Thomas Yeh

Abstract

Power Converter with capacitive input filter is a non-linear load to the Utility AC power lines. There are widely used as Switch-Mode Power Supplies in office equipment applications ranging from Personal Computers to Office Printers and Copiers. The distorted input current waveform extracted by the capacitive input filter of the power converters produces unwanted harmonics which propagates to other line powered equipments. The harmonic pollutes the AC lines and interferes with the operations of sensitive line powered equipments. The distorted current waveform also leads to inefficient utilization of the available power from the AC outlet. This is because the AC line power is transferred to the load only when each frequency component of the line voltage is an in-phase, scaler related quantity with respect to the same frequency component of the extracted current. The problems of poor power factor and harmonic distortion are compounded by the proliferation of Switch-Mode power supplies and the situation is rapidly becoming intolerable. The problem of poor quality input current waveform can be described by two quantitative measurements: Power Factor (pf) and Total Harmonic Distortion (thd). Two general approaches are available to remedy the problem. One approach is to install passive filter networks between the Utility AC lines and the capacitive input filter. The second approach is to design active power processors as dedicated Power Factor Correction (pfc) Converters and installed as the front end to the capacitive input filter to shape the distorted current waveform into waveforms which will yield higher power factor. This Thesis first introduce the general concept of PF and harmonic distortion in Chapter 1. Chapter 2 derive the mathematical description of pf based on the concept of real power (pR) and apparent power {pA). Both sinusoidal and nonsinusoidal cases are studied. For comparison and completeness, two popular passive power factor correction filter networks are analyzed in Chapter 3 to derive the maximum achievable power factor for each network along with the corresponding harmonic distortions. Governing equations are derived and presented graphically as a function of the filter network and load parameters. Chapter 4 provides the analysis of active power factor correction using switch mode Boost Converter. The analysis is carried out for two types of current controllers used as the current modulators for current waveform shaping. The state space averaged modeling approach is employed to derive the mathematical model of the Boost Converter suitable for large signal time domain and small signal frequency domain analysis. The model is further extended to derive the describing equations for the Boost Converter operating as pfc converter. Characteristics of the two current controller functions impacting the pfc operation are studied to expose their relative strength and limitations. The analysis includes the supplementation of the current control loop by an outer voltage control loop to regulate the output capacitor voltage of the pfc converter. The large signal analysis is first investigated forPF, waveform quality and voltage regulation. The small signal analysis follows to extract the frequency domain behavior of the pfc Boost Converter. The limitation of the voltage control bandwidth and its effect on the achievable pf is discussed. Chapter 5 verified the analysis through computer simulation using PSPICE. The original works of Bello[2] based on Berkeley SPICE are modified for PSPICE. The models are extended to implement the PFC control functions and simulated in both large signal time domain and small signal frequency domain. Both the analysis and computer simulation results are compared to a published design of pfc Converter[3].

Library of Congress Subject Headings

Electric current converters

Publication Date

1992

Document Type

Thesis

Department, Program, or Center

Electrical Engineering (KGCOE)

Advisor

Unnikrishnan, R.

Advisor/Committee Member

Ramanam, S.

Advisor/Committee Member

Palmer, James

Comments

Note: imported from RIT’s Digital Media Library running on DSpace to RIT Scholar Works. Physical copy available through RIT's The Wallace Library at: TK7872.C8 Y43 1992

Campus

RIT – Main Campus

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