Perturb and Observe MPPT with slope verification to achieve ZVS in a CLLC resonant converter

Micro-inverters have attracted the attention in photovoltaic (PV) systems because they mitigate the effects of partial shading. A micro-inverter can be implemented with two-stages: DC-DC step-up conversion and DC-AC conversion (inverter). Resonant converters present high potential for the first stage due to their high voltage gain, efficiency, and switching frequency. These characteristics allows a small footprint and low switching losses due to possibility of zero-voltage-switching (ZVS) operation. The DC-DC converter typically performs the maximum power point tracking (MPPT) while the DC-AC injects the power to the grid. This paper proposes a MPPT technique for a resonant CLLC DC-DC converter to guarantee Zero Voltage Switching (ZVS). First, the resonant converter is analyzed to determine the resistance observed by the PV panel and to determinate the frequency range which the converter operates with ZVS. Then, a modified Perturb and Observe algorithm is proposed, by performing perturbations in the DC-DC switching frequency. The proposed MPPT also verifies the derivative of the converter input resistance regarding the frequency to ensure that the converter operates in ZVS when the MPP is tracked. Finally, Matlab/Simulink simulations validate the proposed MPPT technique with a resistive load and a nonlinear load (DC-AC connected to the grid). The results show satisfactory performance, high steady-state efficiency, and good dynamic response.