Experimental validation of quadratic-boost-zeta converter based on coat circuit

ABSTRACT


INTRODUCTION
Because of worries about the environment and a lack of fossil fuels, photovoltaics (PV) and other forms of renewable energy are becoming more popular worldwide [1]- [4].However, the performance of a PV panel depends on several variables, such as temperature, the amount of sunlight, and the amount of shade.As a result, the voltage of a PV panel is typically around 12-60 volts; thus, a high step-up voltage is required to supply energy to the AC grid or DC microgrid [5]- [9].A conventional boost converter with a very highduty cycle operation can produce a high step-up voltage.However, the voltage of the main switch will increase, it must have a large on-resistance, and an increased cost is required.Additionally, issues with the diode's reverse recovery, electromagnetic interference, and conduction losses will accompany the used highduty cycle and cause more loss, decreasing efficiency.
Many ideas have been put forward to get a high step-up voltage, such as using switched capacitors, voltage multipliers (VMs) or coat circuits, cascade structures or stack structures, transformers, and coupled inductors.Some converters use switched capacitors to boost voltage, but these converters have three significant drawbacks: i) The output voltage ratio is dependent on the number of stages of switched capacitors; ii) The input current is large and pulsating, and the current is significant overshoot through the capacitors; and iii) The converters needed two or more switches, which make the circuit and drive are complex [10]- [12].The converters used by voltage multiplier techniques, providing high-voltage gain, have been presented while simultaneously lowering the voltage on the switches; however, when utilizing multistage converters, the high charging currents cause a significant conduction loss when passing through the converter switch [13]- [16].To create powerful step-up converters, magnetic coupling is often used.A higher 2184 turn ratio will result in a higher voltage gain.But the leakage inductance of the connected inductor or the transformer can cause high voltage spikes in the switches, which can cause high voltage stress, increase size, and decrease efficiency [17]- [22].The other approach suggested in [23], [24] uses cascading or stacking.The converters can be connected in series or parallel to increase voltage gain.The cascade or stack converter uses three or more converters to improve the voltage gain.Due to its many converters, the cascade or stack converter also requires numerous components and has low efficiency.Furthermore, in [25]- [27] A new approach has been proposed in the DC-DC converter by combining two or more conventional transformers, in [25] has successfully combined flyback and SEPIC topologies, but the profit is still small and requires an increase in the duty cycle, as the result increase loss.The work of [26] shows that the boost converter has been improved and the profit is fairly good, but it requires two keys, which makes controlling it complex and increases costs and losses.Also, in [27] has successfully combined quadratic boost and SEPIC topologies but to increase voltage gain should be increased duty cycle and used to switch.
High step-up converters based on zeta converters are suggested in [28].In the circumstances described above, it would be good to combine the advantages methods, i.e., the stacking, cascading layers, and voltage multipliers, together while maintaining high voltage gain without reducing the converter's efficiency [29].This research draws from the abovementioned works and presents a high step-up converter by combining stacking and cascading configurations of two well-known quadratic boost and isolated zeta converters with a single stage based on a coat circuit.This converter gets the benefits of both quadratic-boost and isolated zeta converters, which is low input and output current ripples.Additionally, the proposed converter only needs one active switch, and the voltage stress on the switch and the diodes are much lower than the output voltage.Therefore, switches with low on-resistance and low voltage drop diodes can be used.

THE STRUCTURE AND OPERATION PRINCIPLE OF THE PROPOSED CONVERTER
The proposed converter is shown in Figure 1.It combines quadratic boost and isolated zeta converters with a single-stage of coat circuit, making it a relatively simple structure.The quadratic-boost converter is composed of an input source V in , one inductor ( L 1 ), one transformer with magnetizing inductance( L M ), three diodes ( D 1 , D 2 , and D 3 ), two capacitors (C 1 and C 5 ), and one switch (SW).The zeta converter with a single-stage of coat circuit is composed of two diodes (D 4 and D 5 ), four capacitors (C 2 , C 3 , C 4 , and C 6 ), and two inductors ( L 2 and L 3 ).One of the characteristics of the converter that distinguishes it is that the input and output ports are connected to an inductor( L 1 and L 3 ); this may help explain why the proposed converter can have a low ripple current from both the input and output ports.
Before the analysis, the following assumptions are made: i) All of the parts in the circuit are assumed to be ideal; ii) Since the inductors and capacitors are sufficiently large, their current and voltage ripples can be ignored; and iii) The transformer ratio(n) is equal to
∫ So, the final equation of the converter's gain is (15).
In ( 15) is the voltage gain M of the proposed converter.

Voltage and current stress calculation on switch SW and diodes
The voltage stress imposed on the switch SW, the diodes D 1 , D 2 , D 3 , D 4. and D 5 can be represented by the symbols V SW , V D1 , V D2 , V D3 , V D4 , and V D5 ; r, according to Figure 2, the ( 16)-( 21) can be obtained: Assuming that the converter is efficient at 100% makes the analysis easier.The average current I L1 at the input is, thus.
To simplify the diodes' average current calculation, the ripple was neglected.They can be obtained as ( 23)- (27).

PARAMETERS DESIGN 4.1. Design of inductors
In practical uses, the current ripple is always predetermined.As a result, the inductance of the inductors L 1 , L M , L 2 , and L 3 may be found by using the ( 28)-(31).Consequently, the inductances must be suitable for continuous inductor current mode operation.from on state} (31)

Design of capacitances
In practical uses, the voltage ripple is always predetermined.As a result, the capacitance of the capacitors C 1 , C 2 , C 3 , C 4 C 5 and C 6 can be obtained as (32)-(36).

COMPARISON OF THE DESIGNED CONVERTER WITH OTHER CONVERTERS
The suggested converter is compared to current topologies in [19], [20], [22] and [29], which also produce significant voltage gains.Table 1 details the voltage gains and the normalized equations that characterize the voltage stresses.In addition, the number of diodes and switches used in each equivalent circuit is also provided.To ensure that comparisons are valid, the turns ratio (n) is consistently defined as 2. In [19], [20], and [22], the need for two active switches leads to low efficiency because of conduction losses and complex controlling methods.Even though the displayed topology consists of a single switch and a simple method for controlling it, it still needs a lot of diodes.Also, in [29], the voltage stress on the output diode and the switch is higher than in the proposed structure.A low current ripple can extend the service life of renewable energy sources; thus, it is essential to consider that at the input [30].The topology in [19] has a large ripple in the input and output currents.Also, the input ripple current on the input side of the boost converter is less because it has an input inductor.Also, the output inductor of the zeta converter is one of the components that help reduce the ripple current on the output side.So, the proposed converter is different compared with the other converter structure.
Switch voltage Voltage stress on outer diode

EXPERIMENTAL RESULTS
A 240 W experimental prototype has been constructed to test the aforementioned theoretical study's validity, as shown in Figure 3. Table 2 illustrates the values and device type of the used components by considering the current ripple of each inductor as ∆ 1,2,3 = 30% of  L1,2,3 , and the voltage ripple of each capacitor as ∆ 1,6 = 1% of V C1,6 , ∆ 2,3,4,5 = 0.84 % of V C2,3,4,5 .The experimental voltage and current waveforms of the suggested converter for the input voltage of 30 V are shown in Figures 4-9.The gate pulses of the MOSFETs and closed-loop control are produced by the IC TL494, and the switch operates at a duty cycle of 48.25% to achieve the output voltage of 360 V.Besides that, the experimental waveforms were measured using a Rigol DS1104Z oscilloscope.
The experimental waveforms of input voltage V in and input current  in are shown in Figure 4(a), and the average input voltage and current are around 30V and 8.63 A, respectively.Also, the experimental waveforms of output voltage V o and output current  o are shown in Figure 4(b), and the average output voltage and output current are around 360 V and 0.68 A, respectively.
Figure 5 illustrates the voltage for the capacitors C 1 , C 2 , C 3 , C 4 , C 5 , and C 6 .The experimental results of V c1 = 56.6V, V c2 = 132 V, V c3 = 133 V, V c4 = 132 V, V c5 = 117 V, and V c6 = 253 V are sufficiently near to the theoretical results that are as: V c1 = 57.97V, V c2 = 124.3V, V c3 = 124.3V, V c4 = 124.3V, V c5 = 112 V, and V c6 = 248.6V.The voltage stress and average current of the switch SW are 125 V and 8.33 A, respectively, which are illustrated in Figure 7. Also, the voltage spikes it very small in the switch voltage, and its value is much lower than the output voltage.This means that low-voltage MOSFETs with low ON-state resistance can increase the converter's efficiency.Also, the results agreed with the theoretical assessments., showing that the effect of the leakage inductance of the transformer was observed to be minor.The desired voltage gain was reached for all cases, thus validating the theoretical evaluation.Assuming the symbol  ti the peak-to-peak voltage for the primary transformer, and the symbol  to represents the peak-to-peak voltage for the secondary transformer.
Finally, the converter was tested when the input voltage changed.Figure 9 shows the input and output voltage waveforms when the input voltage changes from 30 to 25 volts using a closed-loop circuit based on the IC TL494, and the output voltage (V o ) maintains its reference value.

.
Assuming the symbol   represents the number of turns of the primary transformer, and the symbol   Represents the number of turns of a secondary transformer.

Figure 1 .
Figure 1.The proposed converter circuits

Figure 3 .
Figure 3. Shows the proposed converter, power supply, R load, and oscilloscope in the laboratory

Figure 6
illustrates the voltage stress and current through the diodes D 1 , D 2 , D 3 , and D 4 .The experimental results of V D1 = 58.4V, V D2 = 67.2V, V D3 = 145 V , V D4 = 286 V and V D5 = 285 V, and the average current of I D1 = 4.6 A, I D2 = 4.6 A, I D3 = 0.9 A , I D4 = 0.85 A and I D5 = 0.68 A.The measurements and waveforms for the current and voltage are shown in the figure, showing that the effect of the leakage inductance of the transformer on the diodes was observed to be minor, and the results are close to their equations.Consequently, all of the voltages agreed with the theoretical assessments.

Figure 4 .Figure 5 .
Figure 4.The waveforms of input and output voltage and input and output current: (a) input voltage and current and (b) output voltage and current

Figure 6 . 4 Figure 8
Figure 6.The waveforms of voltage and current for the diodes: (a) for the diode D 1 , (b) for the diode D 2 , (c) for the diode D 3, and (d) for the diode D 4

Figure 7 .Figure 8 .Figure 9 .
Figure 7.The waveforms of the voltage and the current for the switch (SW)

Table 1 .
Comparison between the proposed converter and other converters