Issue link: http://cp.revolio.com/i/760098
22 | THE DATA CENTER JOURNAL www.datacenterjournal.com Ups mission UPS manufacturers attempt to provide uninterruptible, conditioned, reliable power to users that have a critical electrical operation in process. If the critical operation is interrupted, the owner could lose millions of dollars. "Mission-critical" operations rely on the UPS to keep the mission up and running 100% of the time. To supply this reliable power, the UPS employs semicon- ductors to re-generate the power that comes from the power company. With precise instructions and control from the UPS logic, semiconductors create a conditioned output sinewave every minute of every day (see Figure 1). Figure 1: The output sinewave of a UPS module created by a semiconductor. Over the last 50 years, the UPS industry has improved out- put power quality by upgrading components and fine-tuning the semiconductor. Results include smaller footprints, reduced main- tenance requirements, greater reliability and increased end-to-end efficiency. e drive to higher efficiencies has captured most of the attention and direction of the industry. Ups history During the 1960s, 1970s and 1980s, the silicon control rectifier (SCR) was the semiconductor of choice for creating the UPS output. It produced a "square-wave" output that required filtering for harmonic distortion. Output efficiency was around 85%. In 1992 the insulated-gate bipolar transistor (IGBT) arrived, replacing the SCR. e IGBT, also silicon based, improved output efficiency to 94%. In 2008, the industry used the same IGBT in an enhanced configuration known today as three-level switch- ing. e elimination of internal transformers, combined with the new three-level switch topology, provided a combined efficiency gain up to 97%. e latest step in greater efficiency was made in 2015. A silicon-carbide-based (SiC-based) MOSFET replaced the silicon-based (Si-based) IGBT. Efficiency climbed to 98.6%. Ups Basics What Is a Semiconductor? As the name implies, a semiconductor conducts current, but only partly. e conductivity of a semiconductor is somewhere between that of an insulator (which has almost no conductivity) and a conductor (which has almost full conductivity). Later in this article, we'll see that a semiconductor is ideal for turning electric current on and off. How Does Current Flow in a Semiconductor? Electric current is the movement of negatively charged elec- trons from one atom to another. To understand this electron flow, we need to understand the atom. All atoms have: a nucleus in the center and electrons that orbit around the nucleus (see Figure 2). Notice that electrons exist in different orbital paths. ese paths are called electron bands (bands of electrons). e space between electron bands is called a bandgap. e further away the electron is from the nucleus, the less attractive a force the nucleus has on the electron to keep it in its respective electron band. e outermost electron band is called the valence band. To create the flow of electrons from one atom to another atom (and make electricity), we select the electrons in the valence band because the force holding them to the nucleus is the weakest. e electrons in the valence band must cross a bandgap (space Figure 2: A simple model of the atom. + nucleus - electron - electron - electron Level 1 Level 2 Level 3 + nucleus - electron - electron - electron Level 1 Level 2 Level 3 + nucleus - electron - electron - electron Level 1 Level 2 Level 3 Electron Bands are Electrons in orbit around the nucleus Bandgaps exist between Electron Bands The Valence Band is the outer Electron Band