Fast charging techniques and compact integrated implementations for electrochemical double layer capacitors in portable applications

Abstract

The widespread increase in the range and types of portable electronic devices in the past decades has resulted in higher requirements for energy storage and conversion modules. Most of these devices use rechargeable batteries as energy storage elements. No matter what type of batteries are used (Ni-Cd, Ni-MH, Li-Ion, etc.) they all have one serious drawback in common, in terms of charging time. Electrochemical double layer capacitors (EDLCs) also known as ultracapacitors or supercapacitors seem to have overcome this disadvantage, at the cost of lower energy storage capacity.

This work aims to explore the design of fast and compact integrated charging techniques for ultracapacitors using the AC mains network as the source. The main constraints that arise are the power dissipation on-chip and in the magnetic components due to the large amount of energy that has to be transferred in a very short time interval. Two other limitations come from the EDLC side due to the device parasitics and the widely varying voltages over the operational envelope. This will impose the need for a flexible control system providing high efficiency over the whole output voltage range.

The structure of this thesis comprises five main parts: literature review, behavioural modelling of the control system (including matlab simulations); implementation of the device with discrete components; design of an analogue circuit implementation and design of a mixed signal circuit implementation. As ultracapacitors represent one of the newest solutions in the field of electrical energy storage there are very few designs for chargers from the mains network. Therefore the literature review will also examine the properties and the modelling of EDLCs, as well as the choice of converter topologies available and the characteristics of the magnetic devices required for the system. The behavioural model of the control module gives a preview of the system parameters, while the design chapter introduces a series of new control techniques. The simulations and measurements of the breadboard circuit come as a first confirmation of the design approach and make it a viable starting point for an IC implementation. The analogue IC design presents the integration of the algorithms in a medium-voltage process using the current mode approach, as a demonstrator for a fully monolithic high-voltage IC. Once the functionality of the system is demonstrated at IC level, the mixed-signal system aims to optimize the device and provide a broader flexibility for the system parameters and control algorithms.

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