Denso boost converter for the FCEV Toyota Mirai II: the SiC technology screened by Yole SystemPlus

The considerable impetus to move towards clean mobility has fostered the emergence of new technologies. In order to reach higher levels of power and faster charging features, electrical, hybrid, and fuel cell vehicle manufacturers are shifting gradually from traditional Si MOSFET to wide-bandgap technologies such as silicon carbide (SiC). Although exhibiting higher power density and heat resistance, the cost factor slowed SiC adoption, which had remained in the development stage for almost 20 years. In 2017, this technology was propelled to the forefront of the semiconductor scene with the launch of Tesla’s 400 V EV, equipped with a SiC inverter. Since then, other manufacturers have taken advantage of the superior properties of this material, like BYD and Hyundai with the launch of 800 V EVs, and Toyota with the Mirai II Fuel Cell Electric Vehicle (FCEV) equipped with Denso’s boost DC/DC converter. As one of the first converter integrating SiC power modules, Yole SystemPlus could not miss the opportunity to analyze what was behind this innovative system.

The emergence of SiC in xEV power electronics

With a thermal conductivity three times higher than silicon and, therefore, a superior heat resistance, SiC is the composite of choice for xEV under-the-hood applications. In addition, its intrinsic properties allow SiC power modules to have higher power density and foster the design of more compact and powerful systems. As a result, SiC is gradually replacing Si in power modules for inverters, onboard chargers (OBC), and DC/DC converters. Despite its cost (still three times that of Si), these modules will account for 47% of the semiconductor power device EV market by 2027, as stressed by Yole Intelligence in its recent report (Power Electronics for Automotive Focus Passenger and Light Commercial Vehicles, 2022).

Denso Fuel Cell Converter: what’s in the box?

With only two models currently for sale or lease (Toyota Mirai and Hyundai Nexo), FCEV is an exotic product in the xEV portfolio. Cost, hydrogen filling infrastructure deployment, and fuel cell durability are just some of the challenges to overcome before FCEVs become a successful alternative for consumers. In terms of performance, the challenge remains to design a fuel cell system with higher conversion efficiency, reduced size, and increased power density to operate high-voltage motors. For the first time, in 2021, Toyota equipped a vehicle (Mirai II) with a fuel cell converter integrating several hybrid SiC power modules. This boost converter, developed by Denso, aims to step up the output voltage of the fuel cell stack to the DC link voltage level of the traction motor, up to 650 V. In its recent teardown track (Denso Fuel Cell Converter for Toyota Mirai II, 2022), Yole SystemPlus identified, measured, and cost-evaluated 1660 elements of this innovative system.

The main characteristics of Denso’s boost converter are given in the table below. In addition to the boost converter, other systems, such as the DC/AC inverter stage used for the auxiliary pumps using 600 V / 30 A inverter bridges from Mitsubishi Electric, as well as HV contactors and relay, are included. The power control unit that optimally controls both fuel cell stack output and drive battery charging and discharging was the subject of another teardown track (Denso Power Control Unit (Inverter & Converter) for Toyota Mirai II, 2022).

The teardown of the boost converter revealed that the power modules are integrated into a double-sided cooling structure specific to Denso. In this cooling design, already used in Denso’s Si converter for the Toyota Mirai I, coolers and power cards are stacked alternately, minimizing the system size.    

Yole SystemPlus further investigated SiC power modules in the report – Denso SiC Power Module in the Toyota Mirai II, 2022. The analysis highlights that Denso’s power device integrates eight hybrid SiC MOSFET power modules and eight hybrid SiC diode power modules and adopts the same configuration as the Mirai I Si converter.  

The packaging of the module is also unchanged, and it is interesting to note that the overall size of the module has not benefited from the space-saving resulting from the SiC die’s reduced footprint.

Coming back to the complete system, Japanese players rule the design of this DC/DC boost converter and confirm their traditional strategy of favoring domestic trade links.

With the in-depth analysis of Denso’s boost converter, Yole SystemPlus estimated that the 16 hybrid SiC power modules (SiC MOSFETs and SiC diodes with Si Diode) represent more than 30% of the total manufacturing cost of the Fuel Cell Converter.

Further development steps scenario

This new generation of boost converters integrating SiC is seen as a first step to reaching higher levels of efficiency in FCEVs. The next steps could include the development of new forms of module design, die attachment, and bonding processes, to benefit fully from SiC’s properties and die size. What’s more, how can we not wonder how the next generation Mirai would perform with SiC also integrated into Denso’s power control unit?


Wilfried Theron is the Director of the Electronic Systems Department and Quality Manager at Yole SystemPlus, part of Yole Group (Yole).
Co-founder of Yole SystemPlus and based on his over 20 years’ experience in modeling the manufacturing costs of electronics systems and components, Wilfried developed and regularly improves the step-by-step methodology used in teardown and cost analyses.
Wilfried manages a team of specialized analysts to produce teardowns, reverse engineering, and reverse costing analyses for the Automotive Track online service and specific custom projects requested by our international customers in Europe, America, and Asia.
Wilfried holds a master’s degree in Microelectronics from the University of Nantes (France).

Elena Barbarini - System Plus Consulting

Elena Barbarini is Director, Semiconductor Devices Department, at Yole SystemPlus, part of Yole Group (Yole). Based on extensive experience in the semiconductor industry, Elena manages the production of reverse engineering & costing reports and custom projects through a dedicated team of analysts.  The semiconductor device experts daily investigate innovative semiconductor manufacturing processes to reveal the technology choices made by the leading semiconductor companies, determine the process flows, evaluate manufacturing costs, and describe the related supply chain.
Elena is responsible for the development of the industrial and technical expertise of the semiconductor devices team. At the same time, Elena uses her industrial and technical knowledge to define Yole SystemPlus’s product strategy. She also collaborates with the laboratory team to identify objectives and set up relevant methodologies to implement comprehensive physical analyses.
Elena manages the business relationship with key Yole SystemPlus customers, identifying their needs, responding to their queries, and presenting results.
Prior to Yole SystemPlus, Elena had relevant experience at Alten (France), Osai (Italy), Vishay (Italy), and IBN (Singapore), where she developed significant competency and know-how in semiconductor manufacturing and related equipment, business development, and activity diversification.
With a power electronics background, Elena authored numerous reverse engineering & costing reports and presented numerous times at key international conferences, trade shows, and webcasts. She also authored many scientific papers and articles for power electronics media.
Elena holds a Ph.D. in Power Electronics Engineering and a master’s degree in Micro & Nanotechnologies with a focus on integrated systems from Politecnico di Torino (Italy). She also graduated from Ecole Polytechnique Fédérale de Lausanne (EPFL) (Lausanne, Switzerland) and Grenoble’s Polytechnic Institute (France).