The inverter power stage is kept bidirectional to enable power conversion from DC link into grid compatible AC power and vice versa. State of the art string inverters tend to be grid-tied and synchronized to the grid at all times via Phase-Locked Loop (PLL). The inverter or PFC stage can be divided into two broad categories namely whether the grid is single-phase or three-phase. Single-phase further dictates the rating of the devices on whether it is split-phase (110V_RMS in Japan, the USA etc) or 230V_RMS (in Europe, ROA, and so on).

A grid-tied inverter has the additional task of synchronizing in amplitude, frequency, and phase with the existing grid comprised of multiple sources and loads. It also needs to address the situation of detecting and isolating itself from the grid in the event of any faults in the grid like black-out, brown-out, overvoltage, and so forth. This is called anti-islanding protection.

Present-day string inverters which tend to be cost and power density competitive are generally transformer-less in nature. Transformer-less string inverters present low impedance paths for return currents. This leakage current is 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. 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. Issues with safety and Residual Current Detection (RCD) can be found that needs to be addressed by the topology that is being used. Hence the corresponding topology that is selected, needs to work around this problem.

Figure 2-4 Single-Phase DC/AC Stage Block Diagram

Figure 2-4 represents a HERIC-based DC/AC converter topology present in TIDA-010938. This can also be configured into H-Bridge mode when only FETs Q6, Q7, Q8, Q9 operate and Q10 and Q11 are not used. Various buck derived non-isolated topologies modulated with a sine PWM are used for the inverter stage. These include topologies for single-phase such as two-level H-Bridge with bipolar modulation, three-level H-bridge with unipolar modulation, HERIC and totem-pole (TIDA-010933 which is a 1.6kW rated for inverter stage). TIDA-010938 depicts an inverter stage rated up to 4.6kW and can be configured into unipolar, bipolar and HERIC based converters. Table 2-1 lists the benefits and challenges with each of these systems.