# A Transformer-Less Buck-Boost Grid-Tied Inverter with Low Leakage-Current and High Voltage-Gain
###### Abstract  
To improve the efficiency of photovoltaic (PV) grid-tied systems and simplify the circuit structure, many pseudo DC-link inverters have been proposed by combining a sinusoidal pulse-width modulation (SPWM) controlled buck-boost converter and a low-frequency polarity unfolder. However, due to the non-ideal characteristics of power diodes, the voltage-gain of a buck-boost converter is limited. To meet the needs of grid-connected systems with low input voltage and 220 Vrms utility, this paper uses two two-switch buck-boost converters with coupled inductors to develop a transformer-less buck-boost grid-tied inverter with low leakage-current and high voltage-gain. The proposed inverter is charging on the primary side of the coupled inductor and discharging in series on the primary side and the secondary side so that the voltage-gain can be greatly increased. Furthermore, the utility line can be connected to the negative end of the PV array to suppress leakage current, and the unfolding circuit can be simplified to reduce the conduction losses. High-frequency switching is only performed in one metal-oxide-semiconductor field-effect transistor (MOSFET) in each mode, which can effectively improve conversion efficiency. A prototype was implemented to obtain experimental results and to prove the validity of the proposed circuit structure. leakage-current; voltage-gain; transformer-less; buck-boost; inverter; grid-tied +
Footnote †: 2021: 11, 3625. [[https://doi.org/10.3390/app11083625](https://doi.org/10.3390/app11083625)]([https://doi.org/10.3390/app11083625](https://doi.org/10.3390/app11083625))  
Footnote †: 2021: 11, 3625. [[https://doi.org/10.33390/app11083625](https://doi.org/10.33390/app11083625)]([https://doi.org/10.33390/app11083625](https://doi.org/10.33390/app11083625))  
Footnote †: 2021: 11, 3625. [[https://doi.org/10.3390/app11083625](https://doi.org/10.3390/app11083625)]([https://doi.org/10.3390/app11083625](https://doi.org/10.3390/app11083625))
voltage. Since the high-frequency energy processing is only performed once, efficiency can be improved by reducing power losses. Among these inverters, the flyback and forward converters can be used as the front-end circuit to achieve high voltage-gain and electrical isolation [7; 8; 9]. To apply to PV grid-tied systems with wide-ranging input-voltage changes, non-isolated pseudo DC-Link inverters are usually implemented by buck-boost converters [10; 11; 12; 13]. If the power switches of the two-switch buck-boost converter are switched independently, the non-isolated inverter can work with the principle of either buck type or boost type. Part of the energy could be directly transferred to the output, which can improve conversion efficiency. Theoretically, the buck-boost converter has extremely high voltage gain. However, in practice, due to the non-ideal characteristics of the power elements and the reverse-recovery issue of diodes under a high duty ratio, the voltage gain is restricted. When the input voltage is low, these inverters are not feasible for the mains with high AC voltage. Moreover, in the grid-tied applications, because of high efficiency, small size, and low cost, the transformer-less inverters are more attractive than those with transformers [14; 15]. However, there is a parasitic capacitance between the PV panel and the frame ground, which may induce leakage current and cause electromagnetic interference, current harmonics, and power loss [16]. There are three main methods to suppress leakage current [17]: (1) Separating the PV array from utility-line in freewheeling mode; (2) Connecting the mains to the center of two DC-bus capacitors to limit the common mode voltage (CMV); (3) Connecting the mains directly to the negative end of the PV array. Distinguished by the circuit structures, transformer-less inverters with low leakage current can be classified as: (1) Diode-clamped architecture [18], (2) H5 [19], (3) HERIC [20], (4) oH5 [21], (5) H6 [22], and (f) HBZVR [23].