3.1.1. Isolated and Non-Isolated Converters

The magnitude and type of electrical energy changes according to the flow direction of the energy in the battery or in the grid. Power electronics topologies are employed for this change. The bidirectional AC/DC power converter topologies used in the charging–discharging systems of batteries in V2G and V2H technology are classified as illustrated in Figure 9. The converters used for charging or energizing the grid operate in a bidirectional way. Converters have advantages as well as disadvantages in terms of quality of energy compared to preferred topologies [74].

**Figure 9.** Classification of bidirectional AC/DC power converter topologies: the bidirectional AC/DC power electronic converters are mainly of two types, isolated and non-isolated converters.

Non-isolated converters are obtained in structural terms by connecting an active and a passive semiconductor switching power element and an inductance in different ways. The operating mode of inductance converters is based on the transfer of energy that is stored in the inductor. As long as the semiconductor switching power element is actively transmitting, the energy that is provided by the source that is stored in the inductor being transferred to the cut-off by the semiconductor switching element, which results in the transfer to the load. The important disadvantage of these converters is the lack of isolation between the output and the input [75].

Isolated converters are used in situations when the electrical isolation is required in DC/DC transducer applications or where there is a high rate between input and output. Here, a transformer is used to provide isolation. In essence, the working principle of the isolated transformers is the same as the non-isolated converters. In other words, it is based on the logic of transferring the energy that is stored in the inductance. As long as the semiconductor switching element is actively transmitting, the energy that is provided by the source and stored in the inductor is transferred to the cut-off by the semiconductor switching element, which results in the transfer to the load.

The DC/DC or DC/AC power converters that are employed in charging topologies are usually controlled with two different methods, these being pulse width modulation (PWM) and frequency modulation (FM). In the FM technique, the output value is controlled by changing the pulse frequency of the semiconductor switching element, in other words, by changing its period [76]. However, this technique is mostly used in compulsory situations such as in temporary and low load situations. In addition, as a result of using this technique, fluctuations and noises occur in the output voltage. The PWM technique is widely used in industrial applications as it allows filtering of the fluctuations and noises at the input and output and because of its constant frequency operation. The PWM technique is a method in which the output value is controlled by adjusting the operating time of the semiconductor switch by changing the pulse width at constant frequency. Here, the determination of the operating time of the key by producing a control signal that is needed for the semiconductor switching element with PWM is shown in Figure 10 [77].

**Figure 10.** Pulse width modulation technique: a pulse width modulated wave is found by comparing a reference signal, Vc, and a carrier signal, Vst.
