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Discontinuous conduction mode
(DCVM)
• Occurs at heavy load and low
output voltage
• During the “discontinuous”
interval, all four output diodes are
forward-biased
• The capacitor voltage waveform
exhibits an additional subinterval
with vc = 0
ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
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Lecture 26
State plane trajectory: DCVM
As predicted by
CCM analysis
DCVM trajectory
DCVM occurs when
ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
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Lecture 26
DCVM solution
Mode boundary
Steady-state solution
Output characteristics
Solid lines: CCM
Dashed lines: DCVM
ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
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Lecture 26
Design considerations
Let’s now consider how to design a
resonant converter in a dc-dc
converter application in which the
output voltage V is to be regulated
over a range of operating points.
Typical specifications are of the form
Pmax ≤ P ≤ Pmin
Vgmax ≤ Vg ≤ Vgmin
V is regulated
How specifications map into the
converter output plane
M = V / nVg
J = nR0I / Vg
The turns ratio n can be chosen to
map the range of voltage operating
points to any valid range of M:
ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
Current and power are related to J:
Hence
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Lecture 26
Mapping the specifications into the output plane
ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
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Lecture 26
PRC output plane
ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
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Lecture 26
Peak tank capacitor voltage stresses
Proportional to output voltage in CCM
Peak capacitor voltage, CCM:
DCVM:
(above equations are derived
from the state plane trajectory)
(the course notes include Fig.
5.26(b) that shows peak
voltage stresses below the
peak frequency in the double
frequency region)
ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
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Lecture 26
Peak current stresses
Proportional to output voltage in CCM
Peak transistor current, CCM:
DCVM:
(above equations are derived
from the state plane trajectory)
(the course notes include Fig.
5.25(b) that shows peak
current stresses below the
peak frequency in the double
frequency region)
ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
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Lecture 26
Design example
Off-line dc-dc converter
Design a dc-dc converter using a parallel resonant converter, to meet the
following specifications:
It is desired to
operate MOSFET
devices with zerovoltage switching
ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
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Lecture 26
Choice of range of operating points
Here, “converter design” involves
selection of a valid range of M and J,
that is mapped into the range of
converter specifications through
selection of the turns ratio and
characteristic impedance according to
ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
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The values of L and C
are then chosen as
follows:
Lecture 26
One possible design
Corners of the
operating region for
the design Mmax = 1.2,
Jmax = 0.9
Operating region
overlayed on the
converter output
characteristics
A
D
C
ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
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B
Lecture 26
Comparison of some other choices
Choose Mmax, Jmax
close to the boundary
of allowed operation to
minimize peak current
stress
Increasing Mmax leads
to operation closer to
resonance, with
reduced range of
switching frequencies
ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
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Lecture 26