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What you will learn:
- Why SiC power switches in traction inverters are seen as the keystone to help extend the range of electric vehicles.
- The crucial role of gate drivers in SiC power switches.
- Optimized EV drivetrain energy efficiency by neutralizing EMI in surrounding components.
Two major disruptions currently affect the future of automotive transportation and semiconductor technology. We are embracing an exciting new way to power our vehicles cleanly with electric power, while simultaneously re-engineering the semiconductor materials that underpin electric vehicle (EV) subsystems to maximize energy efficiency and, therefore, the range of EVs.
Government regulators continue to require automotive OEMs to reduce global CO2 emissions from their vehicle fleets, with stiff penalties for non-compliance, and electric vehicle charging infrastructure is beginning to proliferate along our roads and parking lots. Despite all of these advances, however, mainstream consumer adoption of electric vehicles remains held back by lingering concerns about electric vehicle range limitations.
To complicate matters, larger EV battery sizes that could extend EV range and neutralize consumer range anxiety threaten to simultaneously increase EV prices. The battery represents more than 25% of the final cost of the vehicle.
Fortunately, the semiconductor revolution occurring in parallel has produced new wide band gap (WBG) devices such as silicon carbide (SiC) MOSFET power switches. They can help bridge the gap between consumer expectations for electric vehicle range and the ability of OEMs to meet them at competitive cost structures.
Making the Most of SiC Technology
The inherent advantages of SiC-based power switches in terms of power density and efficiency are well understood, with key implications for cooling and system size. The move to SiC promises 3x smaller inverters at 800V/250kW, with significant additional size and cost savings over the associated DC bond film capacitors. Compared to conventional silicon, SiC power switches can allow for better range and/or reduced battery, giving the switches a favorable cost comparison from the device level to the system level.
At the intersection of these range and cost considerations, the traction inverter remains at the epicenter of innovations aimed at unlocking further EV efficiency and range gains. And as the most expensive and functionally important element of the traction inverter, SiC power switches must be very precisely controlled to take full advantage of the additional cost of the switch.
Indeed, all the intrinsic advantages of the SiC switch would be negated by common-mode noise disturbances, as well as extremely high and destructive voltage overshoot due to ultra-fast voltage and current transients (dv/dt and di/ dt) generated in the environment of a mismanaged power switch. Generally speaking, the SiC switch has a relatively simple function despite the underlying technology – it is only a 3-terminal device – but it must be carefully interfaced with systems.
Enter the door pilot
The isolated gate driver will take care of defining the best switching sweet spot, ensuring short and accurate propagation delay through the isolation barrier. It also provides system and safety isolation, controls power switch overheating, detects and protects against short circuits, and facilitates the insertion of the sub-block drive/switch function into an ASIL D system.
However, high slew rate transients introduced by the SiC switch can corrupt data transmission across the isolation barrier. Thus, measuring and understanding the susceptibility to these transients is essential. On this front, IProprietary coupler technology developed by Analog Devices (ADI) has demonstrated industry-leading common-mode transient immunity (CMTI) with performance measured up to 200 V/ns and beyond. This releases the full potential of SiC switching time in safe operation.
The high-performance gate drivers have proven their worth in real-world testing with leading SiC MOSFET power switch vendors such as Wolfspeed. Across key parameters, including short-circuit detection time and total fault clearing time, performance can be achieved up to 300 ns and 800 ns, respectively. For added safety and protection, test results have demonstrated the adjustable soft-stop capabilities critical to proper system operation.
Switching energy and electromagnetic compatibility (EMC) can also be maximized to improve energy performance and EV range. Higher drive capacity allows users to have faster peak rates and therefore reduces switching losses. This not only helps with efficiency, but also saves board space and cost by eliminating the need for gate driver allocated external buffers.
Conversely, under certain conditions the system may need to switch more slowly to achieve optimum efficiency, or even in stages, which studies show can further increase efficiency. ADI provides adjustable slew rate so users can achieve this goal, and removing external buffers eliminates other hurdles.
Elements of a system
It is important to note that the combined value and performance of the SiC gate driver and switch solution can be completely negated by trade-offs and/or inefficiencies in the surrounding components.
A holistic view of the electric vehicle reveals additional opportunities to optimize driveline energy efficiency, which are key to harnessing maximum usable battery capacity while ensuring safe and reliable operations. The quality of the Battery Management System (BMS) directly impacts the miles per charge an EV can deliver, maximizes overall battery life, and therefore lowers the total cost of ownership ( TCO).
In terms of power management, the ability to overcome complex electromagnetic interference (EMI) challenges – without compromising BOM costs or PCB footprint – becomes paramount. Power efficiency, thermal performance, and packaging remain key considerations at the power supply layer, whether the layer is intended for an isolated gate driver power circuit or a low-voltage high-voltage auxiliary circuit. dc to dc circuit.
Either way, the ability to neutralize EMI issues is of greater importance to EV designers. Electromagnetic compatibility is a critical point when it comes to switching multiple power supplies. Superior EMC can go a long way in shortening test cycles and reducing design complexities, thereby speeding time to market.
Deeper in the support component ecosystem, advances in magnetic sensing have produced a new generation of non-contact current sensors with no loss of power with high bandwidth and accuracy, as well as precise and robust position sensors. for both end-of-shaft and off-shaft configurations. Between 15 and 30 current sensors are targeted for deployment in a typical plug-in hybrid electric vehicle,1 with rotation and position sensors monitoring traction motor functions. Sensing accuracy and stray field robustness are critical attributes for measuring and maintaining the efficiency of electric vehicle power subsystems.
By holistically examining all elements of the EV powertrain, from the battery to the traction inverter to the supporting components and beyond, ADI sees myriad opportunities to improve EVs in a way that improves overall fuel efficiency and extends range. Digital isolation is one of many important parts of the equation as SiC power switching technology enters the EV traction inverter.
Likewise, automotive OEMs can leverage a multidisciplinary approach to EV optimization to ensure that all available power monitoring and control devices work closely together for maximum performance and efficiency. . In turn, they can help overcome the remaining barriers to consumer adoption of electric vehicles – range and vehicle cost – while helping to secure a greener future for all.
1.Richard Dixon. “MEMS sensors for the car of the future.” 4and Annual Automotive Sensors and Electronics Summit, February 2019.