Research on a soft-start method for LCC-LCC compensated bidirectional wireless power transfer system

The increased popularity of electric vehicles and the pursuit of user convenience makes bidirectional wireless power transfer (BWPT) an attractive technology for charging batteries. To improve the transmission efficiency of BWPT and limit the external radiated magnetic field, it is necessary to ensure the zero-voltage switching (ZVS) operation of the converter on the transmitter-side and limit the transient start-up current of the inverter when the system is working. To solve this problem, this paper proposes a soft-start modulation strategy based on an H-bridge inverter on the transmitter-side. First, the circular characteristics of dual LCC compensation topology are analysed based on an equivalent circuit. Secondly, according to the characteristics of the input impedance angle and current of the transmitter-side, a soft-start control method combining frequency and phase shift control is proposed. Finally, the feasibility of the proposed soft-start modulation strategy is verified on a prototype.


Introduction
Over the past two decades, researchers have conducted extensive studies on wireless charging systems [1]- [3], and the technology has attracted significant interest from both industry and academia.As the demand for grid power continues to increase, it has become a compelling area of research to realize power sharing between the grid and battery loads with large storage capacity (e.g., electric vehicles).For this grid-to-vehicle and vehicle-to-grid power sharing, the wireless charging system must be bi-directionally secure and efficient [4]- [5].
Battery load variations and their changing state of charge (SOC) affect the efficiency and power capability of the BWPT system.Various compensation networks have a variety of features that are sensitive to variations in battery voltage and SOC.Researchers have found that bidirectional LCC compensation topology is less sensitive to coupling coefficients k and load variations, and the bidirectional LCC structure has become a hotspot for compensation topology research in BWPT systems [6]- [9].When the electric vehicle charging is activated, the highly transient start-up current in the inverter may damage the switching tubes of the inverter.It may trip the protection due to overcurrent.Therefore, a soft-start modulation strategy is required for the BWPT systems.The authors of [10] propose a soft-start strategy with dual phase-shift control, which allows the voltage of output during the booting process to maintain an approximately linear increase under any load conditions and alleviates the overvoltage and overcurrent generated during start-up.In [11], researchers determine the optimal switching frequency within the current limiting region by detecting the output voltage.This ensures that the system does not generate excessive current and voltage stresses during soft-starting, which allows the output voltage to build up smoothly.The literature mentioned above has studied the soft-start strategies for conventional converters.However, the study of soft-start strategies for BWPT systems has not been mentioned.
This paper proposes a soft-starting strategy based on variable frequency and phase-shift control to solve the issue of excessive current and hard-switching of the switching tube in the start-up instant of the bilateral LCC compensation topology BWPT system.With this control strategy, all switching tubes of the converter operate in the soft-switching state during soft-start initiation while the current flowing through the switching tubes is within the allowable safety range.

Characterization of bilateral LCC compensation topology
The bilateral LCC compensation topology BWPT is shown in Figure 1.The figure shows that L1 and L2 are the transmitting and receiving coils' self-inductance, respectively.Coupling coils have a mutual inductance of M. R1 and R2 are the values of the transmitter coil and receiver coil resistances.LF1, CF1, and C1 form the system's transmitter-side resonance compensation network, and LF2, CF2, and C2 form the system's receiver-side resonance compensation network.Due to the filtering characteristics of the LCC compensation network, fundamental wave analysis is used to equate the system circuit in this paper, as shown in Figure 2. When the coupling coil parameters and resonant frequency 0 Z are determined, designed values of resonant network element parameters and the current of transmitter-side are calculated by Equations ( 1)- (2).It can be found from the equation that 0 Z is only related to the capacitance and self-inductance of the coil and is independent of the value of load and coupling coefficient k.Current I1 flowing through coil L1 on the transmitter-side is independent of the coupling coefficient k.Output current ILF2 on the receiver-side is proportional to the equivalent voltage of output and mutual inductance M and inversely proportional to operating frequency and self-perception of coils.
UAB and Uab are RMS values of high-frequency output voltages on the transmitter-side and receiver-side, respectively.The harmonic component in resonant current increases when BWPT systems use phase shift control, but the resonant compensation network possesses a good filtering function to remove the higher harmonic components.Therefore, it is possible to consider only the fundamental component of output voltage.Input-output characteristics of the coupled mechanism can be expressed as the following equation, where D and E are inner shift angles of converter control signals on the transmitter-side and receiver-side, respectively.
Equation ( 4) shows the input and output power characteristics of the BWPT system, where G is the external phase shift angle between the transmitter-side and receiver-side H-bridge.When P1<0 and P2>0, electrical energy is transmitted from the transmitter-side to the receiver-side, which realizes electricity from the grid to the battery.When P1>0 and P2<0, electrical is fed back from the receiving side to the transmitting side, which realizes energy flow from the battery side to the grid.The transmitted power of BWPT is related to the operating frequency, external phase shift angle between H-bridges, and the setting of resonant network parameters.

Analysis of system input impedance angle
To realize the soft switching of the H-bridge switching tubes, the system should operate in the inductive region, which has a positive input impedance angle of BWPT.Since the parasitic resistance of the inductance of coils, resonant capacitance and resonant inductance are very small compared to the load resistance.The parasitic resistance is negligible due to the relatively small impact on BWPT system operation.The following equation calculates the input impedance angle of the bilateral LCC compensation topology. ( The input impedance angle characteristics of the system are displayed in Figure 3.It goes to show that the input impedance angle of the system is positive when the frequency is lower than 0 Z .At this time, the H-bridge inverter operates in the inductive region.The inductive region is conducive for the soft switching of the switching tubes.

Simulation of start-up transient current
The BWPT system is equipped with filter capacitors on the output side for voltage stabilization and filtering.During the system start-up instant, the large-capacity filter capacitor is equivalent to a short circuit, which leads to excessive transient currents on the output side of the system, and the soft-start strategy is required to reduce the peak of the starting current.BWPT system without soft-start protection measures will generate a large inrush current during the charging process of the filter capacitor, which will damage the resonant capacitors and power switching devices as well as affect the dynamic characteristics of the BWPT system.BWPT system model with 100 V output is built in Matlab/Simulink for hard start simulation, and the waveform of the current of the H-bridge on the transmitter-side is indicated in Figure 4.It shows that the instantaneous value of the start-up current has a large overshoot, which far exceeds the value of the current during actual operation.

Comparison of soft start strategies
According to the previous analysis of the characteristics of bilateral LCC compensation topology circuits, the soft-start method can be controlled by variable-frequency control or phase-shift control.
Variable-frequency control reduces the peak start-up transient current by changing the input impedance angle and the input current value on the transmitter-side; phase-shift control reduces the peak start-up transient current by changing the voltage of the H-bridge on the transmitter-side.When the BWPT system adopts the variable-frequency method, it generally adopts the reduced-frequency start-up mode.At the beginning of the BWPT system start-up, the frequency is 1.5-2 multiples of the resonant frequency.As start-up proceeds, operating frequency is gradually diminished to 0 Z .Variable-frequency mode requires a higher upper operating frequency for the power devices used in the BWPT system, leading to a higher manufacturing cost.When the BWPT system adopts the phase-shift control soft-start method, it generally adopts a smaller output voltage for start-up.The voltage gain of phase-shift control is greatly affected by the output power of the receiver-side, and this method has limited regulation of voltage output on the transmitter-side, which does not ensure the soft switching characteristics of the switching tubes during the start-up process and affects the transmission efficiency of BWPT system.Due to the preceding analysis, this paper presented the soft-start strategy of variable-frequency plus phase-shift hybrid control.
The start-up method of the BWPT system adopts the variable-frequency plus phase-shift hybrid control.The starting frequency of BWPT is selected around 0 Z to keep the H-bridge operating in the inductive region.The output voltage is varied by phase-shift control at a selected operating frequency to reduce the voltage of the transmitter-side, which reduces the peak start-up transient current.
The start-up frequency of the BWPT system is 79 kHz, which is near 0 Z of system and provides a good start-up performance.It gradually increases the duty cycle from 0.05 to 0.5 at the setting operating frequency, and the operating frequency is increased from 79 kHz to a resonant frequency of 85 kHz when the duty cycle is maximized.This control method enables lower reactive power cycling while ensuring zero-voltage turn-on of all switching tubes of the H-bridge.The soft shutdown method of the system can also be controlled in the same way, but phase-shift control reduces the duty cycle of the H-bridge from 0.5 to 0.05.The process of output voltage variation on the transmitter-side is shown in Figure 5.
Experimental waveforms of output voltage and current on the transmitter-side when the system is hard-started are exhibited in Figure 6(a).It goes to show that the output current of the system has a large overshoot at the start-up instant and far exceeds the resonant current of the system in a steady state.Output voltage decreases with the initiation process, like the simulation results.Experimental waveforms using the strategy proposed in this paper are displayed in Figure 6(b).Compared with Figure 6(a), the strategy proposed in this article can significantly reduce current inrush at the starting moment and does not produce current overshooting.The resonant current is slowly increased from a small value to a stabilized operating value, which allows the output voltage to increase smoothly.The output voltage is stabilized, and no voltage drop occurs.It shows that the proposed strategy can reduce peak start-up transient current, and voltage can be built up smoothly during the initiation process.BWPT realizes the expected soft-start effect.During the soft start of the system, it is not only necessary to limit the peak instantaneous starting current but also to ensure that the soft switching characteristics of the switching tube are realized.The characteristics of ZVS can reduce losses of the switching tubes and minimize electromagnetic interference.Figure 7 shows the voltage and current waveforms of the H-bridge on the launch side during priming.It shows that the system realizes zero-voltage switching for all switching tubes when adopting the strategy proposed in this article.

Conclusion
This paper proposes the start-up strategy of frequency conversion plus phase shift hybrid control to address the large current peak and switching tube hard switching during the initiation process of bilateral LCC compensation topology BWPT system.The BWPT system operates in the inductive range by controlling the frequency to ensure soft-switching characteristics of the switching tubes, which is combined with phase-shift control to reduce the current peak at start-up instant.The experimental results show that the proposed soft-start strategy effectively reduces the peak current at the start-up moment, and at the same time, it can ensure that the soft-switching characteristics of the switching tubes are realized.Thus, the transmission efficiency of BWPT is improved, and it restricts external electromagnetic interference of the BWPT system at the moment of start-up.

Figure 3 .
Figure 3. Input impedance angle characteristics of the BWPT system.

Figure 4 .
Figure 4. Simulation waveform of hard start output current iAB.

Figure 5 .
Figure 5. Proposed soft start control method.

Figure 6 .
Figure 6.Comparison of experimental waveforms of two starting modes.

Figure 7 .
Figure 7. Experimental waveforms of variable-frequency plus phase-shift hybrid control.