Modular and powerful SiC traction inverters Electrification beyond the car

Related Vendors

FAULHABER Drive Systems

Wuxi InfiMotion Technology Co., Ltd.

In addition to motor vehicles, there are other significant emitters involved in greenhouse gas emissions. Over 55 percent of the emissions from internal combustion engines do not fall back on cars. The electrification of these sectors could play a significant role in the fight against climate change.

Pierre Delatte is CTO at Cissoid

Freight transport, off-road and industrial vehicles, shipping, and aviation contribute more than 55 percent [1] to the total greenhouse gas emissions from internal combustion engines. The electrification of these drivetrains could make a big difference in the fight against climate change. However, the large number of drives in use worldwide is associated with a great variety of types and platforms.

To develop new electric drives quickly, engineers need comprehensive support. The differences are not only physical in terms of size, shape, and weight. The requirements for functional and electrical safety, as well as environmental conditions, are highly dependent on the applications and geographical markets, but also take into account that the competitive situation requires rapid market introduction.

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For greater performance and reliability, silicon carbide (SiC) is the power semiconductor technology of choice. Range anxiety is one reason why the car market has moved away from silicon and towards more energy-efficient SiC. Vehicles such as buses operate on known routes, while off-road vehicles cover relatively short distances. In these applications, the high operating voltage of SiC allows faster charging for shorter turnaround times, and the ability to operate at high temperatures contributes to maximum reliability. SiC modules also require fewer components to share the load. In addition, SiC MOSFETs can be smaller relative to their breakdown voltage than their silicon counterparts. Therefore, the modules are also smaller.

However, SiC power components are not a direct replacement for silicon MOSFETs or IGBTs. It is not easy to control a gate in a way that ensures fast and smooth switching operations at high frequencies. Additional challenges include the integration of hardware components, particularly the inverter and intelligent power module (IPM), and the setting up and calibrating of the motor control software.

Faster development

To overcome development challenges and accelerate the market launch of robust and reliable SiC power modules (Figure 1), Cissoid provides a SiC traction inverter platform and reference design. Drive manufacturers can use it to create systems that can operate at battery voltages up to 850 V. The hardware is modular and scalable, in order to create designs with different rated powers.

The reference design addresses aspects of the inverter that are known to be difficult and time-consuming to correct. Key components include a 3-phase 1,200 V IPM (Intelligent Power Module) already equipped with a gate driver optimized for SiC applications, designed for high temperatures (Figure 2). The driver delivers gate peak currents of over 10 A and can operate at ambient temperatures up to 125 °C.

Since the SiC gate driver is already integrated into the power module, users can start directly with their own solution strategy. The design is already validated and optimized for fast switching speeds and low losses. It is immune to high di/dt and du/dt effects and contains robust protection devices for the power stages. This significantly reduces the number of runs required to fine-tune module performance and ensure adequate thermal management. The additional hardware of the reference design includes DC and phase current sensors, EMI filters, a compact liquid cooler, and a compact DC link capacitor. The latter was specifically developed for the inverter platform and covers a wide range of voltage and current options.

Software control and calibration

To complete the reference design, an electric motor control board with an application-specific processor and software is also available, both pre-certified for functional safety according to ISO 26262 (ASIL level D). The motor control software allows numerous adjustments without affecting the certification for functional safety. This allows the motor's behavior to be flexibly optimized according to the requirements. Users can thus run their own application software.

The control board was developed around the field programmable control unit (FPCU) Olea T222 from Silicon Mobility. 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 highest desired engine speed. The inclusion of the control board also helps the reference design to avoid the usual mechanical and electrical challenges of integration when bringing together the control board and the IPM.

The Olea App for Inverters is a flexible and customizable control software (Figure 3). It allows configuration and calibration parameters to be changed offline or in real time. In this way, the software can be adapted to any electric drive configuration and power range. The software also has a debugging and calibration framework including a graphical interface. With the Olea Composer, developers can shorten the time needed to optimize the motor control software (see box for more information about this four-stage process).

Performance of the SiC inverter

Once the parameters are set up, the motor can be tested and the efficiency of the converter-motor combination can be determined. Images 4a and 4b show the comparison of the performance of the SiC-based inverter with that of a silicon IGBT inverter, which was tested under similar real conditions.

With increasing speed and load demand, the motor torque decreases significantly during operation with the IGBT-based drive due to its lower efficiency. The self-heating associated with the energy losses in the module cannot be dissipated without greatly increased cooling. In contrast, the highly efficient SiC-based drive approaches the maximum torque over a much wider speed and load range.

Setting up and calibrating the drive

The Olea Composer tool suite helps the user get the motor running according to customer specifications. It assists in calibrating parameters such as voltage, rated power, speed, and torque to achieve an optimal working range. Once this is completed, the efficiency of the inverter and motor can be represented in the real test environment.

Setting up and calibrating in four steps:

Step 1 - Configuration of software parameters:

Configuration of the Olea App Inverter software according to the parameters of the electric motor.

Step 2 - Setting up the inverter hardware:

Setup of the electric motor including components such as resolver and temperature sensors. Connection of the electronic control unit (ECU) and the inverter's interfaces (e.g. CAN, safety) as well as the power supply and cooling interfaces. Checking the inverter's safety interfaces with the test stand.

Step 3 - Calibration of the control system:

Open-Loop Mode: Calibration of the current and voltage sensor signal conditioning chains through the Olea T222 FPCU.

Partial Open-Loop Mode: Calibration of the position sensor offset, regardless of whether resolvers or inductive sensors are used.

Current Control Mode: internal PI controller tuning of ID and IQ vectors for field-oriented control.

Torque Control: Fine-tuning of the torque control loop for precision and dynamic behavior. Speed Control Mode: Calibration of the speed controller.

Step 4 - Advanced System Optimization:

Scaling the switching frequency: Adjusting the switching frequency depending on speed and phase currents.

Dead time compensation: Adjusting the dead time compensation algorithm to minimize phase current harmonics.

Flux weakening: Optimizing the id/iq set values for efficient operation in the area of maximum torque per voltage (MTPV).

SVPWM/DPWM: Definition of the threshold between space vector pulse width modulation (SVPWM) and discontinuous pulse width modulation (DPWM), which allows higher efficiency at high speeds. With this approach, the reference design can be tuned to achieve over 99 percent efficiency at a 700 V bus and up to 4,000 rpm, as shown in Figure 5.

Conclusion

The bus, truck and agricultural vehicle sectors offer an excellent opportunity for electrification and reduction of emissions. SiC power components contribute to maximizing the reliability and operating time of vehicles and achieving higher efficiency in the usage area compared to silicon IGBTs or MOSFETs.

The complexity of developing with SiC and the need to ensure a quick market launch require a flexible development platform that helps engineers meet the goals for various vehicle categories and types.

A complete reference design that provides solutions to the key challenges of SiC design while allowing flexibility and scalability for various power levels and battery voltages for small to large vehicles, minimizes design risks and helps accelerate market launch. (mr)