Chapter 7
DC-DC Switch-Mode Converters
• dc-dc converters for switch-mode dc power
supplies and dc-motor drives
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-1
Block Diagram of DC-DC Converters
• Functional block diagram
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-2
Stepping Down a DC Voltage
• A simple approach that shows the evolution
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-3
Pulse-Width Modulation in
DC-DC Converters
• Role of PWM
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by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-4
Step-Down DC-DC Converter
• Pulsating input to
the low-pass filter
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by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-5
Step-Down DC-DC Converter: Waveforms
• Steady state; inductor current flows continuously
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-6
Step-Down DC-DC Converter: Waveforms at
the boundary of Cont./Discont. Conduction
• Critical current below which inductor current
becomes discontinuous
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-7
Step-Down DC-DC Converter:
Discontinuous Conduction Mode
• Steady state; inductor current discontinuous
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-8
Step-Down DC-DC Converter: Limits of
Cont./Discont. Conduction
• The duty-ratio of 0.5 has the highest value of the
critical current
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-9
Step-Down DC-DC Converter: Limits of
Cont./Discont. Conduction
• Output voltage is kept constant
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-10
Step-Down Conv.: Output Voltage Ripple
• ESR is assumed to be zero
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-11
Step-Up DC-DC Converter
• Output voltage must be greater than the input
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-12
Step-Up DC-DC Converter Waveforms
• Continuous current conduction mode
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-13
Step-Up DC-DC Converter: Limits of
Cont./Discont. Conduction
• The output voltage is held constant
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-14
Step-Up DC-DC Converter: Discont.
Conduction
• Occurs at light loads
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-15
Step-Up DC-DC Converter: Limits of
Cont./Discont. Conduction
• The output voltage is held constant
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-16
Step-Up DC-DC Converter: Effect of
Parasitics
• The duty-ratio is generally limited before the
parasitic effects become significant
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-17
Step-Up DC-DC Converter Output Ripple
• ESR is assumed to be zero
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-18
Step-Down/Up DC-DC Converter
• The output voltage can be higher or lower than
the input voltage
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-19
Step-Up DC-DC Converter: Waveforms
• Continuation conduction mode
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-20
Step-Up DC-DC Converter: Limits of
Cont./Discont. Conduction
• The output voltage is held constant
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-21
Step-Up DC-DC Converter:
Discontinuous Conduction Mode
• This occurs at light loads
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-22
Step-Up DC-DC Converter: Limits of
Cont./Discont. Conduction
• The output voltage is held constant
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-23
Step-Up DC-DC Converter: Effect of
Parasitics
• The duty-ratio is limited to avoid these parasitic
effects from becoming significant
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-24
Step-Up DC-DC Converter:
Output Voltage Ripple
• ESR is assumed to be zero
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-25
Cuk DC-DC Converter
• The output voltage can be higher or lower than
the input voltage
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-26
Cuk DC-DC Converter: Waveforms
• The capacitor
voltage is assumed
constant
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-27
Converter for DC-Motor Drives
• Four quadrant operation is possible
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by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-28
Converter Waveforms
• Bi-polar voltage switching
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-29
Converter Waveforms
• Uni-polar voltage switching
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-30
Output Ripple in Converters for DC-Motor
Drives
• bi-polar and uni-polar voltage switching
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-31
Switch Utilization in DC-DC Converters
• It varies significantly in various converters
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-32
Equivalent Circuits in DC-DC Converters
• replacing inductors and capacitors by current
and voltage sources, respectively
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-33
Reversing the Power Flow in DC-DC Conv.
• For power flow from right to left, the input
current direction should also reverse
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 7 DC-DC Switch-Mode
Converters
7-34