An Innovative Step-up DC-DC Converter for Fuel Cell-based Power Source to Decrease Current Ripple and Increase Voltage Gain with Low Stress and Switch Counts

: As for the insufficient nature of the fossil fuel resources, the renewable energies as alternative fuels are much needed and highly heeded. To deliver the required electric power to the industrial and domestic consumers from DC renewable energy sources like Fuel Cell (FC), the power converter operates as an adjustable interface device. This paper suggests a new boost structure to provide the required voltage with wide range gain for FC power source. The proposed structure based on the boost converter and the quazi network, so-called SBQN, can effectively enhance the FC functionality against its high operational sensitivity to experience low current ripple, and also propagate voltage and current with low stress across its semiconductors. Furthermore, the switching power losses have been decreased to make this structure more durable. Full operational analysis of the proposed SBQN and its advantages over the conventional and famous structures has been compared and explained. To more evaluate the high-step up voltage capability of the proposed SBQN and validate the simulations results, a prototype of the single-phase converter has been constructed and tested in the laboratory.


Introduction
Renewable based-power systems have been widely used to indicate the environmental challenges and concerns resulting from fossil fuel exhaustion, energy cost growth and greenhouse gas emissions [1][2][3].Among the wide variety of renewable energy sources (RESs), wind, solar, geothermal, biomass, fuel cell, and hydrogen energy are more common and widespread than other kinds [4,5].Due to the intermittent characteristic and wide operating range of these resources, an electric power converter-based network interface is required to adjust the output power [6,7].Since the conventional voltage and prevalent converters have a limited operating range, utilization of dc-dc converters as interface components is not cost-effective and efficient.The ZSC gives a single-stage voltage buck-boost conversion with low switching power losses that is found to be cut-price for DC-based RESs [8,9].
FC source is such the DC-based RESs which provides a low voltage and high current from the direct combination of fuel and oxidizer without environmental pollution, noise emission and health hazard [10].
Direct production of the electricity without the thermodynamic limitations of the Carnot cycle is to convert the chemical energy of fuel into heat and mechanical energy, and accordingly electricity which reduces energy loss and achieves high electrical energy efficiency [11].As a result of the development of the material technology in recent years, the power of FC source can meet the requirement of low voltage domestic components [12].Since the voltage output of FC source is not sufficient to supply the inverter and consequently the output load, ZSC is used to provide the required voltage with wide voltage range [13,14].
A number of studies have been performed to augment the DC voltage gain, and then lead to suggestion of dozens of the cascaded structures and transformers [15][16][17].A mutual connection of threewinding with inductor-based LZSC is proposed as an upgraded dc-dc converter for hybrid output applications [18].An advanced quasi-ZSC coupled with a single-phase three-level inverter is presented in [19] to increase the output voltage, whereas, it is modulated using an alternative phase opposition disposition pulse width modulation strategy to boost the dc-link voltage and balance the two series capacitor voltages via controlling the shoot-through state.A five-level neutral-point-clamped dual quasi-ZSC with a complementary inductor-capacitor is presented in [20] to impede the initial current of ZSC's inductor and appropriately boost the dc-link voltage.A quasi-ZSC with an additional switch and diode is suggested to enhance the boost factor which can be more suitable for the photovoltaic-based power system [21].Since an almost complicated control strategy must be used for modular multilevel converters to balance the voltages of several modules, a disposition sinusoidal pulse width modulation is applied to operate as voltage modulator [22].A Fuzzy-based DC-DC converter integrated with ZSC which is controlled by a modified switching strategy is proposed to create and boost the desired output voltage [23].
The deep and major drawbacks of such these converters are their large size inductor, high peak inverse voltage on switches, high total cost, low electrical energy efficiency and unqualified output voltage.Due to low voltage of DC-based RESs [20][21][22][23][24], their voltage must be increased to the required value without ignoring the essential voltage quality.That is to say, some DC-based RESs are connected in series to each other to obtain a high-level voltage, if so, it is inevitable to meet the high cost-price and complicated control circuits.
This paper suggests a novel SBQN topology to provide the high qualified voltage with wide range gain and low sensitivity to obviate the aforementioned drawbacks.Based on the configuration of the suggested SBQN topology, the rate of components along with the pinch off voltage of semiconductors is decreased, therefore, it will be more cost-effective than other topologies.Among the DC-based RESs, FC source is appropriate choice to test the high-step up voltage capability of suggested SBQN, because of low output voltage and high sensitivity to the current ripples [25,26].
Due to imperfect and non-ideal characteristic of semiconductors, when the switch changes from on state to off state and vice-versa with high frequency, the switching power losses takes place [27][28][29].
Beyond that, the switch has to withstand severe voltage and current stress as the charged energy of capacitors and inductors increase [30].The voltage and current stress on the switch also heighten the switching power losses, undermines the switch longevity and degrades the converter efficiency [31].To make the switch more durable and better performance of SBQN, the switching power losses must be decreased.The important benefit of this switching control strategy is for its low switching frequency, which will lead to mitigation of switching power losses.
At long last, full operational analysis of the proposed SBQN and its advantages over the conventional and famous structures has been compared and explained.The structure of proposed SBQN has been modeled using MATLAB/SIMULINK software.The high-step up voltage capability of the proposed SBQN along with its DC voltage quality has been evaluated under different scenarios.To more validate the simulations results, a prototype of the single-phase converter has been constructed and tested in the laboratory.

Fuel Cell Features and Functions
FC source actually operates as a battery albeit oxygen and hydrogen or methane are used as its fuels, indeed, some important kinds are: Proton Exchange Membrane Fuel Cell (PEMFC), Solid Oxide Fuel Cell (SOFC) [32].The fuels have been electrolytically attained using a single system which operates in either electrolyzer state or fuel cell state [33].Given the low operating temperature, high energy density and quick starting feature, the PEMFC has found significant attraction among different FC types [34].
Likewise, it has been being successfully advanced as a reliable and portable power source to meet the power demands of various domestic applications.Analogous response of anode and cathode in the Membrane Electrode Assembly (MEA) for can be expressed as follows [35]: The anode response: H2 → 2H + + 2e The cathode response: 1/2O2 + 2H + + 2e -→ H2O MEA can be seen in the single PEMFC schematic presented in Fig. 1.MEA is commonly put into two metal platters which are mutually correlated to provide a bipolar platter when cells are stacked for higher voltages.The FC performance has been practically distinguished according to the polarization curve, which depicts the FC output voltage in terms of the load current.Based on the Tafel equation [36], the stack voltage can be presented as follows: (1) Where To more simplification and comprehension, the following terms are assumed.
1) The power electronic components are considered to be ideal, i.e., the conducting or on-state resistance RDS(on) for the power electronic switches are neglected, and also the series resistance of the inductors and capacitors.
2) The ripples of the inductors' currents and the capacitors' voltages have linearly increased and decreased.
Two basic operating modes Mode I and Mode II related to the proposed converter have been illustrated as follows: During the operation of the SBQN in the step-up mode, the power has flowed from the low voltage side to the high voltage side.T1 has here operated as a prime power switch, while D1 and D2 are the synchronous rectifiers.K is considered as the duty cycle of the gate signal T1: Where, to, T and fS are respectively the conducting time, switching time and switching frequency.Hence, the output voltage VO becomes higher than the input voltage Vin.As for Fig. 4(b), the voltage of inductors and capacitors in state 2 can be presented as follows: Based on the principle of inductor volt-second balance in average steady-state operation, the voltage gain has been extracted in terms of the duty cycle K in CCM state.

3.2.Voltage stresses of the capacitors
In accordance with the (8) and ( 9), the capacitors' voltage stresses can be presented as follows:

3.3.Average currents of inductor
In accordance with Fig. 4(a) and 4(b), the average currents of inductor for states 1 and 2 can be presented as follows.
State 1: Ignoring the losses, the inductor current can be calculated by: Based on the principle of capacitor ampere-second balance in average steady-state operation, the average capacitor currents I C1 and I C2 has been extracted in terms of the duty cycle K in CCM: Substituting the equations ( 15) and ( 16) into the ( 14) in ( 17)

3.4.Voltage Stress of the semiconductors
Regardless of the voltage drop on the power switches, the voltage stress of semiconductors can be calculated by:

3.5.Current Stress of the semiconductors
In the same way, the current stress of semiconductors can be also calculated by:

3.6.Current ripple of inductors
According to the voltage drop of inductors in mode 1, their currents ripples can be calculated by: Where, the ripple of input current is equal to the ripple of inductor (L1) current.

3.7.Voltage ripple of capacitors
During mode 1, inductor L2 transfers its energy into the capacitors C1 and C2.Thus, the voltage ripple of the capacitors can be calculated by: The voltage ripple of the capacitor C3 can be also calculated by: The general waveforms of the suggested SBQN in CCM are depicted in Fig. 2(a).

Power Semiconductor Switch Losses
The power electronic converter losses can be aggregated with power losses experienced by all semiconductor switches.As a rule, this problem has been characterized by three kinds of losses: blocking (OFF) state, conducting (ON) state and switching (transition between ON and OFF) state.Due to trivial value of leakage currents during blocking state [37], it is ignored.Thus, the conducting and the switching losses have been calculated as power switch losses.

4.1.Conduction Losses
The instantaneous conduction losses for transistor and diode can be generally presented as follows [38,39]: , () = �  +   ()�.() Where, pc,T(t) and pc,D(t) respectively indicate the instantaneous conduction losses related to transistor and diode.VT and VD respectively indicate the conducting voltage drops related to transistor and diode.RT and RD respectively indicate the equivalent conducting resistances related to transistor and diode.
Also, α indicates the transistor constant.
Let NT (t) and ND (t) be the numbers of transistors and diodes which conduct the current at instant t.
based on Eq. 47 and Eq.48, the average conduction losses is attained:

4.2.Switching Losses
To extract the switching losses, the energy losses during the transition between ON and OFF states must be calculated.According to the Fig. 8, the voltage and current variations in this state are almost linear.The energy loss during the turn-on state is given as follows: Where, Eon,j is the turn-on loss associated with the jth switch; ton is the turn-on time; I is the current through the switch before turning off; Vo,j is off-state voltage on the jth switch.
In the same way, the energy loss during the turn-off state can be presented as follows: ,   (31) Where, toff indicates the turn-off time.I' indicates the current through the switch after turning on.
The switching loss is attained by summing the energy losses during transition state.Let's consider I=I', then, the total switching losses can be presented as follows: Where, fj indicates the switching frequency Finally, the total switching losses can be presented as follows:

5.1.Simulation verification
To verify the operation of the proposed converter, a case study has been structured which is presented in Fig. 6.Providing the required power via FC source for DC load is the main target of the case study.Its relevant parameters are tabulated in Table 1.
Fig. 6.The case study system with presence of SBQN.Such these simulation results have well demonstrated the performance of the proposed structure.As the FC's voltage becomes lower than DC bus, the SBQN will boost it.According to the Fig. 7, the dc/dc converter boosts the input voltage by changing duty cycle.Let duty cycle be 0.64, then FC's voltage becomes 48.Fig. 8 depicts both the output voltage and current: Good thing, the SBQN needs switches and diodes with low standing voltage.According to Eq. 20, their voltages can be attained which are depicted in Fig. 9: Meanwhile, Fig. 10 shows the current of inductors and the voltage of capacitors which respectively satisfy the charge and discharge of inductors and capacitors.According to Eq. 22, the ripple of input current can be attained which is 2.4.Fig. 10

Experimental verification
To verify the performance of the presented dc/dc converter, a prototype circuit is essentially provided in the laboratory.A developed 220 W prototype is shown in Fig. 11, and its relevant values are presented in Table 1.A Texas Instruments microcontroller TMS320F28335 is taken into account for the voltage loop controller.The SBQN includes two inductors, one IXFT36N50P switch, two RURG8060 diodes, and two capacitors.Fig. 12 presents the voltage stress waveforms of T1, D1 and D2 during the operation of the proposed converter in the step-up mode at the rated condition.As can be seen, the voltage stress on T1, D1 and D2 is 134 V which almost equals half of the high-side voltage.It is worth mentioning that, Channel 1 shows the voltage of switch and Channel 2 and 3 shows the voltage of diodes, whereas T1 and D1 or T1 and D2 must not be simultaneously turned on.At the same condition, the current waveforms of L1 and L2 are presented in Fig. 13.The current ripple rate of L1 and L2 are respectively about 2.45 A and 1.2 A which satisfy the inductor design due to its low average current value.As can be seen, the input current i.e.L1 is continuous wave with low ripple.Likewise, Fig. 14 shows the output voltage which keeps at 220 V, while the input voltage is 48 V.At the same condition, the voltage of capacitors VC1 and VC2 are shown in Fig. 14.Voltage of C1 and C2 are respectively about 85 V and 134 V, which satisfy the capacitor design.

5.3.Comparison verification
To validate the high-step up voltage capability of the SBQN, a comparison is done with the conventional boost converter which is shown in Fig. 15.It is obvious that, the gain of the SBQN is higher than the conventional boost converter as for the factor of (1 + D).Assuming the same duty cycle, the proposed topology has been compared with the classical quasi-Zsource, the switched inductor boost converter and the presented dc-dc converter in [40] and [41].Table 2 presents a comprehensive comparison between the proposed and other converters with respect to the component counts, voltage gain, semiconductor voltage stress and maximum duty cycle.As can be seen, the proposed converter contains low number of components as compared with QZS and presented converter in [40], [41] and [42].Table 2 compares the voltage stresses in the converter switches.It is clear that the voltage are increased for the converters presented in [41], [42], [43] and QZS, but the voltage stresses on their semiconductors are increased which are equal to their output voltages.In contrast to all understudy structures, it can be seen that the proposed transducer withstands less voltage stress than them.

Conclusion
This paper aims to propose an innovative boost structure to provide an essential voltage with wide range gain for FC power source.The functionality of FC power source against its high operational sensitivity is realized using so-called SBQN to experience low current ripple, and also propagates voltage and current with low stress across its semiconductors.Meanwhile, the durability of the proposed converter has been increased due to reduction of the switching power losses, and also the switch counts are reduced which make it will be more cost-effective than other topologies.To validate its high-step up voltage capability, low voltage stress and switch counts, it has been compared with the some famous and recently introduced structures.Both the results and comparisons associated with the simulation and prototype structures have been well validated the aforementioned advantages of the proposed converter.The case study system with presence of SBQN.Voltage of semiconductors (Time/div=2.5µs,Volt/div=100 V) Inductor currents (Time/div=2.5µs,Volt/div=1V) Figure 14 Output voltage and voltage of capacitors (Time/div=20ms, Volt/div=50 V) Comparison between the gain of the proposed and classic boost converters.
Voltage gain comparison between the proposed and other dc/dc converters.

,Fig. 2 . 3 .
Fig. 2. Stack voltage along with power in terms of current density

Fig. 15 .
Fig. 15.Comparison between the gain of the proposed and classic boost converters.

Fig. 16 .
Fig. 16.Voltage gain comparison between the proposed and other dc/dc converters.

Figures
Figures

Figure 1 Schematic of a single conventional PEMFC Figure 2
Figure 1

Figure 4 Current
Figure 4

Figure 5 Switching waveforms of transistor Figure 6
Figure 5 Figure 7

Figure 8 Output
Figure 8

Figure 10 Current
Figure 10

Table 1 :
Case study parameters.
validates the accuracy of proposed structure:

Table 2 .
Comparison between the proposed and other dc/dc converters