Freight transportation, aviation, marine applications, and off-road and industrial vehicles contribute more than 55%* of the total greenhouse gas emissions from internal combustion engines. Electrifying these powertrains could make a significant difference in the fight against climate change.
However, the large quantity of units in action globally is associated with several diverse types and platforms.
Engineers need all the help they can get to develop new electric drives quickly. The different types vary more than physically in terms of size, shape, and weight constraints. The functional and electrical safety requirements and environmental conditions are highly dependent on applications and geographical markets.
On the other hand, the competitive situation between manufacturers demands a fast time to market.
For performance and reliability, silicon carbide (SiC) is the power semiconductor technology of choice. Range anxiety is one concern that has moved the passenger car market away from silicon toward more energy-efficient SiC vehicles — such as buses, which operate on known routes, and off-road vehicles that cover relatively short distances.
SiC’s high-voltage capabilities permit faster charging and shorter turnaround times for these applications. It can also operate at high temperatures, maximizing reliability. Modules require fewer SiC devices to share the duty, and SiC MOSFETs can be smaller in relation to breakdown voltage than their silicon counterparts. This means savings in module size are also possible.
However, SiC power devices are not a direct drop-in replacement for silicon MOSFETs or IGBTs. Proper gate control to ensure fast and smooth switching transitions at high frequencies is not straightforward. Further challenges include integrating hardware components, especially the inverter and the intelligent power module, and setting up and calibrating the motor-control software.
Accelerated development
To help overcome the development challenges and accelerate time to market for reliable SiC power modules, SiC traction-inverter platforms and reference designs are now available to build systems that operate from battery voltages up to 850 V. The hardware is typically modular and scalable, so designs with various power ratings can be produced.
The reference design is intended to solve aspects of the inverter that are notoriously challenging and time-consuming to get right. For example, core components can include a 3-phase 1200V intelligent power module (IPM) — which, when integrated with a gate driver optimized for SiC applications, can withstand elevated temperatures. At the same time, the driver can provide peak gate currents in excess of 10A and operate in ambient temperatures up to 125° C.
Since the SiC gate driver is already integrated with the power module in this application, users could start their projects with a solution that has been validated and optimized for fast switching speed and low losses.
This solution would contain robust protections for the power stages while maintaining immunity to high dI/dt and dV/dt effects. As a result, the number of iterations required to fine-tune module performance and ensure proper thermal management would be significantly reduced.
Additionally, it could be enhanced by hardware in the reference design such as dc– and phase-current sensors, EMI filtering, a compact liquid cooler, and a high-density dc-link capacitor. In this instance, the dc-link capacitor must be developed specifically for the inverter platform and cover a range of voltage and current options.
Software control and calibration
To complete the reference design, there must also be an e-motor control board with an application-specific processor and software, both pre-certified to the ISO 26262 standard and ASIL level D for functional safety. The motor control software must be capable of making various adjustments without compromising the functional safety certification, ensuring flexibility to optimize the motor behavior as required in the end-use case. Users could then run their own custom application software.
The reference design could also ensure real-time performance to the highest desired motor speed, combining the software-based flexibility of a conventional processor with hardware acceleration. By including the control board, the reference design could help users avoid the typical mechanical and electrical integration challenges when bringing the control board and IPM together.
Likewise, users would benefit from flexible and fully customizable control software that can match any electric powertrain configuration and power range — made possible through the ability to modify configuration and calibration parameters either offline or in real-time. It would be a bonus if the software were supported by a debugging and calibration framework, including a graphic interface. Access to a reliable development and calibration framework would enable developers to shorten the time required to optimize the motor control software.
Figure 1. SiC inverter performance up to 260kW @ 13500 rpm, showing efficiency across speed and torque range.
SiC inverter performance
If the performance of a SiC-based inverter is compared with a silicon-IGBT inverter tested under similar real-world conditions (see Figures 1 and 2), it’s possible to see how the greater efficiency of a SiC-based drive ensures a superior user experience.
Figure 2. A silicon-IGBT inverter up to 120kW @ 11500 rpm can impair torque capability (see cross-points) under comparable conditions.
With increasing speed and load demand, the motor torque from an IGBT-based drive significantly reduces because of its lower efficiency. The self-heating associated with the energy losses in the device cannot be dissipated without greatly increased cooling. In contrast, a highly efficient SiC-based drive can deliver closer to the maximum torque over a much wider speed and load range.
The set-up and calibration
Setting up and calibrating a SiC-based drive involves four key steps:
1. Configuration of software parameters
2. Inverter hardware set-up
3. Motor-control system calibration
4. Advanced system optimization
Using such an approach would allow tuning of the reference design to achieve efficiency greater than 99%, operating on a 700 V bus up to 4000 rpm (Figure 3).
Figure 3. Setting up and calibrating a SiC-based drive can allow tuning of the reference design to achieve efficiency greater than 99%.
Conclusion
The bus, truck, and agricultural vehicle sectors present an ideal opportunity for electrification, reducing the environmental emissions burden. Silicon carbide power technology can help maximize reliability and vehicle duty cycle, delivering high efficiency compared to silicon IGBTs or MOSFETs.
The complexities of designing with SiC, and the imperative to ensure a fast time to market, demand a flexible development platform to help designers satisfy the targets for various vehicle categories and types. A complete reference design that offers solutions to the SiC-related design challenges, while allowing flexibility and scalability to address different power ratings and battery voltages to handle small to large vehicles, effectively minimizes design risks and helps accelerate time to market.
References
* Source: NESTE: Towards sustainable mobility | April 2023
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