Fundamentals of Power Electronics 1Chapter 19: Resonant Conversion
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
Fundamentals of Power Electronics 2Chapter 19: Resonant Conversion
Model of uncontrolled rectifierwith inductive filter network
Fundamental component of iR(t):
Fundamentals of Power Electronics 3Chapter 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
Fundamentals of Power Electronics 4Chapter 19: Resonant Conversion
Equivalent circuit model of uncontrolled rectifierwith inductive filter network
Fundamentals of Power Electronics 5Chapter 19: Resonant Conversion
Equivalent circuit modelParallel resonant dc-dc converter
Fundamentals of Power Electronics 6Chapter 19: Resonant Conversion
Construction of Zo
Fundamentals of Power Electronics 7Chapter 19: Resonant Conversion
Construction of H
Fundamentals of Power Electronics 8Chapter 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
Fundamentals of Power Electronics 9Chapter 19: Resonant Conversion
Comparison of approximate and exact characteristics
0.5 0.6 0.7 0.8 0.9 1.0
0.0
0.2
0.4
0.6
0.8
1.0
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
F
M = V/Vg
1 2 3 4 5
0.0
0.2
0.4
0.6
0.8
1.0
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
F
M=V/Vg
Series resonant converterBelow resonance:
0.5 < F < 1
Above resonance:
1 < F
Fundamentals of Power Electronics 10Chapter 19: Resonant Conversion
Comparison of approximate and exact characteristics
0.50 1.00 1.50 2.00 2.50 3.00
0.00
0.50
1.00
1.50
2.00
2.50
3.00
M
F
Qe
=5
Q e =2
Q e =1
Qe
=0.5
Q e =0.2
Parallel resonant converter
Exact equation: solid lines
Sinusoidal approximation: shaded lines
Fundamentals of Power Electronics 11Chapter 19: Resonant Conversion
19.3 Soft switching
Soft switching can mitigate some of the mechanisms of switching loss and possibly reduce the generation of EMISemiconductor 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.
Fundamentals of Power Electronics 12Chapter 19: Resonant Conversion
19.3.1 Operation of the full bridge below resonance: Zero-current switching
Series resonant converter exampleL+–Vg CQ1Q2
Q3Q4D1D2
D3D4+vs(t)–is(t)
+vds1(t)–iQ1(t)Operation below resonance: input tank current leads voltage
Zero-current switching (ZCS) occurs
Fundamentals of Power Electronics 13Chapter 19: Resonant Conversion
Tank input impedance
Re|| Zi || f0ωLR0Qe = R0 /Re
Operation below resonance: tank input impedance Zi is dominated by tank capacitor.
Zi is positive, and tank input current leads tank input voltage.
Zero crossing of the tank input current waveform is(t) occurs before the zero crossing of the voltage vs(t).
Fundamentals of Power Electronics 14Chapter 19: Resonant Conversion
Switch network waveforms, below resonanceZero-current switching
tvs(t)Vg– Vg
vs1(t)
tis(t) tβQ1Q4D1D4Q2Q3D2D3Conductingdevices:“Hard”turn-on ofQ1, Q4“Soft”turn-off ofQ1, Q4“Hard”turn-on ofQ2, Q3“Soft”turn-off ofQ2, Q3
L+–Vg CQ1Q2
Q3Q4D1D2
D3D4+vs(t)–is(t)
+vds1(t)–iQ1(t)Conduction sequence: Q1–D1–Q2–D2
Q1 is turned off during D1 conduction interval, without loss
Fundamentals of Power Electronics 15Chapter 19: Resonant Conversion
ZCS turn-on transition: hard switching
L+–Vg CQ1Q2
Q3Q4D1D2
D3D4+vs(t)–is(t)
+vds1(t)–iQ1(t)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 hard-switched PWM, and switching loss occurs.
tids(t) tβQ1Q4D1D4Q2Q3D2D3Conductingdevices:“Hard”turn-on ofQ1, Q4“Soft”turn-off ofQ1, Q4tVgvds1(t)
Fundamentals of Power Electronics 16Chapter 19: Resonant Conversion
Fundamentals of Power Electronics 17Chapter 19: Resonant Conversion
19.3.2 Operation of the full bridge below resonance: Zero-voltage switching
Series resonant converter exampleL+–Vg CQ1Q2
Q3Q4D1D2
D3D4+vs(t)–is(t)
+vds1(t)–iQ1(t)Operation above resonance: input tank current lags voltage
Zero-voltage switching (ZVS) occurs
Fundamentals of Power Electronics 18Chapter 19: Resonant Conversion
Tank input impedance
Re|| Zi || f0ωLR0Qe = R0 /Re
Operation above resonance: tank input impedance Zi is dominated by tank inductor.
Zi is negative, and tank input current lags tank input voltage.
Zero crossing of the tank input current waveform is(t) occurs after the zero crossing of the voltage vs(t).
Fundamentals of Power Electronics 19Chapter 19: Resonant Conversion
Switch network waveforms, above resonanceZero-voltage switching
L+–Vg CQ1Q2
Q3Q4D1D2
D3D4+vs(t)–is(t)
+vds1(t)–iQ1(t)Conduction sequence: D1–Q1–D2–Q2
Q1 is turned on during D1 conduction interval, without loss
tvs(t)Vg– Vg
vs1(t)
tis(t)tαQ1Q4D1D4 Q2Q3D2D3Conductingdevices:“Soft”turn-on ofQ1, Q4“Hard”turn-off ofQ1, Q4“Soft”turn-on ofQ2, Q3“Hard”turn-off ofQ2, Q3
Fundamentals of Power Electronics 20Chapter 19: Resonant Conversion
ZVS turn-off transition: hard switching?
L+–Vg CQ1Q2
Q3Q4D1D2
D3D4+vs(t)–is(t)
+vds1(t)–iQ1(t)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).
tids(t)Conductingdevices:
tVgvds1(t)tαQ1Q4D1D4 Q2Q3D2D3“Soft”turn-on ofQ1, Q4“Hard”turn-off ofQ1, Q4
Fundamentals of Power Electronics 21Chapter 19: Resonant Conversion
Soft switching at the ZVS turn-off transitionL+–VgQ1Q2Q3Q4
D1D2 D3D4+vs(t)–is(t)+vds1(t)– to remainderof converterClegClegClegCleg
Conductingdevices: tVgvds1(t)Q1Q4D2D3Turn offQ1, Q4CommutationintervalX
•Introduce small capacitors Cleg across each device (or use device output capacitances).
•Introduce delay between turn-off of Q1 and turn-on of Q2.
Tank current is(t) charges and discharges Cleg. Turn-off transition becomes lossless. During commutation interval, no devices conduct.
So zero-voltage switching exhibits low switching loss: losses due to diode stored charge and device output capacitances are eliminated.