Novel Voltage Control Methods for quasi-Z-source Network Converters with Focus on Fixed DC-Link Voltage, Constant- Frequency and Soft-Switching Operation

Mohammadsalehian, Shamim

Novel Voltage Control Methods for quasi-Z-source Network Converters with Focus on Fixed DC-Link Voltage, Constant- Frequency and Soft-Switching Operation

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mohammadsalehian_shamim_PhD_2025_thesis.pdf (6.65 MB)

Date

2025-12-15

Authors

Mohammadsalehian, Shamim

Abstract

Power electronic converters reinforce modern energy systems by enabling efficient power transfer among batteries, renewable sources, and the utility grid. quasi-Z-source (qZS) converters are especially attractive for their single-stage buck–boost capability, continuous input current, and inherent reliability. A central challenge, however, is the intermediate DClink capacitor: its voltage varies with operating conditions. Low DC-link voltage drives high full-bridge current, elevating conduction and turn-off losses; high DC-link voltage raises switch stress and device cost. These fluctuations complicate design and control, degrade efficiency, and disrupt consistent soft switching. Conventional pulse-width-modulated (PWM) qZS converters often rely on variable switching frequency or auxiliary circuits to achieve soft switching, which increases cost and control complexity. Resonant qZS converters can enhance gain and facilitate soft-switching yet still exhibit a variable intermediate DC-link between the impedance network and resonant tank, leading to fluctuating electrical stresses, efficiency penalties, and a constrained operating range. Thus, while Z-source–based topologies broaden buck–boost flexibility, they introduce critical trade-offs in efficiency, component stress, and dynamic performance. Single-stage AC–DC qZS implementations also struggle with DC-link regulation. With a DC link capacitor, near-unity power factor typically requires discontinuous current, increasing current stress and limiting power capability; under light load, continuous current can cause DC-link overvoltage. Without a DC-link capacitor, pronounced 120 Hz ripple and switch voltage overshoot appear, often demanding lossy snubbers or added auxiliaries. Consequently, prior approaches either compromise soft-switching or fail to maintain a stable DC-link across wide input and load variations. This thesis proposes three control strategies for single-phase qZS full-bridge converters: a PWM-based DC–DC qZS control that regulates the DC-link and achieves full soft-switching for both top and bottom devices without auxiliary circuit; an extension to a qZS series-resonant converter that maintains soft-switching for all switches at constant frequency; and a control method for the single-phase AC–DC qZS converter that integrates PFC with DC-link regulation to deliver unity power factor, soft-switching, and a fixed DC-link. Simulations and experiments confirm stable DC-link voltage, constant-frequency operation, reliable soft-switching, and high efficiency, and a comparative study of Z-source/qZS variants benchmarks DC-link controllability, soft-switching behavior, and overall performance.

Summary for Lay Audience

Modern life runs on electronics that quietly reshape electricity so devices can use it safely and efficiently—from phone chargers to solar inverters and electric-vehicle stations. This thesis studies a class of these circuits called single-phase converters that use a special front section, the Z-source or qZS network, to raise or lower voltage in a single stage. Doing both jobs at once can make power supplies smaller, simpler, and potentially cheaper. A practical headache, however, sits in the middle of almost every design: the internal DC-link voltage—an intermediate power level inside the converter. When the input voltage or the device’s demand changes, this internal level can drift. If it falls too low, the circuit must push more current, wasting energy and heating components. If it rises too high, switching parts see extra electrical stress and may need costlier ratings. Either way, efficiency drops, and reliability suffers. Previous research often forced a trade-off. Some solutions add extra sensors, or auxiliary hardware to hold the DC-link steady—but that adds complexity and cost. Other solutions keep the hardware simple yet give up some efficiency because the switches can’t always turn on and off at the most favorable instants (called soft switching), or they vary the switching frequency widely, which complicates magnetic design. This thesis shows a way to get the benefits without the trade-offs. It introduces a simple control method that keeps the DC-link close to a chosen target while keeping the switching frequency fixed—good for compact magnetics and easier synchronization—and while preserving softswitching behavior over normal operating ranges. In plain terms, the converter stays efficient and cool without extra conversion stages or intricate add-ons