In Part I we examined the form and function ofthe major families of power electronic converters.Our goal was to show how the intended powerconversion function is achieved in each case byappropriate configuration of the circuitcomponents and by proper operation of theswitches. Throughout those earlier chapters, ourconcern was with nominal operating conditions, that is,the ideal operating conditions in which aconverter is designed to perform its primaryconversion function. As nominal operation in mostpower electronic circuits involves a periodic steady state, wefocused on situations in which circuit operationand behavior are the same from cycle to cycle.

Power magnetics are often constrained by loss.Consequently, the ability to accurately predictthe loss of a magnetic component is extremelyvaluable for design. The techniques for modelingmagnetics loss introduced in the previous chaptersare useful, but do not adequately cover allsituations. In this chapter we introduce refinedmethods to predict winding and core losses inmagnetic components, with particular emphasis onfactors (such as proximity effect) that becomedominant at high frequencies and on cases wherethe waveforms are not purely dc or sinusoidal.

We add transformers to the topology of ahigh-frequency converter for three reasons: toprovide electrical isolation between two (or more)external systems; to reduce the component stressesthat result when the input/output conversion ratiois far from unity; and to create multiple relatedoutputs in a simple manner. (We showed therelationship between switch-stress factor and theconversion ratio in Fig. 5.26.) There are manyways in which we can include the transformer inthe topology of a dc/dc converter; we present anddiscuss some of them in this chapter.

Power electronic circuits change the characterof electrical energy: from dc or ac to ac or dc,from one voltage level or frequency to another, orin some other way. We refer to such circuitsgenerically as converters, staticconverters (because they contain nomoving parts), powerprocessors, or powerconditioners. The part of the system thatactually manipulates the flow of energy is thepower circuit. It isthe scaffold for the system’s other components,such as the control circuit or the thermalmanagement parts.

The rapid switching transitions of a powerconverter are potential sources of electromagneticinterference (EMI), both for the converter itselfand for the systems to which it is connected.Adequate filtering at the input and output of theconverter is important, both to obtain acceptableperformance and to prevent interference with otherequipment. In this chapter, we consider thesources of EMI in a converter, how EMI is measuredand modeled, and how it can be mitigated, with afocus on conducted (rather than radiated) EMI.

The previous chapters on magnetics provided thekey concepts needed to analyze, model, and designmagnetic components such as inductors andtransformers. The purpose of this chapter is torefine and extend the methods introduced there,with a focus on techniques for magnetic componentdesign. In particular, we introduce design methodsand sizing considerations for efficientlyconverging on an appropriate design. Thisincludes, for example, approximate methods forsizing the magnetic core for an inductor ortransformer, and metrics for comparing magneticmaterials.

AC/AC converters take power from one ac systemand deliver it to another with waveforms of thesame or different amplitude, frequency, or phase.The ac systems can be single phase or polyphase,and reactive power can exist at the input, output,or both, depending on how we configure theconverter. The simplest and most common ac/acconverter is the transformer.

In this chapter we describe power electronics andpresent a brief introduction to semiconductorswitching devices and magnetic components. Anintroduction to these circuit elements is necessarybecause we use them in Part I, although we do notdiscuss them in detail until Part III. We alsointroduce in this chapter nomenclature that we usethroughout the book.

An unfortunate consequence of our preoccupationwith things electrical is that the problems ofheat sinking and thermal management are frequentlyignored until forced on us by sound, sight, orsmell. The insatiable need to make things smaller– and the possibility of doing so by using higherfrequencies and new components and materials –aggravates the problem of heat transfer, becausesuch improvements in power densities are seldomaccompanied by corresponding improvements inefficiency. Thus we are stuck with the task ofgetting the same heat out of a smaller volumewhile disallowing any increase in temperature.

The topics we have addressed so far – powercircuits, control, and components – do not coverall the issues encountered in designing a powerelectronic system. Among those we have deferredare: (i) providing gate and base drives to thepower semiconductor switches; (ii) using forcedcommutation to turn off SCRs; (iii) controllingthe transient voltages and currents that accompanyswitching in practical circuits; (iv) contendingwith EMI created by fast switching waveforms; and(v) providing a thermal environment that allowssystem components to operate within theirtemperature ratings. We address these five topicsin Part IV.

Magnetic components such as inductors andtransformers are present in the vast majority ofpower electronic circuits. Inductors store energyin the conversion process, filter switching ripple(as part of input and output filters, forexample), create sinusoidal variations of voltageor current (paired with capacitors, as in resonantconverters), limit the rate of change of current(as in snubber circuits), and limit transientcurrent.

In this chapter we introduce polyphase systems,their advantages relative to single-phase systems,and how they are represented. We then use thesingle-phase bridge circuit as a building block tocreate polyphase rectifiers and inverters. We alsodiscuss the operating characteristics of thesecircuits from both the ac and dc sides.

When a power semiconductor switch is either onor off, its power dissipation is relatively small.However, the transition from one state to theother is often not so benign, and can imposesimultaneous high voltage and high current on theswitching device. Special efforts are oftenrequired to ensure that the device will survivethis most stressful part of its operating cycle.We have already seen in Fig. 22.4(b) how a snubbercircuit consisting of$C_s$ and$R_s$ can moderatethe effects of parasitic inductance.

The phase-controlled dc/ac converter introducedin Chapter 4 requires that the external ac systembe a voltage source, typically the ac utilityline. This condition is necessary because thephase-controlled converter uses the reversal ofthe ac voltage to drive the commutation process.Therefore the ac frequency in these circuits isconstrained to be that of the ac source. In thischapter we remove the restriction that the acsystem be a voltage source, realizing that bydoing so we must use means other than linecommutation to turn devices off. These circuits,which we call switched-modedc/ac converters, use fully controllabledevices such as MOSFETs and IGBTs, and switch atfrequencies that are higher (often much higher)than the ac-side frequency.