# Analysis of AC/DC/DC Converter Modules for Direct Current Fast-Charging Applications

^{1}

^{2}

^{3}

^{*}

## Abstract

**:**

## 1. Introduction

- (1)
- slow charging, low power (up to 4 kW) AC single-phase (during long term stops), available solutions in both OBC and OffBC versions;
- (2)
- fast charging, medium power (up to 25 kW) AC three-phase, available for OffBC and OBC solutions using machine winding in idling drive;
- (3)
- fast charging, high power DC (up to 900 kW planned), available for OffBC solutions directly connected to the battery through BMS.

## 2. Direct Current Fast Charging Power Converter Topologies

#### 2.1. Requirements

#### 2.2. AC/DC Convertergrid Side

_{DC}potential is demanded at phase output. Choosing one of available states, depending on the direction of the output current, makes charging and discharging the floating capacitor possible in each phase. This mechanism allows voltage balancing but makes the modulation technique more complicated and time-consuming than in Neutral Point Clamped Converters (NPCs) [52,53]. Basic parameters of the analyzed topologies are presented in Table 1.

#### 2.3. DC//DC Galvanically Isolated Converter Battery Side

^{3}, this topology is commonly used in automotive, photovoltaic, and other systems [66]. The problems of DAB which require a broader analysis are the optimization of operating parameters for a wide range of output voltages and the operation at low output voltage. A startup procedure is required to limit current pulses at low voltages. As a rule, it is implemented by limiting the width of the transistor control pulses.

_{R1}, C

_{R2}and two resonant inductors L

_{R1}, L

_{R2}form two series of resonant tanks, each placed at one side of the high frequency transformer. In the rectifying stage, all transistors are turned-off and the voltage is then rectified by antiparallel transistor diodes. This topology ensures zero voltage switching (ZVS) conditions for transistors in the inverting stage and zero current switching (ZCS) conditions for diodes in the rectifying stage. As a result, switching losses are reduced, which increases the energy efficiency of the converters. Moreover, soft-switched converters may operate at a higher switching frequency than hard-switched solutions. Due to higher switching frequency, the sizes of magnetic elements and capacitors may be reduced, thus reducing the overall dimension of the converter [68]. Comparing to hard-switched topologies, the time gradients of current and voltage waveforms recorded during switching processes are smaller, what improves EMI properties of resonant converters. However, the high value of the resonant frequency may interfere with certain frequencies. It must be noted that the topologies of resonant converters are more complicated. In some cases, additional switches and complicated control systems are necessary to control the resonant process, hence the total cost of the converter increases [68]. Significant problems of this type of converter include protection of the secondary side against charging voltage in emergency states, the size and weight of passive components, stabilization of resonance, and difficulties with maintaining soft switching conditions at increased frequency.

#### 2.4. Conclusions

## 3. Simulation Model

## 4. Experimental Model

## 5. Experimental Analysis

#### 5.1. Experimental Test Bench

#### 5.2. Unidirectional Charger with IGBT Technology

#### 5.3. Bidirectional Charger with SiC Technology

#### 5.4. Efficiency Measurement

## 6. Discussion and Conclusions

#### 6.1. Conclusions

- −
- galvanic separation between the mains and the vehicle battery;
- −
- modular design, thanks to independent AC/DC and DC/DC converters, which considerably facilitates service work (possibility of replacing individual modules);
- −
- system scalability—the output power can be increased by connecting modules in parallel;
- −
- expansion with additional functionalities, including bidirectional operation and support for V2G systems with the use of a bidirectional DC/DC converter;
- −
- a wide range of output current and voltage regulation (on the vehicle side), thanks to the use of two levels of energy conversion, as a result of which the design can be adapted to various types of batteries.

#### 6.2. Limitations and Future Works

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

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**Figure 2.**Block schemes of active AC/DC converter topologies: (

**a**) 2-level Voltage Source Converter (VSC); (

**b**) 3-level Floating Capacitor Converter (FLC); (

**c**) 3-level T-type converter; (

**d**) Neutral Point Clamped Converter (NPC).

**Figure 3.**Regulation schemes of active AC/DC converters: (

**a**) Voltage Oriented Control (VOC); (

**b**) Direct Power Control with Space Vector Modulation (DPC-SVM).

**Figure 4.**Block schemes of the considered DC/DC converter topologies: (

**a**) single active bridge (SAB); (

**b**) dual active bridge (DAB); (

**c**) DAB-CLLC resonant converter.

**Figure 5.**Simulation model of 2-level AC/DC converter module with VOC control developed in PLECS environment.

**Figure 6.**Simulation result—step load changes of AC/DC converter, from 85 kW of active power consumed from the grid, through 85 kW of active power restored to the grid, up to 85 kW consumed from the grid: (

**a**) grid voltages; (

**b**) grid currents; (

**c**) DC-link current; (

**d**) DC-link voltage.

**Figure 7.**Simulation model of dual active bridge (DAB) converter module with a Phase Shift Controller developed in the PLECS environment.

**Figure 8.**Simulation result—operation of the DC/DC converter during steady state: (

**a**) phase voltage (orange) and transformer current (blue); (

**b**) input voltage (green), output voltage (red).

**Figure 9.**Developed in the PLECS environment, a simulation model of direct-current fast charger composed of: (

**a**) 2-level AC/DC converter module; (

**b**) DC/DC DAB converter module; (

**c**) control algorithms: VOC control for AC/DC converter and a Phase Shift Controller for the DC/DC converter.

**Figure 10.**Simulation result—step change of bi-directional direct-current fast charger from charging operation mode to battery discharging operation mode: (

**a**) DAB’s input voltage (red), DAB’s output voltage (battery voltage)—blue; (

**b**) grid phase voltages; (

**c**) grid phase currents; (

**d**) battery current (green).

**Figure 11.**Block scheme of charger prototypes analyzed in experimental study: (

**a**) unidirectional charger with a 2-level AC/DC converter and a DC/DC converter composed of single-phase bridge and a full bridge diode rectifier in Si technology; (

**b**) bidirectional charger with 2-level AC/DC converter and DC/DC converter composed of a dual active bridge (DAB) in SiC technology.

**Figure 12.**Experimental test bench used during the research: (

**a**) block scheme; (

**b**) view of the test bench: 1.—control platform; 2.—50 kW unidirectional charger made in IGBT technology composed of 2-level AC/DC and SAB converters; 3.—50 kW bidirectional charger made in SiC technology composed of 2-level AC/DC and DAB converters; 4.—LEM NORMA D6000 power analyzer (provided by Warsaw University of Technology, Warsaw, Poland); 5.—energy storage constructed from batteries of electric vehicles (Nissan Leaf).

**Figure 13.**Operation of unidirectional charger under nominal conditions—steady state: (

**a**) current and voltage waveforms, from the top: battery current (purple), battery voltage (light blue), AC phase current (dark blue), AC line-to-line voltage (green); (

**b**) printed screen from the LEM NORMA Power Analyzer.

**Figure 14.**Operation of the unidirectional charger—step load change from 5% up to 80% of nominal power, from the top: battery current (purple), battery voltage (light blue), AC phase current (dark blue), AC line-to-line voltage (green).

**Figure 15.**Operation of the bidirectional charger under nominal conditions—steady state, charging mode: (

**a**) current and voltage waveforms, from the top: battery current (purple), battery voltage (light blue), AC phase current (dark blue), AC line-to-line voltage (green); (

**b**) printed screen from the LEM NORMA power analyzer.

**Figure 16.**Operation of the bidirectional charger under nominal conditions—steady-state, discharging mode: (

**a**) current and voltage waveforms, from the top: battery current (purple), battery voltage (light blue), AC phase current (dark blue), AC line-to-line voltage (green); (

**b**) printed screen from the LEM NORMA power analyzer.

**Figure 17.**Operation of the dual active bridge (DAB) in the bidirectional charger (reduced time scale)—steady state: (

**a**) charging mode, (

**b**) discharging mode, from the top: transformer current at the primary side (dark blue), voltage of the primary side (light blue), voltage of the secondary side (green).

**Figure 18.**Operation of the bidirectional charger—step load change from discharging to charging mode with 80% of nominal load (100 A), from the top: battery current (red), AC phase current (green), AC line-to-line voltage (purple); battery voltage (dark blue).

**Figure 20.**Efficiency characteristics vs. output power (P

_{OUT}) of the analyzed DCFC systems for different operation modes, from the top: efficiency characteristic of bidirectional charger with SiC power transistors during charging mode (dark blue); efficiency characteristic of bidirectional charger with SiC power transistors during discharging mode (dark green); efficiency characteristic of unidirectional charger with IGBT power transistors during charging mode (yellow). Battery voltage range: 360–410 V.

**Figure 21.**Power loss characteristics vs. output power (P

_{OUT}) of the analyzed DCFC systems, from the top: power loss characteristic of the unidirectional charger with IGBT power transistors during charging mode (yellow), power loss characteristic of the bidirectional charger with SiC power transistors during discharging mode (dark green), power loss characteristic of the bidirectional charger with SiC power transistors during charging mode (dark blue). Battery voltage range of 360–410 V.

Topology | Number of Active Switches | Voltage Range | Additional Components |
---|---|---|---|

2-level VSC | 6 | Full U_{DC} (1200 V) | Bigger input filter |

3-level FLC | 12 | Half U_{DC} (650 V) | 3 phase capacitors |

3-level NPC | 12 | Half U_{DC} (650 V) | 6 clamping diodes |

3-level T-type | 12 | Full U_{DC} (1200 V)Half U _{DC} (650 V) | 6 capacitors |

Property | 2-Level VSC | 3-Level FLC | 3-Level NPC | 3-Level T-Type |
---|---|---|---|---|

Price | + | − | − | − |

Reliability | + | +− | +− | − |

Number of switches | + | − | − | − |

Complexity | + | − | − | − |

Filter Volume | − | + | + | + |

Switching losses | − | + | + | + |

EMI | + | − | − | +− |

Volume | + | − | − | +− |

Property | SAB | DAB | DAB-CLLC |
---|---|---|---|

Price | + | +− | − |

Reliability | + | +− | − |

Number of switches | + | − | − |

Complexity | + | +− | − |

Voltage regulation range | +− | + | + |

Volume | + | +− | − |

Switching losses | − | + | + |

Bi−directionality | − | + | + |

AC/DC Converter | Si Technology | SiC Technology |
---|---|---|

Technology | Si | SiC |

Topology | 2-level | 2-level |

Transistor modules | Fuji 2MBI300VN-120 | Infineon FF11MR12W1M1B11BOMA1 |

Nominal current AC | 78 A | 78 A |

Nominal voltage AC | 3 × 400 V | 3 × 400 V |

Line filter | LC | LC |

Switching frequency | 8 kHz | 12 kHz |

Nominal current DC | 80 A | 80 A |

Nominal voltage DC | 670 V | 670 V |

DC/DC Converter | Uni-Directional (Si) | Bi-Directional (SiC) |
---|---|---|

Technology | Si | SiC |

Topology | Single Active Bridge (SAB) | Dual Active Bridge (DAB) |

Transistor modules | Fuji 2MBI450VN-120 | Infineon FF8MR12W2M1B11BOMA1 |

Nominal current IN | 0 … 80 A | 0 … 80 A |

Nominal voltage IN | 650 V | 650 V |

Switching frequency | 10–12 kHz | 20 kHz |

Nominal current OUT | 0–125 A | 0–125 A |

Nominal voltage OUT | 50–500 V | 50–500 V |

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## Share and Cite

**MDPI and ACS Style**

Piasecki, S.; Zaleski, J.; Jasinski, M.; Bachman, S.; Turzyński, M.
Analysis of AC/DC/DC Converter Modules for Direct Current Fast-Charging Applications. *Energies* **2021**, *14*, 6369.
https://doi.org/10.3390/en14196369

**AMA Style**

Piasecki S, Zaleski J, Jasinski M, Bachman S, Turzyński M.
Analysis of AC/DC/DC Converter Modules for Direct Current Fast-Charging Applications. *Energies*. 2021; 14(19):6369.
https://doi.org/10.3390/en14196369

**Chicago/Turabian Style**

Piasecki, Szymon, Jaroslaw Zaleski, Marek Jasinski, Serafin Bachman, and Marek Turzyński.
2021. "Analysis of AC/DC/DC Converter Modules for Direct Current Fast-Charging Applications" *Energies* 14, no. 19: 6369.
https://doi.org/10.3390/en14196369