Engineers need all the help they can get to develop new electric drives quickly. The differences go far beyond their physical characteristics, such as size, shape, and weight limitations. Functional and electrical safety requirements, as well as environmental conditions, depend heavily on the application and geographic location. Furthermore, competition among manufacturers demands a short time to market.

In terms of performance and reliability, silicon carbide (SiC) is the best power semiconductor technology. Concerns about range have driven the automotive market away from silicon and toward SiC, which offers greater energy efficiency in vehicles such as buses traveling on familiar routes and heavy vehicles covering relatively short distances. For these applications, SiC's ability to operate at high voltages allows for faster charging, while its ability to operate at high temperatures helps maximize reliability. Furthermore, modules require fewer SiC devices, and SiC MOSFETs can be smaller, in terms of breakdown voltage, than their silicon counterparts. This opens up the possibility of reducing module size.

However, SiC power devices cannot be directly replaced by silicon MOSFETs or IGBTs. Establishing proper gate control to ensure fast and smooth transmissions during high-frequency switching is not straightforward. Furthermore, other challenges exist, such as integrating hardware components, especially the inverter and the smart power module, as well as configuring and calibrating the motor control software.

Faster Development:
CISSOID has developed a SiC traction inverter platform and reference design to overcome these challenges and accelerate the commercialization of robust and reliable SiC power modules (Figure 1). Drive manufacturers can use these to build systems capable of operating with battery voltages up to 850V. The hardware is modular and adaptable to designs with various power levels.


Figure 1. Reference design of the CISSOID high-voltage SiC inverter.

The reference design addresses aspects of the inverter that are difficult and typically time-consuming. Its main components include a 1200V three-phase intelligent power module (IPM) integrated with a gate driver optimized for SiC applications and designed to withstand high temperatures (Figure 2). The driver supplies a maximum current exceeding 10A and can operate in ambient temperatures up to 125°C.


Figure 2. The Intelligent Power Module (IPM) consists of a SiC power stage with pin fin cooling and the gate driver installed on top [2].

Because the SiC gate driver is integrated with the power module, users can start their projects with a solution that has already been validated and optimized for fast switching speeds and low losses. It is immune to the effects of high dI/dt and dV/dt levels and includes robust protections for the power stages. As a result, the number of iterations required to fine-tune module performance and ensure proper thermal management is significantly reduced. The reference design hardware also includes DC and phase current sensors, EMI filtering, a small liquid cooler, and a high-density DC link capacitor. These capacitors have been specially developed for the inverter platform and offer a wide voltage and current range.

Software Control and Calibration:
To complete the reference design, there is also an electric motor control board with an application-specific processor and software, both pre-certified according to ISO 26262, ASIL Level D for functional safety. The motor control software allows for numerous adjustments without affecting the functional safety certification, thus providing flexibility to optimize motor performance according to the end-user application requirements. Users can run their own application software on top of this.

The control board is based on the Silicon Mobility OLEA® T222 field-programmable control unit (FPCU). This allows the reference design to combine the software-based flexibility of a conventional processor with hardware acceleration to ensure real-time performance up to the motor's maximum desired speed. By incorporating the control board, the reference design also helps users avoid the mechanical and electrical integration issues that typically arise when connecting the control board and the smart power module.

OLEA® APP INVERTER is a flexible and fully customizable control software (Figure 3) that adapts to any type of electric drive and any power range thanks to a set of configuration and calibration parameters that can be modified offline or in real time. The software also comes with a debugging and calibration environment that includes a graphical interface.


Figure 3. The OLEA® APP INVERTER control software provides numerous functions to regulate and optimize motor operation.

Thanks to OLEA® COMPOSER, developers can shorten the time required to optimize motor control software (see box for more information on the four-step process to accomplish this).

SiC Inverter Performance:
After configuring the parameters, the motor can be tested and the efficiency of the inverter-motor assembly assigned. Figures 4a and 4b compare the performance of the SiC-based inverter with an IGBT-based inverter tested under similar real-time conditions.



Figure 4a. Performance of the SiC inverter up to 260kW at 13500rpm showing efficiency across the entire speed and torque range.



Figure 4b. The silicon-based inverter up to 120kW at 11,500rpm has poor torque capability (see crossover points) under comparable conditions.
These performance graphs show how the superior efficiency of the SiC-based drive ensures a better user experience. With increasing speed and load demands, the motor's torque is significantly reduced due to its lower efficiency when operating with an IGBT-based drive; the heat generated by energy losses within the device itself cannot be dissipated without a substantial increase in cooling. In contrast, the high-efficiency SiC-based drive can approach maximum torque over a much wider speed and load range.

Drive Configuration and Calibration:
The OLEA® COMPOSER tool suite helps users configure the motor to meet customer specifications and facilitates the calibration of parameters such as voltage, rated power, speed, and torque to achieve the optimal operating range. Once completed, the inverter-motor efficiency can be assigned.
Adjustment and calibration are performed in four steps:
Step 1: Software Parameter Configuration
• Configure the OLEA® APP INVERTER software according to the electric motor parameters.
Step 2: Inverter Hardware Adjustment
• Adjust the electric motor, including components such as the resolver and temperature sensors. Connect the electric vehicle's electronic control unit (ECU) and test bench interfaces (e.g., CAN, safety), power, and cooling.
• Verify the inverter's safety interfaces with the test bench.
Step 3: Motor Control System Calibration
• Open-loop mode: Calibrate the current and voltage sensor signal conditioning chains with the OLEA® T222 FPCU.
• Partial open-loop mode: calibration of the position sensor offset, whether using resolvers or inductive sensors.
• Current closed-loop mode: PI controller adjustment of ID and IQ vectors for field-oriented control (FOC).
• Torque control mode: fine-tuning of the torque control loop to improve accuracy and dynamic response.
• Speed ​​closed-loop mode: calibration of the speed governor.
Step 4: Advanced System Optimization
• Switching frequency selection: adjustment of the switching frequency depending on the speed and phase currents.
• Dead-time compensation: adjustment of the dead-time compensation algorithm to minimize phase current harmonics.
• Flux reduction: optimization of the ID/IQ setpoints for efficient operation in the maximum torque-by-voltage (MTPV) region.
• SVPWM/DPWM: definition of the threshold between SVPWM (space vector pulse-width modulation) and DPWM (discontinuous pulse-width modulation) to provide greater efficiency at high speeds.
This method allows adjusting the reference design to achieve an efficiency greater than 99% with a 700V bus up to 4000rpm as shown below:


In conclusion
, buses, trucks, and agricultural vehicles represent a significant opportunity for electrification and the reduction of environmentally damaging emissions. Silicon carbide-based power technology can help maximize vehicle reliability and lifespan, while also offering greater efficiency compared to silicon IGBTs or MOSFETs. The complexity of SiC design and the need to accelerate time to market demand a flexible development platform to help designers meet objectives for various vehicle types and categories. A comprehensive reference design that addresses the main challenges of SiC design, while also providing flexibility and scalability to accommodate different battery power and voltages in small and large vehicles, effectively minimizes design risks and accelerates time to market.

References
[1] Source: NESTE: Towards sustainable mobility / April 2023: https://journeytozerostories.neste.com/transportation/towards-sustainable-mobility#885e75ed
[2] CISSOID SiC Intelligent Power Modules: https://www.cissoid.com/sic-power-modules/

About Cissoid - www.cissoid.comCISSOID
is a leader in power semiconductor solutions, supplying multi-phase high-current power switching modules, motor control systems, and software across a broad power range. Our solutions range from systems-on-a-chip to complete inverter reference designs based on silicon carbide (SiC) and gallium nitride (GaN) for a growing number of electromobility and high-power applications. Our leadership and expertise in semiconductors for extremely high temperatures, high-reliability solutions, and packaging enable us to address the challenges of ever-increasing power density in diverse markets.

Author: Pierre Delatte, CTO, CISSOID