Power Electroni
cs
Chapter 3
DC to DC Converters
(Choppers)
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Outline
3.1 Basic DC to DC converters
3.1.1 Buck converter (Step-down converter)
3.1.2 Boost converter (Step-up converter)
3.1.3 Buck-Boost converter (Step-down/step-up converter) and
Cuk converter
3.1.4 Sepic converter and Zeta converter
3.2 Composite DC/DC converters and connection of
multiple DC/DC converters
3.2.1 A current-reversible chopper
3.2.2 Bridge chopper (H-bridge DC/DC converter)bridge DC/D
3.2.3 Multi-phase multi-channel DC/DC converters
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3.1 Basic DC to DC converters
Introduction—Buck converter
SPDT switch changes dc
component
Switch output voltage
waveform
Duty cycle D:
0 ≤ D ≤ 1
complement D’:
D’ = 1 - D
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Dc component of switch output voltage
Fourier analysis: Dc component = average value
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Insertion of low-pass filter to remove switching
harmonics and pass only dc component
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Basic operation principle of buck converter
Buck converter with
ideal switch
Realization using
power MOSFET
and diode
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Thought process in analyzing basic
DC/DC converters
Basic operation principle (qualitative analysis)
– How does current flows during different switching states
– How is energy transferred during different switching states
Verification of small ripple approximation
Derivation of inductor voltage waveform during different
switching states
Quantitative analysis according to inductor volt-second
balance or capacitor charge balance
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Actual output voltage waveform of
buck converter
Buck converter
containing practical
low-pass filter
Actual output voltage
waveform
v(t) = V + v
ripple
(t)
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The small ripple approximation
v(t) = V + v
ripple
(t)
In a well-designed converter, the output voltage ripple is
small. Hence, the waveforms can be easily determined by
ignoring the ripple:
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Buck converter analysis:
inductor current waveform
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Inductor voltage and current
subinterval 1: switch in position 1
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Inductor voltage and current
subinterval 2: switch in position 2
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Inductor voltage and current waveforms
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Determination of inductor
current ripple magnitude
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Inductor current waveform
during start-up transient
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The principle of inductor volt-second
balance: Derivation
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Inductor volt-second balance:
Buck converter example
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The principle of capacitor charge
balance: Derivation
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Boost converter example
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Boost converter analysis
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Subinterval 1: switch in position 1
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Subinterval 2: switch in position 2
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Inductor voltage and capacitor current
waveforms
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Inductor volt-second balance
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Conversion ratio M(D) of
the boost converter
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Determination of inductor current
dc component
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CCM mode
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DCM mode
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Continuous-Conduction-Mode (CCM) and
Discontinuous-Conduction-Mode (DCM) of buck
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b)
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CCM mode DCM mode
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a)
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Continuous-Conduction-Mode (CCM) and
Discontinuous-Conduction-Mode (DCM) of boost
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VD
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Can be considered as a
combination of a Buck and a
Boost
Can realize two-quadrant ( I & II)
operation of DC motor:
forward motoring,
forward braking
3.2 Composite DC/DC converters and ` ` ` `
` connection of multiple DC/DC converters
A current reversible chopper
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Bridge chopper (H-bridge chopper)
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1
VD
1
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3
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2
VD
2
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4
VD
3
VD
4
M
Can be considered as the combination of two
current-reversible choppers.
Can realize 4-quadrant operation of DC motor.
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Multi-phase multi-channel DC/DC converter
tO
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1
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VD
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1
Current output capability is
increased due to multi-channel
paralleling.
Ripple in the output voltage and
current is reduced due to multi-
channel paralleling.
Ripple in the input current is
reduced due to multi-phase
paralleling.
