Aggregated modeling and control of a boost-buck cascade converter for maximum power point tracking of a thermoelectric generator

For a wide output voltage ranged thermoelectric generator, it is necessary to have a dc-dc converter that allows voltage boost and buck functions. In this paper, a boost-buck cascade dc-dc converter is adopted to allow input voltage range from near 0 V to 25 V and output voltage range from 12.3 V to 16.5 V, needed for vehicle battery charging. To ensure a smooth operating mode change, a modified maximum power point tracking controller is used to control the first stage converter duty cycle while regulating the second stage output voltage. A conventional duty cycle controller is then used to control the second stage converter while regulating the middle dc bus voltage. In order to avoid intricate relationship between two controller duty cycles, a novel aggregated modeling approach is proposed. The small signal model is analyzed, and the design guidelines for the power converter and controller are provided. Experimental results indicate smooth transition between changes and load steps, and the effectiveness of the proposed modeling and the controller design is verified.

I. Introduction

A recently developed thermoelectric generator (TEG) has shown promise in vehicle applications [1]–[2]. By integrating the TEG with an exhaust pipe and a heat exchanger system, it is possible to recover engine waste heat, and convert it to electric power for battery charging and subsequently improve a vehicle gas mileage. The generated voltage of a TEG changes dynamically with the temperature and varies in a wide range. For example, the target set by the U.S. Department of Energy FreedomCar Program [2] ranges from 0 to 25 V. This voltage needs to be converted from 12.3V to 16.5V for battery charging. Therefore, a dc-dc converter, especially having step-up/down characteristic is required. In addition, the controller needs to have two operating modes: (a) maximum power point tracking (MPPT) and (b) power matching (PM). When the TEG output is less than the vehicle power demand, the MPPT mode is activated to harness as much power as possible from the waster heat. When the TEG output is higher than the demand, the PM mode is activated to reduce the TEG power production and to avoid overcharging the battery. During PM mode, the vehicle electrical bus is regulated at the float voltage by the power converter.