Unlocking Efficiency with Cryogenic Cooling of GaN Traction Inverters

Author: Dr. Temoc Rodriguez, Global Technical Expert in Electrification, Ricardo

As the mobility sector accelerates toward zero-carbon propulsion, hydrogen fuel cell systems (HFCS) are emerging as a cornerstone technology for aviation, marine, and long-haul road transport. Among the most promising innovations in this space is the use of liquid hydrogen (LH₂) not only as a fuel source but also as a cryogenic coolant for electric powertrains. This dual-purpose approach offers transformative potential in system efficiency, packaging, and weight reduction—especially when paired with Gallium Nitride (GaN) semiconductors.

The cryogenic advantage

Liquid hydrogen offers twice the volumetric energy density of compressed hydrogen, making it ideal for long-range applications. However, LH₂ must be evaporated and heated above 0°C before entering the fuel cell. Traditionally, this requires dedicated evaporators and heaters, adding complexity and mass. Ricardo’s approach proposes a paradigm shift: using the electric powertrain itself as the heat source to warm the hydrogen, thereby eliminating the need for separate evaporators.

This integration not only simplifies the system architecture but also enables cryogenic cooling of the inverter and electric machine. Operating GaN-based inverters at cryogenic temperatures unlocks significant performance gains. Below -200°C, GaN devices exhibit up to 10x lower ON resistance compared to operation at junction temperature of 100°C typical of water/glycol cold plates. This translates to either higher current handling or dramatically improved efficiency.

System-level optimisation

The cryogenic cooling concept is deeply interlinked with the overall system efficiency. The LH₂ flow rate is proportional to the power demand and the efficiency of the HFCS. The heat rejection capacity is governed by the latent heat of hydrogen evaporation, which is highest during the liquid-to-gas phase transition. To avoid surpassing the critical heat flux, which would impair heat transfer, the powertrain must operate at efficiencies exceeding 95–98%.

Ricardo’s design replaces traditional evaporators and cooling loops with LH₂/GH₂ heat exchangers embedded within the power electronics and electric machine. This results in substantial mass savings. For a 250kW system, the optimized configuration reduces total mass from 55 kg to 20 kg, a >50% reduction. The inverter alone drops from 10 kg to 3.3 kg, while the eMachine sheds over 8 kg.

Experimental validation

To validate the cryogenic inverter concept, Ricardo developed a calorimetric test rig using liquid nitrogen (LN₂) as a safer proxy for LH₂. The rig immerses GaN devices in LN₂ and measures the boil-off rate to determine heat losses. This method offers superior accuracy over conventional electrical instrumentation, especially at the ultra-high efficiencies targeted.

The inverter features a triple-stacked PCB design:

• Top board: GaN transistors (GS66516T) with soldered heat spreaders and decoupling capacitors.
• Middle board: Gate drivers (SI8274GB4D), thermally isolated from the cold zone.
• Bottom board: Microcontroller and instrumentation.

Thermal insulation and strategic board cutouts minimise heat leakage between layers.

Performance results

The inverter was tested at 400V and up to 9.5kVA with an inductive load. Efficiency, measured via LN₂ boil rate, ranged from 99.4% to 99.8%.

Concept design and engineering challenges

Ricardo’s concept design for a cryogenic inverter incorporates:

• Copper cooling plates with channel geometry
• Thermal insulation and sealing shrouds
• Hermetically sealed LV connectors
• Integrated hydrogen interface
• GaN power modules and DC/3-phase busbars

The design addresses thermo-mechanical stresses from frequent 200K temperature swings. Off-the-shelf components typically operate down to -40°C or -55°C, far above cryogenic levels. Therefore, custom packaging and materials are essential.

Key engineering challenges include:

• Suppressing oscillation mechanisms (dV/dt and dI/dt)
• Validating die paralleling at cryogenic temperatures
• Ensuring reliable thermal cycling performance

Strategic implications

This research positions Ricardo at the forefront of next-generation power electronics for hydrogen mobility. By leveraging cryogenic cooling, the team has effectively skipped two generations of inverter technology, approaching superconducting-like performance without the complexity of superconductors.

The implications extend beyond technical performance. The simplified architecture reduces system mass, volume, and cost—critical factors for commercial viability in aerospace, marine, and heavy-duty transport sectors.

Conclusion

Ricardo’s cryogenic GaN inverter concept represents a bold leap toward ultra-efficient, lightweight, and integrated hydrogen propulsion systems. While challenges remain in materials, packaging, and reliability, the experimental results are compelling. With efficiencies nearing 99.8% and mass reductions over 50%, cryogenic cooling could redefine the future of electric mobility.

As the hydrogen economy matures, innovations like this will be pivotal in delivering clean, scalable, and high-performance solutions across all mobility sectors.

Dr Temoc Rodriguez, FIET, will be presenting the results of our cryogenic research at the CTI Powertrain symposium in Berlin on 2nd and 3rd December 2025