## 2 Bidirectional DC-DC Converters
### Classification of Bidirectional DC-DC Converter
#### 2.1.1 Non-Isolated Bidirectional _DC_ to _dc_ Converter (_Nibdc_)  
Due to safety concerns, this type of converter cannot be employed in high-frequency transformers to cater for any electrical isolation. _NIBD_Cs are considerably more efficient in low-power applications due to their ease of control and light weight [37]. Non-isolated bidirectional _DC-DC_ converters have been evaluated in terms of ease of control, simplicity of circuit configurations, low _EMI_, and high steeping ratio by several researchers [29, 38, 39]. This type is considered the fundamental circuit of bidirectional _DC-DC_ converters. Figure 4 illustrates the structure of this converter [17, 31]. It is a combination of the buck-boost converter in parallel but moving or oriented in opposite directions. Power flows from the high voltage (_Vb_) side to the low voltage (_Va_) side (buck approach) and operates in the reverse manner as a boost converter [18]. In the buck mode, switch _Q1_ is ON by duty cycle control, while _Q2_ is OFF in this mode. Similarly, when step-down _Q2_ is ON, _Q1_ is OFF. Cross conduction can be avoided by setting a dead time between both switches to ensure safe operation [40]. This topology is basic and has noteworthy effectiveness [41]. Bidirectional SEPIC-ZETA _DC_ to _dc_ Converter  
This topology has two modes of power flow; these are positive and backward modes. In the forward power flow, this converter works as _SEPIC_, while in the negative mode, the converter works as _ZETA_. The structure of a _SEPIC-ZETA DC-DC_ converter is shown in Figure 5; it serves in the adjustment of _Cuk_ converters so that their output will not have inverse polarity, as is the case with _Cuk_ converters. The operation of this converter relies on  
Figure 4: Bidirectional buck–boost converter [18]. Figure 3: Classification of bidirectional _DC-DC_ converters. both buck and boost techniques [42]; positive power flow is achieved by switching ON \(Q1\) and turning OFF \(Q2\) such that it can work as a buck converter (SEPIC mode). However, in the reverse power flow mode, \(Q2\) is switched OFF so that the system will work as a boost converter (\(ZETA\) mode). The use of two inductors, \(L1_{\text{dc}}\) and \(L2_{\text{dc}}\), can reduce the ripple from voltage yield and voltage rating overwork on switches. Bidirectional Cuk Converter  
Cascaded Bidirectional Buck-Boost Converter  
Figure 7 displays another converter topology that can be achieved by cascading a buck converter with a boost converter. This topology is the outcome of cascading two buck-boost _BDCs_[18]. Each of the four quadrants is conceivable with this topology; thus, this topology works in buck and boost modes in both directions. The four-quadrant activity of this topology makes this topology generally adaptable. Despite this, it has some weaknesses, such as the use of a higher number of switches, which causes higher switching losses; it also relies on complex control algorithms and experiences additional operating losses due to the inverse diode recovery [38; 46; 47]. In _ESSs_, the cascaded buck-boost (\(CBB\)) converter is frequently utilized. In comparison to the combined half-bridge (\(CHB\)) converter, the \(CBB\) converter is smaller and more efficient at converting power because it only utilizes one inductor [48]. Figure 5: Bidirectional SEPIC–ZETA \(DC\)/\(DC\) converter [38]. Figure 6: Bidirectional Cuk converter [38]. Switched Capacitor Bidirectional DC-DC Converter  
In bidirectional _DC-DC_ converters, capacitor exchange is achieved by employing integrated circuits (_ICs_) in _DC-DC_ converter circuits (see Figure 8). This integration of _ICs_ is less complex due to the lack of any need for magnetic equipment in the non-isolated _DC-DC_ converter. Due to the huge number of inactive elements that account for electromagnetic interfacing (_EMI_), such converters suffer from high ripple at the output.