Stuck by the electric car OBC design?This article can solve your confusion

[Introduction]On-board chargers (OBCs) provide the critical function of charging high-voltage DC battery packs in electric vehicles (EVs) from the infrastructure grid. The OBC handles charging when the EV is connected to a supported Level 2 Electric Vehicle Supply Equipment (EVSE) via a suitable charging cable (SAE J1772, 2017). Owners can use a special cable/adapter to connect to a wall plug for Level 1 charging as an “emergency power source”, but this provides limited power and therefore takes longer to charge.

OBCs are used to convert alternating current to direct current, but this conversion is not required if the input is direct current (Figure 1). When connecting a DC fast charger to the vehicle, this bypasses the OBC and connects the fast charger directly to the high voltage battery.

Figure 1: OBC Power Path Function Block

OBCs are used in pure electric vehicles (BEVs), plug-in hybrid electric vehicles (PHEVs) and potentially fuel cell vehicles (FCEVs). These three types of electric vehicles (EVs) are collectively referred to as new energy vehicles (NEVs), but have different requirements for system-level charging functions (Table 1).

Table 1: Electric Vehicle OBC System Level Requirements

The core function of accepting AC input and converting it to DC output provides the proper voltage and current for charging the high voltage battery pack. In general, this function is unidirectional as it only provides transmission from the grid to the car. The OBC cell changes voltage and current based on the health and state of charge of the entire battery (Figure 2).

Figure 2: Typical charging curve of a 410 V Li-ion battery pack (Image source: ON semiconductor TND6318-D document “On Board Charger(OBC) LLC Converter”)

Design constraints for an OBC include AC input, target output power level, battery pack voltage, cooling method, space constraints, and whether the design is unidirectional or bidirectional. In addition, in many cases, such modules must support Automotive Safety Integrity Level (ASIL) B or C for functional safety.

Considering the overall hardware functional block of OBC, designers should address the following issues:

● AC rectification and power factor correction (PFC) for AC power input.

● Primary side DC-DC.

● Secondary side rectification (passive or active).

● If it is bidirectional, also carry out secondary DC-DC control.

● Voltage, current and temperature diagnostics.

● In-Vehicle Network (IVN) for communication and diagnostics.

● Communication with Electric Vehicle Supply Equipment (EVSE).

● Isolation between AC power, 12 V battery and high voltage battery, this is a very important safety requirement.

This article focuses on the first four (marked in bold) high-power path sections above.

AC rectification and PFC help minimize reactive power while maximizing actual power transmission and operation in AC-DC conversion mode (Figure 3). In high-power systems such as OBC, without PFC, the power transmission efficiency is not high, and the thermal load will increase. In terms of OBC design, this module has the most versions because it has many implementations depending on AC power input, output power, energy efficiency and cost goals.

Figure 3: Power Triangle (Image source: ON Semiconductor AN-42047 document “Power Factor Correction (PFC) Basics”)

The power factor (PF) specification of the OBC typically achieves PF ≥ 0.9 over the entire operating range and PF ≥ 0.98 over the typical operating range. A high PF value maximizes charging capacity while minimizing line/grid current and apparent power demand. In the future, the industry will pay more attention to various improvements related to line/grid harmonic content, as well as improvement modes under light load conditions. The PFC controller in OBC is used to perform the following functions:

● Make the input phase current consistent with the input phase voltage.

● Reduce the peak current drawn from the AC power source.

● Minimize total harmonic distortion (THD) of line/grid currents as much as possible.

● Make sure that the input current is as close to a sinusoidal waveform as possible.

In Figure 4, both voltage and current are sinusoidal and in phase. This minimises reactive power components, thermal loads and harmonics to provide the maximum amount of real power transmission.

Figure 4: Typical low-power circuit using PFC (Image source: ON Semiconductor HBD853/D document “Power Factor Correction (PFC) Handbook”)

While passive PFCs can be used in general applications, the practical implementation of such systems requires the use of active PFCs because OBCs need to meet higher power levels, space constraints, thermal requirements, and power factor goals (Figure 5).

Figure 5: Typical PFC topology for OBC system power levels

Common active PFC schemes for OBC include (Table 2):

● Traditional boost

● Traditional boost, 2-channel interleaved

● Bridgeless boost

● Totem pole

● Vienna Rectifier

● 3-arm or 4-arm bridge (3-phase totem pole)

Table 2: PFC Typical Device Technology

As OBC output power increases, it is recommended to use PFC topologies that reduce the number of diodes in the power path, or to use SiC Schottky diodes with little reverse recovery characteristics. Designers can also switch to SiC MOSFETs, which allow the PFC stage to switch at higher frequencies while handling higher system voltages, increasing efficiency and energy density.

The next block in the power path is the primary-side DC-DC converter (Table 3). This circuit is used to convert the high voltage DC link from the PFC to an appropriate voltage for charging. The output voltage and current will vary depending on the state of the battery pack.

In a unidirectional design, the typical implementation of this DC-DC is an LLC, but there will also be a PSFB (Phase Shifted Full Bridge) version. For bidirectional designs, the implementation is CLLC or dual active bridge (DAB), and as bidirectional functionality develops, the use of these architectures is expected to increase. SiC MOSFETs are ideal for this situation due to their higher voltages and lower switching losses.

Table 3: DC-DC Device Selection

The secondary side can use diodes for passive rectification, power switches for synchronous rectification, CLLC-enabled full-bridge designs (bidirectional), or the second half of a dual active bridge (bidirectional) (Table 4). Passive rectification does not require control, but only supports one-way power supply from the grid to the vehicle. For higher efficiencies or in the case of 800 V battery packs, SiC diodes offer the best solution.

Synchronous rectification with superjunction MOSFETs (loss of efficiency) or SiC MOSFETs can be used in unidirectional designs, but in many cases these solutions are more expensive than diode solutions. For bidirectional functionality, a full-bridge or multi-arm half-bridge solution design is used.

Depending on the power level, voltage, and efficiency goals of the system, either superjunction MOSFETs or SiC MOSFETs are used. SiC MOSFETs offer higher efficiency in all scenarios and are easier to handle in 800 V systems, while for 400 V systems, for cost optimization, superjunction MOSFETs can be used.

Table 4: Secondary Side Device Selection

The rated output power of an OBC is often related to the size of the battery pack used in the vehicle. The OBC should be able to provide a larger output power for the larger battery in the BEV, and a smaller output power for the smaller battery in the PHEV. This balance prevents over-engineering of the system and helps optimize charging time and cost.

There are several options for BEVs when it comes to the rated capacity of the battery pack. The physical size of the vehicle, cost targets, and expected performance (such as range) all affect this performance. Globally, light-duty passenger vehicles across multiple vehicle segments may have battery pack capacities ranging from 30 kWh to 105 kWh (according to Electric Vehicle Database 2021).

It is more common for light-duty passenger vehicles belonging to the truck or large sport utility vehicle (SUV) segment to have a battery pack capacity of 110 kWh to over 150 kWh (according to the Electric Vehicle Database and Ford Motor Company’s respective 2021 estimates). data).

Two new cars are expected to have battery capacities close to 200 kWh (according to Electric Vehicle Database 2021 and Engineering Explained 2020)! Battery pack capacity ratings are increasing to provide greater range or meet new automotive market segments, while the 800 V specification is being adopted more widely in the industry for faster charging.

PHEVs and FCEVs have battery pack capacities ranging from 5 kWh to 25 kWh. Since PHEVs also rely on additional power sources beyond the battery pack, their capacity is much lower than that of a typical BEV. PHEVs use an internal combustion engine (ICE), while FCEVs use hydrogen fuel cells.

When the battery pack capacity drops below a certain level or other conditions require it, an ICE or fuel cell can provide power to drive a generator that charges the battery. For short distances, such EVs are capable of all-electric drive, but their range is nowhere near as good as BEVs. Such EVs will have more shifts to battery capacities above 15 kWh in order to increase pure electric range.

BEVs have a much larger battery capacity than PHEVs, which affects OBC design and selection, as well as vehicle charging time. Let’s consider a scenario where two different cars (BEV and PHEV) are charged with the same version of OBC and plugged into the same EVSE.

If the battery capacity of the BEV is 4 times that of the PHEV, the charging time of the BEV is about 4 times that of the PHEV. This simplified view does not take into account the many complexities of the charging algorithm, but it is sufficient to estimate this for the purposes of this paper. If both battery packs are depleted, the BEV will take longer to charge.

Charging time is a major consideration for OEMs and customers, and it affects end-user satisfaction. Options that help improve charging time include increasing the power output of the OBC, increasing the efficiency of the OBC, and increasing the system voltage of the battery pack and associated OBC. All of these scenarios help reduce charging time, thereby improving the end-user experience.

The architecture and power levels of OBCs are undergoing rapid transformation. As EV adoption continues to grow, the need for very flexible OBC designs is more important than ever.

Key system considerations:

● The energy density of EV battery packs is increasing.

● Consumers want faster charging times.

● OBCs are migrating to higher power levels.

● OBCs must meet the needs of 400 V and more widely adopted 800 V battery systems.

● To increase end-user functionality, optional bidirectional functionality needs to be provided to support grid-to-vehicle and vehicle-to-grid transmission.

Owners will benefit as they can power their homes with EVs in the event of a blackout, or partner with utility companies to power grid infrastructure (and thus get paid).

Key considerations for PFC:

● SiC-based totem-pole PFC can improve system efficiency and handle higher voltages, while making totem-pole topology popular with Vienna architecture in single-phase and three-phase solutions.

● Vienna rectifier PFC based on superjunction MOSFET or SiC MOSFET and SiC diode can improve system efficiency.

Key considerations for primary/secondary side:

● The primary side DC-DC adopts SiC MOSFET to improve energy efficiency.

● For unidirectional designs, SiC diodes on the secondary side provide the best efficiency.

● For CLLC and DAB topologies, it is easier to implement bidirectional functionality with SiC MOSFETs on the secondary side.

To further reduce the charging time, the output power of the OBC module will start to increase for battery packs with smaller energy density. Another possibility is to add support for DC fast charging, helping PHEVs fully charge in minutes. For larger battery packs, such as those used in BEVs, the trend is to move to 11 kW and 22 kW OBCs, while continuing to support fast chargers and higher voltages.

Finally, Tier 1 suppliers are integrating HV-LV DC-DC module functionality into OBCs. This integrated module design, called a Combo Charger Unit (CCU), provides “2 modules in one” while improving system-level efficiency between the high-voltage power grid and the 12 V power grid.

Electric vehicle architectures (BEVs, PHEVs and FCEVs) that support the use of OBC will account for approximately 46% of total EV sales in 2021 and 57% in 2026. The 5-year compound annual growth rate (CAGR) of OBC is expected to be 25.6%, and the number in 2026 is estimated at 21.4 million units (according to Strategy Analytics 2020 data) (Figure 6).

Figure 6: Growth of vehicles requiring OBC

For power electronics used in inverters, various requirements such as maximum power density, high efficiency, supply chain stability and long-term reliability must be met.

Onsemi offers scalable technology for automotive OBC power stages from 3.3 kW to 22 kW and battery voltages up to 800 V. The product portfolio includes SiC MOSFETs, hybrid IGBTs with co-packaged SiC diodes, superjunction MOSFETs, automotive power modules (APM), SiC diodes, gate drivers, regulated power supplies and in-vehicle networking (IVN) solutions. The partnership with ON Semiconductor enables customers to design flexible OBC and infrastructure charging solutions for a variety of EV applications.

Source: EDN Electronic Technology Design, by Marc Bracken