SiC推动汽车电子进一步向前

简介

混合动力汽车/纯电动汽车的发展对功率电子提出了大量需求。目前，绝大多数功率电子都是基于硅材料。然而，硅材料的使用达到了极限，而碳化硅（SiC）的出现则提供了更多的可能性。SiC能大大解决硅材料在稳定性和成本方面带来的限制：SiC技术已成熟，可用于汽车。

正文

Electrification in automobiles is steadily on the rise. With it, comes the need for highly efficient power semiconductors for plug-in hybrid and all-electric vehicles (xEVs). The advantages of silicon carbide (SiC) over conventional silicon (Si) chips can not only be leveraged in industrial applications, but in cars too. SiC’s increased efficiency and power density allows for higher autonomy (and smaller batteries), reduced system size and weight, fast charging, and ultimately greater customer acceptance for e-mobility. Thanks to technological advances, SiC semiconductors’ penetration in xEV subsystems will increase over the next decade.

Currently power modules and discrete components typically use silicon-based  diodes, MOSFETs, and IGBTs. By comparison, SiC circuits in xEV drivetrains enable smaller chip dimensions for the same power rating. In addition, the SiC technology reduces thermal losses. This means more efficient, lighter and more compact applications are now possible compared to previous systems. Typical applications that benefit from the advantages of SiC are main inverters, onboard charger electronics, boosters or DC/C converters (Figure 1).

Significant progress – SiC is ready for use in vehicles

For over two decades, SiC semiconductor components have been used in various applications. But there remained obstacles to be overcome before using them in automotive electronics. In order to be able to efficiently leverage the new technology in vehicles too, two main aspects had to be fulfilled: high reliability and cost-effectiveness.

In case of SIC MOSFET switches, the development of reliable gate-oxide has been a major road-block over many years. Recent breakthrough in the design (e.g. Trench concept) and manufacturing process has enabled devices meeting the reliability level requested by car manufacturers.

Moreover, SiC  base material production (wafers) is significantly more complex resulting in smaller wafer diameters (Figure 2), higher number of defects per wafer, and higher cost.  While Si crystal can be grown with high purity, the defect density in SiC wafers has been a major challenge. Significant progress has been made in recent years however, and defect density has been markedly reduced. Larger chip areas are finally possible, easing the integration into power packages.

In the past, most SiC providers were small, specialized semiconductor manufacturers with relatively low volumes and little experience in the automotive industry. As a result, economies of scale were limited. But this situation has now fundamentally changed. The availability of high-quality 6-inch SiC wafers led to significant productivity improvements. The growth potential offered by the automotive market for SiC has become a compelling reason for leading semiconductor companies to enter this market. Current market reports confirm this trend.

Reducing losses by up to two thirds

Compared to conventional silicon-based high-voltage IGBTs or MOSFETs (> 600V), SiC devices offer several advantages. For example, compared to IGBTs, SiC MOSFETs show no trail effect, almost no forward recovery nor reverse recovery. This results in temperature-independent switching losses that also are significantly lower than with silicon (Figure 3a).  SiC Schottky diodes should also be noted in this context. High switching speeds or extremely low reverse recovery charges (Q_rr) reduce the switching losses and make it possible to efficiently miniaturize the final product. SiC Schottky diodes are ideal for power factor correction (PFC) circuits in onboard charger systems.

In addition to showing less switching losses, SiC MOSFETs have also advantages when it comes to conduction losses. Indeed, they exhibit a resistor-like output characteristic, compared to the diode-like characteristic of IGBTs. This threshold-free on-state characteristic results in smaller conduction losses in the partial load range (Figure 3b).

The basic advantages of SiC MOSFETs make them ideal for operation in onboard chargers or DC/DC converters where of smaller passive components can be used thanks to higher switching frequencies. They make them also ideal for inverter applications where switching frequencies lower than 20 kHz are typical. Here, the efficiency is substantially determined by the operation at partial load. SiC MOSFETs reduce the average thermal losses in inverters by up to two thirds.

Optimized packaging technology

To make the most of the performance of the SiC chips, the power modules need a correspondingly optimized package technology. Higher power densities require not only improved packaging materials but also consideration for the higher thermal resistances of smaller chips leading to higher thermomechanical stress. Moreover, in order to be able to fully leverage the fast switching capability of the SiC MOSFETs, packages with low parasitic inductance are also required. This then calls for new packaging concepts for power modules. Modern packaging concepts with double-sided cooling are available to optimize thermal resistance of the devices. One example are the power modules  HybridPACK™ DSC (double-sided cooling) from Infineon. They feature double-sided efficient heat dissipation and can thus significantly reduce the thermal resistance R_th at the system level (Figure 4). This makes it possible to build inverter designs with extremely high power density.

Significant cost reductions possible

SiC MOSFETs enable to realize very compact and highly efficient inverters. This is the conclusion of the Fraunhofer Institute for Integrated Systems and Component Technology (IISB) drew in its study “Evaluation of potentials for Infineon SiC MOSFETs in automotive inverter applications”. Under comparable conditions, the SiC MOSFETs significantly reduced the chip area compared to IGBT-based inverters. Thanks to the reduced chip losses, the efficiency for different driving scenarios has been improved by more than 3 percent, especially in city traffic with lots of acceleration phases.

When considering the inverter efficiency, it should be noted that the energy flows in two directions – from the battery to the wheel when generating the torque and back from the wheel to the battery during the energy recovery (recuperation). The efficiency of the inverter is thus essential for battery all-electric vehicles because it has a direct impact on the achievable range or the battery size needed to ensure a certain range. Since the battery is a key cost factor, a reduction of the battery cells by 5 to 10 percent can lead to a significant cost reduction in the system with batteries of more than 40 kWh output. 

Silicon does not support high breakdown field strengths as efficiently as SiC does. A standard 1200 V IGBT has significantly greater losses than its counterpart in the 600 V class, whereas a 1200 V SiC MOSFET allows very efficient operation at higher battery voltages in the range of 850 V. This makes SiC particularly suitable for architectures that also enable fast-charging applications. With the charging station infrastructure currently under development, an 80 kWh battery will be charged to 80 percent in just 15 minutes. This means that one of the biggest obstacles to implementing electro-mobility will have been eliminated.

Conclusion

Although SiC material is more expensive than silicon, it allows to drastically increase power density. At a given power requirement, the semiconductor content can be reduced by factor 5. Infineon has manufactured its Trench-based SiC MOSFETs on 150 mm wafers right from the start. Considering the advantages at the system level – for example, more compact design, lower cooling effort, lighter systems, efficiency benefits – SiC is already competitive in its first applications.

An increasing number of system manufacturers (Tier 1s) and car manufacturers (OEMs) are relying on SiC for future developments. With its 1200 V CoolSiC™ MOSFETs, Infineon intends to reach new efficiencies and performance levels. Studies based on the power module HybridPACK™ Drive CoolSiC (figure 5) have demonstrated the feasibility of having compact inverters providing more than 200 kW power at 850 V.

Infineon offers a broad portfolio of silicon and wideband gap semiconductors for power electronics, coupled with innovative packaging technology and gate drivers. Infineon has been developing for 25 years SiC solutions for photovoltaic inverters, windcrafts and industrial applications. Infineon is fully committed to introduce the advantages of SiC in the xEV world.