# A High-Frequency Isolated Online Uninterruptible Power Supply (UPS) System with Small Battery Bank for Low Power Applications

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## Abstract

**:**

## 1. Introduction

- (1)
- The number of batteries is not restricted to the dc-link voltage. The volume, weight, and backup time of the battery bank should be designed according to the specific application.
- (2)
- Cost reduction as no extra voltage balancing circuit is required.
- (3)
- Damaged batteries can be isolated or replaced in the battery bank leaving the sensitive system operation uninterrupted. This is a prime function of UPS systems.
- (4)
- Since the discharging currents of the batteries can be profiled individually, hence the stored energy in the batteries can be utilized more efficiently.

## 2. Circuit Description

#### 2.1. Modes of Operation

#### 2.1.1. Grid Mode

_{r}. Meanwhile; the boost converter steps up battery bank voltage across C

_{r}and provides it to the inverter. The inverter provides regulated output voltage to the connected load.

#### 2.1.2. Battery Mode

#### 2.2. AC-DC Converter

_{2}and S

_{3}, while duty cycle (1-D) controls switches S

_{1}and S

_{4}. Figure 3 presents the waveforms and operating modes of the AC-DC converter.

#### 2.2.1. Mode 1

_{2}and S

_{3}turn ON under ZVS conditions. The boost inductor L

_{in}is charged by input AC line voltage V

_{in}through the switch S

_{2}and S

_{3}. The inductor current is represented by the following equation:

_{b}is charged by the dc-link capacitor C

_{d}through S

_{3}and the secondary side of the high-frequency transformer is fed by the energy stored in the magnetizing inductor L

_{m}. The magnetizing inductor current i

_{Lm}is represented under the initial condition ${i}_{Lm}\left(0\right)$ by Equation (2):

_{1}is same as the output filter inductor current, and is given as:

#### 2.2.2. Mode 2

_{2}and S

_{3}turn OFF during this mode. The parasitic capacitance C

_{S2}is charged and C

_{S1}is discharged using inductor current i

_{Lin}. Similarly, the capacitance C

_{S3}is charged and capacitance C

_{S4}is discharged by the primary current i

_{P}of the transformer.

_{1}and S

_{4}reduces to zero while the voltage across the switch S

_{2}and S

_{3}increases to V

_{d}. At the secondary side, the output filter inductor current freewheels through the diodes D

_{1}and D

_{2}.

#### 2.2.3. Mode 3

_{d}starts charging by getting energy from the boost inductor and as a result i

_{Lin}starts decreasing. As the voltage across S

_{1}reduces to 0, the body diode D

_{S1}turns ON. Similarly, the blocking capacitor is fed by the magnetizing inductance L

_{m}through the body diode D

_{S4}. As a result, i

_{Lm}will induce the flux in the high-frequency transformer secondary winding, and the power will be fed to the output capacitor C

_{r}.

#### 2.2.4. Mode 4

_{1}and S

_{4}turn ON following the ZVS condition. The DC-link capacitor C

_{d}is charged by the stored energy released by the boost inductor L

_{in}. The inductor current is given by:

_{in}, S

_{1}, C

_{d}, and S

_{2}. Similarly, the magnetising current i

_{Lm}starts decreasing from maximum as represented by the following Equation (5):

_{2}. The output filter inductor current i

_{LO}is represented by Equation (6):

#### 2.2.5. Mode 5

_{1}and S

_{4}turn OFF. The parasitic capacitor C

_{S2}is discharged and C

_{S1}is charged by both the current from the transformer primary windings and the input inductor L

_{in}to and from V

_{d}, respectively. Now the voltage across S

_{2}is zero, and the voltage across S

_{1}is V

_{d}. Similarly, the capacitance C

_{S4}is charged and C

_{S3}is discharged by the primary current i

_{P}of the transformer. Thus, the voltage across S

_{4}is V

_{d}. At the secondary side, the output filter current freewheels through the diode D

_{1}and D

_{2}.

#### 2.2.6. Mode 6

_{S2}turns ON because of charging of the inductor. Similarly, the voltage across the switch S

_{3}is zero. This causes the body diode D

_{S3}to be turned ON and the primary current starts flowing through it. In the end of Mode 6, both the S

_{2}and S

_{3}turn ON under the condition of ZVS.

#### 2.2.7. Continuous condition mode (CCM) of operation of input inductor

_{e}is the emulative resistance of the converter. Equation (9) shows that the inductor peak current is not very high because large value of inductor is selected which maintains the continuous conduction.

#### 2.2.8. Discontinuous conduction mode (DCM) of operation of the input inductor

_{b}should be determined by (10):

_{o}is the maximum output power. For positive line period, the peak boost inductor current during T

_{S}is expressed as (11):

#### 2.3. Boost Converter

_{P}and L

_{S}as primary and secondary winding inductance respectively. The characteristic waveform and modes of operation of the DC-DC converter is shown in the Figure 4.

#### 2.3.1. Mode 1 (t_{0}~t_{1})

_{5}is ON, while the switch S

_{6}is OFF. Low battery bank voltage is applied at the input of the DC-DC boost converter. Capacitor C

_{b2}remains charged before Mode 1 and the magnetizing current i

_{Lm}of the coupled inductor increases linearly, as shown in the Figure 5.

#### 2.3.2. Mode 2 (t_{1}~t_{2})

_{5}turns OFF in Mode 2. The primary current i

_{LP}charges the parasitic capacitance across the switch S

_{5}and the secondary current i

_{LS}discharges the parasitic capacitance across switch S

_{6}. When the voltage across switch S

_{5}equals to the capacitor voltage V

_{Cb1}, this mode finishes.

#### 2.3.3. Mode 3 (t_{2}~t_{3})

_{5}is OFF, the primary current i

_{LP}decreases due to leakage inductance. However, the secondary current i

_{LS}increases, which results in the turning ON of the body diode of switch S

_{6}. As the voltage across the switch S

_{5}is higher than capacitor C

_{b1}, it charges C

_{b1}through diode D

_{b1}. Hence, the voltage stress across the switch S

_{5}has been reduced. V

_{Cb1}is the voltage across the capacitor C

_{b1}, and is represented by Equation (14):

#### 2.3.4. Mode 4 (t_{3}~t_{4})

_{6}turns ON under ZVS conditions. Both the windings of the coupled inductor and the capacitor C

_{b2}series connected together transfer maximum energy to the output capacitor C

_{din}of the converter. The i

_{LS}starts increasing until it reaches the i

_{LP}, then it follows the i

_{LP}till the end of the Mode 4. Thus, the energy stored in both the windings of the coupled inductor discharges across the high voltage side of the circuit. D

_{b1}and D

_{b2}are reverse biased during this mode. Applying voltage second balance, we get Equation (16):

#### 2.3.5. Mode 5 (t_{4}~t_{5})

_{6}turns OFF. The current i

_{LS}charges the parasitic capacitance of the switch S

_{6}. Capacitor C

_{b2}is charged by the capacitor C

_{b1}through the diode D

_{b2}:

_{5}turns ON because of the polarities of capacitor C

_{b2}and inductor L

_{P}.

#### 2.3.6. Mode 6 (t_{5}~t_{6})

_{5}turns to ON state under ZVS conditions. Since no current is derived by switch S

_{5}from the clamped circuit, thus the switching losses remains low due to ZVS, results in increasing the efficiency of the converter. Mode 6 finishes at the point when both the V

_{Cb1}and V

_{Cb2}become equal. The turn ratio N = 4 is selected to satisfy the G

_{boost}gain in order to step up the battery bank voltage to required dc-link of inverter.

## 3. Control Strategy

#### 3.1. Inverter Control

_{d}is the applied DC-link voltage, V

_{out}the filter capacitor C

_{f}output voltage. i

_{Lf}is the inductor L

_{f}current and i

_{o}the output current through the load R, given by i

_{o}= V

_{out}/R

_{Load}. The state equations of the inverter are given as:

_{1}, and its derivative ${x}_{2}={\dot{x}}_{1}$ need to be find:

_{1}and x

_{2}:

_{S}) with robust control of the inverter.

#### 3.2. Battery Charger Control

_{d}is large enough to give approximately constant voltage i.e., $d{v}_{d}/dt=0$. With ${\widehat{V}}_{in}=0,$ the small signal control $\widehat{d}$ to input current ${\widehat{i}}_{L}$ transfer function ${G}_{{i}_{L}d}\left(s\right)$ of the inner current loop is give by:

_{pi}and integral gain K

_{ii}are selected as 2.3 and 1200, respectively, for the stable operation of the current loop. Figure 6a presents the Bode plot of the current loop gain with phase margin of 89° and stable operation of the rectifier.

_{Ref}, a proportional-integral (PI) compensator has been employed. Combining the power stage with the PI controller ${G}_{v}\left(s\right)={k}_{pv}+\frac{{k}_{iv}}{s}$ provides the overall loop gain.

_{pv}and K

_{iv}in voltage loop are selected as 1.2 and 13, respectively. The stability of the voltage loop can be analyzed using the Bode plot obtained by considering the parameters from Table 4, as shown in Figure 6b. The system shows good stability with a positive phase margin.

_{Bat}is forced to follow the reference current i

_{Ref}using a PI compensator in (36):

_{Bat}to follow the reference voltage V

_{ref}. The current limiter is introduced to limit the maximum charging current of the battery. If the i

_{ref}is greater than i

_{limit}, the battery is charged at constant current (CC Mode), in contrast if the i

_{ref}is less than i

_{limit}, the battery is charged at constant voltage (CV mode).

#### 3.3. Boost Converter Control

_{b2}, the transfer equation is given by:

_{din}is regulated by using a suitable voltage compensator [37]. The PI compensator is used to track the DC-link voltage V

_{d}to follow the reference voltage, as shown in Figure 7.

## 4. Experimental Results

_{3}and S

_{4}. Both the switches are operating under the condition of ZVS. Figure 9b shows the drain to source voltage of the switches S

_{5}and S

_{6}of the boost converter. Both switches are operating under the condition of ZVS. Also the voltage stress across the switches is also limited. When the grid power is interrupted, the system switches from grid mode to battery mode. The rectifier is no longer in operation and the boost converter provides regulated dc-link voltage.

## 5. Conclusions

## Authors Contribution

## Acknowledgments

## Conflicts of Interest

## References

- Lahyani, A.; Venet, P.; Guermazi, A.; Troudi, A. Battery/supercapacitors combination in uninterruptible power supply (UPS). IEEE Trans. Power Electron.
**2013**, 28, 1509–1522. [Google Scholar] [CrossRef] - Zhao, B.; Song, Q.; Liu, W.; Xiao, Y. Next-generation multi-functional modular intelligent UPS system for smart grid. IEEE Trans. Ind. Electron.
**2013**, 60, 3602–3618. [Google Scholar] [CrossRef] - Branco, C.G.; Torrico-Bascope, R.P.; Cruz, C.M.; de A Lima, F. Proposal of three-phase high-frequency transformer isolation UPS topologies for distributed generation applications. IEEE Trans. Ind. Electron.
**2013**, 60, 1520–1531. [Google Scholar] [CrossRef] - Hwang, J.-C.; Chen, J.-C.; Pan, J.-S.; Huang, Y.-C. Measurement method for online battery early faults precaution in uninterrupted power supply system. IET Electr. Power Appl.
**2011**, 5, 267–274. [Google Scholar] [CrossRef] - Karve, S. Three of a kind [UPS topologies, IEC standard]. IEE Rev.
**2000**, 46, 27–31. [Google Scholar] [CrossRef] - Vázquez, N.; Villegas-Saucillo, J.; Hernández, C.; Rodríguez, E.; Arau, J. Two-stage uninterruptible power supply with high power factor. IEEE Trans. Ind. Electron.
**2008**, 55, 2954–2962. [Google Scholar] - Aamir, M.; Kafeel, A.K.; Mekhilef, S. Review: Uninterruptible power supply (UPS) system. Renew. Sust. Energy Rev.
**2016**, 58, 1395–1410. [Google Scholar] [CrossRef] - Rodriguez, E.; Vázquez, N.; Hernández, C.; Correa, J. A novel AC UPS with high power factor and fast dynamic response. IEEE Trans. Ind. Electron.
**2008**, 55, 2963–2973. [Google Scholar] [CrossRef] - Torrico-Bascope, R.P.; Oliveira, D.S.; Branco, C.G.C.; Antunes, F.L.M. A UPS with 110-V/220-V input voltage and high-frequency transformer isolation. IEEE Trans. Ind. Electron.
**2008**, 55, 2984–2996. [Google Scholar] [CrossRef] - Nasiri, A.; Nie, Z.; Bekiarov, S.B.; Emadi, A. An on-line UPS system with power factor correction and electric isolation using BIFRED converter. IEEE Trans. Ind. Electron.
**2008**, 55, 722–730. [Google Scholar] [CrossRef] - Hirachi, K.; Yoshitsugu, J.; Nishimura, K.; Chibani, A.; Nakaoka, M. Switched-mode PFC rectifier with high-frequency transformer link for high-power density single phase UPS. In Proceedings of the IEEE 28th Annual IEEE Power Electronics Specialists Conference, (PESC’97 Record), St. Louis, MO, USA, 27 June 1997; Volume 1, pp. 290–296. [Google Scholar]
- De Rooij, M.A.; Ferreira, J.A.; van Wyk, D. A novel unity power factor low-EMI uninterruptible power supply. IEEE Trans. Ind. Electron.
**1998**, 34, 870–877. [Google Scholar] [CrossRef] - Park, H.S.; Kim, C.H.; Park, K.B.; Moon, G.W.; Lee, J.H. Design of a charge equalizer based on battery modularization. IEEE Trans. Veh. Technol.
**2009**, 58, 3216–3223. [Google Scholar] [CrossRef] - Lee, Y.S.; Cheng, M.W. Intelligent control battery equalization for series connected lithium-ion battery strings. IEEE Trans. Ind. Electron.
**2005**, 52, 1297–1307. [Google Scholar] [CrossRef] - Zhang, Y.; Yu, M.; Liu, F.; Kang, Y. Instantaneous current-sharing control strategy for parallel operation of UPS modules using virtual impedance. IEEE Trans. Power Electron.
**2013**, 28, 432–440. [Google Scholar] [CrossRef] - Yang, S.; Chen, S.; Lin, J. Dynamics analysis of a low-voltage stress single-stage high-power factor correction AC/DC flyback converter. IET Power Electron.
**2012**, 5, 1624–1633. [Google Scholar] [CrossRef] - Duarte, J.; Lima, L.R.; Oliveira, L.; Michels, L.; Rech, C.; Mezaroba, M. Single-stage high power factor step-up/step-down isolated AC/DC converter. IET Power Electron.
**2012**, 5, 1351–1358. [Google Scholar] [CrossRef] - Ki, S.; Lu, D. Extension of minimum separable switching configuration modelling to single-stage AC/DC converters with direct power transfer. IET Power Electron.
**2012**, 5, 1154–1163. [Google Scholar] [CrossRef] - Yang, L.-S.; Liang, T.-J.; Chen, J.-F.; Lin, R.-L. Analysis and design of a novel, single-stage, three-phase AC/DC step-down converter with electrical isolation. IET Power Electron.
**2008**, 1, 154–163. [Google Scholar] [CrossRef] - Ma, H.; Ji, Y.; Xu, Y. Design and analysis of single-stage power factor correction converter with a feedback winding. IEEE Trans. Power Electron.
**2010**, 25, 1460–1470. [Google Scholar] [CrossRef] - Mahdavi, M.; Farzaneh-Fard, H. Bridgeless CUK power factor correction rectifier with reduced conduction losses. IET Power Electron.
**2012**, 5, 1733–1740. [Google Scholar] [CrossRef] - Abasian, A.; Farzaneh-fard, H.; Madani, S. Single stage soft switching AC/DC converter without any extra switch. IET Power Electron.
**2014**, 7, 745–752. [Google Scholar] [CrossRef] - Liang, T.-J.; Yang, L.-S.; Chen, J.-F. Analysis and design of a single-phase AC/DC step-down converter for universal input voltage. IET Electr. Power Appl.
**2007**, 1, 778–784. [Google Scholar] [CrossRef] - Lai, C.; Shyu, K. A single-stage AC/DC converter based on zero voltage switching LLC resonant topology. IET Electr. Power Appl.
**2007**, 1, 743–752. [Google Scholar] [CrossRef] - Lin, J.-L.; Yao, W.-K.; Yang, S.-P. Analysis and design for a novel single-stage high power factor correction diagonal half-bridge forward AC/DC converter. IEEE Trans. Circ. Syst. I Regul. Pap.
**2006**, 53, 2274–2286. [Google Scholar] [CrossRef] - Lu, D.-C.; Iu, H.-C.; Pjevalica, V. Single-stage AC/DC boost–forward converter with high power factor and regulated bus and output voltages. IEEE Trans. Ind. Electron.
**2009**, 56, 2128–2132. [Google Scholar] [CrossRef] - Choi, W.-Y.; Yoo, J.-S. A bridgeless single-stage half-bridge AC/DC converter. IEEE Trans. Power Electron.
**2011**, 26, 3884–3895. [Google Scholar] [CrossRef] - Ribeiro, H.S.; Borges, B.V. High-performance voltage-fed AC–DC full-bridge single-stage power factor correctors with a reduced DC bus capacitor. IEEE Trans. Power Electron.
**2014**, 29, 2680–2692. [Google Scholar] [CrossRef] - Das, P.; Pahlevaninezhad, M.; Moschopoulos, G. Analysis and design of a new AC–DC single-stage full-bridge PWM converter with two controllers. IEEE Trans. Ind. Electron.
**2013**, 60, 4930–4946. [Google Scholar] [CrossRef] - Erickson, R.W.; Maksimovic, D. Fundamentals of Power Electronics; Springer Science & Business Media: New York, NY, USA, 2001. [Google Scholar]
- Aamir, M.; Mekhilef, S.; Hee-Jun, K. High-gain zero-voltage switching bidirectional converter with a reduced number of switches. IEEE Trans. Circ. Syst. II Express Briefs
**2015**, 62, 816–820. [Google Scholar] [CrossRef] - Zargari, N.; Ziogas, P.; Joos, G. A two switch high performance current regulated DC/AC converter module. IEEE Trans. Ind. Appl.
**1995**, 31, 583–589. [Google Scholar] [CrossRef] - Cortes, P.; Ortiz, G.; Yuz, J.I.; Rodriguez, J.; Vazquez, S.; Franquelo, L.G. Model predictive control of an inverter with output LC filter for UPS applications. IEEE Trans. Ind. Electron.
**2009**, 56, 1875–1883. [Google Scholar] [CrossRef] - Tamyurek, B. A high-performance SPWM controller for three-phase UPS systems operating under highly nonlinear loads. IEEE Trans. Power Electron.
**2013**, 28, 3689–3701. [Google Scholar] [CrossRef] - Komurcugil, H. Rotating-sliding-line-based sliding-mode control for single-phase UPS inverters. IEEE Trans. Ind. Electron.
**2012**, 59, 3719–3726. [Google Scholar] [CrossRef] - Abrishamifar, A.; Ahmad, A.A.; Mohamadian, M. Fixed switching frequency sliding mode control for single-phase unipolar inverters. IEEE Trans. Power Electron.
**2012**, 27, 2507–2514. [Google Scholar] [CrossRef] - Ullah, W.; Mekhilef, S. Transformer-less 3P3W SAPF (three-phase three-wire shunt active power filter) with line-interactive UPS (uninterruptible power supply) and battery energy storage stage. Energy
**2016**, 109, 525–536. [Google Scholar] - International Electro-technical Commission. Uninterruptible Power Systems (UPS)—Methods of Specifying the Performance and Test Requirements; IEC62040-3 Standard; IEC: Geneva, Switzerland, 1999. [Google Scholar]

**Figure 3.**Single-stage AC-DC Converter: (

**a**) Operation waveforms; (

**b**) Modes of operation during one switching cycle T

_{S}.

**Figure 8.**Experimental waveform of the input output voltage and current of the UPS system. (

**a**) Input line voltage and current; (

**b**) Inverter with linear Load; (

**c**) Inverter with Non-linear load.

**Figure 9.**Drain to source voltage and current of the switches. (

**a**) S

_{3}and S

_{4}of single-stage AC-DC Converter; (

**b**) S

_{5}and S

_{6}of boost DC-DC converter.

**Figure 10.**Transition between two modes of operation, Input voltage V

_{in}and current I

_{in}, Output voltage V

_{out}and current I

_{out}. (

**a**) Grid to battery mode and (

**b**) Battery mode to grid mode.

Reference | Rated Power | Input/Output Voltage | Battery Bank Voltage | Number of Batteries |
---|---|---|---|---|

[8] | 300 VA | 120 | 48 | 4 |

[9] | 2 kVA | 220 | 108 | 9 |

[10] | 150 VA | 120 | 48 | 4 |

[11] | 2 kVA | 110 | 180 | 15 |

[12] | 3.3 kVA | 220 | 120 | 10 |

Parameters | Model Predictive Control [33] | SPWM Control [34] | Rotating SMC [35] | Fix-Freq SMC [36] | Proposed Work |
---|---|---|---|---|---|

V_{DC} | 529 | 405 | 300 | 360 | 350 |

V_{RMS} | 150 | 220 | 200 | 220 | 220 |

C_{f} (uF) | 40 | 202 | 100 | 9.4 | 6.6 |

L_{f} (mH) | 2.4 | 0.03 | 0.250 | 0.357 | 0.84 |

THD (L) | 2.85% | 1.11% | - | 1.1% | 0.45% |

THD (NL) | 3.8% | 3.8% | 2.66% | 1.7% | 1.25% |

T_{S} (ms) | 50 | 60 | - | 0.5 | 0.3 |

Parameters | Symbol | Value |
---|---|---|

Input Voltage | V_{in} | 220 V |

Output Voltage | V_{out} | 220 V |

Grid Frequency | f_{r} | 50 Hz |

Output Frequency | f_{o} | 50 Hz |

Number of Batteries | V_{b} | 2 Parallel connected (24 V/35 Ah) |

Maximum Output Power | P_{o,max} | 1 kVA |

DC-link Voltage | V_{d} | 360 V |

Parameters | Symbol | Value |
---|---|---|

Input Inductor | L_{in} | 1.2 mH |

Switches | S_{1}~S_{4} | SPP11N60C3 |

Fast Diodes | D_{1}, D_{2} | C3D10060A |

Switching frequency | f_{s} | 50,000 Hz |

H. F Transformer | T | L_{m} = 600 uH, TDK core PQ-40/40 |

DC-Link Capacitor | C_{d} | 1900 uF |

Parameters | Symbol | Value |
---|---|---|

DC-Link Voltage | V_{d_INV} | 360 V |

Battery Bank Voltage | V_{b} | 24 V |

Switching Frequency | f_{s} | 30,000 Hz |

Coupled Inductor | L_{P}, L_{S} | Turns ratio N = 4; Magnetizing Inductor L_{m} = 107 uH; PQ-5050 core |

Capacitor | C_{b1}, C_{b2} | C_{b1}, C_{b2} = 2 × 2.2 uF (ceramic), C_{d} = 1900 uF |

Switches | S_{5} ,S_{6} | IPW60R045CP MOSFET (Infineon Technologies, Santa Clara, CA USA) |

Diodes | D_{b1}, D_{b2} | Ultrafast Recovery diode UF5408 |

Properties | Efficiency | Power Ratings | System Specification | Battery Bank | Size & Weight | |
---|---|---|---|---|---|---|

UPS Topology | ||||||

An On-Line UPS System With Power Factor Correction and Electric Isolation Using BIFRED Converter [10] | - | 150 VA | 110 V | 48 V | Small | |

Two-Stage Uninterruptible Power Supply With High Power Factor [6] | 84% | 150 VA | 120 V | 60 V | Small | |

A UPS With 110-V/220-V Input Voltage and High-Frequency Transformer Isolation [9] | 86% | 2 kVA | 110/220 V | 96 V | High | |

Novel AC UPS With High Power Factor and Fast Dynamic Response [8] | - | 300 VA | 110 V | 48 V | - | |

Proposed UPS System | 91% | 1 kVA | 220 V | 24 V | Medium |

© 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Aamir, M.; Ullah Tareen, W.; Ahmed Kalwar, K.; Ahmed Memon, M.; Mekhilef, S. A High-Frequency Isolated Online Uninterruptible Power Supply (UPS) System with Small Battery Bank for Low Power Applications. *Energies* **2017**, *10*, 418.
https://doi.org/10.3390/en10040418

**AMA Style**

Aamir M, Ullah Tareen W, Ahmed Kalwar K, Ahmed Memon M, Mekhilef S. A High-Frequency Isolated Online Uninterruptible Power Supply (UPS) System with Small Battery Bank for Low Power Applications. *Energies*. 2017; 10(4):418.
https://doi.org/10.3390/en10040418

**Chicago/Turabian Style**

Aamir, Muhammad, Wajahat Ullah Tareen, Kafeel Ahmed Kalwar, Mudasir Ahmed Memon, and Saad Mekhilef. 2017. "A High-Frequency Isolated Online Uninterruptible Power Supply (UPS) System with Small Battery Bank for Low Power Applications" *Energies* 10, no. 4: 418.
https://doi.org/10.3390/en10040418