This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
1
Soft-Switching in DC-DC Converters: Principles, Practical Topologies,
Some of the slides in this presentation are used for the course EE5741 Advanced Power Electronics given by Prof Robbins and Prof Mohan at the University of Minnesota
2
Objectives
• What is soft-switching?• Basic principles• Concentration on a few
popular topologies• Design techniques• Computer simulations• New developments
4
What is Soft-Switching
• Switching transitions occur under favorable conditions – device voltage or current is zero
Possible Solutions (combination)• Snubbers to reduce di/dt and dv/dt
• Circuit layout to reduce stray inductances• Gate drive
• Soft switching to achieve ZVS and/or ZCS
usually no change in losses (unless loss recovery)
circuit layoutturn on / off speeds
• Switching losses • Device stress, thermal management• EMI due to high di/dt and dv/dt• Energy loss in stray L and C
12
Snubbers• Passive components (R, L, C) and a diode to shape
switching trajectories
Turn-on snubber (seldom used)
i t VL
tTd
s( )=
• low di/dt• lower turn-on losses in the device• low reverse recovery current
At turn-on
Price to be paid at turn-off• 1/2 LI2 energy dissipated during off interval• off interval > 2 to 3 times LS /RS time constant• switch voltage rating increases by RS IO
iT
Ls Rs
Io
+-
vT
t
Vd
vT
iTt0
0
iT
Ls Rs
Io
+-
vT
t
Vd
vT
iTt0
0
13
Turn-off Snubbers
At turn-off• while builds up
• switch turn-off loss decreases• lower dv/dt
i I iT o CS
i DC SS
= −( flows through )
Issues at turn-on• 1/2 CV2 energy dissipated in RS and switch• switch current rating increases by• ON interval > 2 to 3 times RS CS time constant
vT
Vd /R S
iT
Io
+
-vT CS
RS
Vd
DSiCS
iT
Io
+
-vT CS
RS
Vd
DSiCS
vT
iTIo
Vd0
CS1
CS2CS3
0C C CS S S3 2 1
> >
CS =0
vT
iTIo
Vd0
CS1
CS2CS3
0C C CS S S3 2 1
> >
CS =0
14
Soft-Switching
• ZVS (Zero Voltage Switching)• ZCS (Zero Current Switching)
Advantages - Lower losses (may be !) - Low EMI (may be !) - Allows high frequency operation
15
ZVS (Zero Voltage Switching)
Turn OFFTurn ON
• Switch voltage brought to zero before gate voltage is applied
• Ideal, zero-loss transition
• Low-loss transition• Parallel capacitor as a
loss-less snubber
• Preferred scheme for very high frequency applications using MOSFETs
16
ZCS (Zero Current Switching)
Best suited for converters with IGBTs due to tail current at turn-off
• Switch current brought to zero before gate voltage is removed
• Ideal, zero-loss transition
Turn OFF
Turn ON• Low-loss transition• Series inductor as a loss-less snubber• Energy in junction capacitance is lost
17
ZVS and Hard-Switched Waveforms
Zero-voltage switched Hard-switched
gate sourcev −
drain sourcev −
12V−
12V
gate sourcev −
drain sourcev −
0V 0V
12V−
18
An Example: Zero Voltage Transition (ZVT)
At t =0 is turned off, T +
( )( )
0 0
0-
C
dC
v
v V+ =
=
- dC Cv v V+ + =
dV
+
+
−
−oV
T+
T−
D+
D−
L
Li
C+
-C
Ci +
-Ci
A
Synchronous Buck Converter
Li
0
19
- dC CSince v v V+ + =
0-C C
s s
dv dvC C
dt dt++ =
0-C Ci i+∴ + =
0
-
C
dC
v
v V+∴ =
=0
dV
- LC CAlso, i - i i+ =
2-L
C C
ii -i+∴ = =
• At the end of this charge/discharge interval, positive iL is carried by
• Subsequently, is turned on; iL must reverse direction