An Extended Sepic DC-DC Converter for Micro-Grid Based Photovoltaic (Pv) Applications.


 Background

Brain dural arteriovenous fistulas(bDAVFs) are anomalous connections between dural arteries and cerebral veins or sinuses. Cerebral venous thrombosis(CVT) often precedes or coincides with bDAVFs and is considered a risk factor for these vascular malformations. Recently, vaccine-induced thrombotic thrombocytopenia causing CVTs has been associated with COVID-19 vaccines. Concurrently with the start of massive vaccination in our region, we have observed a fivefold increase in the average incidence of bDAVFs.
Objective

To raise awareness of the potential involvement of COVID-19 vaccines in the pathogenesis of bDAVF.
Methods

 A retrospective review of demographic, clinical, radiological, COVID-19 infection and vaccination data of patients diagnosed with bDAVFs between 2011-2021 was conducted. Patients were divided into two cohorts according to their belonging to pre- or post-COVID-19 vaccination times. Cohorts were compared for bDAVFs incidences and demographic and clinical features.
Results

Twenty-one bDAVFs were diagnosed between 2011- 2021, 7 of which were diagnosed in 2021. The mean age was 57.7 years, and 62% were males. All cases except one were treated; of them, 85% were exclusively managed with surgery. All treated cases were successfully occluded. The incidence in 2021 was significantly higher than that in the prevaccination period (1.72 vs 0.35/100,000/year;p=0.036; 95%Confidence Interval=0.09-2.66). Cohorts were not different in age, sex, hemorrhagic presentation, dural sinus thrombosis or presence of prothrombotic or cardiovascular risk factors.
Conclusion

The significant increase in the incidence of bDAVF following general vaccination policies against COVID-19 observed in our region suggests a potential correlation between these two facts. Our findings need confirmation from larger cohorts and further pathogenic research.


Background
For a sustainable clean city, there is a need to limit the fossil fuel usage and maximize the use of alternative energy resources (renewables) as a source of electricity generation. These abundant resources are promising for the future due to their reversibility, easy operation, less maintenance, and zero pollution of the environment. There are many forms of renewables with solar power the most common we use today [1][2][3][4].
Photovoltaic (PV) panels are solar powers made up of semiconductor materials that absorb heat from sun and convert it into electricity [5][6][7]. The semiconductor cells on the panel are designed in series or parallel arrangement to get the best energy generation [8]. Lack of sufficient area for installation, low sunshine, shades of trees and tall buildings are some of the biggest threats affecting their performances [9,10]. Therefore, PV per voltage generation tends to be low in nature which is why the power converter technology is much-needed [5], [11,12]. This technology can be used to step up the low DC voltage generated by the PV panel into the desired output value [13,14]. There are many types of power converter but at this junction, a DC-DC type is required.
The DC-DC power converters are of many types, transformerless and those with transformers [11]. The transformer types were deemed too expensive and higher voltage gain can be attained by two things, turn-ratio and duty-cycle of the transformer [10]. The transformerless types have duty cycle only as a requisite to achieve higher voltage gain, some of their structures are well exploited in the literature [2], [15][16][17]. A conventional boost converter is transformerless and it has a good duty cycle, but the voltage gain is always lower than expectations. A transformerless SEPIC is promising only when it is integrated with another converter or incorporated with voltage doubler [4], [14]. Zeta and Cuk converters exhibits Buck-Boost behavior which is not viable for step up application [15]. The aforementioned problems have led researchers to quest for more methods.
Different topologies about transformerless DC-DC converters are well exploited in the literature [2], some topologies were built by integrating two or three converters together either by cascade, inversion or by incorporation with voltage multiplier cells to achieve higher voltage gain. Buck boost presented in [16] to raise the voltage gain with reliable duty-cycle. Double boost integrated with SEPIC were proposed in [14], [17][18][19][20]. Cuk and boost converters were integrated in [18,19] to solve similar problems. Boost with voltage multiplier cells was also proposed in [4], [8], [20,21], SEPIC integrated with boost in [22] to optimize the system.
To add to the present solutions, this paper proposed an extended SEPIC for PV panels and other renewable energy resource applications that require a step up conversion gain. The additional components involve one capacitor and two diodes to extend the voltage gain at the expense of the low duty cycle of the new SEPIC converter. The diagrammatic representation of the system is shown in fig. 1. To make the propose converter simple and lighter, the new topology utilizes a single switch. The steady-state and theoretical analysis is discussed in details. Simulation and laboratory validations were taken and presented herein.

Methods
The new topology is presented in figure 3 which consists of two inductance L1 & L2, three diodes D1, D2, & DO, three capacitors C1, C2 & CO and one controllable power switch. As it can be seen in fig. 2, the new converter has additional two diodes and one capacitor ( 1 , 2 & 2 ) than the conventional SEPIC topology in fig. 2. In fig. 3, capacitor 2 is placed between the two diodes such that its discharged energy would be blocked by 1 , as such, 1 will combine the voltage coming from the power switch with the voltage coming from 2 and dissipate it to the inductor 2 . This approach is similar but in modification to the methods adopted in a classical boost converter presented in [7] and SEPIC incorporated with voltage doubler in [4]. The method adopted in the present study is cheaper and friendly than the methods adopted in [22]. After the realization of the new converter, MATLAB/SIMULINK was used to run the simulation and the laboratory set up was prepared with trainer version 3371 connected with DX9 USB socket and signals were generated. The modes of operation of the extended SEPIC converter is given below:

Mode 1:
Here, the power switch is in ON-state therefore the energy from inductor 1 will flow through diode 1 to 2 . The energy in 2 would be delivered to 1 and then to the output.

Mode 2:
In this mode, the power switch is in OFF-state therefore the leaked energy follows through inductorL 2 .

Analysis of the New Converter:
Applying Kirchhoff voltage law to the circuit in fig. 3 yields the following equation; Applying switching signal to the power switch (Q) yields Equation (2) is used to evaluate voltage across the power switch & capacitor 2 , that is, & 2 .
Therefore, 1 will combine the voltage coming from power switch ( ) with the voltage in the second capacitor( 2 ) and delivers it to the inductor 2 . This is to say, 1 = + 2 From equation (3) it can be confirmed that Generally, with the effect of switching signal, the input/output relationship takes the form Therefore, Collecting the like terms in equation (7) yields: According to the equation (2) This implies that equation (8) can be written as: To find the duty cycle, equation (10) can be expressed in terms of equation (2): For the voltage gain, equation (7) can be rewritten as Concerning the voltage stress across the power switch, equation (2) can be considered:

Design Equations for the New Converter.
The following values were assumed in designing each component of the new converter.
For output capacitor Co, Equation (18) is used to evaluate output current Switching frequency can be evaluated using equation (21):

Discussion
Simulations: MATLAB/SIMULINK was used to carry out the simulation. An output voltage of 125. 232 V DC was realized with input and duty-cycle values of 12 V DC and 0.8 respectively. This means a voltage gain value of 10.45 was confirmed which is in line with the equation (13). For a stress-free converter, half of the output voltage is expected to pass through the power switch. A value of about 62.5 V voltage across the active power switch which can also be evaluated using equation (14).

Experiment:
The prototype of the circuit was built in the laboratory as shown in fig. 3. A trainer (version 3371) connected with USB Dx9 was used to generate the signals shown in figures 4 & 5. Figure 4 depicts an input/output signals and it can be seen that 125 V DC was realized from an input of 12 V DC, this means a voltage gain of 10.42 is obtained with 0.8 as a duty-cycle value. The 50 s per division (100 s per 2 divisions) depicted in fig. 5 represents the period per oscillation which was used to evaluate the switching frequency in equation (21). The values obtained from the new SEPIC were compared with conventional SEPIC as shown in the fig. 6.

Conclusions
An extended SEPIC DC-DC converter for micro-grid based PV panel applications is presented in this work. The new topology has shown that addition of one capacitor and two diodes to the conventional SEPIC topology can extend its voltage gain at the expense of low duty-cycle as depicted in fig. 10. The voltage stress across the power switch has been reduced to minimum.