Power trends and the choice between Si, SiC and GaN
Wide bandgap (WBG) semiconductors can deliver performance and power efficiency that are superior to those of silicon devices, and have useful properties such as high operating temperature. This means that WBG devices, using materials such as gallium nitride (GaN) and silicon carbide (SiC), are becoming increasingly popular in many applications.
However, WBG devices are typically more expensive. This is partly due to the fact that, as with any new technology, there are still cost reductions to be achieved as WBG manufacturing processes become refined. Also, SiC is physically harder than silicon, which makes it slower and costlier to cut, grind and polish SiC wafers.
For power switching and supply applications, when should designers choose a WBG alternative, and how can they evaluate the trade-offs compared to silicon devices? There is no single answer, and in practice it depends on the requirements and commercial considerations of each use case. Some design engineers may be reluctant to adopt WBG devices, but the benefits often outweigh any disadvantages.
In this article, we will examine these issues, and the design challenges of some of the most critical power applications – and why you might choose silicon or WBG devices in each case.
WBG and silicon compared
While silicon has dominated the power electronics market for decades, there is increasing interest in WBG alternatives. Their superior efficiency is attractive for applications that are seeking to maximise every watt of power, like electric vehicles and renewable energy generation.
On the other hand, the higher cost of WBG power semiconductors means they are unsuitable for some applications, such as low-cost consumer power supplies, where the efficiency savings from using a SiC or GaN device are unlikely to justify their additional cost.
Keep in mind that enhanced efficiency also means lower heat dissipation, which can help improve mechanical and thermal design, and enable the construction of a smaller product. SiC can also be used at higher operating temperatures (175 to 200ºC) than silicon, which may reduce the need for a heatsink or other cooling mechanisms. High operating temperatures are also appealing for some automotive and industrial applications.
Figure 1 shows which type of device is typically best suited for different use cases, simplified to only consider voltage and switching frequency. In general, you can see that silicon MOSFETs and IGBTs are best suited to lower-frequency applications, while GaN and SiC come into their own at higher frequencies. ‘FOM’ here means ‘figure of merit’ – in other words, applications in this area may suit multiple semiconductor technologies, and specific comparisons are needed to find the best choice in each application.
Broadly speaking, a silicon IGBT will be the right option where the switching frequency is below approximately 20kHz, and/or the power level is above 3kW. Silicon IGBTs are also suitable when low cost is vital, or the product will be supplied by a three-phase grid.
Figure 1: GaN, Si and SiC compared
Source: Avnet Silica whitepaper, “Selecting GaN or SiC devices in high voltage switching technologies”
Power trends and applications
In the sections below, we will look at some of the significant and growing applications for power electronics, and how their requirements affect the choice between silicon or WBG power devices.
Industrial power supplies and motor inverters
Many industrial and motor drive applications meet one or more of the criteria mentioned above: high power, lower switching frequency, and powered by a three-phase supply. This means they are likely to be well served by Si insulated-gate bipolar transistors (IGBTs), although the extended operating temperature range of SiCs is worth considering.
The efficiency benefits of WBG devices are helpful in some industrial applications, particularly where equipment is running 24 hours a day, 365 days a year. There are also lower-power applications where the efficiency of GaN MOSFETs is valuable for building compact, portable machines – such as some of today’s advanced factory robots.
PV inverters / solar
The world’s installed base of solar power continues to grow rapidly, driven by remarkable reductions in the cost of photovoltaic (PV) technology.
To get the most electricity from solar cells, every stage from generation onwards must be as efficient as possible. For example, onsemi's SiC-based boost and inverter power integrated modules (PIMs) help increase the efficiency of critical stages. This includes single-phase and three-phase inverters, providing both DC/DC and DC/AC conversion. GaN devices are also finding a home in smaller solar inverters.
Heat pumps
As we move towards net zero, heat pumps are an important way of reducing carbon emissions for homeowners. In the past, they usually had a fixed output – the heat pump was either on or off.
More recently, heat pumps may include an inverter, which enables them to vary the speed of their compressor and therefore match their power output to the needs of the building. This improves the heat pump’s efficiency, and extends its lifetime due to lower loading of components.
Energy / home storage systems
Home energy storage is a market that is in its early stages. Most of the systems currently available, such as Tesla’s Powerwall, use lithium-ion batteries. However, these are an expensive option, and numerous companies are working on reducing costs for better readability. One approach is reusing EV batteries that have reached the end of their life in a vehicle.
To connect to a home’s solar panels, if any, and to its existing electricity systems, an energy storage system needs a DC-DC converter or a DC-AC inverter. Efficiency in power switching is important to maximise the effective storage capacity and meet the expectation of homeowners, and the high cost of the energy system overall means the extra cost in using WBG devices is likely to be justified. Silicon carbide devices, as well as SiC/Si hybrids, are meeting these needs.
Electric vehicle (EV) charging
Sales of EVs are growing rapidly, and the charging infrastructure is struggling to keep up in some regions. To meet customer demand for fast charging, we need at least six million new chargers by 2030, in the EU alone.[1]
EV chargers need to deliver high power, with the efficiency and reliability to operate 24x7. A new technology area is vehicle-to-grid (V2G), where vehicles can return power to the grid – for example, to help meet peak demands – which again requires a reliable, efficient solution.
SiC devices offer this efficiency, with superior switching performance and higher reliability compared to silicon. For example, the EliteSiC Diodes portfolio from onsemi includes AEC-Q101 qualified and PPAP capable options, which are specifically engineered and qualified for automotive and industry applications.
Conclusions
Selecting the appropriate products always involves trade-offs, and the availability of WBG power semiconductors has expanded the range of options designers need to consider. As the demand for greater efficiency and higher performance in various applications grows, it becomes increasingly important to weigh the benefits and drawbacks of SiC, GaN, and Si devices. Each application is unique, but gaining a thorough understanding of the characteristics of these semiconductor materials, their suitability for different use cases, and the specific requirements of the target application can assist you in making well-informed decisions. By carefully evaluating the trade-offs and aligning your choice with the evolving trends in the power electronics industry, you can optimise your designs for efficiency, reliability, and cost-effectiveness
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