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Chapter 10
Switching DC Power Supplies
• One of the most important applications of power
electronics
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-1
Linear Power Supplies
• Very poor efficiency and large weight and size
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-2
Switching DC Power Supply: Block Diagram
• High efficiency and small weight and size
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-3
Switching DC Power Supply: Multiple
Outputs
• In most applications, several dc voltages are
required, possibly electrically isolated from each other
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-4
Transformer Analysis
• Needed to discuss high-frequency isolated supplies
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-5
PWM to Regulate Output
• Basic principle is the same as discussed in Chapter 8
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-6
Flyback Converter
• Derived from buck-boost; very power at small power
(> 50 W ) power levels
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-7
Flyback Converter
• Switch on and off states (assuming incomplete core
demagnetization)
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-8
Flyback Converter
• Switching waveforms (assuming incomplete core
demagnetization)
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-9
Other Flyback Converter Topologies
• Not commonly used
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-10
Forward Converter
• Derived from Buck; idealized to assume that the
transformer is ideal (not possible in practice)
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-11
Forward Converter:
in Practice
• Switching waveforms (assuming
incomplete core demagnetization)
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-12
Forward Converter: Other Possible Topologies
• Two-switch Forward converter is very commonly used
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-13
Push-Pull Inverter
• Leakage inductances become a problem
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-14
Half-Bridge Converter
• Derived from Buck
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-15
Full-Bridge Converter
• Used at higher power levels (> 0.5 kW )
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-16
Current-Source Converter
• More rugged (no shoot-through) but both switches
must not be open simultaneously
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-17
Ferrite Core Material
• Several materials to choose from based on applications
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-18
Core Utilization in Various Converter
Topologies
• At high switching frequencies, core losses limit
excursion of flux density
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-19
Control to Regulate Voltage Output
• Linearized representation of the feedback control
system
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-20
Forward Converter: An Example
• The switch and the diode are assumed to be ideal
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-21
Forward Converter:Transfer Function Plots
• Example
considered earlier
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by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-22
Flyback Converter:Transfer Function Plots
• An example
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-23
Linearizing the PWM Block
• The transfer function is essentially a constant with
zero phase shift
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-24
Gain of the PWM IC
• It is slope of the characteristic
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-25
Typical Gain and Phase Plots of the OpenLoop Transfer Function
• Definitions of the crossover frequency, phase and
gain margins
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-26
A General Amplifier for Error Compensation
• Can be implemented using a single op-amp
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-27
Type-2 Error Amplifier
• Shows phase boost at the crossover frequency
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by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-28
Voltage Feed-Forward
• Makes converter immune from input voltage
variations
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by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-29
Voltage versus Current Mode Control
• Regulating the output voltage is the objective in both
modes of control
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-30
Various Types of Current Mode Control
• Constant frequency, peakcurrent mode control is used
most frequently
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by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-31
Peak Current Mode Control
• Slope compensation is needed
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-32
A Typical PWM Control IC
• Many safety control
functions are built in
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by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-33
Current Limiting
• Two options are shown
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by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-34
Implementing
Electrical
Isolation in the
Feedback Loop
• Two ways are shown
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by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-35
Implementing Electrical Isolation in the
Feedback Loop
• A dedicated IC for this application is available
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-36
Input Filter
• Needed to comply with the EMI and harmonic limits
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-37
ESR of the Output Capacitor
• ESR often dictates the peak-peak voltage ripple
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 10 Switching
DC Power Supplies
10-38