PartV

Resonant Converters

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Resonant Conversion

Part V of this text deais with a ciass of converters whose operation differs signifioantiy from the PWM converters covered in Parts I to IV. Resonant power converters [l-3fi] contain resonant L-C networlts whose voltage and ctirrent waveforms vary sinusoidally during one or more subintervals of each switching petiod. Tlieie sinusoidal variations are large in magnittide. and hence the small ripple approximation introdticed in Chapter 2 does not apply.

Dc-to-high-frequency-ac inverters are required in a variety of applications, including electronic ballasts for gas discharge lamps [3,4], induction heating, and electrosurgical generators. These applications typically reqtiire generation of a sinusoid of tens or hundreds of ItHz, having moderate or low total harmonic distortion. A simple resonant inverter system is illustrated in Fig. 19.1(a). A switch network produces a square wave voltage vjt). As illustrated in Fig. 19.2, the spectrum of v/f) contains fundamental pitis txid harmonics. This voltage is applied to the input teiiiiinals of a resonant tank network. The tanlt network resonant freqtiency/ц is tuned to the fundamental component of Vj.(f)i that is, to the switching frequency/ and the tanlt exhibits negligible response at the harmonics of/,. In consequence, the tank current iji), as well as the load voltage i(/) and load current /(/), have essentially sinusoidal waveforms offrequency/, with negligible harmonics. By changing the switching frequency/, (closer to or further from the resonant frequency /(,), the niagnittides of ijit), i(f), and /(/) can be controlled. Other schemes for control ofthe output voltage, such as phase-shift control ofthe bridge switch networlt, are idso possible. A variety of resonant tanlt networks can be employed; Fig. 19.1(b) to (d) illustrate the well-known series, parallel, and LCC tank networks. Inverters employing the series resonant tank network are known as the series resonant, or series loaded, inverter. In the parallel resonant or parallel loaded inverter, the load voltage is equal to the resonant tanlt capacitor voltage. The LCC inverter employs tank ciipacitors both in series and in parallel with the load.

Figure 19.3 illustrates a high-frequency inverter of an electronic ballast for a gas-discharge lainp. A half-bridge configuration ofthe LCC inverter drives the lamp with an approximately sinusoidal