1,4.4 Forward converter, Buck-derived transformer-isolated converter Single-transistor and two-transistor versions Maximum duty cycle is limited Transformer is reset while transistor is off,2,Forward converter with transformer equivalent circuit,3,Forward converter: waveforms,4,Subinterval 1: transistor conducts,5,Subinterval 2: transformer reset,6,Subinterval 3,7,Magnetizing inductance volt-second balance,8,Transformer reset,9,What happens when D 0.5,10,Conversion ratio M(D),11,Maximum duty cycle vs. transistor voltage stress,decreasing,12,Flyback converter,13,Derivation of flyback converter,14,The “flyback transformer”,15,Subinterval 1,16,Subinterval 2,17,CCM Flyback waveforms and solution,18,Discussion: Flyback converter, Widely used in low power and/or high voltage applications Low parts count Multiple outputs are easily obtained, with minimum additional parts Cross regulation is inferior to buck-derived isolated converters Often operated in discontinuous conduction mode DCM analysis: DCM buck-boost with turns ratio,19,Boost-derived isolated converters,A wide variety of boost-derived isolated dc-dc onverters can be derived, by inversion of source and load of buck-derived isolated converters: full-bridge and half-bridge isolated boost converters inverse of forward converter: the “reverse” converter push-pull boost-derived converter Of these, the full-bridge and push-pull boost-derived isolated converters are the most popular, and are briefly discussed here.,20,Full-bridge transformer-isolated boost-derived converter,21,Transformer reset mechanism,22,Conversion ratio M(D),23,4.5 Converter evaluation and design,For a given application, which converter topology is best? There is no ultimate converter, perfectly suited for all possible applications. Trade studies. Rough designs of several converter topologies to meet the given specifications. An unbiased quantitative comparison of worst-case transistor currents and voltages, transformer size, etc. Comparison via switch stress, switch utilization, and semiconductor cost,24,Switch stress and switch utilization, Largest single cost in a converter is usually the cost of the active semiconductor devices Conduction and switching losses associated with the active semiconductor devices often dominate the other sources of loss. This suggests evaluating candidate converter approaches by comparing the voltage and current stresses imposed on the active semiconductor devices. Minimization of total switch stresses leads to reduced loss, and to minimization of the total silicon area required to realize the power devices of the converter.,25,Total active switch stress S,26,Active switch utilization U,27,CCM flyback example: Determination of S,28,CCM flyback example: Determination of U,29,Flyback example: switch utilization U(D),30,Comparison of switch utilizations of some common converters,31,32,33,Active semiconductor cost vs. switch utilization,34,Converter design using computer spreadsheet,Given ranges of Vg and Pload , as well as desired value of V and other quantities such as switching frequency, ripple, etc., there are two basic engineering design tasks: Compare converter topologies and select the best for the given specifications Optimize the design of a given converter A computer spreadsheet is a very useful tool for this job. The results of the steady-state converter analyses of chapters 1-6 can be entered,and detailed design investigations can be quickly performed: Evaluation of worst-case stresses over a range of operating points Evaluation of design tradeoffs,35,Spreadsheet design example,Specificat ions maximum input voltage Vg 390V minimum input voltage Vg 260V output voltage V 15V maximum load power Pload 200W minimum load power Pload 20W switching frequency fs 100kHz maximum output ripple v 0.1V, Input voltage: rectified 230Vrms20% Regulated output of 15V Rated load power 200W Must operate at 10% load Select switching frequency of 100kHz Output voltage ripple 0.1V,Compare single-transistor forward and flyback converters in this application Specifications are entered at top of spreadsheet,36,Forward converter design, CCM,37,Flyback converter design, CCM,38,Summary of key points,1. The boost converter can be viewed as an inverse buck converter, while the buck-boost and Cuk converters arise from cascade connections of buck and boost converters. The properties of these converters are consistent with their origins. Ac outputs can be obtained by differential connection of the load. An infinite number of converters are possible, and several are listed in this chapter. 2. For understanding the operation of most converters containing transformers, the transformer can be modeled as a magnetizing inductance in parallel with an ideal transformer. The magnetizing inductance must obey all of the usual rules for inductors, including the principle of volt-second balance.,39,Summary of key points,3. The steady-state behavior of transformer-isolated converters