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Due to its high step-up voltage ratio, high utilization rate, and good stability, the bidirectional half-bridge current-doubler topology is widely used in lithium battery system. This paper will further analyze the bidirectional half-bridge current-doubler topology. Taking into account the fact that the current is not equal to the two times current inductance may lead to a greater transformer magnetizing current leaving the transformer core saturation occurring. This paper will focus on the circuit modeling of steady-state analysis and small signal analysis, analyzing the influence parameters for the inductor current by steady-state model and analyzing the stability of the system by the small signal model. The PID controllers and soft start algorithm are designed. Then the influence of circuit parameters on the steady state and the effect of soft start algorithm is verified, and finally the function of the soft start algorithm is achieved by the experimental prototype.

The bidirectional half-bridge current-doubler circuit is widely used in the lithium batteries formation for it can achieve low-voltage and high-current two-way flow of energy [

The bidirectional half-bridge current-doubler topology is shown in Figure

Circuit topology of bidirectional half-bridge current-doubler converter.

Before the modeling analysis, simplify the circuit. The flyback winding is mainly working in the soft start of the battery discharge mode, and the flyback winding does not work under normal operating mode, so in the analysis of steady state the two-flyback inductor is regarded as two-common inductor. In the steady-state modeling and analysis, the equivalent resistance of the high-voltage side

Here, this paper only analyzes the energy transfer from the high-voltage side to low-voltage side.

Working mode equivalent model.

According to the equivalent model of each mode in Figure

According to the principle of state space averaging method, we analyze four modes’ state equation and can get the transfer matrix of the state variables and the input variables, for

In the same way, if we analyze the energy from the low-voltage side to the high-voltage side of the discharge, the relation equation shown in formula (

From (

To design the controller, the small signal modeling and analysis of the controlled object are needed [

Boost circuit CCM average switching model.

In Figure

Injection disturbance in steady state: the relation equation (

Among them,

Boost circuit CCM small signal model.

In Figure

By equation group (

The design parameters of the circuit are as follows: transformer ratio:

Since the inductance is small, the inductor DC resistance is also small, the conduction resistance of MOSFET is about several milliohms, and, in order to facilitate the calculation, these three small items can be ignored, the system’s transfer function is as follows:

This paper simulates the circuit analysis by PISM software; circuit simulation parameters are as follows: inductance value is 100

Inductor current simulation waveforms.

The current waveform when the inductance equivalent resistances are not equal

Unequal MOSFET on-resistance current waveform

Current waveform when the duty cycles are unequal in size

The PID controller of the system is designed through the system transfer function of small signal [

Then, the step response of the system is compared with that of the system without PI. From Figure

Step response simulation waveforms.

When the circuit starts, initial duty ratio is too high which will lead to the start process being not smooth, and also large duty ratio will make the circuit voltage and current overshoot and oscillation. This will not only make the system instable and damage the whole circuit system, but also cause a huge impact to the power grid, affecting the quality of power supply; this is the so-called hard start [

Soft start algorithm flowchart.

The soft start algorithm has been simulated, and the simulation results are shown in Figure

Low-voltage side current waveform and MOSFET current of soft start.

Laboratory system is built on the lithium battery charge and discharge test; the circuit parameters are as follows: the low-voltage side of the MOSFET model is

Experimental waveforms.

Output current soft start waveforms

Inductor current waveform

The steady-state model is established and the simulation results show the influence of the parasitic parameters of the circuit on the circuit stability. By soft start algorithm and soft start circuitry we can effectively slow down the impact of high-current circuits, thereby improving the stability of the system and increasing the life of the device. The experimental results prove the feasibility and reliability of the soft start algorithm and verify the good stability and dynamic performance of the system.

The authors declare that there is no conflict of interests regarding the publication of this paper.

This research was supported by the National Natural Science Foundation of China (51477125), and the National Key Basic Research Development Plan (973 Plan) (no. 2013CB632505), the Hubei Province Technological Innovation Major Project (2016AAA046), and the Wuhan Youth Morning Project (2016070204010155).