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19.2.3 Parallel resonant dc-dc converter
Differs from series resonant converter as follows:
Different tank network
Rectifier is driven by sinusoidal voltage, and is connected to
inductive-input low-pass filter
Need a new model for rectifier and filter networks
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Chapter 19: Resonant Conversion
Model of uncontrolled rectifier
with inductive filter network
Fundamental component of iR(t):
Fundamentals of Power Electronics
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Chapter 19: Resonant Conversion
Effective resistance Re
Again define
In steady state, the dc output voltage V is equal to the average value
of | vR |:
For a resistive load, V = IR. The effective resistance Re can then be
expressed
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Chapter 19: Resonant Conversion
Equivalent circuit model of uncontrolled rectifier
with inductive filter network
Fundamentals of Power Electronics
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Chapter 19: Resonant Conversion
Equivalent circuit model
Parallel resonant dc-dc converter
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Chapter 19: Resonant Conversion
Construction of Zo
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Chapter 19: Resonant Conversion
Construction of H
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Chapter 19: Resonant Conversion
Dc conversion ratio of the PRC
At resonance, this becomes
• PRC can step up the voltage, provided R > R0
• PRC can produce M approaching infinity, provided output current is
limited to value less than Vg / R0
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Chapter 19: Resonant Conversion
Comparison of approximate and exact characteristics
1.0
Below resonance:
0.8
M = V/Vg
Series resonant
converter
0.5 < F < 1
exact M, Q=2
approx M, Q=2
exact M, Q=10
approx M, Q=10
exact M, Q=0.5
approx M, Q=0.5
0.6
0.4
0.2
0.0
0.5
0.6
0.7
0.8
0.9
1.0
F
1.0
Above resonance:
0.8
M=V/Vg
1<F
exact M, Q=0.5
approx M, Q=0.5
exact M, Q=10
approx M, Q=10
exact M, Q=2
approx M, Q=2
0.6
0.4
0.2
0.0
1
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3
F
4
5
Chapter 19: Resonant Conversion
Comparison of approximate and exact characteristics
3.00
Parallel resonant
converter
Qe=5
2.50
2.00
M
1.50
Q e=2
1.00
Exact equation: solid
lines
Sinusoidal approximation:
shaded lines
Q e=1
0.50
Qe=0.5
Qe=0.2
0.00
0.50
1.00
1.50
2.00
2.50
3.00
F
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Chapter 19: Resonant Conversion
19.3 Soft switching
Soft switching can mitigate some of the mechanisms of switching loss
and possibly reduce the generation of EMI
Semiconductor devices are switched on or off at the zero crossing of
their voltage or current waveforms:
Zero-current switching: transistor turn-off transition occurs at zero
current. Zero-current switching eliminates the switching loss
caused by IGBT current tailing and by stray inductances. It can
also be used to commutate SCR’s.
Zero-voltage switching: transistor turn-on transition occurs at
zero voltage. Diodes may also operate with zero-voltage
switching. Zero-voltage switching eliminates the switching loss
induced by diode stored charge and device output capacitances.
Zero-voltage switching is usually preferred in modern converters.
Zero-voltage transition converters are modified PWM converters, in
which an inductor charges and discharges the device capacitances.
Zero-voltage switching is then obtained.
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Chapter 19: Resonant Conversion
19.3.1 Operation of the full bridge below resonance:
Zero-current switching
Series resonant converter example
Operation below resonance: input tank current leads voltage
Zero-current switching (ZCS) occurs
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Chapter 19: Resonant Conversion
Tank input impedance
Operation below
resonance: tank input
impedance Zi is
dominated by tank
capacitor.
Zi is positive, and
tank input current
leads tank input
voltage.
|| Zi ||
L
R0
Re
f0
Q e = R0 /R e
Zero crossing of the
tank input current
waveform is(t) occurs
before the zero
crossing of the voltage
vs(t).
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Chapter 19: Resonant Conversion
Switch network waveforms, below resonance
Zero-current switching
vs1 (t)
Vg
vs(t)
t
– Vg
is(t)
t
t
Conduction sequence: Q1–D1–Q2–D2
Conducting
devices:
Q1
Q4
D1
D4
Q2
Q3
“Hard”
“Soft”
“Hard”
turn-on of turn-off of turn-on of
Q 1, Q 4
Q 1, Q 4
Q 2, Q 3
Fundamentals of Power Electronics
Q1 is turned off during D1 conduction
interval, without loss
D2
D3
“Soft”
turn-off of
Q2, Q3
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Chapter 19: Resonant Conversion
ZCS turn-on transition: hard switching
vds1 (t)
Vg
t
ids(t)
t
Conducting
devices:
Q1
Q4
t
D1
D4
“Hard”
“Soft”
turn-on of turn-off of
Q 1, Q 4
Q 1, Q 4
Fundamentals of Power Electronics
Q2
Q3
D2
D3
Q1 turns on while D2 is conducting. Stored
charge of D2 and of semiconductor output
capacitances must be removed. Transistor
turn-on transition is identical to hardswitched PWM, and switching loss occurs.
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Chapter 19: Resonant Conversion
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Chapter 19: Resonant Conversion
19.3.2 Operation of the full bridge below resonance:
Zero-voltage switching
Series resonant converter example
Operation above resonance: input tank current lags voltage
Zero-voltage switching (ZVS) occurs
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Chapter 19: Resonant Conversion
Tank input impedance
Operation above
resonance: tank input
impedance Zi is
dominated by tank
inductor.
Zi is negative, and
tank input current lags
tank input voltage.
|| Zi ||
L
R0
Re
f0
Q e = R0 /R e
Zero crossing of the
tank input current
waveform is(t) occurs
after the zero crossing
of the voltage vs(t).
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Chapter 19: Resonant Conversion
Switch network waveforms, above resonance
Zero-voltage switching
vs1 (t)
Vg
vs(t)
t
– Vg
is(t)
t
t
Conduction sequence: D1–Q1–D2–Q2
Conducting D 1
devices: D
4
“Soft”
turn-on of
Q 1, Q 4
Q1
Q4
D2
D3
Q1 is turned on during D1 conduction
interval, without loss
Q2
Q3
“Hard”
“Soft”
“Hard”
turn-off of turn-on of turn-off of
Q 1, Q 4
Q 2, Q 3
Q2, Q3
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Chapter 19: Resonant Conversion
ZVS turn-off transition: hard switching?
vds1 (t)
Vg
t
ids(t)
t
Conducting D 1
devices: D
4
“Soft”
turn-on of
Q1, Q4
t
Q1
Q4
D2
D3
“Hard”
turn-off of
Q 1, Q4
Fundamentals of Power Electronics
Q2
Q3
When Q1 turns off, D2 must begin
conducting. Voltage across Q1 must
increase to Vg. Transistor turn-off transition
is identical to hard-switched PWM.
Switching loss may occur (but see next
slide).
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Chapter 19: Resonant Conversion
Soft switching at the ZVS turn-off transition
• Introduce small
capacitors Cleg across
each device (or use
device output
capacitances).
• Introduce delay
between turn-off of Q1
and turn-on of Q2.
vds1 (t)
Conducting
devices:
Tank current is(t) charges and
discharges Cleg. Turn-off transition
becomes lossless. During commutation
interval, no devices conduct.
Vg
Q1
Q4
Turn off
Q 1, Q 4
X D2
D3
t
So zero-voltage switching exhibits low
switching loss: losses due to diode
stored charge and device output
capacitances are eliminated.
Commutation
interval
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Chapter 19: Resonant Conversion