Four-Level TCM-Operated Inverter with Full Soft-Switching Capability Using a Hybrid Si+SiC Configuration

Triangular current mode (TCM) modulation is a promising technique for achieving zero-voltage switching (ZVS) in dc-ac power inverters, which already inherently offers a high output voltage quality without a sine-wave filter. Despite slightly higher conduction losses than hard-switched inverters using conventional pulse-width modulation (PWM), TCM-operated inverters achieve outstanding efficiency by significantly reducing switching losses, especially under low-load conditions. However, TCM faces the challenge of a wide variation in switching frequency throughout the fundamental cycle, which is often managed by suspending TCM operation near zero-crossings to prevent exceeding frequency limits. This article introduces a novel multilevel TCM inverter topology featuring two cascaded stages and an asymmetrically subdivided dc-link. For optimized cost and performance, the two stages employ a hybrid configuration using SiC mosfets in the high-frequency stage and Si IGBTs in the low-frequency stage. This design reduces the voltage stress on all power semiconductors and ensures full soft-switching capability even near voltage zero-crossings without excessive switching frequency fluctuations. The effectiveness of the presented topology is maximized when paired with a control algorithm that ensures continuous soft-switching across all devices and manages the necessary voltage control for the subdivided dc-link. A single-phase 2 kW hardware demonstrator with a 1.5 kV dc-link was designed and implemented to validate this concept, incorporating the proposed control algorithm. The demonstrator achieves a peak efficiency of more than 98.6% at nominal load, demonstrating the effectiveness of the full soft-switching capability provided by the new topology.