DC-DC BIDIRECTIONAL CONVERTER TOPOLOGIES

BDC is commissioned to action the power transferring processes from battery/ultra- capacitor to attain V2G or G2V purposes with the inverter [24, 25]. The choice of this converter is essential to achieve the overall system efficiency. The two categories of BDCs are: (i) non-isolated, and (ii) isolated bidirectional converters. This section presents the various topologies used in the BDCs.

The main difference between the tw'o converters is that the isolated converter is having a high-frequency transformer between the input and output side. It brings higher power density and causes the leakage inductance stored in the transformer. Here, the stored energy creates voltage spikes on the switches, which causes voltage stress. A resistor-capacitor-diode (RCD) snubber circuit is recommended to control the switch voltage stress [26].

On the other hand, the non-isolated converter topology has a straightforward structure, low cost, high reliability, high efficiency. The non-isolated half-bridge converter is better but has low efficiency with a wide voltage conversion range. To avoid this issue, various PWM techniques are available. Three-level bidirectional DC-DC converters are suggested to the EV applications, and using this topology, the inductor size reduced by one-third of the half-bridge bidirectional converter.

FIGURE 5.12 Conventional buck-boost converter.

Non-Isolated Bidirectional Converter

The non-isolated bidirectional converters have very simple in construction, less cost, and work under different voltage range [27]. It can be classified into multilevel type, switched capacitor type, cuk type and sepic/zeta type, buck-boost type, coupled inductor type, three-level, and conventional buck/boost types. Various nonisolated bidirectional converters are shown in Figure 5.12-5.17. In that, multilevel and switched capacitor types having more switches and capacitor to attain the high voltage gain, also the control circuits of these types are very complicate [28]. Using cuk and sepic/zeta type converter gives low conversion efficiency due to the two power stages also cannot provide the wide voltage conversion range. The coupled inductor type can achieve large voltage gain by adjusting the number of turn ratios in the coupled inductor. The Circuit configuration of this type is complicated [29]. Three-level converter type having the stress on the switches is half compared to the conventional buck/boost type. In this type converter, the conversion range is narrow. The conventional bidirectional buck-boost converter has a simple circuit here the

FIGURE 5.14 Cascaded bidirectional boost converter.

limitation is that the effect of the switches and the equivalent series resistance of the inductor and capacitor will limit the step-up voltage gain. Figure 5.12 represents the modified conventional buck-boost converter. This topology is implemented with various control techniques in references [29-33]. The voltage conversion ratio in this converter is higher than the previous conventional bidirectional buck/boost converter, w'hich has 92.3-94.8% step-down voltage efficiency and step-up efficiency is 91.2-94.1%. Reference [34] presents a modified buck/boost converter in Figure 5.13 and Figure 5.14 presents the circuit, which acts as a cascaded boost converter in step-up mode and a cascaded buck converter in step-down mode. In step-up mode, it operates as CCM and in step-down mode, it operates as DCM [34]. In this topology, step-up voltage gain is high compared to conventional buck/boost topology.

FIGURE 5.16 Multi-input bidirectional converter.

A non-isolated soft-switching bidirectional LCL resonant DC-DC converter is shown in Figure 5.15 [35]. During bidirectional conversion processes, the diode and switches are getting more stress. It results in high reverse recovery loss and electromagnetic interference problems. Also, the conversion ratio is very less. However, the advantages of the non-isolated converters are the compactness and higher efficiency. To improve the power density, the high-frequency operation is required in DC-DC converters, but the switching losses are very high. To overcome this, soft switching is required. Coupled inductor converter design makes the system more complex with high voltage gain. Usually, non-isolated hard switching limits the switching

frequency, and the switched capacitor is given high efficiency compared to other topologies. Still, the number of components increases the converter cost and limits the switching frequency due to the hard switching.

Figure 5.15 represents the non-isolated soft-switching bidirectional LCL resonant DC-DC converter, which is having a half-bridge in the front end and LCL resonant with voltage doublers combined with another end [36]. The main advantages of this topology are the ZVS turn ON in both direction, and ZCS turn ON and turn OFF in both directions, low voltage stress without any additional snubber circuit, high step- up, and step-down ratio and reduced the volume of the magnetic. It attains 95.5% efficiency in boost operation and 95% efficiency in the buck operation.

Reference [37] proposed a multiple single-input DC-DC converter having advantages of simple and more compact design and reduce the overall cost of the system. Also, the regulated voltage output improves system reliability.

Always the connection of an isolated converter is magnetically coupled circuit, but non-isolated converters are coupled based on the electrical circuit connected. Using isolated converter energy transformed from source to load using the technique of time-domain multiphase is a commonly used one. This additional requirement of circuits makes the system bulky, costly. In non-isolated, the electrically coupled system having a modular structure, low cost, and the absence of transformer make it simpler and more attractive [38]. This electrically coupled circuit can combine various input power source either in parallel or series. The main limitation of parallel- connected source topologies is at a time only one input can be connected to avoid the power coupling effect. For multiple power transfer, the series-connected combination gives the solution. In a series connection using a bypass diode, it can be modified to parallel connection. Still, it increases the count of the overall components which increases the cost of the system.

The series connection, shown in Figure 5.16, operates in bidirectional with the buck, boost, or buck/boost operations. The simple construction makes very less fault occurring capability to improve the reliability of the system. In this system, all the input sources are connected to the load through a single inductor [39]. Flere, the operation of the converter is based on a single switching cycle. The passive elements stored the energy for some particular time then discharges to the load in the remaining time. The power flow is controlled by the inductor only. In this topology, the inductor charged by multiple input sources instead of a single input source.

A three-level bidirectional DC-DC converter is suitable for the photovoltaic energy conversion system proposed in [40]. Here, the advantages of these topologies are low voltage stress, and the ripple frequency of the inductor is twice the converter switching frequency. A zero-voltage transition (ZVT) three-level DC-DC converter to induce to operate in very high switching frequency to obtain the high-power density; also, it improves the efficiency. For the low cost and high-efficiency operation, the bidirectional two-quadrant switches are preferred. However, it gives low efficiency when the light load occurs. Also, due to the power losses associated with the parasitic capacitor, the passive components need large volume and increased input current ripple frequency. Interleaving topology reduces the size of those passive elements. Three-level converters, shown in Figure 5.17, are suitable for higher- level application and implementing soft switching techniques without sacrificing the efficiency to attain the power density with higher switching frequency. The interleaving method is a primary way to achieve the soft-switching method. In this topology, two identical soft switching are implemented for each pair of switches, and it achieved the ZVT for every switch during turn-on operation mode. This circuit contains two resonant inductors, one resonant capacitor, and an auxiliary switch to avoid the turn ON switches by creating a resonant between the inductor and capacitor.

For EVs, the commonly used topology is the hard switching cascaded buck-boost converter in CCM mode with conventional PWM technique [41]. This conventional PWM technique causes the switching loss because of the reverse recovery of diodes, which gives low efficiency. The PWM technique is changed from the conventional to constant frequency soft switching modulation, which performs the ZVS improve the efficiency of this converter. Also, the use of silicon carbide diodes reduces the reverse recovery losses by 67% [41].

Isolated Bidirectional Converters

In the BDC converter, usually, isolation is given by a high-frequency transformer, as shown in Figure 5.18-5.22. This method gives additional cost and losses, but it is essential when (i) the high voltage and low voltage side negatives not grounded together, and (ii) the voltage ratio is much more enough to tackle very high current and high voltage simultaneously [42-46]. There are a number of topologies available with voltage fed on both the high voltage and low voltage side. Also, it is essential to implement the current source between the circuit to get the smooth power shift. Fixing the inductor on the low voltage side needs a high current carrying magnetic component, and fixing the inductor on high voltage side needs high voltage switches because of the voltage stress, the bidirectional DC-DC converter acting as an inverter and rectifier. In the rectification mode, the current conducts by diodes and in the inverter mode switches conducts the current. In case the switches are MOSFETs, to obtain less voltage drop under synchronous rectification, it conducts in the reverse direction. For high power application, the magnetic components and interconnect parasitic are quite different and result in significant variation in losses. For low power application, interconnection is not an issue, but the number of

FIGURE 5.19 Bidirectional DC-DC voltage-fed and current-fed full-bridge converter with active clamp circuit.

FIGURE 5.20 Bidirectional DC-DC voltage-fed and current-fed full bridge converter with RCD snubber circuit.

FIGURE 5.22 Cascaded isolated bidirectional converter.

switching devices must be reduced. Half-bridge converter has a problem of floating gate driver, which can be rectified by replacing the push-pull converter. This converter requires a center tapped transformer, which makes the circuit complex in the design of high-power applications [47]. Various half-bridge current source converters were proposed to eliminate the floating gate drive and to minimize the size of the inductor and switches.

Always the current source converters considered as an isolated boost converter. The essential operation is to connect all switches in a short circuit to store energy in the inductor. While opening half of the switches, the energy will be transmitted through the transformer to the output side. From the figures, the voltage fed converter always suffering from various limitations of very high input pulsating current and limited soft-switching range, circulation current through the switches, and the magnetic devices. Also, the efficiency of high voltage amplification is very low for high input current applications [48]. Current-fed converters proved meritorious more than the voltage fed converters. It has the advantages of the less input current ripple, negligible duty cycle loss, easy to control, and RCD snubber circuit or some other snubber circuit to absorb the voltage spikes when the devices turn OFF. The active clamp snubber circuit gives more efficiency and performs zero voltage switching for the switches.

Recently, the usage of the high-frequency transformer replaced the traditional line frequency transformer for the present and future generations [43, 49]. The merits of a high-frequency transformer are light in weight, and the volume is less and cheaper. And this type of transformer avoids the distortion of voltage and current

TABLE 5.2

Comparison of Various Topologies for High Power Design

Abbreviations: IGBT. insulated-gate bipolar transistor; PWM, pulse width modulation.

waveforms, where the switching frequency is higher than the total noise of the power conversion system, such as 20 kHz. A comparison of various design considerations of isolated converter topologies for the high-power applications is given in Table 5.2.