# A Novel Hybrid LDC Converter Topology for the Integrated On-Board Charger of Electric Vehicles

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## Abstract

**:**

## 1. Introduction

## 2. Operation of the Proposed Integrated OBC

_{o1}, an active clamp circuit with C

_{r1}and an active switch Q

_{5}and a PSFB converter with switches Q

_{1,}Q

_{2,}Q

_{3}and Q

_{4}. The transformer of the PSFB converter is TR

_{1}with a magnetizing inductance L

_{m1}, a leakage inductance L

_{LK1}and a turns ratio of n

_{1}:1. The transformer of the forward converter is TR

_{2}with a magnetizing inductance L

_{m2}, a leakage inductance L

_{LK2}and a turns ratio of n

_{2}:1. The secondary side of the PSFB converter includes rectifier diodes D

_{1}and D

_{2}, a passive snubber circuit composed of C

_{r2}, D

_{4}and D

_{5}. The capacitor C

_{r2}connected in parallel with the output inductor L

_{O2}also plays a function as an output capacitor of the forward converter and reduces the current ripple of the output. As mentioned earlier, the focus is on explaining the operation of the LDC of the proposed integrated OBC. Here, a half of the switching cycle of the proposed converter is divided into seven modes and the operation is explained in detail since the other half is symmetric. The key waveforms and the equivalent circuits of each operation mode are shown in Figure 3 and Figure 4, respectively. For the sake of simplicity, all of the circuit components are ideal except the output capacitance of the switch and all of the output capacitances of the switches are assumed to be same.

_{1}, Q

_{4}is on and Q

_{3}is off. Q

_{2}and Q

_{5}turn off, and their parasitic capacitors are charged. The parasitic capacitor of Q

_{1}is discharged and its body diode is forward biased creating ZVS turn-on condition for Q

_{1}. The power is transferred to load through the transformers TR

_{1}and TR

_{2}.

_{2}and D

_{5}are reverse biased. D

_{1}, D

_{3}and D

_{4}are forward biased. The resonant capacitor C

_{r2}resonates with output inductor L

_{O2}and discharges the energy to the output.

_{2}, Q

_{1}turns on while Q

_{4}is already on in mode 1. Q

_{2}, Q

_{3}and Q

_{5}are off. The body diode of Q

_{1}is reverse biased and the current flows through Q

_{1}. The secondary side works in the same fashion as in mode 1. Due to the resonance between C

_{r2}and L

_{O2}, the current flowing through D

_{4}decreases to zero and achieves ZCS turn-off condition for D

_{4}at the end of this mode.

_{3}, Q

_{1}and Q

_{4}are on. Q

_{2}, Q

_{3}and Q

_{5}are off. The input power is delivered to the output by both converters. In the forward converter, the primary current i

_{pri2(t)}flows through the transformer TR

_{2}and charges the resonant capacitor C

_{r2}in the secondary side. In the PSFB converter, the primary current i

_{pri1}flows through switches Q

_{1}, Q

_{4}and transformer TR

_{1}. The currents i

_{pri1}and i

_{pri2}(t) are determined as below.

_{pri1}is the primary winding voltage of the transformer TR

_{1}, V

_{aux_bat}is the auxiliary battery voltage, i

_{pri1}is the primary current of the forward converter, i

_{pr2}is the primary current of the PSFB converter, i

_{Lo2}is the current flow through the output inductor L

_{o2}, and V

_{Cr2}is the voltage of capacitor C

_{r2}.

_{4}is reverse biased and D

_{5}is forward biased to charge the resonant capacitor C

_{r2}and the output capacitor C

_{o2}, respectively. The rectifier bridge voltage V

_{rec}(t) can be calculated as in (3).

_{rec}is the rectifier voltage of the PSFB converter.

_{4}, the switch Q

_{4}turns off. The parasitic capacitor of Q

_{3}is discharged. In the forward converter, the current of transformer TR

_{2}flows through the active clamp circuit including capacitor C

_{r1}and the body diode of Q

_{5}. The capacitor C

_{r1}resonates with the leakage inductor L

_{LK2}. Since the parasitic capacitor of Q

_{4}is charged and that of Q

_{3}is discharged, the ZVS turn-on condition for Q

_{3}is achieved. The voltage V

_{Coss_Q3}across Q

_{3}can be found as shown in (4).

_{1}can be calculated using (5) and (6), respectively.

_{1}to D

_{4}and resonant capacitor C

_{r2}. The current i

_{D1}flowing through D

_{1}can be determined as follows.

_{5}, Q

_{3}and Q

_{5}are turned on while Q

_{1}is on. The body diodes of Q

_{3}and Q

_{5}and diodes D

_{1}, D

_{3}and D

_{4}are forward biased. The other diodes are reverse biased. The resonant capacitor C

_{r2}is discharged through the diodes D

_{1}and D

_{4}. In the forward converter, the resonance between the capacitor C

_{r1}and the leakage inductor L

_{LK2}continues. The power is transferred to the secondary side through the transformer TR

_{2}of the forward converter.

_{pri1}(t) of the PSFB converter can be expressed using (8).

_{m1}is the magnetizing current of the PSFB converter.

_{1}can be expressed using (9).

_{6}, the diode D

_{1}is reverse biased as the current commutation from D

_{1}to D

_{4}is completed. In the primary side of the PSFB converter, Q

_{1}is on and the body diode of Q

_{3}is forward biased. The primary current of the PSFB converter is circulating and kept constant. The forward converter operates the same as in mode 5.

_{3}and D

_{4}are forward biased. Due to the discharge of L

_{O2}, the current through D

_{3}decreases to zero gradually. At the end of this mode, the diode D

_{3}is turned off with ZCS.

_{7}, the body diode of Q

_{5}is reverse biased and the current flows through the Q

_{5}. In the forward converter, the capacitor C

_{r1}resonates with the inductance of the transformer TR

_{2}and the resonant current resets it, thereby eliminating the need for tertiary winding of the forward converter. The primary current of the PSFB converter is still circulated through the Q

_{1}and the body diode of Q

_{3}. In this mode, there is no power transferred to the secondary side. As in mode 6, the diode D

_{4}is still forward biased and the energy in the capacitor C

_{r2}is discharged to the load. After mode 7 the other half of the switching cycle operates in a symmetric fashion.

## 3. Features and Design Consideration

#### 3.1. Features of the Proposed Converter

#### 3.1.1. High Step-Down Voltage Conversion Ratio

_{o}of the proposed converter can be derived as (10).

_{eff}is the effective duty cycle of the PSFB converter and D’ is the time period from mode 4 to mode 6 (t

_{4}–t

_{6}) when the resonant capacitor C

_{r2}resonates with the leakage inductor L

_{LK2}of the transformer TR

_{2}as shown in (11).

_{1}and n

_{2}as shown in Figure 6.

_{pri2}is the primary winding voltage of the transformer TR

_{2}and V

_{Pri1}is the primary winding voltage of the transformer Tr

_{1.}

_{proposed}of the proposed converter can be calculated as in (14).

#### 3.1.2. Elimination of the Circulating Current

_{r2}, D

_{4}and D

_{5}. Figure 7 shows the difference between the circulating current in the conventional PSFB and the proposed converter. As explained in the operation of mode 4 and mode 5, since the resonant capacitor C

_{r2}is discharged and the diodes D

_{1}and D

_{4}are forward biased the primary current is reduced quickly during the freewheeling period thereby reducing the circulating current. Consequently, ZCS turn-off can nearly be achieved for the lagging leg switches of the PSFB converter.

#### 3.1.3. Small Output Current Ripple

_{1(Conventional_PSFB)}of the output inductor can be determined using (15).

_{2(Proposed_Converter)}of the output inductor in the proposed converter can be calculated as in (16).

_{ripple}of the ripple of the current between these two cases can be calculated as in (17).

_{O2}of the proposed converter is just 29% of that of the conventional PSFB converter. When the voltage of the propulsion battery is 250 V, the current ripple of the output inductor L

_{O2}is just 47% of that of the conventional PSFB converter. Therefore, the value and size of the output filter inductor L

_{O2}can be significantly reduced.

#### 3.2. Design Consideration

#### 3.2.1. ZVS Conditions for All of the Switches in the PSFB over the Full Load Range

_{m1}of TR

_{1}needs to be large enough. The magnetizing current and the energy stored in the magnetizing inductance of TR

_{1}can be determined as follows.

_{m}

_{1,peak}is the peak value of the magnetizing current of TR

_{1}. T

_{S}is the switching period and D

_{eff_min}is the minimum effective duty cycle of the PSFB converter.

_{oss}of MOSFETs.

#### 3.2.2. Design of the Clamp and Resonant Capacitors

_{r1}resonates with the leakage inductance L

_{LK2}and the magnetizing inductance L

_{m2}of TR

_{2}. Hence, the value of clamp capacitor C

_{r1}can be calculated as in (23). However, since the voltage applied to the switches of PSFB converter increases due to the resonance, the resonant frequency needs to be selected much lower than the switching frequency in order to reduce it.

_{r2}is the output capacitor of the forward converter. Thus, its value can be calculated using (24).

## 4. Experimental Results

_{1}turns on with ZVS and turns off with nearly ZCS. There is a small negative current flowing through its body diode to maintain the zero voltage during the turn-on period. The turn-off current of Q

_{1}is just 0.5 A, thus turn-off losses of the lagging leg MOSFETs are minimized. Figure 11 shows that MOSFET Q

_{3}is also turned on with ZVS condition.

_{1}at the light load condition (10% load) with an input voltage of 330 V and output power of 100 W. It can be clearly observed from Figure 12 that the lagging leg switches can maintain ZVS turn-on and nearly ZCS turn-off at light load condition.

_{1}. It can be observed that there is nearly no circulating current in the primary side of the transformer TR

_{1}during freewheeling interval.

_{2}. We can see that the current of transformer TR

_{2}is reset by the active clamp circuit Q

_{5}and C

_{r1}. Figure 15 shows that both ZVS turn-on and ZCS turn-off can be achieved at the secondary rectifier diodes, hence there is no reverse recovery. In addition, the voltage at the rectifier diode is clamped around 80 V so that the diode with a lower voltage rating can be used.

_{3}are shown in Figure 16. It can achieve both ZVS turn-on and ZCS turn-off. The voltage across the diode D

_{3}oscillates due to the leakage inductance of the transformer TR

_{2}. Figure 17 shows the current and voltage waveforms at the active clamp switch Q

_{5}with ZVS turn-on.

_{4}are shown in Figure 18. Both ZVS turn-on and ZCS turn-off are achieved at diode D

_{4}, thus there is no recovery loss at this diode.

_{DC}= 400 V, V

_{DC}= 420 V and P

_{O_I}= 2.3 kW. In function II, the maximum efficiency is 97.58% when V

_{DC}= 400 V, V

_{Pro_Bat}= 420 V and P

_{O_II}= 1.8 kW.

## 5. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Conflicts of Interest

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**Figure 5.**Simplified circuit model of the LDC converter: (

**a**) power transfer mode in both converters, (

**b**) power transfer mode in forward converter and freewheeling mode of PSFB converter, (

**c**) freewheeling mode of both converters.

**Figure 7.**Comparison of the circulating current in the conventional PSFB converter and proposed converter.

**Figure 8.**Voltage and current waveforms at the output inductor of the conventional PSFB converter and the proposed converter.

**Figure 9.**The ratio of the current ripple of the proposed converter compared to the conventional topology.

**Figure 19.**Efficiency plots during function III operation with wide range of input voltage variation.

**Figure 21.**Efficiency plots during function II operation with wide range of input voltage variation.

Operation Condition | Parameter | Value [Unit] |
---|---|---|

PFC stage | AC voltage | 220 [V]/60 [Hz] |

DC-link Voltage | 380–420 [V] | |

Rated power | 3.5 [kW] | |

Function I: DC-link to propulsion battery | DC-link voltage | 380–420 [V] |

Propulsion battery voltage | 250–420 [V] | |

Rated power | 3.3 [kW] | |

Switching frequency | 30 [kHz] | |

Function II: Propulsion battery to DC-link (OBC) | DC-link voltage | 380–420 [V] |

Propulsion battery voltage | 250–420 [V] | |

Rated power | 3.3 [kW] | |

Switching frequency | 30 [kHz] | |

Function III: Propulsion battery to auxiliary battery (LDC) | Propulsion battery voltage | 250–420 [V] |

Auxiliary battery voltage | 23–25 [V] | |

Rated power | 1 [kW] | |

Switching frequency | 50 [kHz] |

Components | Value |
---|---|

All Switches $({S}_{1}~{S}_{4}:{Q}_{1}~{Q}_{5}$) | IPW65R041CFD |

Turns ratio of the transformer TR_{1} (1:n:m) | 20:23:3 |

Leakage inductance of transformer TR_{1} (L_{LK1}) | 12.2 [µH] |

Magnetizing inductance of transformer TR_{1} (L_{m1}) | 605 [µH] |

Core size of TR_{1} | PQ72/52 |

Turns ratio of the transformer TR_{2} (n_{2}) | 32:16 |

Leakage inductance of transformer TR_{2} (L_{LK2}) | 16 [µH] |

Magnetizing inductance of transformer TR_{2} (L_{m2}) | 452 [µH] |

Core size of TR_{1} | PQ72/52 |

Clamp capacitor (C_{r1}) | 0.22 [µF] |

Resonant capacitor (C_{r2}) | 100 [µF] |

Diodes $({D}_{1}~{D}_{2}$) | DSSK 70-008A |

Diode $({D}_{3}~{D}_{5}$) | DSSK 60-0045B |

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**MDPI and ACS Style**

Nam, V.-H.; Tinh, D.-V.; Choi, W. A Novel Hybrid LDC Converter Topology for the Integrated On-Board Charger of Electric Vehicles. *Energies* **2021**, *14*, 3603.
https://doi.org/10.3390/en14123603

**AMA Style**

Nam V-H, Tinh D-V, Choi W. A Novel Hybrid LDC Converter Topology for the Integrated On-Board Charger of Electric Vehicles. *Energies*. 2021; 14(12):3603.
https://doi.org/10.3390/en14123603

**Chicago/Turabian Style**

Nam, Vu-Hai, Duong-Van Tinh, and Woojin Choi. 2021. "A Novel Hybrid LDC Converter Topology for the Integrated On-Board Charger of Electric Vehicles" *Energies* 14, no. 12: 3603.
https://doi.org/10.3390/en14123603