## 2 Bidirectional DC-DC Converters
### Classification of Bidirectional DC-DC Converter
#### 2.1.2 Isolated Bidirectional \(DC\)-\(DC\) Converters (IBDCs)
Note that the inductor and the _LVS_ half-bridge are arranged uniquely, with the _LVS_ half-bridge having dual roles, which include (**i**) serving as a boost converter to improve voltage; and (**ii**) serving as an inverter to improve the frequency of the _AC_ voltage [66]. More current is drawn by the _LVS_ boost converter from the load voltage resource when compared to the full-bridge voltage source inverter. A boost function is obtained by merging the _LVS_ half-bridge and the inductor. Dual Active Full-Bridge (_DAFB_) _IBDC_  
One of the most common topologies is to employ back-to-back bidirectional techniques that are isolated by a high-frequency transformer. Back-to-back converters can be voltage-fed or current-fed, half-bridge, or full-bridge configurations. Figure 12 illustrates the configuration of the full-bridge _IB DC-DC_ converter, which utilizes two full-bridge techniques in both sides of the transformer. The power transmission of bidirectional converters is proportional to the number of switches, and the high productivity and high power density of this topology make it appealing to hybrid energy systems [67]. The above description of power capacity with switches generally suggests that the _DAFB-IBDC_ has the greatest power capacity (efficiency of around 95%); thus, this converter is well suited for high power applications with similar hybrid energy systems. In this structure, full-bridge is employed at either end of the isolation transformer, while a soft switched phase shift approach is used to implement the control. To provide an approximate  
Figure 11: Isolated DHB bidirectional _DC-DC_ converter [31]. Figure 12: Isolated DAFB bidirectional _DC-DC_ converter [31]. square wave \(AC\) voltage, in every converter, the diagonal switching couples are switched on at once, with fifty percent duty cycle (excluding a small dead time) and with the two legs of the crossways transformer terminal having a \(180^{\circ}\) phase shift. An important factor, represented as \(\varphi\) (Phi), denotes the observed phase shift between the two \(AC\) voltages; it is important in deciding the quantity and direction of power transfer between the \(DC\) buses. This parameter can be modified to achieve a fixed frequency activity with full control [68]. Figure 13 illustrates the contrast between half- and full-bridge topologies. The half-bridge topology has all the switch-related voltage stresses as twice the \(DC\) input voltage (\(2V_{dc}\)), while the current stress and load current \(I_{ac}\) are equivalent. In the full-bridge topology, each switching device relies on the voltage stress and is equivalent to the DC input voltage (\(V_{dc}\)), while the current stress is equivalent to the load \(I_{ac}\). Figure 13 shows the total device rating (\(TDR\)) in half- and full-bridge topologies. For the full-bridge topology, the \(TDR\) is calculated as:  
\[TDR_{F}=V_{dc}\times I_{ac}\times 4(devices)=4\times P_{o} \tag{1}\]  
where \(P_{o}\) is the output power. Meanwhile, the \(TDR\) of the half-bridge topology is estimated as:  
\[TDR_{F}=2V_{dc}\times I_{ac}\times 2(devices)=4\times P_{o} \tag{2}\]  
Hence, the \(TDR\) is the same for the dual full-bridge and dual half-bridge topologies at the same output power. Additionally, it is true that half-bridge devices are exposed to twice the \(DC\) input voltage; this is beneficial for both \(EVs\) and \(HEVs\), as well as for fuel cell usage due to the low value of the \(DC\) input voltage (12 Volt battery). The dual half-bridge topology also uses fewer devices compared to the full-bridge topology. Half-Bridge-Full-Bridge Bidirectional \(DC\)-\(DC\) Converter  
Considering dual active bridge, in the case of a \(UPS\) design, an isolated bidirectional \(DC\)-\(DC\) converter was proposed, which uses a voltage-fed half-bridge topology in the primary side and a voltage-fed full-bridge topology in the secondary side of the transformer (see Figure 14) [18]. It enables easier control requirements than \(DAB\) because of the lower number of switches.