electric vehicles (xEVs). The xHEVs category is comprised of 2 different kinds of vehicles. These are mild hybrid electric vehicles (MHEVs) and full hybrid electric vehicles (FHEVs).
MHEVs primarily rely on an ICE while incorporating a small battery (usually 48V). However, they cannot solely run on electric power, and the electric motor assists in modestly reducing fuel consumption. FHEVs offer enhanced flexibility, as they can seamlessly combine the ICE and the electric motor, which is powered by a battery (usually operating in the 100V to 300V voltage range). FHEVs also recharge their batteries through regenerative braking, capturing energy when the brakes are applied to improve efficiency. All xEVs, including plug-in hybrids and pure battery electric vehicles (BEVs), are equipped with regenerative braking systems. However, given their larger battery capacities, these vehicles heavily rely on OBCs to replenish their batteries.
The simplest form of charger is little more than a cable to connect the EV’s OBC to a wall outlet (ground fault protection is typically required). While convenient, these mostly residential Level 1 systems (or SAE AC Level 1 as defined in J1772 standard) operate at around 1.2kW and add up to 5 miles of range per hour of charging. Level 2 systems (or SAE AC Level 2) typically use a multi-phase AC feed from the grid and are most commonly found in public buildings and commercial facilities. With power levels up to 22kW, as much as 90 miles of range can be added for every hour of charging.
Both Level 1 and Level 2 chargers deliver AC to the EV, so an OBC is essential to convert the AC input to DC output to charge the battery. Most of the chargers deployed at the moment are Level 2.
High-power DC chargers, known interchangeably as Level 3, SAE Level 1 & 2 DC chargers or IEC Mode 4 chargers, output a DC voltage and can charge the battery directly, eliminating the need for an OBC. The power levels of these DC chargers range from 50kW to over 350kW, allowing for a charge of up to 80% of the battery capacity in around 15 to 20 minutes. Given the high-power levels and the infrastructure changes required in the power grid, the number of fast charger outlets is still relatively limited, although it is rapidly increasing.
Figure 2: Bridgeless totem-pole topology
Many automakers are currently making the transition from 400V to 800V batteries. This shift is aimed at enhancing the EV range by improving system efficiency, boosting performance, enabling faster charging speeds and reducing the weight of cables/batteries.
Anatomy of an OBC
Typically, an OBC is a 2-stage power converter with a power factor correction (PFC) stage followed by an isolated DC/DC converter stage. It’s worth noting that non-isolated configuration is possible but rarely used. The PFC stage rectifies the AC feed, manages the power factor to >0.9, and generates a regulated bus voltage for the DC/DC stage.
Over the past few years, there has been a significant increase in the demand for bi-directional systems. These systems enable EVs to reverse the power flow from the battery back to the source. The key benefits of this will include dynamically balancing the grid load through vehicle-to-grid (V2G) technology or managing grid outages via vehicle-to-load (V2L) technology.
The traditional PFC approach involves using a rectifier diode bridge in conjunction with a boost converter. The rectifier bridge converts AC voltage to DC voltage, while the boost converter increases the voltage level. An enhanced version of this basic circuit is the interleaved boost topology, where multiple converter stages are connected in parallel to reduce ripple current and improve efficiency. These PFC topologies typically utilise silicon technologies such as super-junction MOSFETs and low Vf diode.
The emergence of wide bandgap (WBG) power switches, particularly ones based on SiC, has enabled the development of new design approaches - due to their advantages of lower switching losses, lower RDS(on), and inclusion of low reverse recovery body diodes.
The bridgeless totem-pole topology has gained popularity in medium to high-power PFC applications, typically 6.6kW and above. Figure 2 illustrates this topology, with the slow branch (Q5-Q6) switching at the grid frequency (50Hz to 60Hz) and the fast branch (Q1-Q4) shaping the current, stepping up the voltage and operating at a higher frequency (typically 65kHz to 110kHz) in hard-switching mode. While the bridgeless totem-pole topology significantly improves efficiency and reduces the number of power components, it does introduce complexity in terms of control.
Figure 3: A bi-directional DC/DC allows power to be returned to the grid during peak demand/
The DC/DC stage commonly employs an isolated topology, using a transformer for isolation, with the main objective of regulating the output voltage based on the battery's charge status. Although half-bridge topologies could be employed, the prevailing solutions nowadays mainly rely on dual-active-bridge (DAB) converters, such as resonant converters (e.g. LLC or CLLC) or phase-shifted full bridge (PSFB) converters. Resonant converters, particularly LLC and CLLC, have recently gained significant attention due to their numerous advantages, including a wide soft switching operation range, bi-directional operation capability, plus the ease of integrating the resonant inductor and transformer into a single power transformer.
SiC in OBC applications
650V SiC devices are typically the preferred choice for 400V battery packs. However, for 800V architectures, the higher voltage requirements make using 1200V rated devices necessary.
The adoption of SiC in the OBC field can be attributed to its exceptional performance across various figures-of-merit. SiC exhibits advantages in terms of specific RDS(on) per area, low switching losses and elevated breakdown voltage levels. These advantages allow SiC-based solutions to operate reliably at higher temperatures. By leveraging these superior performance characteristics, more efficient and lightweight designs may be achieved. Consequently, systems can reach higher power levels (up to 22kW) which would be impractical to attain using traditional silicon-based solutions (such as IGBTs or super-junction MOSFETs).
While a higher power OBC in an EV may not have a direct impact on the vehicle's range, it still plays a crucial role in addressing the issue of range anxiety by significantly reducing charge times. The power levels of OBCs are on the rise so as to achieve faster charging. SiC technology will be pivotal in making these systems more efficient, ensuring optimal conversion of power from the grid without energy wastage. As a result, more compact, lightweight and reliable OBC systems can be designed.
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