3.1.3. Bi-Directional Buck–Boost Isolated Converter

The charge level of the battery is required to be equal or more than the voltage converted from AC to DC. Similarly, it is necessary that the current level drawn from the battery is adequate in the conversion process. If these conditions do not exist, voltage collapse and similar negative situations occur. For this reason, the bi-directional buck–boost converter topology is needed in power electronics. In Figure 12, a bi-directional buck–boost converter and bi-directional isolated converter circuits are depicted. In the conversion process, the S1 switch is used in buck operation, and the S2 switch is used in boost operation [80]. Thus, the desired voltage level for the battery charge and the voltage levels needed in the battery to convert the energy to AC are obtained. In the bidirectional isolated converter, the DC voltage converted from the AC is transferred to the other side through an AC voltage

and isolated transformer; then, it is converted into DC to charge the battery. The same processes also apply in transferring from the battery to the grid. Firstly, the voltage that is obtained from the battery is converted into AC and then transferred from the isolated transformer to the other side, thereby being converted into an alternating current through the AC voltage corrector circuit. Here, the grid and the battery part of the circuit are isolated with the isolated transformer circuit to ensure circuit protection [80,81].

**Figure 12.** Bi-directional buck–boost converter and bi-directional isolated converter circuits.

3.1.4. Non-Isolated Charging Topology with PWM and Bi-Directional Buck–Boost DC/DC Converter

In Figure 13, the non-isolated charging topology, which consists of PWM (pulse width modulation) and bidirectional buck–boost DC/DC converter are shown [82]. Firstly, the AC grid signal is converted into DC voltage by the converter circuit and is filtered by the capacitor; then the battery is charged by using the S5 switch (reducing converter). Similarly, the DC voltage obtained from the battery is increased by the switch S6 (increasing converter), and converted into AC by the inverter circuit and given to the grid [83–85].

**Figure 13.** Non-isolated charging topology with PWM (pulse width modulation) and bidirectional buck–boost DC/DC converter.

3.1.5. The Non-Isolated Charging Topology with PWM and Bi-Directional Cascade DC/DC Buck–Boost Converter

Figure 14 illustrates the non-isolated topology, which consists of PWM (pulse width modulation) and the bidirectional cascade DC/DC buck–boost converter. Firstly, the grid signal is converted into DC voltage by the rectifying circuit and is then filtered by the capacitor and the coil. Then, the battery is charged by using the buck–boost converter circuit. Similarly, the DC energy obtained from the battery is increased with a converter circuit, which increases or decreases the voltage to the grid, and is then converted into AC voltage by the inverter circuit [86].

**Figure 14.** The non-isolated charging topology with PWM and bidirectional cascade DC/DC buck–boost converter.

3.1.6. The Two-Stage Topology with PWM Convertor—Active Double Bridge and Series Resonance Convertor

In Figure 15, the bidirectional topology is highlighted, consisting of a PWM converter and active double-bridge. In Figure 16, a two-stage topology is given, consisting of a PWM converter and series resonance converter. In both topologies, the grid and battery sides are isolated by using an isolated transformer [87,88]. In Figure 16, a capacitor is used in the topology, which consists of a series resonance converter, to increase the output voltage and efficiency [20,89]. Full-bridge AC/DC converter with PWM controllers are also widely used in different switching power converters [90,91].

**Figure 15.** The two-stage topology with PWM convertor and active double bridge converter.

**Figure 16.** The two-stage topology with PWM converter and series resonance convertor.

## 3.1.7. Buck–Boost DC/DC Convertors

Buck–boost converters essentially consist of non-isolated type power converters, which consist of a functional combination of a buck converter and a boost converter. The factor, which determines whether such convertors work as buck or boost convertors, is determined by the duty rate (D), which is the rate of the pulse width to the total period [92]. When the semiconductor switching element in its structure is in the transfer position, it is fed only by the inductance, and in this way, the current passing through the inductance increases in a linear way, and the energy level of the inductance is increased [93]. The feeding of the load is provided by a capacitor. When the semiconductor switching element is in the cut-off position, the power diode starts transmission, and the output is fed by the energy that is accumulated in the inductance [94]. After this point, the inductance current decreases in a linear way, and the energy level of the inductance is reduced. Here, the power elements are exposed to the total of the input and output voltages [95]. In addition, since the direction of the output voltage is reverse to the input voltage direction, these converters are also known as inverted convertors [10]. The circuit structures, which show the basic circuits of the semiconductor switching element and the transmission and cutting status of the semiconductor switching element and basic waveforms, are given in Figure 17.

**Figure 17.** (**a**) Basic circuit, (**b**) status with turned-on IGBT, (**c**) status with turned-off IGBT, and (**d**) waveforms of the circuit of the buck–boost convertor.
