Highly Efficient Grid-Tied Micro-Inverter For Photovoltaic System

Compared to centralized inverter and string inverter, microinverter has the feature of “Plug-and-Play” operation, ease of expandability, enhanced reliability and improved energy harvest. In terms of structure, microinverter can be classified into two types: single stage microinverter and two stages microinverter. This paper concentrates on the second power stage – DC/AC conversion, proposing a control strategy based on boundary current mode(BCM), which can realize zero voltage switching(ZVS) of MOSFET and improve the efficiency of micoinverter.The operation principal of the control strategy realizing ZVS based on BCM of inductor Lf is analysed in this paper. Single polar modulation and bi-polar modulation are compared in switching frequency and THD of output current respectively. Compared to bi-polar modulation, the switching frequency in single polar modulation is lower, which leads to higher efficiency. However, the THD of output current in single modulation is a little worse than that in bi-polar modulation. Thus, single polar modulation is a good choice to improve efficiency if single polar modulation meets the requirement of THD.Depending on the boundary shape of the current limits, there are three possible inductor current modulations for achieving ZVS operation: BCM with fixed reverse current, BCM with variable reverse current, and BCM with fixed bandwidth. BCM with fixed reverse current and BCM with fixed bandwidth are two specific cases of BCM with variable reverse current. In practical application, to select a better inductor current mode, a trade off between the RMS current of inductor and switching frequency should be considered.There are two ways to achieve the control strategy based on BCM of inductor Lf : software predictive control and hardware reset control, which are compared in this paper. A new method of ZVS BCM current control is proposed, which is a combination of hardware reset and predictive control and is referred to as Hybrid control. In practical contol system, there are sampling time delay, PLL time delay and drive time delay, which may distort the inductor current and output current. And a corresponding compensation method is proposed to compensate the effect of time delay.The parameter design of main circuit and control circuit for 250 W BCM current controlled inverter is given in this paper. A 250 W BCM current controlled inverter simulation model is built in Matlab/Simlink, and an inverter prototype is built to verify the validity of the contol strategy based on BCM, the peak efficiency is 98.5%. The BCM current contolled strategy can realize ZVS and hence improve the effieciency of inverter, which is sepecially suitable for low power application such as microinverter.