Under varying load current, the voltage imbalance in series-connected IGBTs would be worse due to the slow convergence of voltage balancing control. In this paper, an improved fuzzy Proportional-Integral (PI) control method is proposed for voltage balancing, based on the simplified relationship between the drive time delay, voltage error and its deviation. The method is implemented with field programmable gate array (FPGA) for 1200 V/45 A IGBT module. The circuit and control are experimentally verified by a buck converter under varying output current.