chap05 - 5 The Discontinuous Conduction Mode When the ideal...

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The Discontinuous Conduction Mode When the ideal switches of a dc-dc converter are implemented using current-unidirectional and/or volt- age-unidirectional semiconductor switches, one or more new modes of operation known as discontinu- ous conduction modes (DCM) can occur. The discontinuous conduction mode arises when the switching ripple in an inductor current or capacitor voltage is large enough to cause the polarity of the applied switch current or voltage to reverse, such that the current- or voltage-unidirectional assumptions made in realizing the switch with semiconductor devices are violated. The DCM is commonly observed in dc–dc converters and rectifiers, and can also sometimes occur in inverters or in other converters containing two- quadrant switches. The discontinuous conduction mode typically occurs with large inductor current ripple in a con- verter operating at light load and containing current-unidirectional switches. Since it is usually required that converters operate with their loads removed, DCM is frequently encountered. Indeed, some convert- ers are purposely designed to operate in DCM for all loads. The properties of converters change radically in the discontinuous conduction mode. The con- version ratio M becomes load-dependent, and the output impedance is increased. Control of the output may be lost when the load is removed. We will see in a later chapter that the converter dynamics are also significantly altered. In this chapter, the origins of the discontinuous conduction mode are explained, and the mode boundary is derived. Techniques for solution of the converter waveforms and output voltage are also described. The principles of inductor volt-second balance and capacitor charge balance must always be true in steady state, regardless of the operating mode. However, application of the small ripple approxi- mation requires some care, since the inductor current ripple (or one of the inductor current or capacitor voltage ripples) is not small. Buck and boost converters are solved as examples. Characteristics of the basic buck, boost, and buck-boost converters are summarized in tabular form. 5
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108 The Discontinuous Conduction Mode 5.1 ORIGIN OF THE DISCONTINUOUS CONDUCTION MODE, AND MODE BOUNDARY Let us consider how the inductor and switch current waveforms change as the load power is reduced. Let’s use the buck converter (Fig. 5.1) as a simple example. The inductor current and diode current waveforms are sketched in Fig. 5.2 for the continuous conduction mode. As described in Chapter 2, the inductor current waveform contains a dc component I , plus switching ripple of peak amplitude During the second subinterval, the diode current is identical to the inductor current. The minimum diode current during the second subinterval is equal to since the diode is a single-quadrant switch, operation in the continuous conduction mode requires that this current remain positive. As shown in Chapter 2, the inductor current dc component I is equal to the load current: since no dc current flows through capacitor C. It can be seen that I depends on the load resistance R. The
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5.1
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This note was uploaded on 01/17/2010 for the course EL 5673 taught by Professor Dariuszczarkowski during the Spring '09 term at NYU Poly.

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chap05 - 5 The Discontinuous Conduction Mode When the ideal...

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