This thesis is concerned with series connection issues of IGBTs and methods of voltage balancing (active voltage control in particular). The wide use of high voltage converters in power electronics applications such as high power traction and industrial drives, high voltage dc transmission systems (HVDC) and flexible ac transmission systems (FACTS) in recent years calls for high voltage power semiconductor switches. Insulated gate bipolar transistor (IGBT) as a high power semiconductor switch with attractive features are increasingly being used in the mentioned applications. However, due to some technological issues, the maximum voltage blocking capability of commercially available IGBTs is 6.5kV. Higher voltage ratings for switches in high voltage converters can be achieved by connecting IGBTs in series. The main problem with the series connection of semiconductor switches is unbalanced static and dynamic voltage sharing. Static and dynamic voltage sharing are respectively defined as equal voltage sharing between series-connected switches in off-state and in switching transients (turn-off and turn-on). Unequal voltage sharing can damage the overstressed switch or reduce its lifetime that can in turn cause failure or make the system less reliable. Static voltage sharing can be achieved easily, however, there are more difficulties with dynamic voltage balancement. Spread in device parameters and external circuit conditions are two main causes of voltage unbalancement. Proper selection of devices and symmetrical design reduce the unequal voltage sharing. However, near ideal dynamic voltage sharing can only be attained by special methods and extra circuitry. Snubbers, active voltage clamping and active gate control are three main methods for mitigating voltage sharing. Some parameters such as voltage unbalancement mitigation degree, effective operation region, switching and circuit losses, scalability, reliability, complexity and cost are among the most important criteria for voltage balancing method selection. In this thesis, issues, challenges and practical considerations of series connection of IGBTs are discussed and investigated. Voltage balancing methods are also introduced and a comparison of them is made. Active voltage control (AVC) as a closed-loop control by a high degree of unbalancement mitigation introduced and an AVC-based gate drive designed and implemented. In this method in the simplest way the collector-emitter voltage of IGBT is compared with a reference, the error passes through a proportional controller and after current amplification is applied to IGBT gate resistor. In order to improve AVC performance two extra feedbacks, gate-emitter voltage and collector-emitter derivative are added to form a cascaded active voltage control (CAVC) method. Effect of different parameters on AVC performance to control the voltage of a single IGBT and then to resolve voltage unbalancement of two series-connected IGBTs is discussed and verified using implementation results. Keywords: IGBTs series connection, static voltage sharing, dynamic voltage sharing, active voltage control (AVC), power semiconductor switch