A Hybrid Second Harmonic Current Mitigation Strategy for Two-Stage Single-Phase DC–AC Converters

The instantaneous AC-side output power of a two-stage single-phase inverter pulsates at twice the output voltage frequency, inducing second harmonic current (SHC) in the front-end DC–DC converter. While conventional SHC mitigation methods mainly focus on controller optimization for PWM-controlled DC–DC converters, LLC resonant converters, which have been widely adopted in two-stage single-phase inverters for high efficiency and soft-switching characteristics, lack tailored solutions due to frequency modulation complexities. To address this gap, this paper first analyzes the propagation mechanism of the SHC in terms of converter output impedance. Then, by simultaneously lowering the open-loop gain and increasing the output impedance of the DC–DC converter at 2fN, this paper proposes a hybrid SHC mitigation strategy that achieves low SHC and fast dynamic performance for frequency-modulated LLC converters. Finally, a 28 V DC to 220 V/50 Hz AC inverter was developed, and the experimental results verified the effectiveness of the proposed control strategy.