Study On Modularized Cell Equalizer Using Full Wave Voltage Multiplier

Nowadays,energy storage system are widely used in various applications,e.g.electric vehicles,renewable energy systems.Lithium-Ion batteries and super-capacitors are usually used in energy storage system.However,the voltage of a single Lithium-Ion battery/ super-capacitor is inherently low and cannot provide required voltage for high voltage applications,such as electric-vehicles(EVs)and renewable energy systems.Lithium-Ion batteries/super-capacitors are thus usually connected in series to provide high voltage.The voltages of these cells are gradually unbalanced due to the difference of individual cell properties,such as capacitance,internal impedance,self-discharge rate,etc.Therefore,equalization circuits are required to prevent overcharging and over-discharging of series-connected Lithium-Ion battery/ super-capacitor stack.With the development of power electronics technology,the passive equalization method is gradually being phased out,active equalization circuits have become a hot research topic.According to the circuit structure,active equalizers can be divided into two types: centralized equalizer and distributed equalizer.Traditional centralized battery equalizer is always comprised of coupled-inductor or multi-winding transformer.It has the disadvantages of large size low equalization precision and hard to extend.On the other hand,distributed equalizer has the disadvantages of high cost and too many components.In order to solve the above problems,a novel single-inductor bidirectional cell to cell equalizer based on Buck-Boost converter and switch matrix was proposed.Independent regulation can be achieved in this equalizer by multiplexing the single inductor.Inductor current operates in discontinuous conduction mode(DCM),eliminates the cross-regulation of each cell.Control strategy and corresponding characteristics were analyzed in this paper.Experimental results of a four-battery equalizer are presented to verify the analysis results and control strategy,which also indicate that this type of equalizer benefits with high equalization precision,low cost,easy to extend,small size and light weight of equipment and device.Althouogh active equalizers are widely used,most of conventional active equalizers require multiple switches and/or a multiwinding transformer,resulting in complex circuitry and poor modularity.In terms of circuit and control complexity,battery charger/equalizer using passive components with fewer switches is generally simple and significant.Battery charger/equalizer circuits using half wave voltage multiplier circuit(HW-VM)have been proposed.They consist of passive components(capacitors and diodes)and one or two switches.Without multiple switches or a multi-winding transformer,these equalizers dramatically reduce circuit complexity and increase flexible to extension.However,with HW-VM structure,these circuits have issuies in terms of slow equalization speed and large equalization loss.In this paper,a modularized non-isolated two-switch equalizer for series-connected battery/super-capacitor is proposed.Without using coupled-inductor,multi-winding transformer,and multiple active switches,the proposed equalizer benefits with small circuit volume,low cost,and flexible for extension.Meanwhile,it can work under open-loop,which makes its control simple.High efficiency is achieved by zero voltage switching(ZVS)of switches.In order to further accelerate the equalization speed,reduce equalization loss,and improve the equalization accuracy,a two-switch double-Cuk FW-VM equalizer is proposed in this dissertation.The proposed equalizer is essentially based on the combination of one DC-DC converter and one full wave voltage multiplier circuit.The proposed equalizer needs only two switches and ZVS for all switches are achieved.Furthermore,the current of the proposed equalizer can be limited automatically,even when the equalizer works under open-loop control.Meanwhile,the proposed equalizer benefits with high equalization precision,i.e.,low voltage difference between maximum and minimum battery voltage at the end of the equalization,with the equalization precision is independent of the internal impedance of battery cells and the number of battery cells.In addition,the proposed equalizer can directly transfer energy from the battery pack to the individual battery with the shortest energy transfer path,which reduced the equalization loss.The proposed equalizer benefits with fast speed,high equalization precision and low equalization loss.In many high voltage applications,such as hybrid electric vehicles,numerous of battery packs need to be used in series.Compared with battery cell unbalance,the battery pack unbalance is more serious.In order to realize the voltage balance of battery cells and the balance of battery packs at the same time,the conventional equalization method usually requires two equalization systems: one for equalization of the battery cells and the other for equalization of battery packs.Thus,it has disadvantages of low efficiency and complicated circuit structure.Based on double-Cuk FW-VM,a high efficiency two-switch battery cell/battery pack hybrid equalizer is proposed.The modulation principle of circuit is described and its equalization characteristics are analyzed.The research results show that the proposed circuit achieves high-efficiency equalization in a simple circuit form,and has the advantages of fast equalization speed and small number of components.The corresponding simulation and experimental platform is established in this paper.Simulation and experimental results have a good agreement with the theoretical analysis presented.