A Four-Level Single-Phase Diode-Clamped Inverter with Natural DC-Link Capacitor Voltage Balance

This paper presents research concerning natural balancing of the dc-link voltages in a single-phase full-bridge inverter composed of four-level diode-clamped legs (4L-FBCLD). The converter creates seven levels of output voltage utilizing three capacitors in a dc-link. Under the standard PWM technique such a converter may not operate properly due to unequal discharge of dc-link capacitors. In this paper new concepts of modulation are proposed to achieve more symmetrical discharging of the capacitors and to create the conditions for natural dc-link voltage balance. The natural balancing in the four level converter is a novel achievement and significantly differs from the situation with regard to three-level topology. The paper proposes the concept that the modulation patterns are different for particular levels. In the unbalanced state the output voltage triggers a unique component in the output current which when flowing via the dc-link capacitors varies their average voltages. A proof of this mechanism is also demonstrated in this paper. The concepts are verified by both analytical discussion and simulations.