Var Nostrand Reinheld Electrical/Competer Science and Engmeernng Series
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`bee
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`POWER
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`CIRCUITS
`Rudolf P Severns
`Gordon E. Bloom
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`ANKER 1015
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`1
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`MODERN
`DC-TO-DC
`SWITCHMODE
`POWER
`CONVERTER
`CIRCUITS
`
`Rudolf P. Severns
`Senior Staff Engineer
`Applications Engineering
`SILICONIX, INC,
`
`Gordon (Ed) Bloom
`Senior Partner and Engineering Specialice
`¢/] BLOOM Associates
`
`Van Nostrand Reinhotd Electrical\Computor Science and Engineering Series
`
`[aN VAN NOSTRAND REINHOLD COMPANY
`
`
`
`NewYork
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`2
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`Copyright © L945 Sy Rudolf P. Severs and Gordon Bloom
`
`Litrary of Congress Catalog Card Nowiber: 84-1 2026
`ISBN: 0442.213%-4
`
`hereon may
`AD rights steered. No part of this work covered by ae
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`Litrazy of Congress Cataloging in Publication Dare
`
`Sewers, Redodl P,
`Modern DC-to-DC swhehened: power converter chcuks.
`
`(Ven Nostrand Reinkold electricalcomputer science and
`engineering series)
`
`eeQoaesupply
`Microelectronics—Powe: supply.
`|. Bloom, Gordon.
`IL Tithe, TE, Title: Modera [X°-10-DC switchrmode
`Power comweme: circukte
`IV, Series.
`T7868PESS? 06
`621.3415 32
`§4-13006
`
`ISBN 0-442-21296-4
`
`3
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`

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`COMBINATIONS OF CONVERTERS 158
`
`iL im Fig. 7.44 a5 an essential SPC fearwre and, by processes of rotation and
`or-imaging, used ta logically evolve buck or boost SPC topologies.
`As we have seen, it is posible by a simple manipulation to derive the buck~
`t converter topoloay from the cascade conmection of the two elerpentary
`. The derivation of the bucktous! SPC small-signal model given in
`ex 10 also supports this view, For these reasons, we have chosen here to
`the buck-boost converter as a spécial combination of buck and boost
`Cs, rather than an elementary comverter untoitself. Since this assumption lias
`n to be self-consistent, we have retained it throughout the discussionsof this
`4, but certainty the point of origin of this comverter topology is an arguable
`
`‘When multiple cutpets and/or DC isolation from input to output are desired
`a buck-boost converter, it is mot necessary to incorporate a DC transformer,
`jwe will see. Consider the buck-ovst circuit of Fig. 7 5A, where the inductor
`now has (wo identical windings & parallel. This winding addition does not
`the bacle converter operation. Next, the links commecting the ran indurctne
`dings are removed, a4 shown in Fig. 7.5A. Again, circuit operation remnains
`same except that DC isolation between the inpet and the owtput has been
`j. The final step in this process is shown in Fig. 7.5C, where additional
`sondary windings have been introduced im the inductor assembly with arbiteary
`ms
`ratios relative to the inductor “primary.” Thistransfoemertsolated multiple-
`form of the buck-Doost SPC ts called the ftyback converterand is probably
`put
`t most popular circuit im wee today for ourput power levels below 50 W.
`primary advarsage is simplicity, particularly in that only one magnetic com-
`nent
`is meeded, even when input-to-output DC isolation asd/or multiple out-
`are required from the converter. The primary difficulty associated with
`SPC is the design ofinductor L, which now essumes the roles of both an
`Ky-torage device and 2 transformer element, Its design becomes even more
`ait when multiple oulpets with good cross regulation are required, Success-
`flyback SPC design examples with as many a8 25 isolated ourputs do exist,
`their magnetics designs were very complicated.
`Besides the palating input and output current sharacteristic and the biah
`itch currents mentioned earlier, the comtrol-to-output small-sigral transfer
`tion of the buck-boost has a right half-plane cero, as we will see in Chapcer
`). The presence ofthis trowb)esome zero should comeas no surprase since one
`the elementary comstituents of the buck-boost comverter has # sitilar comtrol-
`owtput transfer function.
`When @ buck SPC is preceded by a boost SPC, as shown in Fig. 746A, quite
`different, and in many ways a more useful comverter can be derived in much
`same manner as was followed to evolve the buck-boost and flyback SPCs. In
`7.6B, the two switch-and-diode sets are replaced by 2 single DPDT switch.
`this case, Cl cannot be omitted aimee It Is required for energy storage. As was
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`capacitor. A practical realization ofthis new circuit Is shown in Fig. 7.6D.
`
` .
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`Pig. 7.5. Derivation of the flyback converter from the buck-Doost comwerter.
`
`the case for the buck-boost,if & polarity inversion ofoutput voltage Is
`the circuit can be further simplified. As indicated im Fig, 7,.6C, the DPDT
`an shunt capacitor network can be replaced by 2 SPDT switch and a
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