A Novel Modulation Method for LLC Converter-Based Semi-Single-Stage Isolated Inverter with Hybrid Sliding Mode Control

The LLC-converter-based pseudo DC link semi-single-stage isolated inverters (PDL-SII) face challenge of achieving low gain outputs within the zero-voltage switching operation region. As a competitive option, the burst-like modulation technique for LLC converters offers a lower gain than conventional pulse frequency modulation, by managing the converter's activation, which is particularly advantageous for LLC converters operation in PDL-SII configuration. Despite its advantages in soft switching and low gain, conventional burst modulation suffers from increased output ripple, potentially affecting the output current quality. This is largely attributed to the sampling time delay inherent in the sampling filters, and the continued energy transmission of the LLC converter post-deactivation due to residual resonant circuit energy. To tackle these issues, a novel hybrid sliding mode control (HSMC) for LLC converters is proposed in this paper. In the proposed HSMC, the filter effect is minimized through an averaged LLC converter model, and the optimal trigger timing for deactivation and activation is accurately predicted by analyzing the phase plane trajectory of LLC converter. When applied to the LLC-converter-based PDL-SII and paired with the full and half bridge topology morphing strategy of LLC converter, the proposed HSMC presents higher efficiency and reduced total harmonic distortion (THD) characteristics. Experimental validations confirm the theoretical propositions, demonstrating that the proposed mixed control can lower THD by up to 1.6% and boost efficiency by 1% compared to conventional approaches