Figure 1-2 shows a block diagram for the DC/AC stage. The inverter stage is bidirectional, enabling power conversion from DC stage to AC stage and vice versa. The topology is constituted by an H-Bridge with each group of diagonal switches operating at high frequency during one half-wave of output voltage. Additional switches placed in parallel to the grid allows an additional voltage-level across the output filter making this power conversion system a three-level topology, thus reducing the switching losses and COSS losses across the FETs. This also enables constant common-mode voltage leading to negligible leakage current since the PV input stage is decoupled from the AC grid in the freewheeling phase.

Figure 1-5 DC/AC Converter Block Diagram

This topology is a good choice for transformer-less string inverter applications where there is no isolation available between the AC grid and the PV panels. The common-mode currents are a well-known challenge in PV applications due to PV surfaces exposed over grounded roof or other surfaces in the proximity. The large surface areas can lead to high values of stray capacitance between the PV panel and ground, which can go as high as 200nF / kWp in damp environments or on rainy days, as seen in Figure 1-6. This parasitic capacitance can cause high common-mode current flowing into the system when common-mode voltage of the converters is not well mitigated and can lead to EMI and issues such as grid current distortion.

Figure 1-6 PV Panel Parasitic Capacitance

Microinverters containing transformers present high impedance return path for current, however, that is not the same case with cost-sensitive applications such as string inverters. String inverters usually present low impedance paths for return currents, hence leading to very high values of currents as shown in Figure 1-7. The leakage currents to the ground thus constitute an important issue in transformer-less concepts. Special single-phase transformer-less topologies with reduced oscillations can be implemented for such purposes and is later discussed. In addition, the introduction of frame-less panels further contributed to the reduction of such problems.

Figure 1-7 Common-Mode Noise

This DC/AC converter stage is operated at a high switching frequency of 87kHz for sinusoidal grid current control, thus allowing the EMI filter design to be compact. With the signle-phase 230V_RMS grid, an output power of 4.6kW can be achieved with an output current of 20A_RMS. The EMI filter is composed of a boost inductor split between both rails for better common-mode rejection capability, two common-mode chokes, C_x capacitors, and C_y capacitors. The EMI filter has been designed to attenuate both the differential-mode and common-mode noise injected into the grid. Additionally, electrolytic capacitors are present at the DC- link to compensate for the 100Hz power ripple present in such single-phase applications. Note that both of the half-bridges need to have a dead-time to avoid shoot-through. The current in the grid is measured and then controlled by the MCU using Proportional Resonant (PR) controllers. High-accuracy measurement of the current flowing in the Point of Common Coupling (PCC) is required to control active and reactive power. The current control requires the implementation of a Phase Locked Loop (PLL) which is synchronized with the grid voltage frequency. A DC-link voltage control loop is used to control the amplitude of the active current sink or source from the grid.