I. Introduction
Converters connected to transmission lines (e.g., FACTS devices) require insulated gate bipolar transistors (IGBTs) with high blocking voltages and low switching losses to reduce number of devices connected in series and capacity of ac filters. However, these performances have a tradeoff relationship. To improve this tradeoff, semiconductor companies have developed several technologies. They are field stop (FS), soft punch through (SPT) technology, and injection-enhanced gate transistor (IEGT) technology. Such technologies make it possible to operate near the silicon semiconductor limit. Nowadays, the FS technology is applied to 6.5-kV IGBTs [1]–[3], as well as the SPT technology [4], [5]. In addition, IEGTs with blocking voltages up to 4.5 kV are available [6]–[8]. However, all these IGBTs and IEGTs still have very high switching losses; thus, in practice, the switching frequency is limited to about 1 kHz. If such devices are used in inverters connected to transmission lines (e.g., STATCOMs), high switching frequencies are necessary to achieve low harmonic distortion of the current without large filters.