Beyond Silicon: Exploring the Efficiency Gains of SiC Modules
Analysing and comparing the performance of Silicon Carbide (SiC) and Insulated-Gate Bipolar Transistors (IGBTs)
Silicon carbide’s advantages over IGBTs include lower conduction losses. The linear characteristics of SiC and switching frequency are key to its value.
To achieve ever-higher efficiency in power electronics, advancements in semiconductor technology are crucial. This article analyses the performance of two common module types: Silicon Carbide (SiC) and Insulated-Gate Bipolar Transistors (IGBTs).
Electric vehicles (EVs) traction inverters rely on these devices to convert the DC supply from the high-voltage batteries into an AC output required to drive a 3-phase AC electric motor. During most of its time, the EV traction inverter operates at a fraction of the full rated load, which is reached only during acceleration. Therefore, a high efficiency at partial load is an essential requirement in EVs to achieve longer range.
While efficiency at full load may seem comparable, a deeper evaluation reveals significant benefits associated with SiC devices. This is particularly true for applications like electric vehicles (EVs) where operation at partial load is the primary mode.
Efficiency and Switching Loss Comparison: SiC Modules vs. IGBTs
To comprehensively evaluate the efficiency of SiC and IGBT modules, both conduction and switching losses will be analysed.
Conduction Losses: Knee Voltage and RDS(on)
IGBTs exhibit a voltage drop across the collector and emitter terminals (VCE) when conducting current (ON state), which can be used to calculate the power dissipation loss of the IGBT. As the Vce value decreases, the power dissipation loss also decreases. Hence, it is imperative to engineer the IGBT with the lowest possible Vce value.
While this voltage should ideally remain constant with increasing current, in reality, VCE increases more rapidly at lower current levels, creating a “knee” in the characteristic curve. Figure 1 shows a comparison between the output characteristics of two commercial power devices, a SiC MOSFET and a silicon IGBT, at 25°C and 175°C.
Observing the graphs at 25°C, we see that Id/Ic increases linearly with Vds/Vce for the SiC MOSFET. However, due to its threshold voltage (“knee”), the IGBT exhibits a higher Vce compared to the SiC MOSFET’s Vds. This area of advantage at 25°C for the SiC MOSFET is maintained for currents up to 25A. At higher temperatures, the SiC MOSFET still maintains its linearity, while the area of advantage moves up to 35A.
The Vds/Id and Vce/Ic characteristics provide valuable information about the conduction in the ON state (Rds(on)). If the Vds/Vce is low for a specific Id/Ic, it indicates that the on-resistance is also low. This suggests that the steeper the slope of the characteristic curve, the lower the on-resistance.
While SiC MOSFETs maintain a low Rds(on) over a wide voltage range, IGBTs have a low-voltage region where Vce, and therefore conduction losses, are higher than those of SiC MOSFETs. This is a major drawback for applications like electric vehicle (EV) drivetrains where operation often occurs at partial load, which falls below the knee of the VCE curve.
Moreover, some IGBTs feature a built-in "tail current" that flows when the power device is turned off. Since this occurs when the applied VCE is high, it contributes to the losses at low currents. One example to mention here is Nexperia’s SiC MOSFETs, designed to minimise RDS(on) while maintaining stability across temperature variations, making them an ideal choice for high-efficiency applications.
Figure 1: Comparison of IGBT with SiC output characteristics at different temperatures (Source: STMicroelectronics)
Switching Losses: Turn-on/Turn-off Energy
IGBTs exhibit higher turn-on and turn-off energy due to their inherent properties. The carrier injection and removal processes within an IGBT are slower compared to SiC MOSFETs, leading to higher energy dissipation during switching events.
Due to their lower switching losses, SiC modules can operate at higher switching frequencies, enabling a reduction in the size and weight of passive components like transformers and inductors. The lower RDS(on) and improved switching characteristics of SiC devices translate to lower overall losses, particularly in partial load conditions.
Application Challenges and Benefits of SiC Modules in eMobility
The electrification of transportation, or eMobility, presents a significant opportunity to reduce greenhouse gas emissions and dependence on fossil fuels. However, optimizing the performance of EV powertrains requires addressing key challenges.
One of the primary advantages of SiC for eMobility applications lies in its superior efficiency. Unlike silicon (Si) devices, which experience significant switching and conduction losses at high frequencies, SiC exhibits lower electrical resistance and can operate at higher temperatures. This translates to reduced energy losses within the powertrain, particularly during inverter operation where the conversion of direct current (DC) from the battery to alternating current (AC) for the electric motor occurs.
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With the ongoing shift to renewable energy and the electrification of transport and industry, the demand for higher power and greater energy efficiency in electronics is increasing. Wide bandgap technologies including SiC and GaN bring many advantages.
WBG OVERVIEWThe benefit of SiC efficiency in EVs becomes most apparent when considering real-world driving scenarios. During stop-and-go traffic or highway cruising at constant speeds, the powertrain operates at partial load conditions. Traditional Si-based inverters experience increased inefficiency at these partial loads, leading to higher energy consumption and reduced driving range.
Figure 2: ST offers a family of power modules targeting a range of power demands. (Source: ST)
Achieving optimal efficiency at partial loads in SiC designs requires careful consideration of Figures of Merit (FoMs). These are performance metrics used to evaluate the trade-off between various device characteristics. In the context of SiC inverters, key FoMs include on-resistance (Ron), switching losses (Eon, Eoff), and reverse recovery charge (Qrr). Minimizing Ron reduces conduction losses while optimizing Eon, Eoff, and Qrr minimizes switching losses. However, achieving the best possible performance in each FoM can be counterproductive. For example, excessively reducing Ron might come at the expense of higher switching losses.
Effective SiC inverter design involves strategically selecting devices and employing circuit topologies that achieve a balance between these FoMs. This optimization process often involves trade-offs based on the specific application requirements. For eMobility applications, where maximizing partial load efficiency is crucial, engineers might prioritize lower Ron and Eoff values, even if it means slightly higher switching losses compared to a purely performance-focused design.
Market Solutions and Industry Players in SiC Modules
The growing demand for high-efficiency power electronics in eMobility has spurred significant development in the SiC module market. Leading companies, including Nexperia, onsemi and STMicroelectronics are at the forefront of this technological advancement. These companies offer a diverse range of SiC module solutions tailored to address the efficiency demands of various eMobility applications, paving the way for a future of cleaner and more efficient electric transportation.
onsemi recently launched nine EliteSiC Power Integrated Modules (PIMs) that allow for bidirectional charging in DC ultra-fast electric vehicle (EV) chargers and energy storage systems (ESS). These SiC-based devices can significantly enhance system cost-effectiveness through increased efficiency and simplified cooling systems. These solutions can reduce size by up to 40% and weight by up to 52% compared to conventional silicon-based IGBT solutions.
STMicroelectronics offers another strong presence in the SiC module market. An example of a product targeting eMobility applications is the SCT070HU120G3AG, an automotive grade 1,200V, 63mΩ typ., 30A SiC power MOSFET.
Developed with ST’s third-generation SiC MOSFET technology, the device exhibits a low RDS(on) over the entire temperature and high switching performance, making it a suitable choice for eMobility applications like traction inverters, DC/DC converters for EVs, and onboard chargers (OBCs).
To summarize, SiC power technology delivers significant efficiency gains in power conversion and has proved to be reliable in the most demanding applications. Manufacturing capacity has grown considerably in recent years and SiC investment from all the manufacturers mentioned above demonstrates a strong ongoing commitment to this technology.
Check out our other technical resources to learn more about power solutions available from Avnet Silica or contact your local representative.
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