chap08

# chap08 - 8 Converter Transfer Functions The engineering...

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Converter Transfer Functions 8 The engineering design process is comprised of several major steps: 1. 2. 3. 4. 5. 6. 7. Specifications and other design goals are defined. A circuit is proposed. This is a creative process that draws on the physical insight and experience of the engineer. The circuit is modeled. The converter power stage is modeled as described in Chapter 7. Components and other portions of the system are modeled as appropriate, often with vendor-supplied data. Design-oriented analysis of the circuit is performed. This involves development of equations that allow element values to be chosen such that specifications and design goals are met. In addition, it may be neces- sary for the engineer to gain additional understanding and physical insight into the circuit behavior, so that the design can be improved by adding elements to the circuit or by changing circuit connections. Model verification. Predictions of the model are compared to a laboratory prototype, under nominal oper- ating conditions. The model is refined as necessary, so that the model predictions agree with laboratory measurements. Worst-case analysis (or other reliability and production yield analysis) of the circuit is performed. This involves quantitative evaluation of the model performance, to judge whether specifications are met under all conditions. Computer simulation is well-suited to this task. Iteration. The above steps are repeated to improve the design until the worst-case behavior meets specifi- cations, or until the reliability and production yield are acceptably high. This chapter covers techniques of design-oriented analysis, measurement of experimental transfer func- tions, and computer simulation, as needed in steps 4, 5, and 6. Sections 8.1 to 8.3 discuss techniques for analysis and construction of the Bode plots of the con- verter transfer functions, input impedance, and output impedance predicted by the equivalent circuit

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266 Converter Transfer Functions models of Chapter 7. For example, the small-signal equivalent circuit model of the buck-boost converter is illustrated in Fig. 7.17(c). This model is reproduced in Fig. 8.1, with the important inputs and terminal impedances identified. The line-to-output transfer function is found by setting duty cycle varia- tions to zero, and then solving for the transfer function from This transfer function describes how variations or disturbances in the applied input voltage lead to disturbances in the output voltage v ( t ). It is important in design of an output voltage regulator. For exam- ple, in an off-line power supply, the converter input voltage contains undesired even harmonics of the ac power line voltage. The transfer function is used to determine the effect of these harmonics on the converter output voltage v ( t ).
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chap08 - 8 Converter Transfer Functions The engineering...

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