Date of Award

Summer 7-13-2014

Document Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Electrical, Computer & Energy Engineering

First Advisor

Dragan Maksimović

Second Advisor

Regan Zane

Abstract

Point-of-load (POL) power supplies are high-output-current, low-output-voltage, DC-DC converters that are placed near the electronic components, such as memory chips and microprocessors, on a computer motherboard. They have to meet challenging requirements of high efficiency over a wide load range, and fast transient responses to very dynamic load profiles. The most popular POL topology is based on single-phase or multi-phase buck converters. Buck converters have limitations in large step-down applications due to very low duty cycle requirements for the control MOSFET(s), and relatively high switching losses at high frequencies.

This work proposes a new converter architecture and control method for POL applications - a modular converter based on active-clamp LLC resonant modules, designed to work with an on/off digital controller. The active clamp LLC converter inherits advantages of the standard LLC resonant converter, including soft-switching and 50%-duty-cycle operation of all switching devices. The active clamp addresses the voltage oscillation across the rectifier devices caused by transformer secondary-side leakage inductances and MOSFET output capacitances by clamping the voltage to approximately twice the output dc voltage. In addition, the active clamp helps to reduce the output capacitor current ripple. The converter is well suited for a multiple-parallel-module configuration in which each module, when on, operates at its maximum efficiency. The output voltage is regulated by turning on/off one module in a pulse-width-modulation (PWM) manner while the other modules are either fully on or fully off, depending on the load power demand.

Analysis, modeling, design and control methods are described for the modular active-clamp LLC converter and the results are verified on experimental prototypes. It is found that the proposed converter and the corresponding control approach yield high overall efficiency and fast step-load transient responses. The approach is suitable for single or multi-module high-frequency high-step-down low-voltage point-of-load applications where secondary-side devices and control circuitry can be integrated in a low-voltage CMOS process.

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