Simplified modeling of ultracapacitors for bidirectional DC-DC converter applications

This paper investigates the simplified modeling of the ultracapacitors (UC) as variable voltage sources and quantifies the errors involved therein. Although modeling the energy storage devices such as Lead-acid, Li-ion, NiMH, NiCd batteries as variable voltage sources are popular, the available literature reports and treats UC as RC networks. This paper extends the voltage source model to UC which helps simplify the system design in comparison to other models based on RC networks. This paper also compares the proposed voltage source model with widely used series RC model. This is done using metrics such as the z-domain bode plots and F and Q parameters. The z-domain current loop plant transfer functions are derived from the continuous time state space models using exact discretization technique. Utilizing the derived comparison metrics, this paper investigates the limiting power and voltage operating levels of the proposed voltage source model against series RC model. Based on the proposed model, the controller design has been carried out for inner current loop control. This was experimentally verified at a power level of Po=l20W and a voltage level of Vg=26V with the dc-dc converter switching at fsw=100kHz.

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