[Guide]There are currently two major factors affecting the future of vehicle transportation and semiconductor technology. The industry is embracing an exciting new method of driving our cars with clean electricity while redesigning the semiconductor materials that support electric vehicle (EV) subsystems to maximize the power efficiency ratio and thereby increase the driving range of electric vehicles .
Government regulators continue to require automobile OEMs to reduce the overall carbon dioxide emissions of their vehicles, impose severe penalties on violations, and start adding electric vehicle charging infrastructure along roads and parking areas. However, despite these advances, mainstream consumers still have doubts about the mileage of electric vehicles, which hinders the promotion of electric vehicles.
To make matters more complicated, although large-size electric vehicle batteries can increase their driving range and relieve consumers’ anxiety about driving range, it will increase the price of electric vehicles—battery costs account for more than 25% of the overall vehicle cost. %.
Fortunately, the semiconductor technology revolution in the same period gave birth to new wide bandgap devices, such as silicon carbide (SiC) MOSFET power switches, which made consumers’ expectations for the mileage of electric vehicles and the actual mileage achievable by OEMs under the cost structure between. The gap was narrowed.
One of Wolfspeed SiC power device leaders and power platform manager Anuj Narain said, “Compared with existing silicon-based technologies, SiC MOSFETs have their own advantages and are widely considered to increase the driving cycle of standard electric vehicles by 5% to 10% cruising range. “Based on this, they are an important part of the new generation of traction inverters in the electric vehicle drive system. If properly developed together with supporting devices, its energy efficiency improvement will represent a substantial increase in consumer confidence in the field of electric vehicles, and will help accelerate the popularization of electric vehicles.
Figure 1. Power conversion components in electric vehicles. The motor inverter converts the DC voltage of the high-voltage battery into an AC waveform to drive the motor and drive the car forward.
Take full advantage of SiC technology
As we all know, SiC-based power switches themselves have advantages in power density and efficiency, which are of great significance for system heat dissipation and device size reduction. The use of SiC is expected to reduce the size of the inverter by a factor of three at 800 V/250 kW. If the DC link film capacitor is used in conjunction, the size and cost can be further reduced. Compared with traditional silicon power switches, SiC power switches can help achieve better driving range and/or smaller battery size, making the switch cost more advantageous at both the device level and the system level.
Figure 2. The battery-to-motor signal chain. In order to increase the mileage, each module should be designed to provide the highest energy efficiency.
When considering driving range and cost factors at the same time, it is still necessary to continue to innovate focusing on motor inverters, aiming to further improve the efficiency and driving range of electric vehicles. As the most expensive and most important component in motor inverters, SiC power switches need to be precisely controlled to give full play to the value of additional switching costs.
Figure 3. Voltage and current waveforms at turn-on (left) and turn-off (right). In the SiC environment, the dv/dt will exceed 10 V/ns, which means that the time to switch the 800 V DC voltage will not exceed 80 ns. Similarly, when the di/dt is 10 A/ns, it means that the current is 800 A in 80 ns, and the change in di/dt can be observed.
In fact, all the inherent advantages of SiC switches will be disturbed by common mode noise and extremely high and destructive caused by ultra-fast voltage and current transients (dv/dt and di/dt) in a poorly managed power switching environment. Voltage overshoot effects. Generally speaking, regardless of the underlying technology, the function of the SiC switch is relatively simple. It is only a 3-terminal device, but it must be connected to the system carefully.
About the gate driver
The role of the isolated gate driver is related to the optimal switching point of the power switch, ensuring a short and accurate propagation delay through the isolation barrier, while providing system and safety isolation, avoiding overheating of the power switch, detecting and preventing short circuits, and prompting the ASIL Insert sub-module drive/switch function into D system.
Figure 4. Isolated gate drivers bridge the signal world (control unit) and power world (SiC switch). In addition to isolation and signal driving, the driver also performs telemetry, protection and diagnostic functions, making it a key element of the signal chain.
However, the high slew rate transients caused by SiC switching can disrupt the data transmission across the isolation barrier, so measurement and understanding of the susceptibility to these transients is crucial. ADI’s proprietary iCoupler® technology has excellent common-mode transient immunity (CMTI), with measurement performance up to 200 V/ns and above. In a safe operating environment, this can fully release the potential of SiC switching time.
Figure 5. For more than 20 years, ADI has been at the forefront of the development of digital isolation technology and introduced the iCoupler® digital isolation IC. This technology uses a transformer with a thick polyimide insulation layer. The digital isolator uses a wafer CMOS process. The transformer adopts a differential architecture and has excellent immunity to common-mode transients.
Considering the smaller die size and strict thermal packaging, short-circuiting is another major challenge for SiC-based power switches. The gate driver provides the necessary short-circuit protection for the reliability, safety and life cycle optimization of the electric vehicle transmission system.
In actual tests conducted by leading SiC MOSFET power switch providers such as Wolfspeed, high-performance gate drivers have proven their value. The performance of key parameters, such as short-circuit detection time and total fault clearing time, can be as low as 300 ns and 800 ns, respectively. In order to improve the safety and protection level, the test results show that the adjustable soft shutdown capability is essential to the smooth operation of the system.
Similarly, switching energy and electromagnetic compatibility (EMC) can be maximized to maximize power performance and electric vehicle mileage. When the driving ability is higher, the user can obtain a faster edge rate, thereby reducing the switching loss. This not only helps to improve efficiency, but also eliminates the need to allocate external buffers for each gate driver, saving board space and cost. On the contrary, under certain conditions, the system may need to reduce the switching speed to achieve excellent efficiency, and even requires hierarchical switching. Studies have shown that the above can further improve efficiency. ADI provides an adjustable slew rate that allows users to do this, and the removal of external buffers further reduces obstacles.
It should be noted that the comprehensive value and performance of the gate driver and SiC switching solution may be completely offset by the compromise and/or inefficiency of the surrounding components. The combination of ADI’s experience in power control and sensing and our system-level performance optimization methods can cover a variety of design considerations.
From a holistic perspective, electric vehicles reveal additional opportunities for optimizing the power efficiency of the transmission system, which is essential for maximizing the use of battery capacity while ensuring safe and reliable operation. The quality of the battery management system directly affects the mileage that an electric vehicle can travel per charge. A high-quality battery management system can maximize the overall life of the battery, thereby reducing the total cost of ownership (TCO).
As far as power management is concerned, being able to overcome complex electromagnetic interference (EMI) problems without reducing BOM costs or reducing PCB size will become critical. Whether it is the power supply circuit of an isolated gate driver or a high-voltage to low-voltage DC-DC circuit, high power efficiency, thermal performance, and packaging are still key considerations in the power domain. In all cases, the elimination of electromagnetic interference is extremely important to electric vehicle designers. When it comes to switching multiple power supplies, electromagnetic interference is a very critical pain point. If the EMC performance is excellent, it will greatly help reduce the test cycle and reduce the design complexity, thereby speeding up the time to market.
If you study the ecosystem of supporting components in depth, you will find that the advancement of electromagnetic sensing technology has promoted a new generation of contactless current sensors that can provide high bandwidth, high accuracy, and no power loss. In addition, it has also promoted the production of precision and Reliable position sensor, suitable for shaft end and off shaft arrangement. A typical plug-in hybrid electric vehicle deploys 15 to 30 current sensors, and uses rotation and position sensors to monitor the traction motor. Accuracy and reliability under interference electromagnetic fields are important attributes for measuring and maintaining performance across electric vehicle power systems.
From the battery to the motor inverter to the supporting components, looking at all the components of the electric vehicle transmission system as a whole, ADI has found countless opportunities to improve the electric vehicle, which can increase its overall energy efficiency and increase the driving range of the electric vehicle. With the penetration of SiC power switching technology into electric vehicle motor inverters, digital isolation has become an important part of it.
Similarly, automotive OEMs can use a multidisciplinary approach to optimize electric vehicles to ensure that all available power detection and control devices work closely together to maximize performance and efficiency. At the same time, they can help eliminate the last obstacle for mainstream consumers to purchase electric vehicles, namely mileage and cost, while helping to create a more environmentally friendly future.
1 Richard Dixon. “MEMS sensors for future cars.” The 4th Annual Automotive Sensors and Electronics Summit, February 2019.