# Multicell Power Supplies for Improved Energy Efficiency in the Information and Communications Technology Infrastructures

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

- Input in Series Output in Series (ISOS);
- Input in Series Output in Parallel (ISOP);
- Input in Parallel Output in Series (IPOS);
- Input in Parallel Output in Parallel (IPOP).

## 2. Structure of AC to DC 48 V Power Supplies

_{module}≤ (P

_{rack-max}/2). The rectifier stage of the PSU consists of a bridge rectifier with switches (MOSFETs, IGBT, Thyristors, etc.) or diodes. The power factor correction system is used to ensure conformity with EN61000 and that there are no excessive harmonics in the AC supply from the UPS system. The third stage of a PSU is needed to step-down the voltage from the rectification stage to the desired 48 V DC. The rectification is common in all systems but there are two options used to produce the desired 48 V DC and to satisfy the input power quality requirements. The first option is to use a two-stage process with boost PFC and DC to DC step-down converter. The second option is to use the buck PFC single-stage circuit solution (PFC and DC to DC step-down circuits combined). In a single-stage conversion circuit it is very difficult to control both the input current quality and the output voltage level at different loads, and at the same time achieve high conversion efficiency. The two-stage conversion is therefore preferred as a solution for loads above 1 kW [16,17,18].

- P
_{rec}: power losses from the rectification stage; - P
_{pfc−loss}: losses of the power factor correction stage; - P
_{dc−dc loss}: losses of the DC-DC converter circuit; - P
_{sw loss}: MOSFET switch losses of the DC converter circuit (conduction and switching losses); - P
_{trans−loss}: transformer losses; - P
_{ind loss}: inductor losses; - P
_{cap−loss}: capacitor losses in DC-DC converter circuit; - P
_{tr−con−loss}: transformer load and frequency dependent losses; - P
_{no load loss}: transformer no load loss.

#### 2.1. Single-Stage PSU

#### 2.2. Two-Stage PSU Solution

## 3. Multicell Structures

#### 3.1. The Multicell AC to DC Rectification Stage

_{DS−On}). For the diode case, the conduction losses are given by:

_{d}is the diode voltage drop and I

_{rms}is the average current passing through the bridge leg. For the MOSFET switch case, conduction losses are given by:

_{DS−On}is the on-resistance of the MOSFET switch.

_{peak}of AC input). This would require a very high gain to reduce it to 48 V and there would thus be higher power losses in the secondary circuits to step down the voltage. Additionally, a high voltage at the output of the bridge rectification stage would need switches that could withstand a higher voltage. Since R

_{DS−On}is proportional to the voltage, this would result in higher power losses. Therefore, the multicell bridge would not have better efficiency if connected in ISOS or IPOS. The ISOP structure with MOSFET switches would not improve the efficiency either, since the series switches would increase the conduction losses. Considering however that the breaking voltage is proportional to R

_{DS−On}, switches that can withstand a lower voltage would have lower conduction losses. Replacing, therefore, one MOSFET switch with two switches of smaller R

_{DS−On,}could in theory result in better efficiency. Examining however the characteristics of available MOSFET switches, those with breaking voltage of 50 V have an R

_{DS−On}as low as 3.3 mΩ (IFR3805), while switches with a breaking voltage of 200 V have an R

_{DS−On}as low as 4 mΩ (IXFK300N20X 3). Therefore, energy efficiency improvement could not be achieved with ISOP and MOSFETs for this type of power supply. Analyzing the circuit further, it can be seen that the ISOP structure of the bridge rectification part would not provide any energy efficiency improvement with diodes either, since in each cycle of the AC source only two switches/diodes conduct, as seen in Figure 4, as in the single cell structure, namely D

_{1a}and D

_{2b}in the positive cycle and D

_{3b}and D

_{4a}in the negative cycle. In the IPOP structure with diodes, the voltage drop increases as the number of cells increases. Efficiency is therefore slightly improved as a result of lower heat dissipation. For the case of the IPOP structure with MOSFETs, the voltage stress on the switches remains the same and the current passing through the system is divided in the parallel cell bridges, something that may lead to significant energy efficiency improvements.

_{DS−On}) and since 2 (I/2)

^{2}< I

^{2}, it can be easily proven that parallel bridge structures lead to overall energy savings. For N cells, it can be shown that:

#### 3.2. The Multicell PFC Stage

#### 3.2.1. Multicell Buck PFC

_{sw}is the switching frequency, t

_{r}is the rising time the transistor needs to switch on and t

_{f}is the falling time needed to switch off.

_{r}and t

_{f}, they could potentially lead to better efficiency. However, using switches with lower t

_{r}and t

_{f}and at the same lower maximum drain current capability may negatively affect the R

_{DS−On}(i.e., conduction losses).

_{L}is the ripple current and is proportional to the output current of each cell.

_{DS−On}resistance during the ON state. The switching losses, on the other hand, are multiplied by the number of cells but the voltage input is divided by the number of cells (assuming the case of exactly the same R

_{DS−On}). The rising and falling times of the switches in the multicell ISOP and ISOS structures can be minimized since switches with lower voltage breaking capacity tend to have lower falling and rising times. Silicon MOSFETs can have R

_{DS−On}as low as 3.5 mΩ, with rising and falling times of about 170 nS (IXFN300N20X3), while GaN MOSFETs can have 12.4 ns rising and 24 ns falling time with R

_{DS−On}of 25mΩ (GS66516). On the other hand, inductor losses increase if the inductor size is not reduced from the single cell design. Since voltage gain is reduced in the ISOP and ISOS structures, the inductor size can also be significantly reduced. Therefore, ISOP or ISOS structures lead to reduced inductor losses but with insignificant switching loss improvement. The overall improvements are minor in relation to the complexity and increase in cost.

#### 3.2.2. Multicell Boost PFC

_{DS−On}MOSFET switches are used. Since the inductors are in series in ISOP and ISOS structures, the overall DC resistance and hence inductor losses will increase. Considering the inductor size needed to form a multicell ISOP or ISOS structure compared to a single cell, it can be concluded that the ISOP case would lead to reduced losses compared to ISOS, but both cases would not lead to a significant reduction in the inductor’s overall size and power losses (6), (7) and (8). The single cell boost PFC inductor value is given by [24]:

#### 3.3. The Multicell DC to DC Voltage Step-Down in Two-Stage Conversions

^{2}R of windings. The core losses are given by:

_{e}and K

_{h}are the eddy and hysteresis loss constants, P

_{eddy}is the eddy current power loss (W), B is the flux density (Wb/m

^{2}), f is the frequency of magnetic reversals per second (Hz), t is the core material thickness (m), V is the volume of core (m

^{3}) and n is the Steinmetz exponent (ranging from 1.5 to 2.5 depending on the material).

## 4. Experimental Verification of Multicell Structures

_{oc}

_{1}and V

_{oc}

_{2}are the cell type A and cell type B, open circuit voltage, V

_{out}is the output voltage of the system, r

_{1}is the internal resistance of type A buck-cell and w & q are the number of buck-cells of type A and B connected to the system, respectively. The power output of the system consisting of “w” cells of type A and “q” cells of type B is given by:

## 5. Proposed Multicell Buck PFC Converter

#### 5.1. Justification of AC to DC Multicell Selection

_{indutor}and R

_{DS−On}, respectively), the three-cell buck PFC has significantly lower losses (since N(I/N)

^{2}× R < I

^{2}× R).

#### 5.2. Multicell Buck PFC

_{DS−On}).

_{DS−On}switches. The solution with the lowest system cost is selected from the product of the number of cells and the cost of the switches.

## 6. Discussion

## 7. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Conflicts of Interest

## References

- Miller, R. What 5G Will Mean to the Data Center Industry. Datacenter Frontier, 19 February 2019. [Google Scholar]
- Tuffaha, N. Evolution of 5G and the Impact on Data Center Infrastructure. Digital Reality, 29 January 2019. [Google Scholar]
- Shehabi, A.; Smith, S.; Sartor, D.; Brown, R.; Herrlin, M.; Koomey, J.; Masanet, E.; Horner, N.; Azevedo, I.; Lintner, W. United States Data Center Energy Usage Report; LBNL-1005775; Lawrence Berkeley National Laboratory: Berkeley, CA, USA, 2016.
- Xiao, Z.; Liu, H.; Havyarimana, V.; Li, T.; Wang, D. Analytical Study on Multi-Tier 5G Heterogeneous Small Cell Networks: Coverage Performance and Energy Efficiency. Sensors
**2016**, 16, 1854. [Google Scholar] [CrossRef] [PubMed][Green Version] - Zeng, F.; Li, Q.; Xiao, Z.; Havyarimana, V.; Bai, J. A Price-Based Optimization Strategy of Power Control and Resource Allocation in Full-Duplex Heterogeneous Macrocell-Femtocell Networks. IEEE Access
**2018**, 6, 42004–42013. [Google Scholar] [CrossRef] - Jiang, H.; Xiao, Z.; Li, Z.; Xu, J.; Zeng, F.; Wang, D. An Energy-Efficient Framework for Internet of Things Underlaying Heterogeneous Small Cell Networks. IEEE Trans. Mob. Comput.
**2020**. [Google Scholar] [CrossRef] - Energystar. Reduce Energy Losses from Uninterruptable Power Supply (UPS) Systems. Available online: https://www.energystar.gov/products/ (accessed on 11 October 2021).
- EPRI, 80 Plus Certified Power Supplies and Manufacturers. Available online: https://www.plugloadsolutions.com/80PlusPowerSuppliesDetail.aspx?id=0type=1 (accessed on 11 October 2021).
- STMicroelectronics. 48 v Direct Power Conversion. 2018. Available online: https://www.st.com/en/power-management/48-v-direct-power-conversion.html (accessed on 11 October 2021).
- Taranovich, S. Data Center Next Generation Power Supply Solutions for Improved Efficiency. EDN Network, 16 April 2016. [Google Scholar]
- Kasper, M.; Bortis, D.; Deboy, G.; Kolar, J.W. Design of a highly efficient (97.7%) and very compact (2.2 kw/dm_3) isolated ac–dc telecom power supply module based on the multicell isop converter approach. IEEE Trans. Power Electron.
**2016**, 32, 7750–7769. [Google Scholar] [CrossRef] - Fekik, A.; Azar, A.T.; Kamal, N.A.; Serrano, F.E.; Hamida, M.L.; Denoun, H.; Yassa, N. Maximum Power Extraction from a Photovoltaic Panel Connected to a Multi-cell Converter. In Proceedings of the International Conference on Advanced Intelligent Systems and Informatics, Cairo, Egypt, 19–21 October 2020; pp. 873–882. [Google Scholar]
- Yuan, J.; Chen, Y.; Yang, Y.; Blaabjerg, F.; Chen, M. High Frequency Multicell Cascaded Quasi-Square-Wave Boost Converter. In Proceedings of the 2020 IEEE 21st Workshop on Control and Modeling for Power Electronics (COMPEL), Aalborg, Denmark, 9–12 November 2020. [Google Scholar]
- Luo, X.; Kang, L.; Lu, C.; Linghu, J.; Lin, H.; Hu, B. An Enhanced Multicell-to-Multicell Battery Equalizer Based on Bipolar-Resonant LC Converter. Electronics
**2021**, 10, 293. [Google Scholar] [CrossRef] - Gupta, R.; Kumar, A. Control of Multi-cell AC/DC and Cascaded H-bridge DC/AC-basedAC/DC/AC Converter. Taylor and Francis. IETE J. Res.
**2021**. [Google Scholar] [CrossRef] - Hosseinabadi, F.; Adib, E. A soft-switching step-down pfc converter with output voltage doubler and high power factor. IEEE Trans. Power Electron.
**2019**, 34, 416–424. [Google Scholar] [CrossRef] - Rashid, M.H. Power Electronics Handbook; Butterworth-Heinemann: Oxford, UK, 2017. [Google Scholar]
- Chrysostomou, M.; Christofides, N.; Ioannou, S.; Marouchos, C. Design Guidelines for Energy Efficient AC to DC Power Supplies. In Proceedings of the IEEE IECON, Toronto, ON, Canada, 13–16 October 2021. [Google Scholar]
- Sharifi, S.; Babaei, M.; Monfared, M. A high gain buck PFC synchronous rectifier. In Proceedings of the Iranian Conference on Electrical Engineering (ICEE), Mashhad, Iran, 8–10 May 2018. [Google Scholar]
- Axelrod, B.; Berkovich, Y.; Ioinovici, A. Switched capacitor/switched-inductor structures for getting transformerless hybrid dc–dc pwm converters. IEEE Trans. Circuits Syst.
**2008**, 55, 687–696. [Google Scholar] [CrossRef] - Fei, C.; Lee, F.C.; Li, Q. High-efficiency high-power-density llc converter with an integrated planar matrix transformer for high-output current applications. IEEE Trans. Ind. Electron.
**2017**, 64, 9072–9082. [Google Scholar] [CrossRef] - Kim, D.K.; Moon, S.; Yeon, C.O.; Moon, G.W. High-efficiency llc resonant converter with high voltage gain using an auxiliary lc resonant circuit. IEEE Trans. Power Electron.
**2016**, 31, 6901–6909. [Google Scholar] [CrossRef] - Mu, M.; Lee, F.C. Design and optimization of a 380–12 v high-frequency, high current llc converter with gan devices and planar matrix transformers. IEEE J. Emerg. Sel. Top. Power Electron.
**2016**, 4, 854–862. [Google Scholar] - Abdel-Rahman, S.; Stückler, F.; Siu, K. Infineon Technologies: PFC Boost Converter Design Guide; Infineon Technologies AG: Munich, Germany, 2016. [Google Scholar]
- Ioannou, S.; Argyrou, M.C.; Marouchos, C.; Darwish, M. Efficiency investigation of a grid connected pv system with power smoothing. In Proceedings of the 54th International Universities Power Engineering Conference (UPEC), Bucharest, Romania, 3–6 September 2019; pp. 1–6. [Google Scholar]
- Ioannou, S.; Marouchos, C.; Darwish, M.; Putrus, G. Efficiency investigation of a protection and correction solid state device for low-voltage distribution networks. In Proceedings of the 54th International Universities Power Engineering Conference (UPEC), Bucharest, Romania, 3–6 September 2019; pp. 1–6. [Google Scholar]
- Bertoldi, P.; Avgerinou, M.; Castellazzi, L. Trends in Data Centre Energy Consumption under the European Code of Conduct for Data Centre Energy Efficiency; JRC Technical Report; Publications Office of the European Union: Luxembourg, 2017; ISBN 978-92-79-76445-5. [Google Scholar]

**Figure 10.**Efficiency against power output of 1, 2 and 3 PSUs working in parallel (with transformer).

Power Input Cells (W) | Input Current to Cell 1 (A) | Input Current to Cell 2 (A) | Input Current to Cell 3 (A) |
---|---|---|---|

10.9 | 0.16 | 0.17 | 0.17 |

21 | 0.31 | 0.33 | 0.3 |

30.7 | 0.46 | 0.46 | 0.43 |

Power Output (W) | Total Losses (W) | Switch 1 Losses (W) | Switch 2 Losses (W) | Rectifier Losses (W) | PFC Losses (W) | L1 Loss (W) | L2 Loss (W) |
---|---|---|---|---|---|---|---|

200 | 8.38 | 1.46 | 0.51 | 1.28 | 4.91 | 0.19 | 0.03 |

300 | 12.51 | 1.89 | 1.08 | 2.04 | 6.98 | 0.44 | 0.08 |

400 | 16.85 | 2.39 | 1.88 | 2.88 | 8.79 | 0.78 | 0.13 |

500 | 21.89 | 3.02 | 2.93 | 3.8 | 10.7 | 1.23 | 0.21 |

1000 | 55.15 | 8.56 | 11.07 | 8.68 | 20.93 | 5.08 | 0.83 |

Power Output (W) | Single Cell Rectifier Losses (W) | 3 Cell Rectifier Losses (W) | Rectifier Efficiency Improvement | Overall PSU Efficiency Improvement % |
---|---|---|---|---|

1000 | 8.68 | 0.00288 | >99% | 0.79% |

500 | 3.8 | 0.0144 | >99% | 0.70% |

400 | 2.88 | 0.01872 | >99% | 0.66% |

300 | 2.04 | 0.0261 | >98% | 0.62% |

200 | 1.28 | 0.03636 | >97% | 0.58% |

PSU Brand | PSU Model | Power Output | PSU Price | Cost/Watt | Reference |
---|---|---|---|---|---|

Mean well | RCP-1000-48 | 1000 | $241.90 | $0.2419 | Jameco.com |

Mean well | RCP-2000-48 | 2000 | $436.69 | $0.2183 | Jameco.com |

Mean well | RST-5000-48 | 5000 | $1193.00 | $0.2386 | Jameco.com |

Mean well | RST-10000-48 | 10,000 | $2366.00 | $0.2366 | Jameco.com |

Jetpower | SPS60-48/CR4830 | 3000 | $360.00 | $0.1200 | Alibaba.com |

Jetpower | SPS200-48/SR4850 | 10,000 | $1250.00 | $0.1250 | Alibaba.com |

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. |

© 2021 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 (https://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Chrysostomou, M.; Christofides, N.; Ioannou, S.; Polycarpou, A. Multicell Power Supplies for Improved Energy Efficiency in the Information and Communications Technology Infrastructures. *Energies* **2021**, *14*, 7038.
https://doi.org/10.3390/en14217038

**AMA Style**

Chrysostomou M, Christofides N, Ioannou S, Polycarpou A. Multicell Power Supplies for Improved Energy Efficiency in the Information and Communications Technology Infrastructures. *Energies*. 2021; 14(21):7038.
https://doi.org/10.3390/en14217038

**Chicago/Turabian Style**

Chrysostomou, Michael, Nicholas Christofides, Stelios Ioannou, and Alexis Polycarpou. 2021. "Multicell Power Supplies for Improved Energy Efficiency in the Information and Communications Technology Infrastructures" *Energies* 14, no. 21: 7038.
https://doi.org/10.3390/en14217038