Still developing at an extraordinary rate, the IT industry has become one of the largest industries in today’s world economy. A characteristic of this industry, and one which also helps explain its phenomenal growth, is extremely high investment in R&D. The spectacular developments in software and microelectronics technologies have also become key driving forces in ‘knock-on’ applications, such as electrical engineering.
The MOS-transistor (Metal Oxide Semi-conductor) is one of the cornerstones of state-of-the-art microelectronics. This device makes it possible to control, with high precision, a current in a semiconductor by applying a voltage to an insulated gate electrode. What is more, the power required to do this is extremely low. The MOS-transistor lends itself to very cost-efficient manufacturing, all the small features building up the transistor function being created in a planar process using photolithographic methods similar to those used in the printing industry.
Handling high power – the traditional approach
Traditionally, the electronic conversion of electrical power in the high-power region has made use of the principle of line-commutated frequency conversion, with thyristors used to control the current flow. The thyristor is the equivalent of a ‘binary current valve’ with two discrete states, one conducting and one blocking the current. Turn-on is accomplished by injection of a gate current, with turn-off determined by the 50/60Hz line voltage passing through zero. However, the fact that the thyristor cannot be turned off with the gate terminal, limits the range of applications for this device.
Boosting thyristor performance Gate-controlled turn-off was introduced in the late 1970s with the Gate Turn-Off (GTO) thyristor. By making it possible to build efficient converters for controlling the output frequency, the GTO opened the door to variable-speed AC motor drives and other similar applications. However, power losses are higher with the GTO than with classical thyristors, and elaborate units for supplying the high gate currents became necessary.
A remarkable improvement in GTO thyristor performance came with the introduction by ABB in 1997 of a new device concept – the Integrated Gate Commutated Thyristor (IGCT). This new technology featured precisely controlled injection and extraction of gate currents in thyristors by means of an integrated gate drive unit. Using this concept, the freewheeling diode can be integrated on the same semiconductor wafer, simplifying the mechanical design of the converter. The homogeneous switching across the device area results in lower losses than with the GTO. Typical uses include large drive systems and traction power supply systems.
Merging power with micro electronics
Numerous attempts have been made to combine the microelectronics used for precise control of the low-voltage signals in integrated circuits, with the high power handling capabilities needed for power semi-conductor devices. The most successful to date has been the Insulated Gate Bipolar Transistor (IGBT), which combines a high-impedance, low-power gate input with the power-handling capacity of normal bipolar transistors and thyristors.
Control of the IGBT is accomplished by means of a pattern of MOS transistors, distributed on the surface of the device. These MOS transistors allow high-impedance control of the current flow through the device, so that only an extremely low power has to be supplied to the control gate.
IGBT performance is related directly to the properties of the surface MOS transistor cells, and the success of these devices is largely due to the continuous development of the cell structures, in many cases using technologies that were developed for microelectronics circuits.
Although the 1980s saw substantial progress made in the development and production of IGBTs for lower voltages (600-1200 V), it was not until the beginning of the 1990s that it was realised that the same concept could also be used for higher voltages. The MOS transistors on the surface of the wafers, like the silicon wafer thickness, are optimised for high performance when the IGBT is conducting current and for very low losses when the device switches to the off-state to prevent current flowing.
Power semiconductor losses are proportional to the square of the device thickness, so reducing this thickness is an obvious choice when considering what to optimise. ABB has made a ‘quantum’ leap, reducing the thickness of 1200-V IGBTs to less than 70% of the thickness of previous devices. Being able to manufacture extremely thin silicon wafers is key to this performance, as it minimises the silicon material in the current path and hence the electrical losses.
Although the performance of power semiconductors based on silicon will continue to improve, fundamental limitations inherent in this material are within sight. Maximum power-handling density (robustness) and thermal stability (losses, cooling) are important performance parameters that will be limited by silicon. Silicon high-power diodes are already approaching these limits. Converters capable of much higher switching frequencies would be an attractive option for the high (>10 MW) power levels in typical transmission and distribution applications.
One very promising alternative is to build devices based on silicon carbide (SiC). Due to the high atomic binding energy (bandgap) and high specific electric field strength of this semiconductor material, optimised devices made of SiC potentially offer one or two decades of performance improvement when compared with silicon-based devices.
Also, as SiC can be operated at considerably higher temperatures than silicon, it is possible for the power semiconductor to be integrated directly in electrical equipment such as generators and motors. Commercial silicon carbide Schottky diodes for low-voltage applications (600 V), for use in power supply products and power factor correction circuits, are now coming onto the market.
ABB committed to silicon carbide power devices some five years ago. As a first step, silicon IGBTs can be combined with these SiC power diodes to form hybrid modules. Using techniques known from silicon devices, power loss savings of 40-60% have been measured in typical converter configurations. ‘All-SiC’ power modules, including switching devices made of silicon carbide, have the potential to reduce the total converter losses to as little as 10-20% of today’s technology.
A growing field of applications
Another area in which power electronics is driving growth is microgrid technology, where it is essential for connecting small-scale distributed generation units to individual consumers and the utility network.
Electricity from fuel cells, wind turbines and solar panels is generated at low DC voltage levels, and power electronics solutions are needed to convert it to usable voltages and frequencies. As renewable energy resources are usually located some distance from the large cities, new power electronics-based technologies will allow the energy to be fed into the power grid for transmission.
Another advantage of the new power electronics-based technologies is that they allow the reduction of electricity infrastructures, minimising environmental, especially visual, impact and freeing valuable space and resources for other uses.