*3.5. Half Bridge Converter Topology*

The concept of two inductor boost converters was introduced by [46]. The boost converter topology is the boost version of the abovementioned current double topology, also called the HY-Bridge rectifier. Many papers have already been published on the high-power low-Vin application of the two inductor boosts [46–51], representing some important works on this topic. Two inductor isolated boost converters are often referred to as half-bridge converters, as shown in Figure 15. A 1 kW isolated current fed half-bridge LLC resonant DC-DC converter of 24–28 V input and 400 V output was presented in [52]. An un-regulated LLC converter is implemented, which acts as isolated voltage amplifier having constant voltage gain. Experimental efficiency of 90.2 % was achieved with 24 volts input under full load conditions. The LLC converter has inherent bi-directional power flow capability. A 1.2 kW isolated current fed active clamped half-bridge circuit with a Vin range of 28–43 V and an output of 380 Volts is presented in [53]. The proposed converter in this paper is compared with the existing converter topologies. The converter also tested for high power rating, and overall efficiency of 94% was achieved with better

component utilization. Here a 200 W active clamped L-L current fed half-bridge isolated DC-DC converter with a 22 V input and 350 V output. [54]. The topology shown in this study achieves a wide-ranged ZVS of primary side switches from full load to light load conditions. Moreover, the auxiliary active clamp circuit absorbs the turn-off voltage spikes and also assists in achieving soft switching of primary devices [55]. Represents a 1 kW modified isolated two-inductor boost by active clamping and reset. The two transformers integrated by the individual rectifiers are connected in parallel on the input and output sides. Triangular switch currents can be observed due to active clamping. The Vin range is 26–50 V. The obtained Vout is 400 V. At 600 W output power, the maximum efficiency value is 95.6%. For the measured efficiency, the Vin condition is not published. [56] have used a 1 kW two-inductor boost converter with an active clamping. The Vin is 48 V. The observed Vout is 350 V. At a power rating of 500 W, an approximate peak efficiency of 87% is observed. The efficiency value drops to 77% at 1 kW output. A full-bridge boost converter reports 6–10% less efficiency on a comparative basis. As a part of the two-stage DC-DC converter for FC applications, a 1 kW two-inductor boost stage is designed in [57] as depicted in Figure 16. *Energies* **2022**, *15*, x FOR PEER REVIEW 11 of 22 **Table 3.** Parameter analysis of various DC-DC converters. **Parameter [42] [21] [43] [44]** MOSFET voltage stress <sup>0</sup> 1 + 2 − 0 2(1 + ) 0 1 + 0 2 MOSFET Soft switching ZVS ZVS Hard switching ZVS No. of MOSFETs 2 2 1 2 The voltage stress on output diode <sup>0</sup> 1 + 2 − 0 2 <sup>0</sup> 1 + 0 2 Soft switching of diodes ZCS Hard switching Hard switching ZCS Diodes 3 4 3 2 Number of magnetic components 1 1 1 2 Voltage gain 1 + 2 − 1 − 2(1 + ) 1 − 1 + 1 − 2 1 −

**Figure 14.** Asymmetrical forward cells of stacked multiple output topology. **Figure 14.** Asymmetrical forward cells of stacked multiple output topology.

The converter efficiency is about 94% for a 100 kHz frequency of operation, and the output power is 300 W. The conventional isolated converter with N-outputs requires 2 N primary switches. The above circuit requires *N* + 1 primary switches to independently regulate the secondary side N output voltages [45]. *3.5. Half Bridge Converter Topology* Ref. [50] represent a 1.5 kW bi-directional two-inductor boost for a bi-directional interface between a 28 V and a 270 V aircraft power bus. On the low voltage side, active clamping and the rest is used to clamp the switching overvoltage. The range is 22–32 V. At Vin = 32 V and 750 W output, a peak efficiency value of 96% is achieved in the boost mode. Efficiency drops below 89% at 22 Vin and 1.5 kW output. A typical layout of DC-DC dual active bridge converter is shown in Figure 17.

The concept of two inductor boost converters was introduced by [46]. The boost converter topology is the boost version of the abovementioned current double topology, also called the HY-Bridge rectifier. Many papers have already been published on the high-power low-Vin application of the two inductor boosts [46–51], representing some important works on this topic. Two inductor isolated boost converters are often referred to as half-bridge converters, as shown in Figure 15. A 1 kW isolated current fed halfbridge LLC resonant DC-DC converter of 24–28 V input and 400 V output was presented in [52]. An un-regulated LLC converter is implemented, which acts as isolated voltage

with 24 volts input under full load conditions. The LLC converter has inherent bi-directional power flow capability. A 1.2 kW isolated current fed active clamped half-bridge circuit with a Vin range of 28–43 V and an output of 380 Volts is presented in [53]. The proposed converter in this paper is compared with the existing converter topologies. The converter also tested for high power rating, and overall efficiency of 94% was achieved with better component utilization. Here a 200 W active clamped L-L current fed halfbridge isolated DC-DC converter with a 22 V input and 350 V output. [54]. The topology shown in this study achieves a wide-ranged ZVS of primary side switches from full load to light load conditions. Moreover, the auxiliary active clamp circuit absorbs the turn-off voltage spikes and also assists in achieving soft switching of primary devices [55]. Represents a 1 kW modified isolated two-inductor boost by active clamping and reset. The two transformers integrated by the individual rectifiers are connected in parallel on the input and output sides. Triangular switch currents can be observed due to active clamping. The Vin range is 26–50 V. The obtained Vout is 400 V. At 600 W output power, the maximum efficiency value is 95.6%. For the measured efficiency, the Vin condition is not published. [56] have used a 1 kW two-inductor boost converter with an active clamping. The Vin is 48 V. The observed Vout is 350 V. At a power rating of 500 W, an approximate peak efficiency of 87% is observed. The efficiency value drops to 77% at 1 kW output. A full-bridge boost converter reports 6–10% less efficiency on a comparative basis. As a part of the two-stage DC-DC converter for FC applications, a 1 kW two-inductor boost

with 24 volts input under full load conditions. The LLC converter has inherent bi-directional power flow capability. A 1.2 kW isolated current fed active clamped half-bridge circuit with a Vin range of 28–43 V and an output of 380 Volts is presented in [53]. The proposed converter in this paper is compared with the existing converter topologies. The converter also tested for high power rating, and overall efficiency of 94% was achieved with better component utilization. Here a 200 W active clamped L-L current fed halfbridge isolated DC-DC converter with a 22 V input and 350 V output. [54]. The topology shown in this study achieves a wide-ranged ZVS of primary side switches from full load to light load conditions. Moreover, the auxiliary active clamp circuit absorbs the turn-off voltage spikes and also assists in achieving soft switching of primary devices [55]. Represents a 1 kW modified isolated two-inductor boost by active clamping and reset. The two transformers integrated by the individual rectifiers are connected in parallel on the input and output sides. Triangular switch currents can be observed due to active clamping. The Vin range is 26–50 V. The obtained Vout is 400 V. At 600 W output power, the maximum efficiency value is 95.6%. For the measured efficiency, the Vin condition is not published. [56] have used a 1 kW two-inductor boost converter with an active clamping. The Vin is 48 V. The observed Vout is 350 V. At a power rating of 500 W, an approximate peak efficiency of 87% is observed. The efficiency value drops to 77% at 1 kW output. A full-bridge boost converter reports 6–10% less efficiency on a comparative basis. As a part of the two-stage DC-DC converter for FC applications, a 1 kW two-inductor boost

*Energies* **2022**, *15*, x FOR PEER REVIEW 12 of 22

**Figure 15.** Two inductors isolated boost converter or Half Bridge converter. **Figure 15.** Two inductors isolated boost converter or Half Bridge converter. **Figure 15.** Two inductors isolated boost converter or Half Bridge converter.

stage is designed in [57] as depicted in Figure 16.

stage is designed in [57] as depicted in Figure 16.

**Figure 16.** Two inductors isolated boost converter with active clamp. **Figure 16.** Two inductors isolated boost converter with active clamp. **Figure 16.** Two inductors isolated boost converter with active clamp. DC dual active bridge converter is shown in Figure 17.

**Figure 17.** DC-DC converter with Dual active bridge. **Figure 17.** DC-DC converter with Dual active bridge.

A current fed hybrid dual active bridge DC-DC converter reduces the input highfrequency ripple current. While Power MOSFETs are switched with the ZVS technique. Low-voltage FC power conditioning systems employ two active bridge converters. Four power MOSFETs (<sup>1</sup> , 1, <sup>2</sup> , and 2) and two inductors L <sup>1</sup> and L<sup>2</sup> make up the input side. 1 , 2 , 3 , and 4 are the four MOSFETs that make up the output side. The auxiliary half-bridge consists of 5 and 6 . The power MOSFETs and ( and ) capacitors make up the auxiliary half-bridge. The transformer T is used to link the input A current fed hybrid dual active bridge DC-DC converter reduces the input highfrequency ripple current. While Power MOSFETs are switched with the ZVS technique. Low-voltage FC power conditioning systems employ two active bridge converters. Four power MOSFETs (*T*1, *T*1*<sup>a</sup>* , *T*2, and *T*2*a*) and two inductors L <sup>1</sup> and L<sup>2</sup> make up the input side. *Sw*1, *Sw*2, *Sw*3, and *Sw*<sup>4</sup> are the four MOSFETs that make up the output side. The auxiliary half-bridge consists of *Sw*<sup>5</sup> and *Sw*6. The power MOSFETs and (*C<sup>d</sup>* and *Cu*) capacitors make up the auxiliary half-bridge. The transformer T is used to link the input and output sides. Here, the ratio of the transformer turns to the leakage inductance L <sup>k</sup> is 1: n [58]

#### and output sides. Here, the ratio of the transformer turns to the leakage inductance L <sup>k</sup> is 1: n [58] *3.6. Full Bridge Converter Topology*

*3.6. Full Bridge Converter Topology* This section listed the published solutions of full-bridge converter topology for high gain DC-DC converters, as shown in Figure 18. A 500 W current fed full bridge isolated ZVS

This section listed the published solutions of full-bridge converter topology for high

cations is presented in [59]. This converter uses active clamp switch to clamp the voltage spikes across the full bridge switches in the turn-off mode. Moreover, this active clamp switch helps to achieve soft switching of primary side devices. For example, a 100 W full-bridge isolated ZVS DC-DC converter with an input range of 48 V and an output range of 380 V is presented in [60]. The proposed converter uses an integrated magnetic concept to utilize the transformer better. Though the converter is unsuitable for high power grid applications, soft switching is claimed for 100 kHz switching operation. A 1.2 kW current-fed full-bridge topology with an input of 30 V and an output of 600 V was observed. The presented converter topology uses the current fed full bridge topology for FC applications. Based on the theoretical limitations of transferrable power, the

optimized converter is designed for the given specifications.

**Figure 18.** Active clamp full-bridge current fed converter.

active clamp full-bridge converter with 22 V input and 350 V output for FC applications is presented in [59]. This converter uses active clamp switch to clamp the voltage spikes across the full bridge switches in the turn-off mode. Moreover, this active clamp switch helps to achieve soft switching of primary side devices. For example, a 100 W full-bridge isolated ZVS DC-DC converter with an input range of 48 V and an output range of 380 V is presented in [60]. The proposed converter uses an integrated magnetic concept to utilize the transformer better. Though the converter is unsuitable for high power grid applications, soft switching is claimed for 100 kHz switching operation. A 1.2 kW current-fed full-bridge topology with an input of 30 V and an output of 600 V was observed. The presented converter topology uses the current fed full bridge topology for FC applications. Based on the theoretical limitations of transferrable power, the optimized converter is designed for the given specifications. cations is presented in [59]. This converter uses active clamp switch to clamp the voltage spikes across the full bridge switches in the turn-off mode. Moreover, this active clamp switch helps to achieve soft switching of primary side devices. For example, a 100 W full-bridge isolated ZVS DC-DC converter with an input range of 48 V and an output range of 380 V is presented in [60]. The proposed converter uses an integrated magnetic concept to utilize the transformer better. Though the converter is unsuitable for high power grid applications, soft switching is claimed for 100 kHz switching operation. A 1.2 kW current-fed full-bridge topology with an input of 30 V and an output of 600 V was observed. The presented converter topology uses the current fed full bridge topology for FC applications. Based on the theoretical limitations of transferrable power, the optimized converter is designed for the given specifications.

*Energies* **2022**, *15*, x FOR PEER REVIEW 13 of 22

DC dual active bridge converter is shown in Figure 17.

**Figure 17.** DC-DC converter with Dual active bridge.

, 1, <sup>2</sup>

auxiliary half-bridge consists of 5 and 6

power MOSFETs (<sup>1</sup>

, 2

, 3

*3.6. Full Bridge Converter Topology*

side. 1

is 1: n [58]

mode. Efficiency drops below 89% at 22 Vin and 1.5 kW output. A typical layout of DC-

A current fed hybrid dual active bridge DC-DC converter reduces the input highfrequency ripple current. While Power MOSFETs are switched with the ZVS technique. Low-voltage FC power conditioning systems employ two active bridge converters. Four

capacitors make up the auxiliary half-bridge. The transformer T is used to link the input and output sides. Here, the ratio of the transformer turns to the leakage inductance L <sup>k</sup>

This section listed the published solutions of full-bridge converter topology for high gain DC-DC converters, as shown in Figure 18. A 500 W current fed full bridge isolated ZVS active clamp full-bridge converter with 22 V input and 350 V output for FC appli-

, and 2) and two inductors L <sup>1</sup> and L<sup>2</sup> make up the input

. The power MOSFETs and ( and )

, and 4 are the four MOSFETs that make up the output side. The

**Figure 18.** Active clamp full-bridge current fed converter. **Figure 18.** Active clamp full-bridge current fed converter.

A 5 kW isolated full bridge topology is proposed to apply FC vehicles [60]. The voltage clamping concept was introduced using a passive circuit to clamp the primary side switch blocking voltage. The proposed converter was analysed with a 24 V input and an output of 300 V. The calculated efficiency at peak power was 94%. Limited design data are provided to validate the converter. A soft switched 1 kW full-bridge isolated converter is demonstrated in [61]. During the switching, an overlap period of slow resonant commutation is achieved with the proposed converter. For the primary side switches, ZCS turn-off and ZCS turn-on is achieved. With the Vin = 22–27 V, Vout = 1 kV, a peak efficiency of 88% was achieved with 22 V input. An isolated full bridge converter for a 1.4 kW power level is proposed with a resonant LC circuit [62]. The resonant circuit is formed by connecting resonant capacitors parallel to the primary side switches and the LC tank circuit, forming a complete resonant circuit. With a Vin of 100 V and an output of 374 V with a narrow band frequency regulation, the maximum efficiency achieved is 90%.

#### *3.7. Resonant Converters*

Below, Figure 19 shows the Series Resonant Converter topology (SRC). This paper [63] uses a parallel tank circuit formed by an (L-C)||L combination to achieve soft switching of high-frequency switches. The important feature of these converters including (1) Achieving an improved efficiency even at varying load and line conditions. (2) A wide range of soft switching ZVS can be achieved. (3) The Peak current capability of the switch varies with the input current variation and not with the load current changes.

is 90%.

*3.7. Resonant Converters*

**Figure 19.** Series resonant converter having inductive output filter. **Figure 19.** Series resonant converter having inductive output filter.

Here Figure 20 shows is a full bridge phase shifted converter having an inductive output filter configuration. The Soft switched converter configuration for high-power applications vividly uses it. The proposed converter configuration uses a constant frequency capable of realizing ZVS of the main switches on the primary side with a minimal circulating circuit configuration. The ZVS is realized with a filter inductance, a leakage inductance of the transformer, a parasitic capacitance of the switches, and a snubber capacitance. The phase-shifted technique achieves control over the Vout with constant fre-Here Figure 20 shows is a full bridge phase shifted converter having an inductive output filter configuration. The Soft switched converter configuration for high-power applications vividly uses it. The proposed converter configuration uses a constant frequency capable of realizing ZVS of the main switches on the primary side with a minimal circulating circuit configuration. The ZVS is realized with a filter inductance, a leakage inductance of the transformer, a parasitic capacitance of the switches, and a snubber capacitance. The phase-shifted technique achieves control over the Vout with constant frequency. The important characteristics of the proposed converter include: *Energies* **2022**, *15*, x FOR PEER REVIEW 15 of 22

A 5 kW isolated full bridge topology is proposed to apply FC vehicles [60]. The voltage clamping concept was introduced using a passive circuit to clamp the primary side switch blocking voltage. The proposed converter was analysed with a 24 V input and an output of 300 V. The calculated efficiency at peak power was 94%. Limited design data are provided to validate the converter. A soft switched 1 kW full-bridge isolated converter is demonstrated in [61]. During the switching, an overlap period of slow resonant commutation is achieved with the proposed converter. For the primary side switches, ZCS turn-off and ZCS turn-on is achieved. With the Vin = 22–27 V, Vout = 1 kV, a peak efficiency of 88% was achieved with 22 V input. An isolated full bridge converter for a 1.4 kW power level is proposed with a resonant LC circuit [62]. The resonant circuit is formed by connecting resonant capacitors parallel to the primary side switches and the LC tank circuit, forming a complete resonant circuit. With a Vin of 100 V and an output of 374 V with a narrow band frequency regulation, the maximum efficiency achieved

Below, Figure 19 shows the Series Resonant Converter topology (SRC). This paper [63] uses a parallel tank circuit formed by an (L-C)||L combination to achieve soft switching of high-frequency switches. The important feature of these converters including (1) Achieving an improved efficiency even at varying load and line conditions. (2) A wide range of soft switching ZVS can be achieved. (3) The Peak current capability of the switch varies with the input current variation and not with the load current changes.

	- tion. 3. The parasitic ringing problem on the secondary side transformer. 4. For a wide range of ZVS, a large inductor is needed, but the transformer needs to

A 200 W interleaved current fed ZVS active clamp full-bridge, and a Vout of 200 V is presented in [64]. The input current stress will be reduced due to interleaving and Vout

in [65]. The overall efficiency at full load achieved 92.8% with maximum converter utilization. The switching frequency is very low for the designed converter. Two current-fed full-bridge isolated converters are connected parallel to make an interleaved topology [66] and depicted in Figure 21. The voltage doubler circuit is connected in series to form a parallel input and a serial output configuration on the secondary side. The voltage clamping is carried out for the primary side switches using an active clamp circuit. With

The bidirectional isolated DC-DC converter technique reduces the input ripple current. The converter increases the conversion ratio and also the efficiency. Generally, a passive resistor capacitor-diode (RCD) snubber is required to store the energy in leakage inductance and clamp down the voltage spikes. Due to the use of the dual-inductorcapacitor-diode (LCD) snubber instead of the RCD snubber, the recycling of the leakage inductance, which is presented in the energy storing devices, is possible. Hence, the ef-

the Vin = 33 V, Vout = 400 V and the efficiency is 90.5% at 1.2 kW power levels.

**Figure 20.** Phase shifted full bridge converter. **Figure 20.** Phase shifted full bridge converter.

ficiency of the system increases [67].

*3.8. Interleaved Isolated Topology (ITLD Isolated Boost Converter)*

#### *3.8. Interleaved Isolated Topology (ITLD Isolated Boost Converter)*

A 200 W interleaved current fed ZVS active clamp full-bridge, and a Vout of 200 V is presented in [64]. The input current stress will be reduced due to interleaving and Vout extracted up to 700 V for three-phase grid-connected applications. A 1 kW interleaved current fed half-bridge topology with a Vin of 22–41 V and an output of 350 V is presented in [65]. The overall efficiency at full load achieved 92.8% with maximum converter utilization. The switching frequency is very low for the designed converter. Two current-fed full-bridge isolated converters are connected parallel to make an interleaved topology [66] and depicted in Figure 21. The voltage doubler circuit is connected in series to form a parallel input and a serial output configuration on the secondary side. The voltage clamping is carried out for the primary side switches using an active clamp circuit. With the Vin = 33 V, Vout = 400 V and the efficiency is 90.5% at 1.2 kW power levels. *Energies* **2022**, *15*, x FOR PEER REVIEW 16 of 22

**Figure 21.** Interleaved isolated bidirectional DC-DC converter circuit. **Figure 21.** Interleaved isolated bidirectional DC-DC converter circuit.

Figure 22 shows the full bridge converter topology for high-gain DC-DC converters. The bidirectional isolated DC-DC converter technique reduces the input ripple current. The converter increases the conversion ratio and also the efficiency. Generally, a passive resistor capacitor-diode (RCD) snubber is required to store the energy in leakage inductance and clamp down the voltage spikes. Due to the use of the dual-inductor-capacitor-diode (LCD) snubber instead of the RCD snubber, the recycling of the leakage inductance, which is presented in the energy storing devices, is possible. Hence, the efficiency of the system increases [67].

Figure 22 shows the full bridge converter topology for high-gain DC-DC converters.

**Figure 22.** Interleaved Isolated Full Bridge Converter Topology.

**Figure 21.** Interleaved isolated bidirectional DC-DC converter circuit.

Figure 22 shows the full bridge converter topology for high-gain DC-DC converters.

**Figure 22.** Interleaved Isolated Full Bridge Converter Topology. **Figure 22.** Interleaved Isolated Full Bridge Converter Topology.

#### **4. Summary of the Analysis**

The comparison includes an analysis of high-gain converters meant for PV applications. The published performance details are provided in Table 4. It should be noted that the critical test conditions, such as Vin and Vout levels and the measurement tolerances, are usually not provided, making it difficult to compare the efficiency achieved wherever provided. The optimum comparison was achieved by considering 1. Worst-case efficiency, 2. The number of active devices, 3. Switching frequency, and 4. Size of the converter data. Beyond the performance data, the papers reveal an analysis of different types of converters. The boost converters do not deliver high step-up ratios efficiently in continuous conduction mode due to the switch's high current and voltage stress and the diode reverse recovery loss. The non-isolated converter topologies are the suboptimal solution because it is directly connected to output high voltage side and the high boost ratios make it difficult to develop in non-isolated single-stage converter. The greater the differences in the voltage between the output of the FC (low voltage) and the DC link (high voltage), the greater there is need for electrical isolation between the two circuits [17,18,68]. Push-pull converters are typically unsuitable for FC power generation, especially at high power, due to the difficulty in overcoming transformer saturation [38,39]. The modified fly-back converters [69,70] suffer from voltage stress across the rectifier diode. The single winding carries a current, operates in a discontinuous mode (to avoid core saturation), and has high off-state voltage and poor core utilization. Current-fed full bridge converter operates at 10 kHz [66] (as it is a hard-switched converter), resulting in a larger converter due to the greater size of magnets and filters. The voltage clamping requirements [59] show that these circuits are necessary to reduce the switch stress. An active clamp (or reset) circuit requires greater switches and results in greater conduction losses due to the formation of the triangular current waveforms.

The comparison of measured efficiency in converter [18] and conventional boost converter is given in Figure 23. The conversion of 40 V to 380 V DC-DC gives the maximum efficiency of 92.6%.

The comparison of measured efficiency in converter [18] and conventional boost converter is given in Figure 23. The conversion of 40 V to 380 V DC-DC gives the maxi-

**Figure 23.** Measured efficiency Vs power [18]. **Figure 23.** Measured efficiency Vs power [18].

mum efficiency of 92.6%.




**Table 4.** *Cont.*
