Download Fundamentals of Power Electronics

Reduction of power converter size
through increase of switching frequency
• Increasing switching frequency reduces value and
size of filter inductances and capacitances
• Up to a point, increasing switching frequency reduces
transformer size
• Increasing switching frequency increases switching
loss: Psw = fsw ∆vds Qsw
• Much R&D effort has been devoted to increasing the
switching frequency and reducing the loss in highdensity power supplies
• Approaches to achieve these goals include use of
resonant converters and soft switching techniques
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Chapter 19: Resonant Conversion
4.3. Switching loss
• Energy is lost during the semiconductor switching transitions,
via several mechanisms:
• Transistor switching times
• Diode stored charge
• Energy stored in device capacitances and parasitic
inductances
• Semiconductor devices are charge controlled
• Time required to insert or remove the controlling charge
determines switching times
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Chapter 19: Resonant Conversion
4.3.1. Transistor switching
with clamped inductive load
Buck converter example
transistor turn-off
transition
Loss:
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Chapter 19: Resonant Conversion
4.3.4. Efficiency vs. switching frequency
Add up all of the energies lost during the switching transitions of one
switching period:
Average switching power loss is
Total converter loss can be expressed as
where
Fundamentals of Power Electronics
Pfixed = fixed losses (independent of load and fsw)
Pcond = conduction losses
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Chapter 19: Resonant Conversion
Efficiency vs. switching frequency
Switching losses are equal to
the other converter losses at the
critical frequency
This can be taken as a rough
upper limit on the switching
frequency of a practical
converter. For fsw > fcrit, the
efficiency decreases rapidly
with frequency.
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Chapter 19: Resonant Conversion
Soft switching:
Zero-voltage
and zero-current switching
vs1 (t)
Vg
vs(t)
Soft switching can mitigate some
t 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
or current waveforms
–voltage
V
g
is(t)
t
Conducting D 1
devices: D
4
“Soft”
turn-on of
Q 1, Q 4
t
Q1
Q4
D2
D3
Q2
Q3
Conduction sequence: D1–Q1–D2–Q2
Q1 is turned on during D1 conduction
interval, without loss
“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
Soft switching in a PWM converter
Example: forward converter with active clamp circuit
Forward converter
Switching transitions are resonant, remainder
of switching period is not resonant
Transistors operate with zero voltage
switching
Beware of patent issues
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Chapter 19: Resonant Conversion
Analysis of resonant converters
Series resonant dc-dc converter example
• Complex!
• Small ripple
approximation
is not valid
Need new
approaches:
• Sinusoidal
approximation
• State plane
analysis
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Chapter 19: Resonant Conversion
Outline of course
1. Analysis of resonant converters using the sinusoidal approximation
• Classical series, parallel, LCC, and other topologies
• Sinusoidal model
• Zero voltage and zero current switching
• Resonant converter design techniques based on frequency response
2. Sinusoidal analysis: small-signal ac behavior with frequency modulation
• Spectra, beating, and envelope response
• Phasor transform method
3. State-plane analysis of resonant, quasi-resonant, and other soft-switching
converters
• Fundamentals of state-plane and averaged modeling of resonant circuits
• Exact analysis of the series and parallel resonant dc-dc converters
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Chapter 19: Resonant Conversion
Outline, p. 2
4. Resonant switch and related converters
• Quasi-resonant topologies and their analysis via state-plane
approach
• Quasi-square wave converters
• Zero voltage transition converter
• Soft switching in forward and flyback converters
• Multiresonant and class E converter
5. Server systems, portable power, and green power issues
• Modeling efficiency vs. load, origins of loss
• Variable frequency approaches to improving light-load efficiency
– DCM
– Burst mode
• Effects of parallel modules
• DC transformers
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Chapter 19: Resonant Conversion