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Chapter 10
Switching DC Power Supplies
• One of the most important applications of
power electronics
• overview of switching power supplies
• dc-dc converters with electrical isolation
• control circuits
• electrical isolation in the feedback loop
Copyright © 2008
by Jose Bastos.
Chapter 10 Switching
DC Power Supplies
10-1
Linear Power Supplies
• Very poor efficiency and large weight and size
Copyright © 2008
by Jose Bastos.
Chapter 10 Switching
DC Power Supplies
10-2
Switching DC Power Supply: Block Diagram
• High efficiency and small weight and size
Copyright © 2008
by Jose Bastos.
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 © 2008
by Jose Bastos.
Chapter 10 Switching
DC Power Supplies
10-4
PWM to Regulate Output
Copyright © 2008
by Jose Bastos.
Chapter 10 Switching
DC Power Supplies
10-5
Flyback Converter
Vo N 2 D

Vd N1 1  D
• Derived from buck-boost; very power at small power
(> 50 W ) power levels
Copyright © 2008
by Jose Bastos.
Chapter 10 Switching
DC Power Supplies
10-6
Forward Converter
Vo N 2

D
Vd N1
• Derived from Buck; idealized to assume that the
transformer is ideal (not possible in practice)
Copyright © 2008
by Jose Bastos.
Chapter 10 Switching
DC Power Supplies
10-7
Push-Pull Inverter
• Derived from step-down converter
Copyright © 2008
by Jose Bastos.
Chapter 10 Switching
DC Power Supplies
10-8
Half-Bridge Converter
• Derived from step-down converter
Copyright © 2008
by Jose Bastos.
Chapter 10 Switching
DC Power Supplies
10-9
Full-Bridge Converter
Copyright © 2008
by Jose Bastos.
Chapter 10 Switching
DC Power Supplies
10-10
Control to Regulate Voltage Output
Copyright © 2008
by Jose Bastos.
Chapter 10 Switching
DC Power Supplies
10-11
Control to Regulate Voltage Output
• Linearized representation of the feedback control
system
Copyright © 2008
by Jose Bastos.
Chapter 10 Switching
DC Power Supplies
10-12
Forward Converter: An Example
• The switch and the diode are assumed to be ideal
Copyright © 2008
by Jose Bastos.
Chapter 10 Switching
DC Power Supplies
10-13
Forward Converter:Transfer Function Plots
02
s  z
Tp ( s)  Vd
2
2
z s  20 s  0
Copyright © 2008
by Jose Bastos.
Chapter 10 Switching
DC Power Supplies
10-14
Linearizing the PWM Block
• The transfer function is essentially a constant with
zero phase shift
Copyright © 2008
by Jose Bastos.
Chapter 10 Switching
DC Power Supplies
10-15
Gain of the PWM IC
• It is slope of the characteristic
Copyright © 2008
by Jose Bastos.
Chapter 10 Switching
DC Power Supplies
10-16
A General Amplifier for Error Compensation
• Can be implemented using a single op-amp
Copyright © 2008
by Jose Bastos.
Chapter 10 Switching
DC Power Supplies
10-17
Type-2 Error Amplifier
• Shows phase boost at the crossover frequency
Copyright © 2008
by Jose Bastos.
Chapter 10 Switching
DC Power Supplies
10-18
Voltage versus Current Mode Control
• current mode control has several advantages
Copyright © 2008
by Jose Bastos.
Chapter 10 Switching
DC Power Supplies
10-19
A Typical PWM Control IC
• Many safety control functions are built in
Copyright © 2008
by Jose Bastos.
Chapter 10 Switching
DC Power Supplies
10-20
Current Limiting
• Two options are shown
Copyright © 2008
by Jose Bastos.
Chapter 10 Switching
DC Power Supplies
10-21
Implementing Electrical Isolation in the
Feedback Loop
Copyright © 2008
by Jose Bastos.
Chapter 10 Switching
DC Power Supplies
10-22
Implementing Electrical Isolation in the
Feedback Loop
• A dedicated IC for this application is available
Copyright © 2008
by Jose Bastos.
Chapter 10 Switching
DC Power Supplies
10-23