Ch10 Switch Power Supplies

5/24/2018 Ch10 Switch Power Supplies

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Bridging Theory in Practice

Transferring Technical Knowledge

to Practical Applications


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Chapter 10

Introduction to Switching Regulators

Objective of Chapter 10 is to answer the followingquestions:

1. What is a switching power supply?

2. What types of switchers are available?

3. Why is a switcher needed?

4. How does a switcher operate in general?

5. How does a buck converter operate?

6. How to calculate power loss?7. How to select external components?


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Introduction to Switching Regulators

Intended Audience: Electrical engineers with limited power supply

background

A simple, functional understanding of inductors and

capacitors is assumed

A simple, functional understanding of transistors is

assumed

Expected Time:

Approximately 60 minutes


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Outline1. Switching Regulator Overview

What is a Switching Regulator?

Why is a switcher needed?

What are the main differences between a switching and linear regulator?

Buck, Boost, Buck-Boost (Inverting)

2. Switching Regulator Operation

How does a Switching Regulator Operate?

3. Buck Converter Design Example

Qualatative explanation

Quantative explanation

Volt-Second princple (CCM)

Discountinous Conduction mode (DCM)

4. Practical Guidelines

How to select components (Transistor, Inductor, Diode, Capacitor)?

Integration vs. Mixed

What could go wrong? Troubleshooting suggestions.

How to calculate total power loss (Switching + Conduction)?

Stability Pointers

5. Control Mechanisms

Voltage Mode Control

Current Mode Control

Pulse Skipping Mode

Soft Start


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What is a Switching Regulator?

Switching Regulator

Converts an input voltage into desire output voltage.

The power transistor operates as a switch, completely on or

off.

An energy storage part (inductor) is used in the architecture

12 V 5 V

Control


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Choosing Between Linear and

Switching Regulators

When possible, most designers would prefer to use alinear voltage regulator rather than a switching

voltage regulator

Linear regulators are usually lower in price

Linear regulators are usually simpler to implement

Linear regulators do not have associated noise/ripple

problems apparent in switching regulators


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When to use a switching regulator #1:

When the minimum input voltage is at or below

the desired output voltage

Linear regulators cannot provide an output voltage greater

than the input voltage

VIN< VOUT


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The heatsinking of a linear regulator is prohibitive in

price or space

h d


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The efficiency of a linear regulator cannot maintain

the junction temperature below the specifiedmaximum

The maximum junction temperature is usually 150C

The efficiency of linear regulators often prohibit their

use in high voltage, high current applications


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Why are switching regulators needed?

The power dissipation is too high for a linear regulator

The efficiency of a linear regulator cannot maintain the junction

temperature below maximum (150 C)

The heat sinking of a linear regulator is prohibitive in price or space

OutputPower Switching Regulator

Linear Regulator

Linear Regulator

Maximum Power Dissipation


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Why are switching regulators needed?

The desired output voltage is greater than the input voltage

Linear regulators cannot provide an output voltage greater than the input

voltage

The desired output voltage is opposite polarity than the input voltage

Linear regulators cannot invert an input voltage

1.5 V

Battery Power Supply

5 V

Required

Power Supply12 VBattery

-12 VRequired


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Types of Switching Regulators AC-DC,

AC-AC, DC-AC, and DC-DC Converters

AC-DCDC-AC DC-DC

12 Vdc

t

110 Vac

t

12 Vdc

t

12 Vdc

t

5 Vdc

t

AC-AC

110 Vac

t

220

Vac

t

110 Vac

t


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Types of DC-DC Converters

Step Down, Step Up and Inverting

Step DownBuck

V

t

V

t

Vin = 12 V Vout = 5 V

Step UpBoost

V

t

Vin = 5 V

V

t

Vout = 12 V

InvertingBuck-Boost

V

t

Vin = 5V

V

tVout = -10 V


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Basic Circuit Configuration

VOUT

VIN

VM

VGATE

L

C

ISW

ILVOUT

VIN

VMCVGATE

LIL

ISW

VOUT

VIN

VMC

VGATE

LI

L

ISW

Buck-BoostVIN< -VOUT< VINBoostVIN< VOUT

BuckVIN> VOUT

All topologies consists of the same basic components but

are arranged differently


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Buck Configuration

The input voltage is always greaterthan the output

voltage

VOUT

VIN

VM

VGATE

LC

ISW

IL

VIN

time

20V

15V

10V

5V

0V

VOUT

time

7.5V

5V

2.5V

0V

10V


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Boost Configuration

The input voltage is always lessthan the output

voltage

VOUT

VIN

VMCVGATE

LIL

ISW

VIN

time

20V

10V

5V

VOUT

time

10V

0V 0V

20V

5V

15V 15V

24V


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Buck-Boost Configuration

The input voltage is always not constrainedby the

output voltage

VOUT

VIN

VM C

VGATE

LIL

ISW

VIN

time

20V

15V

10V

5V

0V

VOUT

time

-10V

-20V

0V

-15V

-5V

Oth S it hi V lt R l t


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Other Switching Voltage Regulator

Topologies

SEPIC

Push-Pull and Forward Converter

Flyback Converter


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VIN

Switching Regulator

Duty Cycle

Controller

Output

Monitor

VOUT

time

5V

Voltage

OK50%Filte

rNetwo

rk

VOUT

How a Switching Regulator Works


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VIN

Voltage Regulator

Duty Cycle

Controller

Output

Monitor

VOUT

time

5V

Voltage

OK50%Filte

rNetwo

rk

VOUT



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VIN

Voltage Regulator

Duty Cycle

Controller

Output

Monitor

VOUT

time

5V

Voltage

OK50%Filte

rNetwo

rk

VOUT



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VIN

1V

Voltage Regulator

Duty Cycle

Controller

Output

Monitor

VOUT

time

5V

Voltage

Low60%Filte

rNetwo

rk

VOUT



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VIN

1V

Voltage Regulator

Duty Cycle

Controller

Output

Monitor

VOUT

time

5V

Voltage

Low60%Filte

rNetwo

rk

VOUT



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VIN

Switching Regulator

Duty Cycle

Controller

Output

Monitor

VOUT

time

5V

Voltage

Ok50%Filte

rNetwo

rk

VOUT



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10.5 Switching Regulator Components

Switching Power Supply Block


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VIN VOUT

Switching Power Supply


Diagram

Switch

ErrorAmplifier

BandgapReference

PWMController

Network Network


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Bandgap Reference Voltage Need very small temperature coefficient

Balances negative temperature coefficient of pn

junction's VBE with positive temperature coefficient of

thermal voltage, Vt= kT/qV

INPUT

VBE

+

-

T

VBE

-2mV/C

Vt= kT / qT

kT/q

+0.085mV/C

A0

VREF= VBE+ A0Vt


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Internally generated with tight tolerance, traditionally ~ 1.2V VOUTis built from this voltage reference by zener zapping

TARGET

VREF

R1

R2

R3

VREF

1.24V

R3= + 3%R4= - 2%

R5= - 1%1.22V

1.20V

1.18V

1.16V

R3= +3%

R4= -2%

R5= -1%

VINPUT

R5R4

Bandgap Reference Voltage


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Error Amplifier

The error amplifier determines if VOUTis valid

VOUTis divided down and compared to the reference voltage

2

1 2

OUT DIV

RV V

R R

VOUT

VREF

R1

R2

PWMController


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PWM Controller In a switching voltage regulator, the pass transistor is used as

a switch - it is either on or off

The output voltage, however, is an analog value

PWM controller senses error in VOUTvia the error amplifier

PWM controller updates the duty cycle of the of transistor

adjusting the output voltage

Error

Amplifier

PWM

Controller

0-100%VOUT

Switching Transistor


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Switching Transistor

Bipolar and MOSFET

Bipolar MOSFET

Switch Speed Slow Fast

Drive Method Current Voltage

Drive Circuit Complex Simple

ESD Robustness High Low

Collector

Emitter

Base

Drain

Source

Gate



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VIN VOUT



Diagram

Switch

ErrorAmplifier

BandgapReference

PWMController

Network Network


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External Network An external network (consisting of an inductor, capacitor, and

diode) transforms the energy from the PWM controlled powerswitch into a desired output voltage

NetworkSwitch

VIN VOUT

VIN = 12 V

VOUT = 5 V



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VIN VOUT



Diagram

Switch

ErrorAmplifier

BandgapReference

PWMController

Network Network

Step Down Switching Regulator


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VOUT

VIN

VM

VGATE

+ VL-

COUT

ISW

IL

VGATEgoes high

VM~ VIN

VL= VMVOUT

t

VM

t

VGATE

t

IL

VOUT

t

ISW

t

RLOAD

-VF


Steady State Operation

-

VF+



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VOUT

VIN

VM

VGATE

COUT

ISW

IL

VL Constant

t

VM

t

VGATE

t

IL

VOUT

t

ISW

L LdI V

= Constantdt L

ILand ISWincrease

t

RLOADCOUTis charged by IL

and

VOUTincreases

-VF


Steady State Operation

-

VF+

+ VL-



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VOUT

VIN

VM

VGATE

COUT

ISW

IL

VGATE= 0VThe pass transistor

is turned off

ISW= 0A

t

VM

t

VGATE

t

IL

VOUT

t

ISW

t

RLOAD

L

V=

dt

dI LL

ILcannot go to

0A instantly:

VMgoes negative

VL= VMVOUT

L LdI V= < 0 A/sdt L

-VF

-

VF+

+ VL-

Step Down Switching RegulatorSteady State Operation



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VOUT

VIN

VGATE

COUT

ISW

IL

But, VMis clampedto -VF

and ILdecays

through the diode

t

VM

t

VGATE

t

IL

VOUT

t

ISW

t

RLOAD COUTstabilizes

the output voltage

so VOUTwill only

slowly decay

-VF


VM= -VF

-VF+

+ VL-



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VOUT

VIN

VGATE

COUT

ISW

IL

The MOSFET isturned on and off

to repeat

the sequence

RLOAD

t

VM

t

VGATE

t

IL

VOUT

t

ISW

t

-VF


VM= -VF

-VF+

+ VL-


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Volt-Second Principle

VOUT

VIN

VM

VGATE

COUT

ISW

RLOAD IL

t

VGATE

t

+ VL-

IL

ConstanLV

dtdi LL


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In steady state, the inductor currentripples about an average, IL,AVG:

Therefore, the total area (or volt-

seconds) under the inductor voltage

waveform is zero.

Voltage-Second Principle

1-D TT DT

L L L

0 0 DT

V (t)dt = V (t)dt + V (t)dt

VL

t

VIN- VOUT

-VOUT

TDT

(1-D)T

T

L IN OUT OUT

0

V (t)dt = (V - V )DT +(-V )(1-D)T = 0

+ VL -+ VL -

Voltage-Second Principle


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IN OUT OUT OUTV DT +(-V D+ V D- V )T = 0

OUT

IN

V= D

V

Voltage-Second Principleand the DC Transfer Function

From:

we can calculate the transfer function of the step down switching voltageregulator

T

L IN OUT OUT

0

V (t)dt = (V - V )DT +(-V )(1-D)T = 0

IN OUTV DT+(-V )T =0

VIN vs VOUT


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VOUT

VIN

L

COUT

ISW

IL

RLOAD

SIN

SGND

During steady state:

VL,AVG= 0V

+ VL -

VL

time

VIN

- VOUT

-VOUT

TDT

(1-D)T

IN OUT OUTV - V D = V (1-D)

OUT IN

V = DV

VINvs. VOUTand Duty Cycle, D

VOUT Increases wit D


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VOUT

VIN

VM

VGATE

COUT

ISW

IL

VOUT

RLOAD

t

VL

t

VGATE

VIN- VOUT

t

+ VL - -VOUT

SIN

SGND

VOUTIncreases wit DVOUT= DVIN

VOUT Decreases wit D


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VOUT

t

VL

t

VGATE

VIN- VOUT

t

-VOUT

VOUT

VIN

VM

VGATE

COUT

IL

RLOAD

+ VL -

SIN

SGND

ISW

VOUTDecreases wit DVOUT= DVIN


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In practice, voltage drop

across the top switch (VSIN)

and the bottom switch

(VSGND):

VOUT

VIN

L

COUT

IL

RLOAD

SIN

SGND

+ VL -

+

VSIN-

-

VSGND+

ISW

Duty Cycle and Switch Loss


47/83

VL

time

VIN- VSIN- VOUT

-VSGND- VOUT

TDT

(1-D)T

OUT SGND

IN SIN SGND

V + VD =

V - V + V

IN SIN OUT SGND OUTV - V - V D = V + V (1-D)

Duty Cycle and Switch Loss


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Recall, ILis the sum of thecurrent flowing through SIN

and SGND

VOUT

VIN

COUT

IL

RLOAD

SIN

SGND

IGND

time

ISW IGNDIL

IL,AVG

ISW

Ripple Current


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Inductor Ripple Current

IL(and IOUT) has both DC and AC components

The maximum value of the deviation from the DC value

is given by (I/2)+IL,AVG

IL

time

IL,AVG IOUT

(I/2)+IL,AVG


50/83

Selection of I

The amount of allowed inductor ripple current, I, isone of the key decisions made in designing a power

supply

It is an important factor in correctly sizing the other

components in the power supply

Typical values of I are 30% to 50% of IL,AVG

Small values of I might be desired, but can result in

more complex and expensive power supplies


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Selection of Inductance

The minimum inductance of a step-down switching voltage

regulator is given by

If I (ripple current) has already been selected and VOUTis a

constant:

L will vary based upon the regulator's switching frequency

(fSW) and the input voltage range

IVf

VVVL

INSW

OUTOUTIN

MIN

In uctor Se ection Examp e


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40

50

60

7080

90

100110

9 12 15 18 21 24 27

Input Voltage (V)

Inductance

(H) Unsafe Inductance(I Violation)

In uctor Se ection Examp eVOUT=5V, I =0.4A, f=100kHz

IVf

VVVL

INSW

OUTOUTIN

MIN

Inductor Selection


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Inductor SelectionSaturation Current

Inductors store energy in their magnetic field proportional to

their inductance:

The inductance of a toroid is determined by several factors:

= Permeability of inductor core

N = Total number of turns in the wire coil

A = Area of a single loop in the coil

l = Length of the coil wrapped around the toroid

2

L IL

2

1E

l

ANL 2

Inductor Selection


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Inductor SelectionSaturation Current

An inductor, however, can only store a finite amount of energy in its

magnetic field

Above an inductor's saturation current (Isat), the inductor's permeability

decreases significantly, reducing its inductance

This results in an increase in the regulator's ripple current:

Therefore, an inductor's Isatmust be greater than the switching regulator'speak inductor current:

2

III

II

L(AVG)SAT

L(PK)SAT

LVf

VVVI

INSW

OUTOUTIN


55/83

Inductor Technology There are a number of inductor technologies to

choose from Drum core

Flat coil

Toroid

Bead

Wirewound

Planar

In addition to inductance and saturation current, the

inductor technology will also affect: Inductor resistance and impedance

Size (length, width, height)

Cost


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The selection of the output capacitor affects the voltageripple

The output voltage ripple is a function of the output

capacitor's value and its equivalent series resistance (ESR)

Low ESR capacitors (ceramic and tantalum) are

recommended to minimize the output voltage ripple

RIPPLE OUT

switching OUT

1V = I ESR +

8f C

Output Capacitor Selection

Output Capacitor Example


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0

1

2

3

4

0.1 1 10

ESR ()

VRIPPLE

(V)

RIPPLE OUT

switching OUT

1V = I ESR +

8f C

p p p

COUT=1F, I=.4A, f=100kHz

Output Capacitor


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Output CapacitorCurrent Rating

The output capacitor must be able to handle a worst

case ripple current equal to IOUT

Capacitors are rated for different maximum ripple

currents as a function of their ESR, package (thermalresistance), and ambient temperature

Example Capacitor Parameters

Ripple Current (A rms)

Cap (F) Part # 25C 85C 125C100 1 6.0 5.4 2.4

220 2 8.0 7.2 3.2

680 3 10.6 9.6 4.2

Output Capacitor


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Output CapacitorBreakdown Voltage

Capacitors have a specifiedbreakdown voltage

Below the breakdown voltage,

the dielectric material between

the electrodes is an insulator -the device has capacitance

Above the breakdown voltage,

the dielectric material conducts

resulting in a (catastrophic)

capacitor failure

Dielectric


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Output Capacitor Technology

There are dozens of capacitor technologies to choosefrom...

Typically, output capacitors are tantalum or ceramic

In addition to capacitance and maximum current, the

capacitor technology will also affect:

Equivalent series resistance (ESR) and inductance (ESL)

Size (length, width, height)

Cost

Changes in performance vs. variations in temperature

Output Capacitor ESR vs.


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1

10

100

1000

-50 -10 30 70 110 150

Tantalum

Ceramic

Output Capacitor ESR vs.Technology and Temperature

Temperature (C)

ESR

(m)


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Recirculation Diode

VOUT

VIN

VOUT

VIN

SIN

SGND

VM VM


63/83

Recirculation Diode

A Schottky diodes reverse recovery time

(switching from a forward bias state to a

reverse bias state) is very fast

This fast reverse recovery time minimizes the

power loss in the recirculation path

The recirculation diode must have a stand-offvoltage (break-down voltage) higher than the

maximum positive voltage seen at VM


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Recirculation Diode

The voltage drop across the recirculationdiode affects the switching regulator's duty

cycle and efficiency

OUT SGND

IN SIN SGND

V + VD =V - V + V

OUT

OUT DISSIPATED

P=P +P


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Recirculation Diode Replacement

Therefore, in many efficiency designs, therecirculation diode is replaced with a low RDSON

MOSFET

The recirculation MOSFET must be turned on/off180oout of phase with the pass transistor

The voltage drop across the recirculation MOSFETshould be smaller than an available Schottky diode (

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Ch10 Switch Power Supplies