## 4 Grid-Connected PV Inverter Topologies
### Single-Stage Buck-Boost Inverters  
Generally, topologies can only offer the same input voltage at the output, for example, traditional buck inverter-based topologies. However, many applications require to boost the input voltage for proper operation. The main differences between the buck-boost inverter topology with traditional full-bridge buck inverter and boost inverter are summarized in Reference [83]. The conventional full-bridge inverter is a converter that does not produce a larger output at the input but is easily controlled. For its part, the boost inverter, like the buck-boost inverter, generates an AC output higher than the input, controls the duty cycle. Recent research focuses on designing a single-stage buck-boost inverter topology [84; 85; 86], basically to reduce the number of components such as switches; also, the two-stage power conversion results in the complex circuit structure and control algorithm [87]. Generally, single-stage buck-boost inverters consist of distinct circuits operating individually in buck/boost or positive/negative modes, the latter leading to crossover distortion in the output current [88]. Usually, in a single stage, an output peak ac voltage higher than the input dc voltage is obtained by using a full-bridge inverter followed by a low-frequency step-up transformer. However, the bulky transformer increases the volume, loss, and system cost [89]. This type of converter compensates for variations in renewable energy sources, such as: shading, irradiance, cloudiness and temperature. Other typologies may offer similar results to these natural variations, for example, the traditional full-and-bridge inverters followed by a boost-type DC/DC converter. However, it requires multiple inductors and capacitors for the same result. The differential boost inverter presented in Reference [90] provides buck-boost operation in a single-stage from two identical DC/DC boost converters. This topology employs two inductors; all its switches operate at high frequency, and it has no common ground. In Reference [91], a single-stage bidirectional buck-boost transformerless inverters using a single inductor and eliminating the common-mode leakage current is proposed. The scheme is shown in Figure 11 has two input supplies, these supplies can have the same or different voltage value, so the topology allows to obtain the energy from several types of clean sources. The authors highlight that due to its buck-boost ability, one of the advantages of the proposed inverter is its capability of generating a sinusoidal output voltage even if \(Vin_{1}\
eq Vin_{2}\). Another variant is also described in the same work, an inverter from a single input source. This scheme is similar to the traditional half-bridge inverters [92], noting that it does not require a higher input voltage, with the use of two capacitors \(C_{1}\) and \(C_{2}\). The modes of operation are presented in Figure 12. The inverter has 3 modes of operation, which make it bidirectional and can provide reactive power. The paper points out that the leakage current is reduced since the output voltage \((V_{0})\) is low frequency sinusoidal ac voltage, \(Vin_{1}\) and \(Vin_{2}\) are dc constant voltages and the values of the parasitic capacitances are extremely small. The scheme presented requires a higher number of devices compared to the other topologies presented up to this point. Besides, it is valid to specify that, although two of its switches operate at a low-switching frequency \(S_{1}\) and \(S_{2}\) and conduction losses are negligible in this case, two other switches \(S_{3}\) and \(S_{4}\) operate at one high-switching frequency. In the latter case, the losses are due to the discharge of the junction capacitor \(C_{oss}\). The work shows an important comparison of the buck-boost inverters. The comparison summarizes some of the essential features of this type of converter: common-mode leakage current, bidirectional operation, components amount, operated as grid-connected or stand-alone.