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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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Choosing Between Linear and
Switching Regulators
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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Choosing Between Linear and
Switching Regulators
When to use a switching regulator #2:
The heatsinking of a linear regulator is prohibitive in
price or space
h d
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Choosing Between Linear and
Switching Regulators
When to use a switching regulator #3:
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
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
How a Switching Regulator Works
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VIN
1V
Voltage Regulator
Duty Cycle
Controller
Output
Monitor
VOUT
time
5V
Voltage
Low60%Filte
rNetwo
rk
VOUT
How a Switching Regulator Works
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VIN
1V
Voltage Regulator
Duty Cycle
Controller
Output
Monitor
VOUT
time
5V
Voltage
Low60%Filte
rNetwo
rk
VOUT
How a Switching Regulator Works
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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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10.5 Switching Regulator Components
Switching Power Supply Block
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VIN VOUT
Switching Power Supply
Switching Power Supply Block
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
Switching Power Supply Block
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VIN VOUT
Switching Power Supply
Switching Power Supply Block
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
Switching Power Supply Block
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VIN VOUT
Switching Power Supply
Switching Power Supply Block
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
Step Down Switching Regulator
Steady State Operation
-
VF+
Step Down Switching Regulator
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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
Step Down Switching Regulator
Steady State Operation
-
VF+
+ VL-
Step Down Switching Regulator
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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
Step Down Switching Regulator
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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
Step Down Switching RegulatorSteady State Operation
VM= -VF
-VF+
+ VL-
Step Down Switching Regulator
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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
Step Down Switching RegulatorSteady State Operation
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
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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
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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
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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
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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 (