Research On Variable Mode LLC Based On Micro-grid Solar Inverter

This paper analyses and does research into the character of DC/DC converter in Small-scale grid-connected solar inverter. On account of the fact that the output voltage of PV module changes in a large range, this paper studies topological structure appropriate for this situation and finally chooses the LLC resonant converter as a research object.Firstly, this paper introduces operation modes of the LLC resonant converter when pulse frequency mode (PFM) conversion control or phase shifting control with fixed frequency is adapted respectively. The condition of LLC voltage gain is analyzed based on the sinusoidal analysis according to PFM or phase shifting control with fixed frequency. On the strength of the above-mentioned work, this paper proposes a variable modal LLC which can change the topological structure or control method of LLC according to the variation of input voltage, since the range of input voltage is particularly wide. This paper certainly analyzes the voltage gain of the variable modal LLC. Digital signal processor (DSP) is applied to make digital control in the circuit, so that the control process is quite easy and convenient with no need for extra analog circuit.With the wide input voltage range as a precondition, this paper analyzes the different work situation of LLC resonant converters and compares them with each other when different kinds of control method is adopt, and finally find out that the effective duty cycle maximizes and the circulation time of the resonant cavity minimizes when the work frequency and phase shifted angle can follow up the change of input voltage and change accordingly. Meanwhile, the LLC efficiency increases effectively.