Thompson - DC-DC Converters for Renewable Energy.PPT

DC-DC Converters

1. 7-1
Power Electronics Chapter 7 Introduction to DC/DC Converters
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: [email protected]
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. 7-2
Power Electronics Chapter 7 Introduction to DC/DC Converters
Summary
•Non-isolated (i.e.no transformer) DC/DC converters

3. 7-3
Power Electronics Chapter 7 Introduction to DC/DC Converters
Block Diagram of Typical AC Input,
Regulated DC Output System

4. 7-4
Power Electronics Chapter 7 Introduction to DC/DC Converters
Stepping Down a DC Voltage
•In this example, the average value of the output voltage =
DV
inwhere D is the DUTY CYCLE in PWM (pulse-width
modulation) control
•D = t
on/T
s

5. 7-5
Power Electronics Chapter 7 Introduction to DC/DC Converters
Step-Down (Buck) DC-DC Converter
•Add LC filter to reduce
switching ripple
•Flyback diode also
needed

6. 7-6
Power Electronics Chapter 7 Introduction to DC/DC Converters
Buck Converter: Waveforms
•Steady state; inductor current flows continuously
•Waveform below for buck in continuous conductionmode

7. 7-7
Power Electronics Chapter 7 Introduction to DC/DC Converters
Buck Converter: SPICE Circuit
•Circuit shown: fsw = 200 kHz, D = 0.5

8. 7-8
Power Electronics Chapter 7 Introduction to DC/DC Converters
Buck Converter: Startup Waveforms

9. 7-9
Power Electronics Chapter 7 Introduction to DC/DC Converters
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 Tseconds
•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. 7-10
Power Electronics Chapter 7 Introduction to DC/DC Converters
Buck Converter in Continuous ConductionV
o
D
i
L
L
V
cc
v
c
+
-
C R V
o
i
L
L
V
cc
v
c
+
-
C
R V
o
i
L
L
v
c
+
-
Switch closed (for time DT) Switch open (for time (1-D)T)di
dt
V v
L
L CC o

 di
dt
v
L
L o

•In continuous conduction, buck converter has 2 states ---
switch OPEN and switch CLOSED

11. 7-11
Power Electronics Chapter 7 Introduction to DC/DC Converters
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( ) ()VVDT
L
V DT
L
VDV
CC o o
o CC





1
0

12. 7-12
Power Electronics Chapter 7 Introduction to DC/DC Converters
Buck Converter: Waveforms at the Boundary
of Cont./Discont. Conduction
•I
LB= critical current below which inductor current becomes
discontinuous

13. 7-13
Power Electronics Chapter 7 Introduction to DC/DC Converters
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 currentmax,
2
2
25.0
LB
od
o
I
I
D
D
V
V



14. 7-14
Power Electronics Chapter 7 Introduction to DC/DC Converters
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. 7-15
Power Electronics Chapter 7 Introduction to DC/DC Converters
Buck: Limits of Cont./Discont. Conduction
•In regulated power supply, V
dmay fluctuate but V
ois kept
constant by control of D

16. 7-16
Power Electronics Chapter 7 Introduction to DC/DC Converters
Buck Conv.: Output Voltage Ripple
•ESR is assumed to be zero; continuous conduction mode

17. 7-17
Power Electronics Chapter 7 Introduction to DC/DC Converters
Buck Conv.: Output Voltage Ripple
•ESR is assumed to be zeroLf
DV
L
TDV
i
sw
oo
ppL
)1()1(
,



 Lf
DViT
Q
sw
oppL
2
,
8
)1(
222
1 





















 LCf
DV
C
Q
v
sw
o
ppo 2,
8
)1(




18. 7-18
Power Electronics Chapter 7 Introduction to DC/DC Converters
Buck Conv.: Calculations
•Shown for SPICE example with f
sw= 200 kHz, D = 0.5, L =
33 µH, C = 10 µF, Io = 1AA
Lf
DV
i
sw
o
ppL 38.0
)1033)(102(
)5.01)(5()1(
65, 





 mV
LCf
DV
C
Q
v
sw
o
ppo 24
)1010)(1033()102(8
)5.01)(5(
8
)1(
66252, 









19. 7-19
Power Electronics Chapter 7 Introduction to DC/DC Converters
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. 7-20
Power Electronics Chapter 7 Introduction to DC/DC Converters
Pulse-Width Modulation (PWM) in DC-DC
Convertersst
control
V
v
D
ˆ


21. 7-21
Power Electronics Chapter 7 Introduction to DC/DC Converters
Step-Up (Boost) DC-DC Converter
•Output voltage must be greater than the input

22. 7-22
Power Electronics Chapter 7 Introduction to DC/DC Converters
Boost Converter Waveforms
•Continuous current conduction mode
Switch closed:di
dt
V
L
L CC

Switch open:di
dt
Vv
L
L CC o


Inductor Volt-second balance:VDT
L
VV DT
L
V
V
D
CC CC o
o
CC

 



( )()1
0
1

23. 7-23
Power Electronics Chapter 7 Introduction to DC/DC Converters
Boost: Limits of Cont./Discont. Conduction
•The output voltage is held constant
•For low load current, current conduction becomes
discontinuous

24. 7-24
Power Electronics Chapter 7 Introduction to DC/DC Converters
Boost Converter: Discont. Conduction
•Occurs at light loads

25. 7-25
Power Electronics Chapter 7 Introduction to DC/DC Converters
Boost: Limits of Cont./Discont. Conduction
•The output voltage is held constant

26. 7-26
Power Electronics Chapter 7 Introduction to DC/DC Converters
Boost Converter: Effect of Parasitics
•The duty-ratio D is generally limited before the parasitic
effects become significant

27. 7-27
Power Electronics Chapter 7 Introduction to DC/DC Converters
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. 7-28
Power Electronics Chapter 7 Introduction to DC/DC Converters
Step-Down/Up (Buck-Boost) Converter
•The output voltage can be higher or lower than the input
voltage
•Note output phase inversion

29. 7-29
Power Electronics Chapter 7 Introduction to DC/DC Converters
Buck-Boost Converter: Waveforms
•Continuation conduction mode
Switch closed:di
dt
V
L
L CC

Switch open:di
dt
v
L
L o

Inductor Volt-second balance:VDT
L
V DT
L
V
DV
D
CC o
o
CC





()1
0
1

30. 7-30
Power Electronics Chapter 7 Introduction to DC/DC Converters
Buck-Boost: Limits of Cont./Discont. Conduction
•The output voltage is held constant

31. 7-31
Power Electronics Chapter 7 Introduction to DC/DC Converters
Buck-Boost: Discontinuous Conduction
•This occurs at light loads

32. 7-32
Power Electronics Chapter 7 Introduction to DC/DC Converters
Buck-Boost Converter: Limits of
Cont./Discont. Conduction
•The output voltage is held constant

33. 7-33
Power Electronics Chapter 7 Introduction to DC/DC Converters
Buck-Boost Converter: Effect of Parasitics
•The duty-ratio is limited to avoid these parasitic effects
from becoming significant

34. 7-34
Power Electronics Chapter 7 Introduction to DC/DC Converters
Buck-boost Converter: Output Voltage Ripple
•ESR is assumed to be zero

35. 7-35
Power Electronics Chapter 7 Introduction to DC/DC Converters
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. 7-36
Power Electronics Chapter 7 Introduction to DC/DC Converters
Cuk DC-DC Converter: Waveforms
•The capacitor
voltage is assumed
constant (very large)
•Note phase inversion
at the outputD
D
V
V
d
o


1

37. 7-37
Power Electronics Chapter 7 Introduction to DC/DC Converters
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 inversionD
D
V
V
d
o


1

38. 7-38
Power Electronics Chapter 7 Introduction to DC/DC Converters
Converter for DC-Motor Drives
•Four quadrant operation is possible
•For:
•DC motor drives
•DC to AC inverters for UPS

39. 7-39
Power Electronics Chapter 7 Introduction to DC/DC Converters
Converter Waveforms
•Bi-polar voltage switching

40. 7-40
Power Electronics Chapter 7 Introduction to DC/DC Converters
Converter Waveforms
•Uni-polar voltage switching

41. 7-41
Power Electronics Chapter 7 Introduction to DC/DC Converters
Output Ripple in Converters for DC-Motor
Drives
•Bi-polar and uni-polar voltage switching

42. 7-42
Power Electronics Chapter 7 Introduction to DC/DC Converters
Switch Utilization in DC-DC Converters
•It varies significantly in
various converters
•P
T= V
TI
Twhere V
Tand I
T
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. 7-43
Power Electronics Chapter 7 Introduction to DC/DC Converters
Equivalent Circuits in DC-DC Converters
•Replacing inductors and capacitors by current and voltage
sources, respectively

44. 7-44
Power Electronics Chapter 7 Introduction to DC/DC Converters
Reversing the Power Flow in DC-DC Conv.
•For power flow from right to left, the input current
direction should also reverse

45. 7-45
Power Electronics Chapter 7 Introduction to DC/DC Converters
Real-World Issue: Capacitor ESR
•Real-world capacitors have equivalent series resistance (ESR)
•This ESR may have dominant effect on output ripple

46. 7-46
Power Electronics Chapter 7 Introduction to DC/DC Converters
Effects of Capacitor ESR
•Without ESR, output ripple is 24 mV-pp
•ESR has increased ripple to approximately 30 mV-pp