A Fast Hexagonal Sigma-Delta Strategy with DC-Bus Balancing for Three-Level Wide-Bandgap T-Type Converters

The efficiency of wide-bandgap (WBG) power converters can be significantly enhanced using high-frequency modulation techniques. This article presents a novel hexagonal sigma-delta (H-$\Sigma\Delta$) modulation designed for high-frequency three-level T-type converters employing WBG semiconductors. To simplify implementation and reduce computational burden, a fast-processing quantizer is proposed. The proposed H-$\Sigma\Delta$ strategy operates with a variable switching frequency, minimising switching losses compared to conventional pulse-width modulation (PWM) techniques, while suppressing low-frequency neutral-point voltage oscillations. The behaviour is analysed via simulation and experimentally validated on a T-type converter using silicon carbide (SiC) MOSFETs and gallium nitride (GaN) e-HEMTs. Compared against Carrier-Based PWM (CB-PWM) and Modified Sinusoidal PWM (M-SPWM) under identical conditions, experimental results demonstrate that the proposed H-$\Sigma\Delta$ modulation achieves superior performance, reaching a peak efficiency of 95.92\%. Additionally, it virtually eliminates low-frequency DC-link oscillations. Regarding power quality, the modulation mitigates Common-Mode Voltage (CMV) spikes and exhibits a favourable spectral profile when compared to the limits suggested by IEC~61000-3-2 and IEC~55025 emission standards. The analysis confirms the proposed strategy as a highly effective solution for maximising the efficiency and electromagnetic compatibility of WBG-based T-type converters.