7. Introduction to DC/DC Converters

1
7. Introduction to DC/DC Converters

• Marc T. Thompson, Ph.D.
• Adjunct Associate Professor of Electrical
  Engineering
• Worcester Polytechnic Institute
• Thompson Consulting, Inc.
• 9 Jacob Gates Road
• Harvard, MA 01451
• Phone [phone]
• Email marctt_at_aol.com
• Website http//members.aol.com/marctt/index.htm

Portions of these notes excerpted from the CD ROM
accompanying Mohan, Undeland and Robbins, Power
Electronics Converters, Applications and Design,
3d edition, John Wiley 2003
2
Summary

• Non-isolated (i.e. no transformer) DC/DC
  converters

3
Block Diagram of Typical AC Input, Regulated DC
Output System
4
Stepping Down a DC Voltage

• In this example, the average value of the output
  voltage DVin where D is the DUTY CYCLE in PWM
  (pulse-width modulation) control
• D ton/Ts

5
Step-Down (Buck) DC-DC Converter

• Add LC filter to reduce switching ripple
• Flyback diode also needed

6
Buck Converter Waveforms

• Steady state inductor current flows
  continuously
• Waveform below for buck in continuous conduction
  mode

7
Buck Converter SPICE Circuit

• Circuit shown fsw 200 kHz, D 0.5

8
Buck Converter Startup Waveforms
9
Analysis for DC/DC Converter in Continuous
Conduction and Steady State

• In steady state, the inductor current returns to
  the same value every switching cycle, or every T
  seconds
• Therefore, the inductor ripple current UP equals
  ripple DOWN
• Several assumptions to simplify analysis
• Periodic steady state --- all startup transients
  have died out
• Small ripple --- ripple is small compared to
  average values

10
Buck Converter in Continuous Conduction

• In continuous conduction, buck converter has 2
  states --- switch OPEN and switch CLOSED

Switch closed (for time DT)
Switch open (for time (1-D)T)
11
Buck Converter in Continuous Conduction

• The inductor ripple current UP equals ripple
  DOWN
• We already knew this result from first
  principles, but this methodology of inductor
  Volt-second balance can be used to evaluate other
  more complicated DC/DC converters

12
Buck Converter Waveforms at the Boundary of
Cont./Discont. Conduction

• ILB critical current below which inductor
  current becomes discontinuous

13
Buck Converter Discontinuous Conduction Mode

• Steady state inductor current discontinuous
  (i.e. it goes zero for a time)
• Note that output voltage depends on load current

14
Buck Limits of Discontinuous Conduction

• The duty-ratio of 0.5 has the highest value of
  the critical current
• For low output current, buck goes discontinuous

15
Buck Limits of Cont./Discont. Conduction

• In regulated power supply, Vd may fluctuate but
  Vo is kept constant by control of D

16
Buck Conv. Output Voltage Ripple

• ESR is assumed to be zero continuous conduction
  mode

17
Buck Conv. Output Voltage Ripple

• ESR is assumed to be zero

18
Buck Conv. Calculations

• Shown for SPICE example with fsw 200 kHz, D
  0.5, L 33 µH, C 10 µF, Io 1A

19
Buck SPICE Result in Periodic Steady State

• Analysis shows inductor ripple 0.38 A-pp,
  output voltage ripple 24 mV-pp, confirmed by
  SPICE

20
Pulse-Width Modulation (PWM) in DC-DC Converters
21
Step-Up (Boost) DC-DC Converter

• Output voltage must be greater than the input

22
Boost Converter Waveforms

• Continuous current conduction mode

Switch closed
Switch open
Inductor Volt-second balance
23
Boost Limits of Cont./Discont. Conduction

• The output voltage is held constant
• For low load current, current conduction becomes
  discontinuous

24
Boost Converter Discont. Conduction

• Occurs at light loads

25
Boost Limits of Cont./Discont. Conduction

• The output voltage is held constant

26
Boost Converter Effect of Parasitics

• The duty-ratio D is generally limited before the
  parasitic effects become significant

27
Boost Converter Output Ripple

• ESR is assumed to be zero
• Assume that all the ripple component of diode
  current flows through capacitor DC component
  flows through resistor

28
Step-Down/Up (Buck-Boost) Converter

• The output voltage can be higher or lower than
  the input voltage
• Note output phase inversion

29
Buck-Boost Converter Waveforms

• Continuation conduction mode

Switch closed
Switch open
Inductor Volt-second balance
30
Buck-Boost Limits of Cont./Discont. Conduction

• The output voltage is held constant

31
Buck-Boost Discontinuous Conduction

• This occurs at light loads

32
Buck-Boost Converter Limits of Cont./Discont.
Conduction

• The output voltage is held constant

33
Buck-Boost Converter Effect of Parasitics

• The duty-ratio is limited to avoid these
  parasitic effects from becoming significant

34
Buck-boost Converter Output Voltage Ripple

• ESR is assumed to be zero

35
Cuk DC-DC Converter

• The output voltage can be higher or lower than
  the input voltage
• Capacitor C1 is primary means of storing and
  transferring energy from input to output
• When switch is ON, C1 discharges through the
  switch and transfers energy to the output
• When switch is OFF, capacitor C1 is charged
  through the diode by energy from the input and L1

36
Cuk DC-DC Converter Waveforms

• The capacitor voltage is assumed constant (very
  large)
• Note phase inversion at the output

37
SEPIC Converter

• Single-ended primary inductance converter
  (SEPIC)
• Can buck or boost the voltage
• Note that output is similar to buck-boost, but
  without a phase inversion

38
Converter for DC-Motor Drives

• Four quadrant operation is possible
• For
• DC motor drives
• DC to AC inverters for UPS

39
Converter Waveforms

• Bi-polar voltage switching

40
Converter Waveforms

• Uni-polar voltage switching

41
Output Ripple in Converters for DC-Motor Drives

• Bi-polar and uni-polar voltage switching

42
Switch Utilization in DC-DC Converters

• It varies significantly in various converters
• PT VTIT where VT and IT are peak switch
  voltage and current
• In direct converters (buck and boost) switch
  utilization is good in indirect converter
  (buck-boost and Cuk) switch utilization is poor

43
Equivalent Circuits in DC-DC Converters

• Replacing inductors and capacitors by current
  and voltage sources, respectively

44
Reversing the Power Flow in DC-DC Conv.

• For power flow from right to left, the input
  current direction should also reverse

45
Real-World Issue Capacitor ESR

• Real-world capacitors have equivalent series
  resistance (ESR)
• This ESR may have dominant effect on output
  ripple

46
Effects of Capacitor ESR

• Without ESR, output ripple is 24 mV-pp
• ESR has increased ripple to approximately 30
  mV-pp