# Design and Implementation of a Flexible Photovoltaic Emulator Using a GaN-Based Synchronous Buck Converter

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

^{2}. In addition, solar PV generation systems require relatively low maintenance costs. Increasing the usage of distributed PV systems to a certain level can substantially reduce transmission losses, thus increasing energy utilization. Based on the data reported in [3], until 2017, the total share of global RE-based generation was only roughly 4%, but it has been expected that the solar PV generation level will catch up in the share of future RE-based generation.

## 2. PV Module Modeling and Design of Synchronous Buck Converter

#### 2.1. PV Module Modeling

_{ph}represents equivalent current source (A); D

_{j}represents the P-N junction diode; R

_{sh}represents equivalent parallel resistance of the material (Ω); R

_{s}represents equivalent series resistance of the material (Ω), and V

_{p}and I

_{p}represent output voltage (V) and current (A), respectively.

_{rs}represents equivalent reverse saturated current of D

_{j}(A); q represents elementary charge (1.60 × 10

^{−19}C); A represents ideal factor (ranging from 1 to 5); k represents Boltzmann constant (1.38 × 10

^{−23}J/K), and T represents surface temperature (K).

_{p}represents the number of paralleled diode paths, and n

_{s}represents the number of series-connected diodes in each path.

_{sh}is large, and R

_{s}is small. Ignoring these two parameters, we can simplify (1) to obtain (2).

_{rr}represents reverse saturated current (A) at reference temperature (T

_{r}), and

_{scr}represents short-circuit current (A) at reference temperature and irradiance; α represents short-circuit temperature coefficient of a PV cell (mA/°C), and S represents solar irradiance (kW/m

^{2}). The output power of a PV cell can then be expressed as follows:

_{sh}and R

_{s}can be ignored so that the relationship between output voltage and current can be expressed as follows:

_{mp}= 300 W, number of PV cells = 60, rated current I

_{mp}= 7.87 A, rated voltage V

_{mp}= 38.4 V, short-circuit current I

_{sc}= 8.7 A, open-circuit voltage V

_{oc}= 48 V, and idea factor A = 1.5. The ambience temperature is set at 25 °C. To evaluate the V-I characteristics of multiple PV modules connected in series under normal and partial shading scenarios, two PV modules are used, as shown in Figure 3, where the output terminals of each module are shunted with a bypass diode.

^{2}, respectively). Next, Figure 5 shows theoretical and measured P-V characteristics of two PV modules connected in series under the same load conditions and one module receives 500 W/m

^{2}irradiance, and the other receives 1000 W/m

^{2}irradiance.

#### 2.2. Design of Synchronous Buck Converter

#### 2.2.1. Operating Principle and Mathematical Model of Synchronous Buck Converter

_{1}and S

_{2}, an inductor L, an input capacitor C

_{in}, and an output capacitor C

_{o}. The operating principle of this converter is as follows: S

_{1}acts as the main switch; when it is on, V

_{in}charges L, and when S

_{1}is off, the energy stored in L is transmitted through S

_{2}to C

_{o}.

_{1}is on, and S

_{2}is off, the voltage across L can be expressed as follows:

_{1}is off, and S

_{2}is on, the voltage across L can be expressed as follows:

#### 2.2.2. Design of Controllers for the Proposed PV Emulator

_{s}and k

_{v}represent current and voltage sensing scales, respectively; I

_{L}and i

_{L}

^{*}represent current feedback and command, respectively, and v

_{con}represents PWM voltage.

_{L}represents load resistance. Substituting the related design specifications into (22) gives the following:

## 3. Simulation and Implementation

_{PV}: 5 A/div; V

_{o}: 40 V/div; T: 2 s/div; V

_{PV}: 48 V/div; P

_{PV}: 270 W/div).

_{PV}: 5 A/div; V

_{o}: 40 V/div; T: 2 s/div; V

_{PV}: 48 V/div; P

_{PV}: 270 W/div).

_{PV}: 5 A/div; V

_{pv}: 40 V/div; T: 20 ms/div).

## 4. The Analysis of System Efficiency

_{out}and measuring the corresponding P

_{in}of the converter, the system efficiency at a specific power level and switching frequency can be readily calculated. In this paper, two switching frequencies, i.e., 50 and 80 kHz, are tested at five load levels. The calculated results are graphically shown in Figure 19. As can be seen in Figure 19, a maximum efficiency of 99.05% appears at about 80% of the converter’s rated capacity (1 kW), with a switching frequency of 50 kHz, and it is found that, when the switching frequency increases, the efficiency decreases. This is mainly due to the increase in switching losses. In practice, other factors may also affect the performance of efficiency; typical factors include layout construction, the noise level of sensing devices, and driving and control techniques used.

## 5. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 3.**Schematic configuration of two PV modules connected in series, (

**a**): S = 1000/1000, (

**b**): S = 1000/500.

**Figure 4.**Comparison of calculated and measured P-V characteristics of a single PV module under 0–150 Ω load conditions and with the same irradiance conditions (S = 1000 and 500).

**Figure 5.**Comparison of calculated and measured P-V characteristics of two PV modules connected in series under 0–150 Ω load conditions and irradiance condition of S = 1000 and 500 for the two modules, respectively.

**Figure 11.**Complete simulation model of the proposed PV emulator and the arrangement of control signals. (

**a**): buck-boost converter, (

**b**): analog to digital module, (

**c**): current controller, (

**d**) and (

**e**): PV model blocks, (

**f**) and (

**g**): voltage command blocks.

**Figure 13.**Experimental results of the proposed PV emulator under S = 1000 & 1000: (

**a**) simulated power and current outputs and PV voltage; (

**b**) implemented power and current outputs and PV voltage.

**Figure 14.**Output results of the proposed PV emulator under S = 500 & 1000 with theoretical PV values: (

**a**) simulated power and current outputs and PV voltage; (

**b**) implemented power and current outputs and PV voltage.

**Figure 15.**Output results of proposed PV emulator under S = 500 & 1000 with practical measured PV data: (

**a**) simulated power and current outputs and PV voltage; (

**b**) implemented power and current outputs and PV voltage.

**Figure 16.**Results of transition from normal condition (A) to shading condition (B): (

**a**) simulation; (

**b**) implementation.

**Figure 17.**Results of transition from shading condition (B) to normal condition (A): (

**a**) simulation; (

**b**) implementation.

**Figure 19.**Efficiencies of the proposed GaN-based synchronous buck converter at different switching frequencies.

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**MDPI and ACS Style**

Ma, C.-T.; Tsai, Z.-Y.; Ku, H.-H.; Hsieh, C.-L.
Design and Implementation of a Flexible Photovoltaic Emulator Using a GaN-Based Synchronous Buck Converter. *Micromachines* **2021**, *12*, 1587.
https://doi.org/10.3390/mi12121587

**AMA Style**

Ma C-T, Tsai Z-Y, Ku H-H, Hsieh C-L.
Design and Implementation of a Flexible Photovoltaic Emulator Using a GaN-Based Synchronous Buck Converter. *Micromachines*. 2021; 12(12):1587.
https://doi.org/10.3390/mi12121587

**Chicago/Turabian Style**

Ma, Chao-Tsung, Zhen-Yu Tsai, Hung-Hsien Ku, and Chin-Lung Hsieh.
2021. "Design and Implementation of a Flexible Photovoltaic Emulator Using a GaN-Based Synchronous Buck Converter" *Micromachines* 12, no. 12: 1587.
https://doi.org/10.3390/mi12121587