This paper proposes a voltage balancing technique for series-connected power devices in voltage source converters (VSCs). Each main power device has an active clamping module (ACM), consisting of a capacitor and an auxiliary switch. The ACM can clamp the main power device voltage, and voltage sharing among series-connected power devices can be realized by balancing the ACM capacitor voltages. The balancing circuit does not affect the normal operation of the devices and the introduced power losses can be ignored. The possible charging and discharging principles of the ACM capacitors are summarized. Then, the control strategy to balance the ACM capacitor voltages is introduced. The operating principles of a VSC with series-connected devices and the proposed balancing circuits are presented, including Charging Stage, Discharging Stage and Steady Voltage Stage. A switching sequence of auxiliary switch and main power switch is proposed to ensure ZVS-on of the auxiliary switch. Finally, experimental results are provided to verify the effectiveness and feasibility of proposed method. The voltage imbalance degree of the 6 series-connected devices is less than 3% under different operating conditions. The proposed method shows good balancing performance, good scalability and low power losses.