JOHN D. LENK
`SIM .214FIED
`DESIGN
`OF SWITCHING
`P• R
`SOU e PLIES
`YcHt4rt:i1.2,-,. LINEAR TECHNOLOGY
`•
`
`(408)-432-1900
`
`VOUT
`
`VIN
`
`01 DI
`
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`
`943)
`SAL:,
`•T)48
`
`RT2101
`(RT2201)
`
`SHDN
`
`II0
`
`GND
`
`R2
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`C4 NW..
`RI NW -
`c311101p-c6
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`0.60 IN 2
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`GND •
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`LTC1147-3.3
`HIGH EFFICIENCY SWITCHING REGULATOR
`DEMONSTRATION CIRCUIT 042A
`
`REKVA SERIES FOR DESIGN ENGINEERS
`
`Petitioners
`Ex. 1058, p. Cover
`
`

`

`Simplified Design of
`Switching Power
`Supplies
`
`John D. Lenk
`
`Butterworth-Heinemann
`Boston Oxford Melbourne Singapore Toronto Munich New Delhi Tokyo
`
`Petitioners
`Ex. 1058, p. 1
`
`

`

`Cover photograph courtesy of Linear Technology Coipocation.
`
`Copyright O 1995 by Butterworth-Heinemann
`
`-a- A member of the Reed Elsevier group
`
`All rights reserved.
`
`No part of this publication may be Icy oduced, stored in a retrieval system, or transmitted, in any
`form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the
`prior written permission of the publisher.
`
`Recognizing the importance of preserving what has been written, it is the policy of Butterworth-
`Heinemann to have the books it publishes printed on acid-free paper, and we exert our best
`efforts to that end.
`
`Library of Congress Cataloging-in-Publication Data
`
`Lenk, John D.
`Simplified design of switching power supplies / by John D. Lenk.
`p.
`Includes bibliographical references and index.
`ISBN 0-7506-9507-2 (hardcover)
`ISBN 0-7506-9821-7 (paperback)
`1. Electronic apparatus and appliances—Power supply—Design
`and construction. 2. Switching power supplies—Design and
`I. Title.
`construction.
`TK7868.P61.A56 1995
`621.381'044—dc20
`
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`A catalogue record for this book is available from the British Library.
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`Transferred to digital printing 2005
`
`Petitioners
`Ex. 1058, p. 2
`
`

`

`Switching Power-Supply Basics 3
`
`1.1.4 Transistor and Diode Characteristics
`Switching regulators must use transistors with a gain-bandwidth product (IT)
`of at least 4 MHz to operate efficiently (an fT of 30 MHz is even better). Darlington
`transistors and MOSFETs are also used in switching regulators.
`A fast-recovery rectifier, or a Schottky barrier diode, is used as a free-wheel-
`ing clamp diode to keep the switching-transistor load line within safe operating lim-
`its and to increase efficiency. Other solid-state devices used in some switching
`regulators include gates, flip-flops (FFs), op-amp comparators, timers, and recti-
`fiers.
`
`1.2 Typical Switching-Regulator Circuits
`Figure 1-2 shows four typical PNP/NPN switching-regulator circuits. All of
`the circuits have the following common elements: switching transistor, clamp diode,
`LC filter, and a logic or control block. None of the circuits provide full isolation be-
`tween the line and load, as would be the case if more than one series transistor is
`used. However, the one-transistor design is the simplest and most economical.
`It is usually desirable to have at least one line in common with the input and
`output to reduce ground loops. The one-line approach also determines whether the
`output voltage is considered positive or negative. However, most circuits can oper-
`ate from either supply because the input and output grounds are usually isolated.
`The one-transistor, one-line approach is the most popular switching-regulator de-
`sign.
`
`In the circuits of Figs. 1-2(a) and 1-2(b), the logic or control operates, from
`the load voltage. Such circuits are not self-starting, and provisions must be made to
`operate from the line during start-up (and in the event of short circuits).
`In the circuits of Figs. 1-2(c) and 1-2(d), the logic operates continuously from
`the line and is isolated from the load voltage. The sense and feedback elements must
`be electrically isolated (sometimes with an optocoupler).
`The circuits of Figs. 1-2(b) and 1-2(d) are generally used in line-operated
`supplies because economical high-voltage NPN transistors are available whereas
`PNP types are not. Of the two, the circuit of Fig. 1-2(d) is most popular, because the
`logic is tied directly to the series switch and switching is more efficient. Driver
`transformers are used in some designs to interface between the logic and switching
`transistors. In such a case, the switching transistor may be either PNP or NPN.
`Figure 1-3 shows three typical MOSFET switching-regulator (or -converter)
`circuits, representing the three basic configurations: buck, boost, and buck-boost (all
`of which are described more fully in Section 1.5). In brief, each of the three config-
`urations meets a particular need. When output voltage is greater than the input, the
`converter is usually operated in the positive voltage-boost circuit (also known as a
`step-up converter). The buck circuit is used when the input voltage is always greater
`than the desired output voltage (and is also known as a step-down converter). The
`
`Petitioners
`Ex. 1058, p. 3
`
`

`

`4 SIMPLIFIED DESIGN OF SWITCHING POWER SUPPLIES
`
`Logic
`
`(a) Positive output, common logic and load
`
`Logic
`
`(b) Negative output, common logic and load
`
`Logic
`
`Remote
`sense
`
`(c) Positive output, isolated logic and load
`
`Remote
`sense
`—
`
`Logic
`
`V0
`
`Ground
`
`Ground
`
`Vo
`
`0
`
`Ground
`
`Ground
`
`Vo
`
`Id) Negative output, i olated logic and load
`
`Figure 1-2. Four typical switching-regulator circuits
`
`Petitioners
`Ex. 1058, p. 4
`
`

`

`Switching Power-Supply Basics 5
`
`Vb.,,
`
`Control
`section
`
`Va > Veei
`
`Boost converter
`
`Control
`section
`
`Buck converter
`
`Control
`section
`
`< Vb.,,
`
`either
`
`> Vwa
`or
`< Van
`
`Figure 1-3. Three typical MOSFET switching-regulator circuits (Maxim Seminar Appli-
`cations Book, 1989, p. 115)
`
`Buck-boost converter
`
`buck-boost circuit inverts the input voltage and can be used with an input voltage
`that is either greater or less than the desired output. For this reason, the buck-boost
`circuit is sometimes called an inverter.
`
`1.3 Switching-Regulator Theory
`Figure 1-4 shows a theoretical switching-regulator circuit (buck configura-
`tion) and the related waveforms. The high efficiency of switching regulators is the
`result of operating the series transistor in a switching mode. When the transistor is
`switched on, the full input voltage is applied to the LC filter. When the transistor is
`switched off, the input voltage is zero. With the transistor turned on and off for
`equal amounts of time (50% duty cycle) the DC load voltage is half the input volt-
`age. The output voltage Vo is always equal to the input voltage VIN, times the duty
`cycle D, or Vo = DVIN.
`
`Petitioners
`Ex. 1058, p. 5
`
`

`

`12 SIMPLIFIED DESIGN OF SWITCHING POWER SUPPLIES
`
`+Vs
`
`C Vow
`
`Figure 1-9. Theoretical buck-boost or inverting configuration (Raytheon Linear Inte-
`grated Circuits, 1989, p. 9-54)
`
`The following paragraphs describe how both theoretical and practical func-
`tions are performed and how the practical functions (Fig. 1-10) relate to the theoret-
`ical (Fig. 1-9). Notice that switch S in the theoretical circuit is replaced by transistor
`Q1 in the practical IC circuit. Capacitor C, diode D, and inductor L in Fig. 1-9 are
`replaced by CF, Di, and Lx in Fig. 1-10.
`As shown in Fig. 1-9, when switch S is closed, charging current from the bat-
`tery flows through inductor L, which builds up a magnetic field that increases as S is
`held closed. When S is opened, the magnetic field collapses, and energy stored in
`the magnetic field is converted into a current that flows through L in the same direc-
`tion as the charging current. Because there is no path for this current to flow through
`the switch, the current must flow through diode D to charge capacitor C. The key to
`inversion is the ability of the inductor to become a source when the charging current
`is removed.
`In the practical circuit of Fig. 1-10, the feedback circuit and the output capac-
`itor decrease the output voltage across the inductor to a regulated fixed value. When
`power is first applied, the ground-sensing comparator (pin 8) compares the output
`voltage to the +1.25-V reference. Because CF is initially discharged, a positive volt-
`age is applied to the comparator, and the output of this comparator gates the square-
`wave oscillator. The gated square-wave signal turns the switch transistor Q1 on and
`off.
`
`The turning on and off of the switch transistor Q1 performs the same function
`as opening and closing of the switch (S in Fig. 1-9). That is, energy is stored in the
`inductor during the on-time and is released into output capacitor CF during the off-
`time. The comparator continues to gate the oscillator square wave to Q1 until
`enough energy is stored in CF to make the comparator input voltage decrease to less
`than 0 V. The voltage applied to the comparator is set by the output voltage, the ref-
`erence voltage, and the ratio of R1 to R2.
`
`1.5.4 Buck or Step-Down
`Figure 1-12 shows the theoretical buck or step-down configuration. Figure
`1-13 shows a typical IC switching regulator (the Raytheon RC4391) connected in a
`practical step-down converter configuration. Figure 1-14 shows the corresponding
`waveforms.
`
`Petitioners
`Ex. 1058, p. 12
`
`

`

`14 SIMPLIFIED DESIGN OF SWITCHING POWER SUPPLIES
`
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`
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`
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`yr
`
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`
`Figure 1-11. Inverter waveforms (Raytheon Linear Integrated Circuits, 1989, p. 9-55)
`
`+Vs
`
`Figure 1-12. Theoretical buck or step-down configuration (Raytheon Linear Integrated
`Circuits, 1989, p. 9-55)
`
`The following paragraphs describe how both theoretical and practical func-
`tions are performed and how the practical functions (Fig. 1-13) relate to the theoret-
`ical (Fig. 1-12). Notice that switch S in the theoretical circuit is replaced by a
`transistor (within the IC) in the practical IC circuit. Capacitor C, diode D, and induc-
`tor L in Fig. 1-12 are replaced by CF, DI, and Lx in Fig. 1-13. Also notice that the
`ground lead of the IC (pin 4) is not connected to circuit ground. Instead, pin 4 is tied
`to the output voltage. Using this rearrangement of the feedback system, it is possible
`
`Petitioners
`Ex. 1058, p. 14
`
`

`

`Switching Power-Supply Basics 15
`
`to regulate a nonnegative output voltage (as the feedback system senses voltages
`more negative than the ground lead).
`As shown in Fig. 1-12, when switch S is closed, current flows from the bat-
`tery, through the inductor L, and through the load resistance to ground. After S is
`opened, stored energy in L causes current to keep flowing through the load. The cir-
`cuit is completed by catch diode D. Because current flows to the load during charge
`and discharge, the average load current is greater than in an inverting circuit. The
`significance is that for equal load currents, the step-down circuit requires less peak
`inductor current than an inverting circuit. As a result, the inductor of a step-down
`circuit can be smaller than for inverting, and the switch transistor in a step-down IC
`is not stressed as heavily for equal load currents.
`In the practical circuit of Fig. 1-13, output filter capacitor CF is discharged, so
`that the ground lead (pin 4) potential starts at 0 V. The reference voltage is forced to
`+1.25 V above the ground lead, pulling the feedback input (pin 8) more positive
`than the ground lead. This positive voltage forces the control network to start puls-
`ing the output transistor.
`As the switching action pumps up the output voltage, the ground lead rises
`with the output until the voltage on the ground lead is equal to the feedback voltage.
`At that point, the control network reduces the on-time of the switch to maintain a
`constant output.
`
`R1
`
`R2
`
`-125V
`Ref/Bias 11
`
`— 4:1 Vs
`
`VREF
`
`OSC
`1131.
`
`2
`
`3
`
`Cx
`
`•Vout
`
`RC4391
`0
`
`Lx
`
`*BOUT'
`
`25! (
`
`01 1N914
`
`Important Note. This circuit must have a minimum load ≥ 1 mA always connected
`
`Figure 1-13. Practical step-down converter configuration (Raytheon Linear Integrated
`Circuits, 1989, p. 9-56)
`
`Petitioners
`Ex. 1058, p. 15
`
`