A new report from the RHODaS project provides the analysis and validation of the switching behaviour of the 150-kW hybrid T-Type power converter, a key building block of the RHODaS Integrated Motor Drive.
This research forms part of the integration, testing, and technical validation of the project’s core technologies. Its primary objective is to assess the feasibility and robustness of combining Wide Band Gap (WBG) semiconductors—Gallium Nitride (GaN) and Silicon Carbide (SiC)—in a high-voltage, high-power inverter architecture suitable for heavy-duty electric vehicles.
From Initial Design to Robust Redesign
The first iteration of the converter aimed to exploit the fast-switching capabilities of GaN devices while minimising parasitic effects through a careful Printed Circuit Board (PCB) layout and current path optimisation. Although early switching tests showed promising results, the initial design ultimately failed during inverter operation. The root cause was traced to the limitations of pre-series GaN devices with integrated drivers, particularly the lack of a permanent negative gate-off voltage, which increased sensitivity to parasitic turn-on under high-voltage conditions.
These findings led to a comprehensive redesign of the GaN power stage. The revised architecture replaced the pre-series devices with commercially mature GaN Systems transistors and independent gate drivers, enabling the application of a negative turn-off voltage. While this redesign introduced minor trade-offs—such as slightly higher conduction losses—it significantly improved robustness and operational safety.
Switching Tests and High-Voltage Validation
Extensive experimental testing was carried out to characterise switching behaviour and validate safe operation. Double Pulse Tests, endurance tests, and inverter-level experiments were performed across a wide voltage range, culminating in stable operation at DC link voltages up to 1000 V. The tests demonstrated that controlling parasitic inductance and deliberately limiting switching rise times to the 70–100 ns range was essential to keeping voltage overshoots within the safe operating limits of the GaN devices.
Thermal imaging and waveform measurements further confirmed balanced current sharing between parallel GaN devices and stable behaviour under both two-level and three-level operation modes. While some measurement constraints affected the precision of high-voltage overshoot data, the overall results confirmed the reliability of the redesigned power stage under demanding operating conditions representative of heavy-duty applications.
Control Strategies and Digital Implementation
In parallel with hardware validation, the report documents the development and simulation of advanced modulation strategies, including Carrier-Based Pulse-width Modulation (PWM) for three-level operation and Sinusoidal PWM for two-level operation. These control techniques were implemented as an IP block on the System-on-Chip Field-Programmable Gate Array (FPGA) platform used for the inverter control.
Although time constraints prevented full experimental validation of all custom modulation strategies on the high-power test bench, the two-level modulation was successfully applied, and high-power characterisation was completed using an alternative Space Vector PWM approach. Simulation results confirmed that the advanced strategies are technically sound and aligned with previous validations on lower-power RHODaS prototypes.
Contribution to Heavy-Duty Electric Mobility
The report represents a significant milestone for RHODaS, demonstrating that high-voltage hybrid T-Type converters based on GaN and SiC technologies can be operated reliably when supported by appropriate driver design and switching strategies. The work provides valuable insights into the practical limits, design trade-offs, and validation requirements associated with next-generation power electronics for electric trucks and other heavy-duty vehicles.
By addressing critical challenges such as parasitic turn-on, voltage overshoot, and high-frequency switching stability, the results contribute directly to the broader objective of enabling more efficient, compact, and reliable electric powertrains—supporting Europe’s transition toward zero-emission heavy-duty transport.
Read the full report here >