# Variable Multiple Interleaved Bi-Directional DC/DC Converter with Current Ripple Optimization
## 1 Introduction  
Energy storage systems have been widely used to improve the power distribution between renewable power generation, large power grids, and user loads [1; 2; 3]. As the power transmission channel between the DC bus and the energy storage system, the bidirectional DC/DC converter can realize bidirectional energy flow and improve energy distribution efficiency [4; 5; 6]. However, the DC/DC converter may introduce a large current ripple to the DC bus, which affects the quality of the power supply [7; 8]. Many researchers have proposed different methods to reduce the current ripple, each lack has its advantages [9; 10; 11]. At present, the current ripple optimization methods of DC/DC converter mainly include: input parallel output series (IPOS) DC/DC converter, ripple current injection DC/DC converter, resonant ripple absorption DC/DC converter, DC/DC converter using coupled inductor technology, multiple DC/DC converter, etc. Reference [12] studied an IPOS DC/DC converter, which could offset the current ripple to a certain extent, but has the problem of low voltage gain. The DC/DC converter with ripple current injection proposed in [13] artificially injects ripple current at both ends of the inductor to eliminate the ripple current. The ripple suppression effect is good, but the circuit is more complicated. Reference [14] proposed a resonant ripple absorption DC/DC converter that uses the LC devices to generate resonance to optimize the current ripple, but the ripple suppression capability for dynamic loads is poor due to the fixed system parameters. The DC/DC converter with coupled inductor technology [15] replaces the inductor in the traditional DC/DC converter with a coupled inductor. When the coupling coefficient of the coupled inductor is selected reasonably, a better ripple suppression effect can be achieved, however, the switching stress is larger due to the influence of leakage inductance. The multiple DC/DC converter adopts multiple structure and hashigh power grade. The increase in converter multiplicity can reduce the current ripple, reduce the switching stress, improve the equivalent switching speed, improve the dynamic performance, and then improve the power quality [16; 17; 18; 19; 20; 21; 22; 23]. The above current ripple optimization methods of DC/DC converters mainly have the problems of large switching stress and low voltage gain, so they are difficult to be applied to high power energy storage systems, while the traditional multiple DC/DC converters can not only reduce switching stress, but also achieve better current ripple suppression through continuous improvement, so they are widely used in high power energy storage systems [18; 19]. Energy storage systems (especially battery energy storage systems) are highly sensitive to current ripples, and excessive current ripples will affect the service life of the system. Reducing the current ripple of the converter is of great significance for improving the quality of the power supply, prolonging the service life of the system. However, the existing research mostly studies improving the circuit structure of the multiplexed DC/DC converter to optimize the current ripple, such as using the coupled inductor instead of the traditional inductor [15] and using the new SiC device [24], the working model of the multiplexed converter is also limited to the fixed multiplicity model, and there are few studies on the influence of changing the parallel multiplicity on the current ripple. In fact, related researches find the multiplicity of parallel circuits also has an important influence on the magnitude of current ripple, and there is always a parallel multiplicity corresponding to the minimum current ripple under different duty cycle [20; 21; 22; 23]. The main contribution of this paper is to propose a current ripple optimization method for bidirectional DC/DC converters by changing the multiplicity of parallel circuits.