Application of a Bidirectional DC/DC Converter to Control the Power Distribution in the Battery–Ultracapacitor System

Abstract

The article presents the use of the Texas Instruments LM5170EVM-BIDIR bidirectional DC/DC converter to control power distribution in a hybrid energy storage system based on a battery–ultracapacitor system. The paper describes typical topologies of connecting a battery with an ultracapacitor. The results of tests for calibration and identification of converter parameters are presented. The main innovation of the solution presented in this paper is the appropriate selection of the nominal voltage of the ultracapacitor so that the converter can be operated only in the constant current mode, in a cascade connection, excluding the low-efficiency constant voltage mode. This article demonstrated that such control allows for high efficiency and reduction of losses in the DC/DC converter, which is necessary in the case of mobile solutions. The amount of losses was determined depending on the control voltage in the operation modes of the converter: in the Step Up mode by increasing the voltage from 12 V to 24 V, from 12 V to 36 V, and from 12 V to 48 V and in the Step Down mode by decreasing the voltage from 48 V to 12 V, from 36 V to 12 V, and from 24 V to 12 V. For a calibrated converter in a semi-active topology, bench tests were carried out in a cycle with pulsating load. The tests were carried out using LiFePO4 cells with a voltage of 12 V and Maxwell ultracapacitors with a package voltage of 48 V. Power distribution in the range of 10% to 90% was achieved using the myRIO platform, which controlled the operation of the DC/DC converter based on an external current profile.