lecture # 12&13 switching-mode power supplies 1. switching-mode power supplies 2
TRANSCRIPT
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Lecture # 12&13
SWITCHING-MODE POWER SUPPLIES
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Switching-Mode Power Supplies
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Introduction to Switching RegulatorsObjective of topic is to answer the following questions:
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 does a boost converter operate?7. How does a buck-boost converter operate?8. How many design topologies of a switching power supplies?
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Introduction to Switching RegulatorsIntended 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 120 minutes
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Outline of the presentation1. 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)
Five designing topologies.. Flyback converter. Forward converter. Push-pull converter. Half Bridge converter. Full-Bridge converter.
3. Switching Regulator Operation . How does a Switching Regulator works? Switching Regulators components.
2. Switching mode DC power supplies
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What is Switching Power Supply?
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What is Switching Power Supply?
The advantage of a switching-mode power supply is that the relatively high frequency oscillator allows the use of small, lightweight and low-cost transformers.
This makes them considerably smaller and lighter than linear power supplies. Almost all modern powers supplies, including those in PCs, are switching mode power supplies. Their disadvantages are complexity and RF egress (interference).
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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 VControl
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Choosing Between Linear and Switching Regulators• When possible, most designers would prefer to use a
linear 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 :
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:
• The efficiency of a linear regulator cannot maintain the junction temperature below the specified maximum
• 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 RegulatorMaximum 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 VBattery 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-DC DC-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 Down Buck
V
t
V
t
Vin = 12 VVout = 5 V
Step Up Boost
V
t
Vin = 5 VV
t
Vout = 12 V
InvertingBuck-Boost
V
t
Vin = 5V
V
tVout = -10 V15
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Basic Circuit Configuration
VOUT
VIN
VM
VGATE
LC
ISW
ILVOUT
VIN
VM
CVGATE
LIL
ISW
VOUT
VIN
VM C
VGATE
LIL
ISW
Buck-Boost VIN < -VOUT < VIN
Boost VIN < VOUT
BuckVIN > VOUT
• All topologies consists of the same basic components but are arranged differently 16
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Buck Configuration
• The input voltage is always greater than 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 less than the output voltage
VOUT
VIN
VM
CVGATE
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 constrained by the output voltage
VOUT
VIN
VM C
VGATE
LIL
ISW
VIN
time
20V
15V
10V
5V
0V
VOUT
time
-10V
-20V
0V
-15V
-5V
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Switched-Mode DC Power Supplies
• Five Designing Topologies.– Flyback converter.– Forward converter.– Push-pull converter.– Half Bridge converter.– Full-Bridge converter.
• Operate at high frequencies– Easy to filter out harmonics
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Flyback Converter
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Mode 1 Operation -- Q1 ON• Current builds up in the primary winding• Secondary winding has the opposite polarity D1 OFF• C maintains the output voltage, supplies load current
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Mode 2 Operation -- Q1 turned OFF• Polarity of the windings reverses• Diode D1 conducts, charging C and providing current to
the load RL
• Secondary current falls to 0 before the next cycle begins
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Waveform Summary
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Double-ended Flyback Converter
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Forward Converter
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Features• Includes a “reset” winding to return energy.• Secondary “dot” so that D2 forward biased when Q1 is
ON – no energy stored in the primary.• Operates in continuous mode.
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Mode 1 Operation -- Q1 ON• Current builds up in the primary winding• Energy transferred to the load
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Mode 2 Operation -- Q1 turned OFF• Polarity of transformer voltages reverses• D2 turns OFF, D1 and D3 turn ON
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Waveform Summary
Vo
D3
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Double-ended Forward Converter
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Push-Pull Converter
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Push-Pull Operation• Q1 ON, Vs across the lower primary winding
• Q2 ON, Vs across the upper primary winding
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Half-Bridge Converter
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Mode 1 Operation • Q1 ON, D1 conducting
• Energy transferred to the load
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Mode 2 Operation• Both transistors are OFF• D1 continues to conduct due to current in L1
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Mode 3 Operation• Q2 ON, D2 conducting
• Energy transferred to the load
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Mode 4 Operation• Both transistors OFF• D2 continues to conduct due to current in L1
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Full-Bridge Converter
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Mode 1 Operation• Q1,Q4 ON, Q2,Q3 OFF
• D1 conducting, energy transferred to the load
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Mode 2 Operation• All transistors are OFF• D1 continues to conduct due to current in L1
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Mode 3 Operation• Q2,Q3 are ON, Q1,Q4 OFF
• D2 conducting, energy transferred to the load
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Mode 4 Operation• All transistors are OFF• D2 continues to conduct due to current in L1
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VIN
Switching Regulator
Duty Cycle Controller
OutputMonitor
VOUT
time
5V
VoltageOK50%
Filt
er N
etw
ork
VOUT
How a Switching Regulator Works
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VIN
Voltage Regulator
Duty Cycle Controller
OutputMonitor
VOUT
time
5V
VoltageOK50%
Filt
er N
etw
ork
VOUT
How a Switching Regulator Works
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VIN
Voltage Regulator
Duty Cycle Controller
OutputMonitor
VOUT
time
5V
VoltageOK50%
Filt
er N
etw
ork
VOUT
How a Switching Regulator Works
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VIN – 1V
Voltage Regulator
Duty Cycle Controller
OutputMonitor
VOUT
time
5V
VoltageLow60%
Filt
er N
etw
ork
VOUT
How a Switching Regulator Works
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VIN – 1V
Voltage Regulator
Duty Cycle Controller
OutputMonitor
VOUT
time
5V
VoltageLow60%
Filt
er N
etw
ork
VOUT
How a Switching Regulator Works
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VIN
Switching Regulator
Duty Cycle Controller
Output Monitor
VOUT
time
5V
VoltageOk50%
Filt
er N
etw
ork
VOUT
How a Switching Regulator Works
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Switching Regulator Components
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VIN VOUT
Switching Power Supply
Switching Power Supply Block Diagram
Switch
Error Amplifier
Bandgap Reference
PWMController
Network Network
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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 VOUT via the error amplifier
• PWM controller updates the duty cycle of the of transistor adjusting the output voltage
ErrorAmplifier
PWMController
0-100%VOUT
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Switching TransistorBipolar and MOSFET
Bipolar MOSFET
Switch Speed Slow Fast
Drive Method Current Voltage
Drive Circuit Complex Simple
ESD Robustness High Low
Collector
Emitter
BaseDrain
Source
Gate
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VIN VOUT
Switching Power Supply
Switching Power Supply Block Diagram
Switch
Error Amplifier
Bandgap Reference
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 power switch into a desired output voltage
NetworkSwitchVIN VOUT
VIN = 12 VVOUT = 5 V
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VIN VOUT
Switching Power Supply
Switching Power Supply Block Diagram
Switch
Error Amplifier
Bandgap Reference
PWMController
Network Network
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VOUT
VIN
VM
VGATE
+ VL -
COUT
ISW
IL
VGATE goes high
VM ~ VIN
VL = VM – VOUT
t
VM
t
VGATE
t
IL
VOUT
t
ISW
t
RLOAD
-VF
Step Down Switching RegulatorSteady 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= Constant
dt L
IL and ISW increase
t
RLOAD
COUT is charged by IL
and
VOUT increases
-VF
Step Down Switching RegulatorSteady State Operation
-VF
+
+ VL -
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VOUT
VIN
VM
VGATE
COUT
ISW
IL
VGATE = 0V
The pass transistor
is turned off
ISW = 0At
VM
t
VGATE
t
IL
VOUT
t
ISW
t
RLOAD
LV
=dtdI LL
IL cannot go to 0A instantly:
VM goes negativeVL = VM – VOUT
L LdI V= < 0 A/s
dt L
-VF
-VF
+
+ VL -
Step Down Switching RegulatorSteady State Operation
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VOUT
VIN
VGATE
COUT
ISW
IL
But, VM is clamped
to -VF
and IL decays
through the diode t
VM
t
VGATE
t
IL
VOUT
t
ISW
t
RLOADCOUT stabilizes
the output voltageso VOUT will only
slowly decay
-VF
Step Down Switching RegulatorSteady State Operation
VM = -VF
-VF
+
+ VL -
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VOUT
VIN
VGATE
COUT
ISW
IL
The MOSFET is
turned 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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For student study :
Read Chapter 21 discrete & integrated voltage regulators topics (21.5) from the book introductory electronic devices and circuits by (Robert T .paynter)conventional flow version.
Wikipedia & world wide web.
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REFERENCES
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THANK YOU!
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