Research on High Power Charging Power Supply Based on LLC and LCL-T Resonant Converter

Electric vehicle charging uses constant current followed by a constant voltage charging method, the MOSFETs operate over a large range of frequencies, which will cause trouble to the design of magnetic components, and the magnetizing current is circulated in the resonant tank, which will lead to more core loss. In this paper, a new resonant converter based on inductive multiplexing of LLC and LCL-T is studied to reduce the switching frequency range by using the inherent output characteristics of the converter. During the constant voltage charging phase, the LLC resonant tank operates as a constant voltage source to provide a stable charging voltage. In the LLC resonant tank, the resonant inductor is equated to the secondary side, and this resonant inductor is reused to develop the LCL-T resonant tank for the constant current charging stage, which is equivalent to a constant current source to provide a stable charging current. The resonant converter in this paper has lower losses and higher efficiency compared to other LLC resonant converters. A prototype with 3.3 kW is created to test the theoretical analysis.


Introduction
Under the strategic background of promoting the double carbon target, the development trend of electric vehicles, as an essential part of new energy vehicles, is irreversible.With the rapid development of power electronics technology, the research of battery charging technology has also been greatly supported.On this basis, with the continuous breakthrough of soft switching technology, the efficiency of high-power charging power supply has been further improved.LLC resonant converters have outstanding characteristics [1] and superior conversion efficiency.They are also adapted to high frequency and high power devices and are widely used in high-power charging power supplies.
LLC resonant converter has been well studied in the power converter application field by its high frequency, good soft switching characteristics, and high conversion efficiency.In past studies, researchers have focused on the following aspects: changing the control strategy [2], changing the topology [3], using voltage-doubling rectification techniques [4], and module integration techniques [5].In addition, researchers have spent much effort in improving the light-load efficiency [6].However, the problem of reactive power loss due to circulating current must be addressed.
Secondly, the conventional LLC resonant converter requires an extensive scope of variation in the frequency of the MOEFETs during constant current charging, which will result in a more significant circulating current and reduced efficiency.In [6], the primary-side inverter circuit is a staggered parallel structure with the addition of phase shift and frequency control, which reduces the switching frequency range.However, there is an immense circulating current in the circuit.[8] describes the hybrid rectifier structure by changing the configuration of the rectifier structure to realize a lower switching frequency range.But the additional devices undoubtedly increase the circuit cost and control complexity.[9] presents LLC resonant converters selected by using equivalence relations.
In this paper, we propose to equate the resonant inductor to the secondary side and reuse this inductor and then solve the problem of magnetizing current circulating in the resonant cavity.The resonant inductor is then multiplexed to build an LCL-T resonant network, which takes advantage of their inherent output voltage and current characteristics to reduce the switching frequency of the MOSFETs.

Main Topology
Figure 1 (a) is the central topology of the proposed converter, where Q1 -Q4 is the input-side MOSFETs, VD1 -VD4 and C1 -C4 are the body diode and parasitic capacitor of the corresponding switching tube, and the transformer secondary rectifier diode.C1 and C2 are the filter capacitors.Lr, Cr, and L2 are the resonant components of the LLC resonant network.C1, Lr, and L1 are the resonant capacitance and inductance of the LCL-T resonant network respectively.
Since the two-stage charging curve fits the charging curve of the energy storage battery too well, the charging method is usually applied to the charging technology.The charging curve is shown in Figure 1 (b) with constant current charging at the rated current first and constant voltage charging at the rated voltage when the voltage rises to the rated voltage.1.The main topology and stage charging characteristic curve

LLC mode
The LLC mode is operating when the auxiliary switch S1 is closed and S2 is open for the constant voltage charging stage.Figure 2 (a) shows the LLC mode topology.
Because the LLC resonant cavity has filtering characteristics, the high-frequency component of the square wave signal is ignored.However, the fundamental wave analysis method can equate the nonlinear LLC resonant converter to a linear topology, thus greatly simplifying the analysis of the converter.Although the fundamental analysis method is an approximate equivalent method, it can meet the accuracy requirements of most resonant converters, so the fundamental analysis method is used to analyze this converter.Figure 2 (b) below presents the AC simplified model for the constant voltage phase of the topology.

LLC Mode and its AC equivalent circuit in LLC mode
In the constant voltage charging phase, the switching tubes alternate conduction, and the magnitude of Vab is Vin.
By ignoring the higher harmonics above the fundamental wave, its practical value is obtained.
Again by ignoring the higher harmonics in the equation, the RMS value of the transformer secondary voltage is obtained.
Io and the Icd are where Icd (t) is secondary current.Therefore, the AC equivalent resistance of the secondary side is where Rac is the output DC load.When the turns ratio is n:1, the equivalent resistance to the original side is The above-mentioned fundamental wave analysis method analyzes the converter to obtain the AC simplified model of this converter in the constant voltage phase.In the constant voltage phase operation, G1 is defined as the DC gain of the circuit in the constant voltage phase, and its expression is as follows.
The concept of normalization is introduced, and the following variables are defined.
Introducing the concept of normalization, fn = fs/fr (7) Inductance factor: The above parameters are brought into the DC gain equation to obtain the proposed converter DC voltage gain expression.

LCL-T mode
For the proposed converter, the LCL-T resonant converter operates when the auxiliary switch S1 is open, and S2 is closed for the constant current charging stage.Figure 3 (a) shows the LCL-T mode topology.Figure 2 (b) below presents the AC simplified model for the constant current phase of the topology.
(b) Figure 3. LCL-T Mode and its AC equivalent circuit in LCL-T mode According to the fundamental wave analysis method, the equivalent resistance Req1 can be expressed as Vs1 is a sinusoidal AC voltage, and its magnitude is Introducing the concept of normalization, The ratio between the inductance is defined as Quality factor Also, the ratio between the inductance is .The characteristic impedance of the LCL-T resonant network is Therefore, the current flowing through inductor L1 is It is evident from the above equation that Io is load independent when fn1 = 1.The LCL-T resonant network rectifier behaves as a current source.

Analysis of the experimental result
To test the proposed topology, a prototype power supply with a rated power of 3.  In the MOSFETs turn-on phase, the drive voltage Vgs starts to conduct only when Vds drops to zero level.From Figures 5 and 6, it can be seen that, during the rectifier diode turn-on process, the voltage Vd at both ends starts to build only when its on-state current Id is reduced to zero.Therefore, the designed power supply prototype can realize soft switching.1 gives the results of the comparison between the proposed method and other methods.The number of magnetic elements should be reduced in the design because of the complexity of the design and the fact that magnetic elements bring more losses.The switching frequency range affects the immunity to electromagnetic interference, the design of magnetic components, etc.Therefore, the range of switching frequency should also be minimized.Among the many conversion analysis methods, fundamental analysis is simple and highly accurate.Through the above aspects, the converter proposed is better.

Conclusion
This paper proposes a power converter built with LLC and LCL-T resonant converters, equating the resonant inductor to the secondary side while reusing this resonant inductor to build an LCL-T resonant tank and using the fixed current and constant voltage characteristics of this converter to reduce the switching frequency operating range.Compared with other resonant converters, this topology has a more considerable magnetizing inductance value and reduced circulating current.In addition, experimental data are given to test the constant current characteristics of the resonant tank as well as the soft switching characteristics.The experimental data correspond to the theoretical derivation and prove the correctness of this topology.
3 kW was built.The parameters were set as follows: Vi = 380 V -400 V, Vo = 310 V -330 V, Io = 10 A, fr = fr1 = 100 kHz, Lr = 32 mH, Lm = 245 mH, Cr = 41 nf, Lr1 = 32 mH, Cr = 78 nf. 4 gives the experimental results of the topology at different loads during the constant current charging stage.Vab is equal to Vi. Vo and Io are the output voltage and current of topology, and Ir is the resonant current.In the LCL-T mode, Io is kept at 10 A. Second, the operating frequency range of the MOSFETs is low.Although the output current is load independent, in practice the operating frequency will vary due to parasitic parameters.

Figure 4 .
Figures 5 and 6 are the operating waveform of the soft switch of the converter, Vds is the voltage across the MOSFETs, Vgs is the driving voltage, Vd and Id are the voltage and current of the diode respectively.In the MOSFETs turn-on phase, the drive voltage Vgs starts to conduct only when Vds drops to zero level.From Figures5 and 6, it can be seen that, during the rectifier diode turn-on process, the voltage Vd at both ends starts to build only when its on-state current Id is reduced to zero.Therefore, the designed power supply prototype can realize soft switching.

Figure 5 .Figure 6 .
Figures7 and 8show the ZVS waveform, resonant current waveform, and ZCS waveform at the primary load level in the constant voltage charging stage.Similarly, it shows that the converter achieves soft switching.Therefore, regardless of the operating stage, the designed power supply prototype can realize soft switching.

Table 1 .
Comparative Results