Ali Husain, Sr. Manager, Corporate Strategy & Marketing, onsemi
Electricity demand is only set to increase as the world middle class grows and automobiles, HVAC, and industrial drives become more electrified. The efficiency that can be achieved at each power stage (generation, distribution, conversion and consumption) will determine the level of increased burden on the overall power infrastructure. In each stage, inefficiency leads to heat generation as the main by-product. Often, removing or otherwise dealing with heat requires expending even more energy. Reducing waste heat generation in each of the stages therefore has a considerable impact.
Primarily in the conversion stages, heat generated by power electronics is mainly due to conduction losses and switching losses; more efficient semiconductors means less heat and, therefore, less energy wasted. The heat generated by inefficient semiconductors is unusable and mostly unwanted, but thanks to continued improvement in semiconductor technologies and materials to increase efficiency, it is also becoming more avoidable.
Power semiconductors continue to evolve, which is driven in no small way by the needs of its end markets. Today, no vertical sector, market or application is without its own specific power requirements. Even in recent times, these differing needs would have had to have been met with largely the same underlying semiconductor technology: the silicon FET. Using the same technology for everything inevitably leads to some applications exhibiting greater losses than others based purely on the limitations of the device; one size never fits all.
There are currently several different options for the power switch. The power MOSFET is the most basic device and mostly used for applications with breakdown voltage below 200V. The superjunction MOSFET is an extension of it designed to reach higher voltages with relatively fast switching characteristics. The IGBT can be seen as a hybrid of the bi-polar junction transistor and the MOSFET which has a slower switching waveform but is well suited to hard switching topologies and is used for high power applications.
Today, technologies broadly classified as Wide Bandgap (WBG), including Silicon Carbide (SiC) and Gallium Nitride (GaN), have matured to the point that they can challenge silicon FETs and IGBTs on every metric as they apply to specific power applications. One of the main advantages WBG brings is their ability to switch efficiently at high frequencies, which can directly translate to smaller passive, magnetic components in power supplies. Another benefit is their relative lack of a reverse recovery current, allowing them to be used in place of diodes in various power supply topologies, which not only improves overall efficiency but presents the opportunity for entirely new configurations.
The development of WBG power transistors give OEMs a much wider choice in switching solutions, leading to alternative topologies to deliver higher efficiency and greater power density. This level of choice is not only advantageous in existing applications; it is actually enabling entirely new ways to employ electricity. A good example is the totem-pole PFC topology, which is much more useful and feasible with the choice of WBG devices.
Looking at many application areas today, almost all of them exhibit strong demand for power electronics. In the automotive industry the trend towards electrification of the drivetrain continues. With more hybrid and full-electric vehicles in development comes the need for AC-DC and DC-DC conversion to address on-board battery charging and electric motor drives. Renewable energy now represents a larger part of the overall power infrastructure, and that portion is set to grow. In photovoltaic power the requirement is for inverters that can take relatively low voltage but high current DC and convert it to AC for use by the wider grid. A complementary application area is the use of battery storage to stabilize the demand on power grids. This technology is displacing less efficient coal and gas generators that need to be physically rotated up to speed each time they are brought online, often for relatively short periods of time.
The way electricity is created is moving away from fossil fuels and more towards renewable 'green' sources such as solar, wind and wave. These natural sources of energy are less compliant than the resources the energy sector has grown up with, such as gas and coal. This has traditionally meant the cost per Watt is higher, though this is now changing. More efficient power electronics is one of the reasons why solar and wind are becoming lower in cost than fossil fuels. The advent of WBG and the continuing advancement of conventional semiconductor technologies means renewable energy is now much more viable and will play a greater part in powering tomorrow's electrified systems.
Efficiency is a driving force
The voltage and current levels required by electric loads vary hugely, which is apparent from the fact that these quantities are expressed using prefixes ranging from micro to mega. When electrical energy reaches its end application, its power level will be at its lowest. This controlled reduction requires conversion, described earlier as the penultimate stage in its journey. It is also, arguably, the most important in terms of efficiency.
The ratio of power sources (generators) in operation, to the number of energy consumers (electronic devices) in use is too large to be meaningful. The industry is expecting many tens of billions of new devices to come online as part of the IoT and we are not seeing a commensurate increase in the number generators. Driving up efficiency at every stage in the power lifecycle is becoming critical in order to sustain this trend.
The IoT will undoubtedly bring a significant number of new devices but, in reality, there are many more already in service and consuming electricity, with comparable numbers in development or production. While not all of these will be connected to the global data network they will, in some way, be a load on a national power network. The inefficiencies every one of these devices exhibits contributes to the total power lost, or rather energy that is dissipated as heat, every minute of every day. By selecting the best switching technology for the application, these losses can be minimized.
The main characteristics of WBG technologies that makes them suitable for power applications include their relatively high electron mobility, high breakdown voltages, their tolerance to high temperatures and their high bandgap energy. These features contribute to their ability to switch on and off at higher frequencies than conventional silicon-based power transistors. They also exhibit a lower resistance when they are on, which is crucial in a power circuit as this is where the majority of losses occur in the form of heat.
As an example, GaN transistors can switch tens of kW at speeds in the MHz. Their high switching frequency makes GaN transistors attractive for other applications such as RF transmitters and amplifiers, however it is the ability to switch high voltages quickly that really makes GaN suitable for power circuits. Similarly, SiC also out-performs silicon FETs and IGBTs in terms of switching speeds and voltage. With limited crossover in their figures of merit, SiC complements, rather than competing with, GaN technology and both are finding design wins in high power switching applications.
The advantages of WBG aren't completely 'free'. As well as a relatively higher cost both SiC and GaN require different gate driver solutions than silicon FETs and IGBTs. Fortunately, the supply chain for these technologies is developing quickly. onsemi now has strategies for all of these technologies, including silicon FETs, IGBTs, SiC and GaN, along with the corresponding gate drivers specifically designed with support for both SiC and GaN.