Medium-Voltage Solid-State Transformer: Technology for a Smarter and Resilient Grid

The most important impact of power electronics on our society in the last 50 years has been the elimination of the 60-Hz ac power delivery system for consumer electronic products. Central to this achievement is the use of silicon (Si) power devices and pulsewidth modulation (PWM) techniques in delivering regulated ac and dc powers to low-voltage (LV) loads such as light-emitting diodes and computers. These solid-state power electronic converters have provided our society numerous benefits, including high-quality power and substantial energy savings. They also form the core technology for integrating renewable energies such as wind and solar into our power grid. Figure 1 shows a typical power delivery architecture commonly found in computer supplies and data centers. The incoming universal ac grid power is converted by a power factor correction circuit to 400 V dc before it is stepped down to a lower voltage dc intermediate bus, such as 12 V, and then it powers the digital loads at voltages as low as 1 V by a point-of-load converter. Si power metal-oxide-semiconductor field-effective transistor (MOSFET) transistors from 20 V to 700 V are almost exclusively used in this application with switching frequencies from tens of kilohertz to one megahertz. Emerging devices based on gallium nitride (GaN) heterojunction field effect transistors reduce the switching and conduction losses when compared with Si power MOSFETs and are, therefore, poised to compete in these applications, driven by the need for higher energy efficiency and higher power density.