The potential of Silicon carbide (SiC) for automotive applications
From the building technology to the transport of passengers and freight, we are facing substantial upheavals in all areas of the daily life. A brand-new topic is electromobility. Automotive manufacturers, industrial companies and research institutes work hand in hand to implement entirelly electrically driven vehicles and the necessary infrastructure. The prospects for electric vehicles have improved significantly in the recent past. New drive concepts have been researched and tested by various companies during the last few years. The first models of hybrid and electric vehicles are now available on the market. New components such as power electronics systems are integrated into the car which have not existed in conventional diesel / gasoline vehicles.
Examples include systems such as:
- Drive inverters to drive the drive motor (up to 300 kW)
- Battery chargers (onboard chargers) from 3,6 kW to 22 kW
- Inductive charge (wireless charging) from 3,6 kW to 22 kW
- DC/ DC converter up to 5 kW
- Inverter for auxiliary units such as air conditioning, steering support, water pumps, etc.
For the abovementioned systems, power electronics plays a decisive role in ensuring the functionality of hybrid respectively electric vehicles.
SiC- efficient semiconductor material
The requirements of the automotive OEMs placed on power electronics systems are a great challenge for the developers of such systems. In particular, space requirements, weight and efficiency play a significant role. In addition, the entire system costs and the effort in the product design phase are to be kept low while at the same time, product quality and operational safety have to be guaranteed as well.
The efficiency of conventional power electronics is based on silicon semiconductor technologies and generally varies between 85% and 95%. This means that during each power conversion about 10% of the electrical energy gets lost as heat. Generally, it can be said that the efficiency of power electronics is mainly limited by the performance characteristics of power semiconductors. Due to its physical properties, the semiconductor material SiC has great potential to meet the requirements of these market trends.
Compared to silicon semiconductor devices, the electrical field strength of SiC is nearly ten times higher (2.8MV/cm vs. 0.3MV/cm). The higher electric field strength of this very hard SiC substrate makes it possible to apply a thinner layer structure, the so-called epitaxial layers, to the SiC substrate. This corresponds to one tenth of the layer thickness of Si epitaxial layers. The doping concentration of SiC can reach two orders of magnitude higher than that of their Si counterparts for the same blocking voltage. Thus, the surface resistance (RonA) of the component is reduced which results in a considerable reduction in pass-through losses.
The thermal design plays a decisive role in power electronics systems in order to design high power density and in consequence, compact systems. As a semiconductor material, SiC is excellently suited for these applications because its thermal conductivity is almost three times higher than that of Si semiconductor devices. SiC is also suitable for higher operating temperatures compared to Si semiconductors.
Power dissipation of the semiconductor devices
During the operation of power electronics systems, there is a loss of power during the current flow and the switching of the semiconductor components. The total loss of power in power electronics systems consists of static losses and switching losses. The static losses mainly occur during the transmission state of a power component. The switching losses result from switching on and off the semiconductors. The higher the switching frequency is during operation, the higher are the switching losses.
The switching frequency in power electronics systems is often defined by the application and system specific constraints. For example, the switching frequency of an electric drive is determined by the required output frequency to the motor. Furthermore, other factors such as the resonance behaviour of the entire system, electromagnetic compatibility (EMC) and heat management play an important role in defining the switching frequency to be used. In addition to the power loss in the power semiconductor device, there is also a loss in the passive components such as transformers, inductors and intermediate circuit capacitors.
The interaction between power semiconductors and the passive components such as inductivities and transformers became meanwhile the decisive factor in achieving high power density in the overall system. Therefore, the physical properties of both, the passive components and the semiconductors, should be taken into account when designing the power electronics systems.
The static losses and switching losses as well as passive components add up to the total power loss in the system which in turn can be converted into heat. The emerging heat must be dissipated through a suitable cooling medium in order to ensure the reliability of the components and the system used. In principle, the switching losses are produced by a single switching process, e.g. when switching on or switching off the semiconductors. An increase in the switching frequency leads to an increase in the total switching losses, which in turn strongly influences the total power loss. In certain power electronics systems in the vehicle, a high switching frequency is preferred in order to meet the system requirements or the specification. In such a system, the switching losses will account for a large portion of the total power loss in the system.
If Si semiconductors are used in this kind of high switching frequency applications, the high power dissipation and the resulting heat force system developers to limit the load current in order to ensure the functionality and reliability of the system. This means in other words, a high switching frequency leads to less power. However, if high load currents in these applications are indispensable, the overall volume of the system must be increased in consequence. This measure would be unavoidable at this point but does not correspond to the expectation of the end users. One can say that the Si semiconductors have almost reached their limits.
Comparing a SiC-MOSFET and a Si-GIBT in applications featuring a high switching frequency, one can state that the output current has to be reduced due to the high switching losses of the Si-IGBT and the resulting heat. This is the only way to not exceed the maximum chip temperature, and to secure the functionality of the semiconductor.
The picture looks different when using SiC. The SiC semiconductor features a betters switching behaviour than the Si-IGBT. Thus, SiC causes less switching losses at a high switching frequency. As a result, more load current can be obtained in the application at a high switching frequency compared to a Si-IGBT.
Figure 2 show a comparison between a raw SiC half-bridge module (BSM300D12P2E001) and four different IGBT modules from a market player.
This picture clearly displays that at a high switching frequency, the SiC MOSFET is more efficient than the Si-IGBT. When using a 300A IGBT module and a switching frequency of 40 kHz, no more than 80 arms load current could be obtained in the application. In contrast, a load current of 200 arms would be achievable with the use of a Rohm SiC 300A module. This corresponds to 120% more load current than the Si-IGBT.
In order to be able to develop compact power electronics systems, power electronics developers must use optimal cooling. Several new cooling concepts have been introduced in recent years in the market to meet such challenges. These cooling concepts are usually cost-intensive and sometimes cause problems within the applications. This kind of issues or challenges not only occurs in the development phase but also while handling the system during the production phase and during service operation. By using an efficient semiconductor material such as SiC, complicated cooling can be omitted. This results in a reduction of cooling costs and a simple system handling.
Miniaturization of power electronics systems
Based on the application scenarios of an electric vehicle, various requirements are imposed by the automobile manufacturers on the power electronics systems. These are e.g. resistance to temperature changes, vibration resistance, operational reliability at different temperatures as well as a long lifetime. In addition, a requirement like high power density of the integrated systems is now considered self-evident by the automobile manufacturers. However, all these requirements are a major challenge for power electronics.
The range of the high-voltage battery is one of the biggest hurdles for the spread of hybrid and electric vehicles. In order to convince the end customer (i.e. the car owner) of the electric mobility, a number of car manufacturers currently rely on charging systems with fast charging times. This is to simplify the usage of electric cars. But a fast charge means for the technical implementation that a higher charging performance is required within a short time frame in order to charge the battery. Since the volume of available space within the vehicle is always limited, the battery charger system must feature a high-power density. This is the only way that such systems can be integrated into the vehicle in order to meet the market requirement.
Onboard Chargers are complicated systems consisting of different components for the power conversion. Several components are integrated in such systems. Examples include: semiconductors (such as diodes, MOSFETs), passive components (e.g., inductors and capacitors), and transformer with adapted translation ratios to charge the battery with the required voltage. In addition, the transformer is used to galvanically decouple the high-voltage battery during charging.
One of the options to miniaturize power electronics is the more compact design of passive components such as inductors and transformers. This is usually only possible if the deployed semiconductors in the same circuit can be controlled at a high switching frequency. In the case of Si semiconductors, the thermal load at a high switching frequency will limit this approach. Due to its excellent switching characteristics, the SiC-MOSFET is ideally suited for these cases.
Figure 3 shows the following example: For a DC / DC converter system with Si semiconductors, the switching frequency is limited to 25 kHz. If a SiC MOSFET is used, a switching frequency of 160 kHz is possible. This led to a major miniaturization of the winding quality in the entire system. High power density and significant overall weight reduction can be achieved.
The advantages of SiC semiconductors have finally been recognized by automotive manufacturers. The first products consisting of Rohm SiC diodes are used for onboard charging in various vehicles in mass production worldwide. The worldwide first SiC MOSFET qualified for Automotive will soon be available from Rohm. In addition to battery charging devices, the SiC semiconductor promises great potential for applications such as DC / DC converters as well as drive inverters. Concrete solutions from Rohm are available for this kind of applications.
Rohm’s second-generation of SiC SBD currently includes products for 650V from 5 to 100A as well as for 1200V and 1700V with current carrying capacities up to 50A. Rohm’s range of SiC MOSFETs is even more comprehensive. Rohm offers two different technologies: planar technology and double trench technology. The planar technology already offers discrete products and modules in the voltage range of 650V, 1200V and 1700V with a current carrying capacity of up to 300A.
ROHM is launching mass production of Third Gen SiC MOS for discrete components as well as full SiC modules featuring proprietary double trench technology, which extends the existing MOSFET product family and contributes to the further development of highly efficient and reliable power electronics.
About the author:
Aly Mashaly is Manager Power Systems Department, ROHM Semiconductor GmbH, Willich, Germany.