Using SiC-based bidirectional on-board chargers to feed back power to the grid

Range anxiety and limited charging equipment have long been major barriers to EV adoption. Even as automakers have demonstrated that their batteries can support longer distances and the number of charging stations has proliferated, EV charging remains a challenge, but it also presents an opportunity to balance grid loads.

Using SiC-based bidirectional on-board chargers to feed back power to the grid

The transition to electric vehicles also means looking at how to better interact with the grid. Electric vehicles are equipped with lighter, more power-dense batteries that can not only increase driving range, but may also be used to support independent loads. At the same time, the update and evolution of the on-board charger (OBC) to the direction of bidirectional energy transmission enables the OBC to obtain power from the grid and to feed back the power to the grid.

Wolfspeed’s award-winning 6.6 kW bidirectional OBC, based on its new 650 V Silicon Carbide (SiC) MOSFETs, can play a vital role in the evolution of electric vehicles and the grid that powers them.

OBC’s One-Way Challenge

Except in the roughest and most remote areas, petrol drivers rarely worry about running out of gas between gas stations. But since the advent of electric vehicles, there have been concerns about driving range. While charging stations are becoming more commonplace and being integrated into new residential developments, how far EVs can go, battery capacity and how long they stay charged are areas for improvement.

The number of OBCs is growing, and with the development of electric vehicles themselves, but OBCs are not as powerful as fast chargers. A fast charger can charge a car in about an hour, while an OBC takes six to seven hours. An even bigger disadvantage of one-way OBC is that a parked vehicle is slowly discharged, wasting both power and money. But this problem also forced a solution. It opens the door for electric vehicles to feed power back to the grid, rather than allowing energy to “leak” slowly. The OBC for bidirectional energy transfer can not only obtain power from the grid, but also feed back energy to the grid. This enables an electric vehicle to help balance the load of a city’s overall electrical infrastructure.

Bidirectionality is also beneficial for on-board batteries that require charge-discharge cycles, rather than always being charged at 80 percent. Car batteries are ideally also fully discharged occasionally, much as is the case with smartphones. In most cases, keeping the battery fully charged also means that all your components are always charged, which shortens their lifespan. That also means replacing the battery ahead of time, which, like a smartphone, is expensive. Under ideal conditions, the OBC should be able to intelligently sense when 30% of the car’s charge is left, and then recharge the battery by feeding this remaining power back to the home’s grid and recharging the car. charge-discharge cycles.

The purpose of using a bidirectional OBC is to efficiently transfer power back and forth with minimal losses during the transfer process. Although there are multiple solution options, Wolfspeed SiC MOSFETs still have more advantages than other devices in optimizing bidirectional OBC.

Grid Power Opportunities for OBC

OBC addresses concerns arising from limited infrastructure for charging stations and off-board chargers. Off-board chargers, while fast, can only be used at charging stations, and sometimes they are proprietary or limited in use. In addition, the time spent going to the charging station and waiting each day is a bit of a loss when it comes to commuting.

While OBC has advantages over off-board charging at charging stations, its slower charging speed means it needs to be charged at night at home or at work during the day, the same way most people charge their smartphones. This is why the batteries in OBC vehicles need to be recycled, which also allows for bidirectional charging.

In China, two-way OBC actually turns the car into a power bank, which becomes a valuable selling point for customers.

Another possible application scenario for bidirectional OBC is to interconnect multiple vehicles in a power network to generate large amounts of electricity to power the grid. Individuals can “buy” electricity at a low rate at night and “sell” it back at a high price during the day.

Today’s bidirectional OBCs can be based on insulated gate bipolar transistors (IBGT-) or silicon carbide (SiC-). SiC devices are the best solution for OBC due to their smaller size, lower overall system cost and higher efficiency compared to Si devices.

SiC Solutions for Efficient Bidirectional OBC

Given the many advantages of SiC, Wolfspeed, a Cree company, set out to design a 6.6 kW bidirectional electric vehicle OBC based on SiC MOSFETs.

The goal of the design is to develop an efficient bidirectional OBC with high power density that can be used to support independent loads and feed back grid energy. A digitally controlled reference design achieves this with a continuous conduction mode (CCM) totem pole PFC with a switching frequency of 67 kHz and a CLLC resonant converter with a switching frequency of 150-300 kHz. 54 W/in3 power density and higher than 96.5% peak efficiency.

Given the need to optimize electric vehicles for space and weight, maximizing high density and efficiency is critical. Wolfspeed’s OBC solution consists of a bidirectional AC-DC converter and an isolated bidirectional DC-DC converter that provides high efficiency and a wide output voltage range in both charge and discharge modes.

To reduce transmission losses, Wolfspeed eschews traditional PFC boost converters because diode bridge rectifiers are lossy and do not support bidirectional operation. Due to the good reverse recovery performance of the body diode of SiC MOSFETs, an interleaved CCM totem-pole PFC can be used as a pre-stage for a 6.6 kW OBC.

Thermal management is also critical when designing an OBC. Typically, TO-247 packaged MOSFETs are reverse assembled on a PCB and mounted on a flat cooling substrate. However, since the MOSFETs are bent down, the PCB area increases. This can adversely affect the overall power density of the system. Therefore, tooled heat sinks are used to accommodate semiconductor and magnetic materials.By applying the power

The semiconductors are mounted on the outside of the heat sink, enabling vertical MOSFET assembly to reduce PCB area. Then, the magnetic material is potted with thermally conductive glue in the slot of the heat sink. The result is a low thermal resistance from the tooled aluminum heat sink to the system cooling substrate.

Experimental results of a 6.6 kW bidirectional OBC converter based on SiC MOSFETs in charge and discharge modes show that its efficiency and power density are high, so that the bidirectional OBC can not only charge and discharge the battery efficiently, but also more efficiently The electrical energy is fed back to the grid.

Energy Efficient Prototype

By designing and evaluating its new 650 V SiC MOSFETs in a 6.6kW bidirectional OBC scheme, Wolfspeed demonstrates how the DC bus voltage range can be optimized to 385-425 V over the common battery voltage range of 250-450 V for OBC development .

Furthermore, an experimental prototype validates the performance and thermal integrity of the design. Due to the low power losses of 650 V SiC MOSFETs, and by integrating power semiconductors and power magnetics on the same tooled heat sink, we can achieve high power density and high efficiency in bidirectional high power conversion applications such as OBC.

For more information on this reference design and others, visit the Wolfspeed Reference Designs page.

The Links:   LQ10D013 LM150X04-TL01

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