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Soft Switching Control and Loss Analysis for High Frequency Power Converters


Type

Thesis

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Authors

Abstract

Efficiency and power density stand as pivotal considerations in the realm of power converters, driving the momentum towards sustained electrification for the realization of a low-carbon economy. Historically, Silicon (Si) power semiconductors dominated applications ranging from 100 W to several hundred kW. Nonetheless, the Si-based technology is progressively nearing its inherent physical constraints. Silicon-based power converters typically operate at switching frequencies below 100 kHz to mitigate switching losses. This imposed limitation poses a challenge in attaining elevated power density, primarily due to the need for bulky filter inductors and capacitors.

Wide bandgap (WBG) power semiconductor devices are considered game-changing devices to overcome the limitation posed by traditional Si counterparts, enabling us with much higher switching speed and lower switching loss. However, when the switching frequency is pushed even higher to hundreds of kHz, soft-switching solutions will be necessary to decrease the switching loss.

This dissertation focused on the implementation of a new family of soft-switching schemes based on parallel switching cells in depth. Chapter 1 presents a review of state-of-art soft-switching schemes, featured by Triangular Current Mode (TCM) with zero-voltage-switching (ZVS) turn-ON but varying switching frequency. Based on this discussion, the limitation of existing soft-switching technology has been revealed and it was identified that a new solution suitable for high-power applications but with constant switching frequency is still missing.

Based on the discussion in Chapter 1, Chapter 2 details a generic ZVS soft-switching scheme based on paralleled half-bridge (HB) switching cells. The scheme, named Quadrilateral Current Mode (QCM), deliberately creates delay time between parallel switching cells to facilitate ZVS. Both mathematical description and experimental verification of the scheme have been provided. Chapter 3 further extends the QCM scheme to semi-bridge switching cells where the switching unit is formed by one MOSFET and one diode. Furthermore, Chapter 3 also proposes a magnetic integration solution which is able to integrate the ZVS inductor into the filter inductor. The proposed magnetic integration design is also applicable to HB switching cells and addresses the issues caused by additional ZVS inductors.

A further contribution investigates the application of the QCM scheme in a DC-AC inverter and proposes a Hybrid Quadrilateral and Continuous Current Mode (HQCCM) modulation in Chapter 4 for general high-frequency single-phase DC-AC conversion based on paralleled SiC MOSFETs. The proposed HQCCM adaptively operates in soft-switching Quadrilateral Current Mode (QCM) or hard-switching Continuous Conduction Mode (CCM) in one AC line cycle depending on the instantaneous AC load current. Thus, high efficiencies can be achieved over the full power range.

Apart from WBG power semiconductors, advanced passive component technologies like high-power-density ferroelectric Class II multi-layer ceramic capacitors (MLCCs) promise even more compact and efficient power conversion. Ferroelectric Class II MLCCs have been widely applied as the DC-link capacitor or resonant capacitors in the WBG-based power converters. However, in literature, little attention has been paid to their loss behavior, especially when high-frequency excitation and DC-bias voltage are present. Accordingly, Chapter 5 comprises a toolset to model the loss of Class II MLCCs when complex electrical excitations (high-frequency, large-signal and DC-bias) are present. The loss model, which is based on the proposed Steinmetz’s Pre-electricized Graph (SpeG) and other material-level estimation tools, is able to make the loss prediction of a Class II MLCC as easy as that of an inductor.

Chapter 6 of the dissertation presents a conclusion of the achieved results and an outlook on topics for the continuation of research on advanced soft-switching schemes. It provides a comprehensive study of the proposed solutions and outlines future research directions.

Description

Date

2023-06-27

Advisors

Teng, Long

Keywords

capacitor, high frequency, loss, soft switching

Qualification

Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
Sponsorship
Jardine Foundation