Every two years the Power Sources Manufacturers Association (PSMA) forms a team of industry experts to compile the latest trends shaping power conversion technologies. The goal is to stimulate discussion of trends that are driving new technology in power conversion, in particular for the following specific product families expected to be the areas of largest market growth: Ac-dc front end power supplies; external ac-dc power supplies; isolated dc-dc converters; and non-isolated dc-dc converters. The findings of the expert panel are compiled in a report called the PSMA Power Technology Roadmap which is published by the organization and, in this latest version, aims to prepare users and manufacturers for changes in the industry through the year 2017.
While the full outlook--which consists of numerous presentations, commentary, tables, charts and reports on market drivers and applications trends-- is beyond the scope of this column, I will focus on component trends identified in a summary presentation developed for the Roadmap. These trends are expected to shape power conversion products for the next few years. I’ve listed the specific PSMA power component trends in italics below (listed in no particular order) followed by my comments discussing the trend and its expected impact.
Nanocrystalline magnetic core materials better than ferrite
at more than 100kHz high flux
Magnetic components are essential for most power electronic circuits. The introduction of commercial wide-band-gap semiconductors (more on this shortly) is allowing radical increases in power-converter switching frequency, the design requirement here points to the need for low loss materials with low, linear permeability that remain constant at high frequencies. Advanced soft magnetic materials can match the high-power density and switching frequencies made possible by advances in wide band-gap (SiC and GaN) semiconductors.
At high frequencies ferrite core material traditionally used for switched mode transformers and chokes does not perform very well. Nanocrystalline cores offer a design option because these materials combine the high permeability of amorphous materials and the low losses of ferrite materials, thus they are very promising in power electronics. As a result, over the past several years soft magnetic cores of nanocrystalline FeCuNbSiB alloys have supplemented and to some degree even replaced soft magnetic ferrites, amorphous and NiFe cores in industrial electronics. Nanocrystalline soft magnetic metals are a Japanese invention made by Hitachi Metals. Nanocrystalline metals are produced like amorphous metals by using a special rapid quenching technology. The melt is chilled into a solid state within a thousandth of a second.
For a transformer for a welding inverter at 100kHz studies comparing ferrite cores and nanocrystalline cores show that using a nanocrystalline core leads to a much lower component size (volume) and weight, the improvement in size can be more than 60% and the improvement in weight is over 55%.
SiC MOSFETs look promising at 1,200V and up
Considerable focus in the power industry is on the emergence of wide bandgap devices − gallium nitride (GaN) and silicon carbide (SiC) − for use in power devices. Bandgap is the energy required for an electron to jump from the top of the valence band to the bottom of the conduction band within the semiconductor. Stated another way, it is the amount of energy needed to free an electron so that it can become mobile, typically on the order of a few electron volts (eV).
Semiconductor devices that enable reduced power losses are important for improving the performance, size and cost of power electronics. Power semiconductors made from silicon carbide (SiC) are demonstrating dramatic improvements at both the component and the system level compared to their silicon counterparts. The advantages of silicon carbide make it possible to produce power transistors that block high voltages and have low series resistance, leading to low conduction losses. The high band gap also allows power transistors to switch higher voltages and current at higher temperatures. SiC power transistors are also electrically robust with excellent short circuit capabilities.
The market research firm Yole Developpement (Lyon, France) in its report “SiC Market 2013: Technology and Market for SiC Wafers, Devices and Power Modules” notes that despite a quite depressed market last year, PV inverters proved their appetite for SiC devices. They are the biggest consumer of SiC devices together with PFCs. Yole said the SiC device market grew +38% year-to-year and reported there are now more than 30 companies worldwide which have established a dedicated SiC device manufacturing capability with related commercial and promotion activities.
As an example of current SiC devices consider the Cree Z-FET 1200V Silicon Carbide (SiC) Power MOSFETs, which enable engineers to replace silicon transistors (IGBTs) and develop high-voltage circuits with extremely fast switching speeds and very low switching losses. This product also reduces the size of magnetics and filter components and significantly reduces cooling requirements.
1,200V SiC BJTs may begin to encroach on IGBT territory
As just noted SiC technology is becoming accepted as a reliable and pertinent alternative to silicon. Many power module and power inverter manufacturers have already included it as a current or future option. SiC bipolar junction transistors (BJTs) offer very fast switching speed, very low conduction losses, easy high temperature operation, very good robustness and relative ease of manufacturing. Working in conjunction with a properly designed gate driver, SiC BJTs also offer safe and reliable operation. Silicon Carbide BJTs becoming commercially available are featuring very high current density levels, high gain, and excellent switching performance. For example, among the first products to be released in Fairchild's SiC portfolio is a family of SiC BJTs. By leveraging these efficient transistors the SiC BJTs enable higher switching frequencies due to lower conduction and switching losses (ranging from 30-50 percent) that are said to provide up to 40 percent higher output power in the same system form factor.
GaN on Si Cascode FETs (600V) entering commercialization
In practical terms GaN devices also operate at higher switching frequencies and because of its wide bandgap have a higher breakdown voltage and higher power efficiency than Si devices. Because GaN has brighter emission characteristics than silicon it is also being used more often for optical components (LEDs, laser diodes, optocouplers).
GaN- based power devices are said to have the potential to provide 10-100x Improvement in both conduction (Rdson) and switching (Qr) performance compared to Si. Development of the first generation of 600 V GaN-on-Si based power devices using International Rectifier’s GaNpowIR platform has now been completed. IR reports that compared to IGBTs a GaN motor drive has 6x lower conduction losses and 2x lower switching losses. GaN power density (module level) is said to be 10x higher than an equivalent IGBT module, according to the company.
Other notable GaN points in the component summary: The lateral GaN based HEMT likely has a practical limit of about 1200 V. Continuing effort will be required to bring this technology in line with the expectations for quality and reliability set by silicon incumbents and it is considered essential to bring costs to < 2x of silicon based alternatives to achieve widespread adoption of GaN- based power device technology.