Dissertation/Thesis Abstract

Control architectures for spur-free operation in switching power regulators
by Tao, Chengwu, Ph.D., Iowa State University, 2011, 134; 3473106
Abstract (Summary)

The gap between battery voltage and the lower supply voltage required for integrated circuits requires designers of battery-operated devices to seek voltage regulators with high power efficiency, efficient area and cost, as well as low noise. However, while buck regulators can achieve extremely high power efficiency, the spurious noise associated with periodic switching used in buck regulators becomes problematic for many RF/analog circuits. This thesis focuses on developing efficient spurious noise reduction techniques to improve the performance of buck regulators without significantly impact on the power efficiency, chip area and cost. By studying the operation of buck regulators, small signal models for analyzing the spectrum of buck regulators are derived. Based on these models, convenient and easy-to-use expressions describing spectrum of buck regulator output voltage are derived to predict the nature of switching spurs. These predictions are confirmed by spectrum measurement results. This provides an easy way for an RF/analog designer to account for spurious noise in the cases where commercially available buck regulators lacking detailed information with respect to the switching spurs are used to power their circuits. These small signal models also provide a theoretical basis for developing spur reduction techniques. Based on the discussion on the advantages and disadvantages of previous spurious noise reduction techniques, a frequency hopping technique is proposed that would be a good candidate for spurious noise reduction except for the transients and low frequency noise floor peaking associated with the duty cycle disturbance that occur each time the switching frequency is hopped. By mandating the completeness of the switching cycle to reduce this duty cycle disturbance, an improved frequency hopping technique with much better spur reduction is implemented in buck regulator. This implementation also eliminates the synchronous requirement and allows flexibility in setting the hopping rate to be independent from the internal clocks of the regulator. To maximize spectrum spreading and completely eliminate the switching spurs without significant impact on power, area and cost, a spur-free Fixed-Cycle Frequency Hopping (FCFH) technique is proposed. By custom tailoring the hopping period to each frequency such that all frequencies are always applied for the same number of cycles, duty cycle disturbance is eliminated to allow a maximum hopping rate while the phase is randomly chopped to fully eliminate the switching spurs. Therefore this proposed FCFH buck regulator achieves spur-free performance without transients or significant low frequency noise floor peaking as in frequency hopping. The proposed buck regulator is also used to directly power an off-the-shelf GSM PA to mimic average power level tracking PA characteristics. Spectral measurements demonstrate that the out of band mixing spurs, 9.3dB above the limits of the GSM spectral mask, are reduced below the appropriate compliance level without intermediate filtering or linear regulation between the regulator and the PA. This application provides convincing proof of the attractive role to be played by the proposed buck regulator with FCFH control methodology in battery-operated devices.

Indexing (document details)
Advisor: Fayed, Ayman
Commitee: Chen, Degang, Dong, Liang, Kim, Jaeyoun, Neihart, Nathan
School: Iowa State University
Department: Electrical and Computer Engineering
School Location: United States -- Iowa
Source: DAI-B 72/12, Dissertation Abstracts International
Source Type: DISSERTATION
Subjects: Electrical engineering
Keywords: Average power level tracking, Buck regulators, Frequency hopping, Pulse width modulation, Spur-free
Publication Number: 3473106
ISBN: 978-1-124-87243-8
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