SiC vs. GaN for EV Applications: Choosing the Right Material

SiC vs. GaN in EV Powertrains: Voltage and Frequency Dictate 80% of the Choice

The adoption of 800V battery systems is accelerating. Porsche Taycan, Hyundai IONIQ 6, Kia EV6 – what they have in common is the transition from traditional 400V architectures to double the voltage band to balance shorter charging times with improved driving efficiency. This trend is directly impacting power device selection. The question of "SiC or GaN" is no longer a matter of technical curiosity but a choice that must be made from both design and procurement perspectives.

So, how do we decide? In conclusion, the majority of EV applications will be segmented by a combination of "voltage level" and "operating frequency." Understanding these two axes makes the selection reasons for each application, discussed later, naturally apparent.

Main Inverters Remain a SiC Monopolyn – and It's Not Just About Withstand Voltage

The main inverter, at the heart of an EV's drivetrain, converts DC power from the battery to AC for motor drive. 400V systems require a breakdown voltage of approximately 650-750V, while 800V systems demand around 1200V. The mainstream GaN devices currently available on the market are in the 650V breakdown voltage class, with many 1200V and above products still in the research and development stages. In contrast, SiC MOSFETs are commercially available across a range from 650V to 1700V, giving them an advantage in terms of voltage coverage.

However, withstand voltage is not the only reason to choose SiC. Main inverters handle high currents, typically tens to hundreds of amperes, and protecting the device during a short circuit is a significant design challenge. SiC MOSFETs have a metric called "short-circuit withstand time" (SCWT, or Tsc), which indicates the time margin before the device fails after a load short circuit occurs. The protection circuit must operate within this margin. For example, Microchip's 700V/1200V SiC MOSFETs specify a typical SCWT of 3µs under specific conditions in their datasheets.

While 3µs may seem ample at first glance, it imposes design constraints when considering the parasitic inductance of the gate-drain wiring and the response delay of the protection IC. Furthermore, this withstand capability varies with operating conditions – the short-circuit withstand time changes with variations in drain voltage, gate voltage, and junction temperature. The tendency for withstand capability to increase at higher temperatures is due to increased on-resistance (RDS(on)) limiting the saturation current.

Additionally, SiC devices have higher current density and smaller die sizes compared to silicon, leading to faster temperature rise. Engineers with extensive design experience emphasize that this necessitates a different approach to protection circuit response time design than with silicon IGBTs.

Automotive OBC & DC-DC Converters – Where GaN Shines

In contrast to main inverters, automotive onboard chargers (OBCs) and DC-DC converters are areas where GaN tends to exhibit its strengths. The reason lies in the operating frequency.

OBCs are responsible for charging the battery from an AC power source, and to achieve smaller size and weight, it is desirable to set a high switching frequency. This is because increasing the switching frequency allows for the miniaturization of transformers and passive components. GaN devices have lower switching losses compared to silicon and SiC, allowing them to maintain high efficiency in operating ranges from hundreds of kHz to over 1MHz.

In terms of voltage levels, automotive OBCs typically have output voltages around 400V even in 400V battery systems, making 650V GaN products well-suited. DC-DC converters often handle even lower voltages (12V-48V), and designs in this range are also common with 650V or below.

This segmentation is not fixed, and as research into higher voltage GaN progresses, the overlapping areas are likely to expand in the future. Current selections should be considered in light of a 5-year technology roadmap.

"Either Choice Results in the Same Loss" Isn't True – Numbers Show the Difference

While it's common to hear that "both SiC and GaN have low losses," this alone is insufficient for decision-making. What truly matters is a specific comparison of "where each excels under which operating conditions."

Switching losses increase proportionally with operating frequency. Compared to silicon IGBTs, SiC significantly reduces switching losses. However, GaN may exhibit even lower loss characteristics, particularly in the high-frequency range above hundreds of kHz. On the other hand, conduction losses due to on-resistance (RDS(on)) can favor SiC under high-current, low-frequency conditions.

The graph below illustrates typical operating frequency ranges for EV-related applications. Main inverters typically operate at 10-20kHz, OBCs at 100-400kHz, DC-DC converters at 50-500kHz, and wireless charging at 85kHz to several MHz. Overlaying this frequency map with voltage requirements reveals the contours of device selection.

This graph highlights that the operating frequency of main inverters is one-tenth to one-twentieth that of OBCs or DC-DC converters. This structure, observable from the numbers, indicates that SiC's on-resistance characteristics are advantageous in the low-frequency, high-current domain, while GaN's switching characteristics directly impact efficiency in the high-frequency, medium-current domain.

SiC Technology Advancements: Overcoming the Short-Circuit Withstand Time vs. On-Resistance Trade-off

A design challenge that cannot be avoided when adopting SiC devices is the "trade-off between short-circuit withstand time and on-resistance." Reducing on-resistance increases current density per chip area, leading to more rapid heating during short circuits. Conversely, prioritizing short-circuit withstand time tends to degrade the on-resistance, affecting conduction losses.

Device manufacturers are addressing this issue through structural improvements. Mitsubishi Electric has significantly enhanced short-circuit withstand capability by introducing a p-type protective layer in trench-type SiC MOSFETs. Rohm's 4th-generation SiC MOSFETs aim to achieve both low on-resistance (RonA) and high short-circuit withstand capability through their proprietary device structure.

Approaches are also being taken on the protection circuit side. A representative method is the DESAT (desaturation) function. This mechanism monitors the drain-source voltage (VDS) in the ON state and activates the protection circuit to turn off the power transistor when overcurrent is detected, widely used for SiC MOSFET short-circuit protection. In protection circuit design, the DESAT trigger threshold (VDESAT), DESAT current (IDESAT), and short-circuit blanking time are crucial design variables.

The combination of these device structure improvements and refined protection circuit design ensures reliability in actual systems – this structure can serve as a reference for evaluating SiC selection.

What to Verify in Practice – The Intersection of Device Selection and Procurement Evaluation

Once the technical direction is set, it's beneficial to organize the points to be confirmed from both design and procurement perspectives.

Regarding short-circuit withstand time, verify the conditions (drain voltage, gate voltage, temperature) under which the datasheet values were measured. Even with the same 3µs, the margin in actual operation can vary significantly depending on the measurement conditions. While withstand capability tends to improve at higher temperatures, confirming temperature margin design with actual equipment evaluation increases the accuracy of judgment.

onsemi offers a lineup covering SiC MOSFETs, SiC diodes, and SiC modules from 650V to 1700V, providing a wide range of product selection options according to voltage class and application. Whether dependence on a single vendor poses a risk should be evaluated in conjunction with supply stability during mass production and confirmation of alternative products.

This graph illustrates the correspondence between applications and breakdown voltage. Main inverters for 800V battery systems require 1200V breakdown voltage, while DC-DC converters and 400V system OBCs are covered by the 650-750V class. This breakdown voltage map serves as one criterion defining the boundary between areas where GaN is applicable and where SiC is necessary.

For GaN evaluation, gate drive voltage handling is a critical point. Many GaN devices have a narrower gate voltage margin compared to silicon, making gate driver selection and wiring design directly impact performance and reliability. Specifically, suppressing gate ringing during high-frequency operation is a recurring topic in OBC design.

The selection of power devices for EVs does not follow a simple "SiC or GaN" dichotomy but rather a structure where the optimal solution changes based on the combination of voltage, frequency, and application. The current segmentation, with SiC for main inverters and GaN for OBCs, is a result of the breakdown voltage, loss, and protection design characteristics of both materials. How this segmentation will shift with the advancement of higher voltage GaN is a point to continue following alongside the design direction of next-generation EV architectures.