ECEN5817 Lecture 6
19.2.3 Parallel resonant dc-dc converter
Differs from series resonant converter as follows:
Different tank network
Rectifier is driven by sinusoidal voltage
voltage, and is connected to
inductive-input low-pass filter
Need a new model for rectifier and filter networks
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Equivalent circuit model
Parallel resonant dc-dc converter
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Ways to construct transfer function H in
terms of impedances
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Construction of Zo – resonant (high Q) case
C = 0.1 μ
μF, L = 1 mH, Re = 1 kΩ
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Construction of H = V / Vg – resonant (high Q) case
C = 0.1 μ
μF, L = 1 mH, Re = 1 kΩ
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Construction of ||H||
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Dc conversion ratio of the PRC
At resonance, this becomes
• PRC can step up the voltage, provided R > R0
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Comparison of approximate and exact PRC characteristics
Parallel resonant converter
Exact equation:
solid lines
Sinusoidal approximation:
shaded lines
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19.3 Soft switching in resonant converters
Soft switching can mitigate some of the mechanisms of switching loss and possibly
reduce the generation of EMI
Losses due
L
d to
t high
hi h voltage
lt
andd high
hi h currentt presentt in
i switch
it h during
d i transitions,
t
iti
e.g. due to diode reverse recovery
Losses due to charging or discharging device capacitances through semiconductor
devices
Soft switching: semiconductor devices are switched on or off at the zero crossing of
their voltage or current waveforms:
Zero current switching: transistor turn-off
Zero-current
turn off transition occurs at zero current.
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
g switching:
g transistor turn-on transition occurs at zero voltage.
g
Diodes may also operate with zero-voltage switching. Zero-voltage switching
eliminates the switching loss induced by diode stored charge and device output
capacitances.
Z
Zero-voltage
l
switching
i hi is
i usually
ll preferred
f
d in
i converters based
b d on MOSFETs
MOSFET
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19.3.1 Operation of the full bridge below resonance:
Zero-current switching
Series resonant converter example
+
Q1
Vg
+
–
vds1(t)
D1
Q3
D3
iQ1(t) –
L
C
+
vs(t)
Q2
D2
Q4
–
is(t)
D4
Current bi-directional switches
ZCS vs.
vs ZVS depends on tank current zero crossings with respect to
transistor switching times = tank voltage zero crossings
Operation below resonance: input tank current leads voltage
Zero-current switching (ZCS) occurs
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Tank input impedance
Operation below
resonance: tank input
impedance Zi is
dominated by tank
capacitor.
Zi is negative, and
tank input current
leads tank input
voltage
voltage.
|| Zi ||
1
C
L
R0
Re
f0
Qe = R0 /Re
Zero crossing of the
tank input current
waveform is(t) occurs
before the zero
crossing of the voltage
vs(t) – before switch
t
transitions
iti
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Switch network waveforms, below resonance
Zero-current switching
+
Q1
Vg +
–
vds1(t)( ) D1
Q3
D3
iQ1(t) –
L
C
+
vs(t)
Q2
D2
Q4
–
is(t)
D4
Conduction sequence: Q1–D1–Q2–D2
Q1 is turned off during D1 conduction
interval, without loss (same for Q4/D4)
Q2 is turned off during D2 conduction
interval, without loss (same for Q3/D3)
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Turn-on transitions: significant switching losses
+
Q1
Vg +
–
Q
vds1(t) D1 3
D3
iQ1(t) –
L
C
+
vs(t)
Q2
D2
Q4
–
is(t)
D4
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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More on Diode Stored Charge and Reverse Recovery
Typical test circuit
and parameter
d fi iti
definitions
in
i diode
di d
data sheets
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“Snappy” and “soft-recovery” diodes
Also see textbook Section 4.3.2 and HW 1 problem 3 solution
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