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Soft Switching

In addition to tiie resonant circuits introduced in Ciiapter 19, there has been tnuch interest in reducing the switching loss ofthe PWM converters ofthe previous chapters. Several ofthe more popular approaches to obtaining soft switching in buck, boost, and other converters, are discussed in this chapter.

Mechanisms that catise switching loss are discussed in Chapter 4, including diode reverse recovery, semiconductor otitput capacitances, and IGBT current tailing. Soft switching involves mitigation of one or more of these switching loss mechanisms in a PWM converter. The energy that wotild otherwise be lost is recovered, and is transferred to the converter source or load. The operation of a semiconductor device, during a given turn-on or turn-off switching transition, can be classified as hard-switched, zero-ctirrent switched, or zero-voltage switched. Operation of diodes and transistors with soft switching is examined in Section 20.1. In particular, it is preferable to operate diodes with zero-voltage switching at their turn-off transitions, and to operate MOSFETs with zero-voltage switching during their turn-on transitions. However, zero-voltage switching tomes at the expense of increased conduction loss, and so the engineer must consider the effect of soft switching on the overall converter efficiency.

Resonant switch converters are a broad class of converters in which the PWM switch network of a conventional buck, boost, or other converter is replaced with a switch cell containing resonant elements. These resonant elements are positioned such that the semiconductor devices operate with zero-current or zero-voltage switching, and such that one or more ofthe switching loss mechanisms is reduced or eliminated. Other soft-switching approaches may employ resonant switching transitions, but otherwise exhibit the approximately rectangular waveforms of hard-switched converters. In any case, the resulting hybrid converter combines the properties of the resonant switching network and the parent hard-switched PWM converter.

Soft-switching converters can exhibit reduced switching loss, at the expense of increased condticlion loss. Obtaining zero-voltage or zero-current switching requires that the resonant elements have large ripple; often, these elements are operated in a manner similar to the discontinuous condttction

7 а Soft Switching

modes of the series or parallel resonant converters. As in other resonant schemes, the objectives of designing such a converter arc: (1) to obtain smaller transformer and low-pass filter elements via increase of the switching frequency, and/or (2) to reduce the switching loss induced by component nonidealities such as diode stored charge, semiconductor device capacitances, and transformer leakage inductance and winding capacitance.

The resonant switch and soft-switching ideas are quite general, and can be applied to a variety of topologies and applications. A large numberof resonant switch networks have been documented in the literature; a few basic approaches are listed here [1-24]. The basic zero-current-switching quasi-resonant switch network is analyzed in detail in Section 20.2. Expressions for the average components of the switch network terminal waveforms are found, leading to determination of the switch conversion ratio f.. The switch conversion ratio Д performs the role of the duty cycle rfofCCM PWM switch networks. For example, the buck converter exhibits conversion ratio ,V/equal to jl. Both half-wave and full-wave ringing of the tank network is considered; these lead to different switch conversion ratiofunctions fl. In general, given a PWM CCM converter having conversion ratio M(d), we can replace the PWM switch network with a resonant switch network having switch conversion ratio pt. The resulting quasi-resonant converter will then have conversion ratio M(fi). So we can obtain soft-switching versions of all of the basic converters (buck, boost, btick-boost, forward, flyback, etc.), that exhibit zert)-voltage or zero-current switching and other desirable properties.

In Section 20.3, the characteristics of several other resonant switch networks are listed: the zero-vohage-switching quasi-resonant switch network, the zert)-current-switching and zero-voltage-switching quasi-square-wave networks, and the multiresonant switch network. One can obtain zero-voltage switching in all transistors and diodes using these networks.

Several related soft-switching approaches are now popular, which attain zero-voltage switching of the hansistor or hansistoiii in commonly-used converters. The zero-voltage transition approach finds application in full-bridge buck-derived converters. Active-clamp snubbers are often added to forward and flyback converters, to attain zero-voltage switching and to reset the transformer. These circuits lead to zero-voltage switching of the transistors, but (less-than-optimal) zero-current switching of the secondary-side diodes. Nonetheless, high efficiency can be achieved. An auxiliary resonant-commutated pole can achieve zero-voltage switching in voltage-source inverters. These converters are briefly discussed in Section 20.4.

20.1 Sf)FT-SWITCHINf; MECHANISMS OF SEMICONDUCTOR DEVICES

When loosely used, the terras zero-current switching and zero-voltage switching normally refer to one or more switching transitions of the transistor in a converter. However, to fully understand how a converter generates switching k)ss, one must citjsely examine the switching transitions of every semiconductor device. As described in Section 4.3, there are typically several mechanisms that arc sources of significant Switching loss. At the turn-off transition t)f a diode, its reverse-recovery process can induce loss in the transistor or other elements of the converter. The energy stored in the output capacitance of a MOSFET can be lost when the MOSFET turns on. IGBTs can lose significant energy during their turn-off transitit)n, owing to the current-tailing phenomenon. The effects of zero-current switching and zero-voltage switching on each of these devices is discussed in detail below.