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Snubbers - 1W.P. Robbins © 1997
Snubber Circuits
A. Overview of Snubber Circuits
B. Diode Snubbers
C. Turn-off Snubbers
D. Overvoltage Snubbers
E. Turn-on Snubbers
F. Thyristor Snubbers
William P. RobbinsProfessor
Dept. of Electrical EngineeringUniversity of Minnesota
200 Union St. SE.Minneapolis, MN 555455
Copyright 1997
Snubbers - 2W.P. Robbins © 1997
Function of Snubber Circuits
• Protect semiconductor devices by:
• Limiting device voltages during turn-off transients
• Limiting device currents during turn-on transients
• Limiting the rate-of-rise (didt
) of currents through the
semiconductor device at device turn-on
• Limiting the rate-of-rise (dvdt
) of voltages across the
semiconductor device at device turn-off
• Shaping the switching trajectory of the device as itturns on/off
Snubbers - 3W.P. Robbins © 1997
Types of Snubber Circuits
1. Unpolarized series R-C snubbers
• Used to protect diodes and thyristors
2. Polarized R-C snubbers
• Used as turn-off snubbers to shape the turn-on switching trajectory of controlled switches.
• Used as overvoltage snubbers to clamp voltages applied to controlled switches to safe values.
• Limit dvdt
during device turn-off
3. Polarized L-R snubbers
• Used as turn-on snubbers to shapte the turn-off switching trajectory of controlled switches.
• Limit didt
during device turn-on
Snubbers - 4W.P. Robbins © 1997
Need for Diode Snubber Circuit
-
+
Df
R s
Cs
Lσ
Vd
I o
Sw
I rr
i Dfd
d t
VdLσ
=Io
t
tVd
i (t)Df
v (t)Df
• Diode voltage without snubber
d
d tLσ
i Lσ
L = stray inductanceσ
S closes at t = 0w
R - C = snubber circuits s
•
•
•
• Diode breakdown if Vd + Lσ
diLσdt
> BVBD
Snubbers - 5W.P. Robbins © 1997
Equivalent Circuits for Diode Snubber
Cs
R s
Lσ
+
-
Vdanode
cathode Diodesnap-off
• Worst case assumption- diode snaps off instantaneously at end of diode recovery
i Df
Cs
+
-
Vd
Lσ• R = 0s
v Cs
+
-
• v = -v Cs Df
• Simplified snubber - the capacitive snubber
t
• Governing equation - d2vCs
dt2 +
vCsLσCs
= Vd
L σCs
• Boundary conditions - vCs(0+) = 0 and iLσ(0+) = Ir r
Snubbers - 6W.P. Robbins © 1997
Performance of Capacitive Snubber
• vCs(t) = Vd - Vd cos(ωot) + Vd Cbase
Cs sin(ωot)
• ωo = 1
LσCs ; Cbase = Lσ
I r r
Vd
2
• Vcs,max = Vd
1 + 1 + Cbase
Cs
0
1
2
3
4
5
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
d
VCs,max
V
C
Cs
base
Snubbers - 7W.P. Robbins © 1997
Effect of Adding Snubber Resistance
• Equivalent circuit with snubber resistance Rs
-
Vd
Cs
R s
Lσ
+
+
-
v (t)Df
• Governing equation LσCs d2vDf
dt2 + RsCs
dvDfdt
+ vDf = -Vd
• Boundary conditions
vDf(0+) = - Ir rRs and
dvDf(0+)
dt = -
I r rCs
- RsVd
Lσ +
I r rRs2
Lσ
• Solution for vDf(t)
vDf(t) = - Vd - Vd e- αt
CbaseCs
sin(ωat - φ − ξ)
ωa = ωo 1 - α2
ωo2
; ωo = 1
LσCs ; α =
Rs2Lσ
tan(φ) = - Rb
ωaLσ -
αωa
; tan(ξ) = αωa
; Rbase = VdI rr
, Cbase = Lσ
(Vd/Irr)2
Snubbers - 8W.P. Robbins © 1997
Performance of R-C Snubber
• At t = tm vDf(t) = Vmax
• tm = tan-1(ωa/α)
ωa +
φ - ξωa
≥ 0
•Vmax
Vd = 1 + 1 + CN
- 1 - R N exp(-αtm)
• CN = Cs
Cbase and RN =
RsRbase
• Cbase = Ls I r r
2
Vd2
and Rbase = VdI rr
0
1
2
3
0 1 2
C = Cs base
R Is rr
Vd
V
V
max
d
R sR base
R s,opt
R base= 1.3
Snubbers - 9W.P. Robbins © 1997
Diode Snubber Design Nomogram
0
0
0
00000000000000000 0 0 00
0
1
2
3
0 1 2 3
R
R s,op
base
V
V
max
ds,opt
for R = Rs
L I /2s rr2
WR
Wtot
L I /2s rr2
base/ CCs
Snubbers - 10W.P. Robbins © 1997
Need for Snubbers with Controlled Switches
t o t1
t3 t4
t5 t6
L didt
L didt
VdIoIo
I o
Vd
L 1 L 2
L 3Sw
vsw
+
-
isw
vsw
isw
turn-on
turn-off
idealized switching loci
t ot1
t3t4
t 5t6
Vd
isw
vsw
• Overvoltage at turn-off due to stray inductance
• Overcurrent at turn-on due to diode reverse recovery
L1 L2 L3• , , = stray inductances
L1 L2 L3L = ++•
Snubbers - 11W.P. Robbins © 1997
Turn-on Snubber for Controlled Switches
• Circuit configuration
Df
Ds
Cs
Rs
Vd
Io+
-
iDF
iCs
Turn-off snubber
Sw
• Equivalent circuit during switch turn-off
Cs
Io - iV
d
i sw
DfI o
sw
• Assumptions
1. No stray inductance.
2. i sw(t) = Io(1 - t/tf i )
3. i sw(t) uneffected by
snubber circuit.
Snubbers - 12W.P. Robbins © 1997
Turn-off Snubber Operation
• Capacitor voltage and current for 0 < t < tf i
• i Cs(t) = I ot
tf i and vCs(t) =
I ot2
2Cstf i
• For Cs = Cs1, vCs = Vd at t = tf i yielding Cs1 = I otf i2Vd
• Circuit waveforms for varying values of Cs
iDf
iCs
Vd
vCs
C = Cs s1
t f i
isw
iDf
iDf
Io
t f it f i
iswi sw
C < Cs s1 C > Cs s
Snubbers - 13W.P. Robbins © 1997
Benefits of Snubber Resistance at Sw Turn-on
IoDf
Cs
Io
RsVd
DsSw
t rrt ri +
i sw
t rr
0
discharge of C s
t ri t2
VdIo
vsw
vsw
Irr
Io
I rr
iDf
t rr
VdRsi sw
• Ds shorts out Rs during Swturn-off.
• During Sw turn-on, Dsreverse-biased and Csdischarges thru Rs.
• Turn-on with Rs = 0
• Energy stored on Csdissipated in Sw.
• Extra energy dissipation in Swbecause of lengthenedvoltage fall time.
• Turn-on with Rs > 0
• Energy stored on Csdissipated in Rs
rather than in Sw.
• Voltage fall time keptquite short.
Snubbers - 14W.P. Robbins © 1997
Effect of Snubber Capacitance
• Switching trajectory
Cs < Cs1
Cs = Cs1
Cs > Cs1
Io
Vdvsw
i sw
RBSOA
• Energy dissipation
0
0.2
0.4
0.6
0.8
1
0 0.2 0.4 0.6 0.8 1 1.2 1.4
WT / base
WR / Wbase
Wtotal / base
Cs / Cs1
W
W
WR = dissipation in
resistor
WT = dissipation in
switch Sw
Cs1 = I otf i2Vd
Wtotal = WR + WT
Wbase = 0.5 VdI otf i
Snubbers - 15W.P. Robbins © 1997
Turn-off Snubber Design Procedure
• Selection of Cs
• Minimize energy dissipation (WT) in BJT at turn-on
• Minimize WR + WT
• Keep switching locus within RBSOA
• Reasonable value is Cs = Cs1
• Selection of Rs
• Limit icap(0+) = VdRs
< Ir r
• Usually designer specifies Ir r < 0.2 Io so
VdRs
= 0.2 Io
• Snubber recovery time (BJT in on-state)
• Capacitor voltage = Vd exp(-t/RsCs)
• Time for vCs to drop to 0.1Vd is 2.3 RsCs
• BJT must remain on for a time of 2.3 RsCs
Snubbers - 16W.P. Robbins © 1997
Overvoltage Snubber for Controlled Switches
• Circuit configuration - Dov, Rov, and Cov form overvoltage snubber
+
-
Vd
L σ
Df I oRov
CovDovSw
• Overvoltage snubber limits magnitude of voltage developed across Sw as it turns off.
• Switch Sw waveforms without overvoltage snubber
• tf i = switch current fall time ; kVd = overvoltage on Sw
o tfi
VdI o
kVdi sw
vsw
• kVd = Lσ
diLσdt
= LσI otf i
• Lσ = kVdtf i
I o
Snubbers - 17W.P. Robbins © 1997
Operation of Overvoltage Snubber
• Dov,Cov provide alternate path for inductor current as Sw turns off.
• Switch current can fall to zero much faster than Lσ current.
• Df forced to be on (approximating a short ckt) by Io after Sw is off.
• Equivalent circuit after turn-off of Sw.
+
-
Vd
L σRov
Cov
Dov
iLσ
vCov
+
-
vCov(0+) = Vd
• Equivalent circuit while inductor current decays to zero
+
-
Vd
L σCov
i Lσ
+
-
iLσ(0+) = Io
0π Lσ Cov
4
iLσ
vsw
vCov
I o Vd
Charge-up of Cov from Lσ
Cov (∆Vsw,max)2
2 = Lσ ( Io)2
2
∆Vsw,max
• Energy transfer from Lσ to Cov
Discharge of Cov thru Rovwith time constant R C ovov
iLσ
(t) = I cos[ ] Lσ Cov
to
.
• Dov on for 0 < t < π LσCov
2
• tf i << π LσCov
2
Snubbers - 18W.P. Robbins © 1997
Overvoltage Snubber Design
• Cov = Ls I o
2
(∆vsw,max)2
• Limit ∆vsw,max to 0.1Vd
• Using Ls = kVd tfi
I o in equation for Cov yields
• Cov = kVdtfiIo
2
I o(0.1Vd)2 =
100k tf i Io
V d2
• Cov = 200 Cs1 where Cs1 = tfiIo 2Vd
which is used
in turn-off snubber
• Recovery time of Cov (2.3RovCov) must be less
than off-time duration, toff, of the switch Sw.
• Rov ≈ toff
2.3 Cov
Snubbers - 19W.P. Robbins © 1997
Turn-on Snubber Circuit
• Circuit topology
Vd
+
-
LsD
Ls
Df
R Ls
Io
Sw
Vd
-
LsD
Ls
Df RLs Io
Sw
Df
+
Snubber circuit
• Circuit reduces Vsw as switch Sw turns on. Voltage drop Ls
diswdt
provides the voltage reduction.
• Switching trajectories with and without turn-on snubber.
swi
vswVd
I o
L sdiswdt
Without snubber
With snubber
Snubbers - 20W.P. Robbins © 1997
Turn-on Snubber Operating Waveforms
• Small values of snubber inductance (Ls < Ls1)
tr i t rr
rrI
I oVd
vsw
i sw
• Large values of snubber inductance (Ls > Ls1).
I o
Vd
vsw
i sw
rrI
reduced
t ≈ > t + t IL os
Vd
on r i rr
• I rr reduced when Ls > Ls1 because Ir r proportional to disw
dt
•disw
dt controlled by
switch Sw and
drive circuit.
• ∆vsw = LsI otr i
•disw
dt limited by circuit
to VdLs
< I otr i
• Ls1 = Vdtr i
I o
Snubbers - 21W.P. Robbins © 1997
Turn-on Snubber Recovery at Switch Turn-off
Vd
+
-
LsD
Ls
Df
RLs
Io
Sw
vsw
i sw V
d
I o R Ls
I o
t rv
IoRLsexp(-RLst/Ls)
• Overvoltage smaller if tf i smaller.
• Time of 2.3 Ls/RLs required for inductor current to decay to 0.1 Io
• Off-time of switch must be > 2.3 Ls/RLs
• Assume switch current fall timetri = 0.
• Inductor current must discharge thru DLs- RLs series segment.
• Switch waveformsat turn-off with turn-on snubber in circuit.
Snubbers - 22W.P. Robbins © 1997
Turn-on Snubber Design Trade-offs
• Selection of inductor Ls
• Larger Ls decreases energy dissipation in switch at turn-on
• Wsw = WB (1 + Ir r/ Io)2 [1 - Ls/Ls1]
• WB = VdI otf i /2 and Ls1 = Vdtf i / Io
• Ls > Ls1 Wsw = 0
• Larger Ls increases energy dissipation in RLs
• WR = WB Ls / Ls1
• Ls > Ls1 reduces magnitude of reverse recovery current Ir r
• Inductor must carry current Io when switch is on - makes
inductor expensive and hence turn-on snubber seldom used
• Selection of resistor RLs
• Smaller values of RLs reduce switch overvoltage Io RLs at
turn-off
• Limiting overvoltage to 0.1Vd yields RLs = 0.1 Vd/Io
• Larger values of RLs shortens minimum switch off-time of
2.3 Ls/RLs
Snubbers - 23W.P. Robbins © 1997
Thyristor Snubber Circuit
id
1 3 5
24 6
+
+
+-
-
-
van
vbn
Rs
Cs
A
B
C
P
L
L
L
vcn
• van(t) = Vssin(ωt), vbn(t) = Vssin(ωt - 120°), vcn(t) = Vssin(ωt - 240°)
• Phase-to-neutral waveforms
vanvbn = v
ba
vanvbn
t1=vLL
• vLL(t) = 3 Vssin(ωt - 60°)
• Maximum rms line-to-line voltage VLL = 32
Vs
Snubbers - 24W.P. Robbins © 1997
Equivalent Circuit for SCR Snubber Calculations
• Equivalent circuit after T1 reverse recovery
Cs
Rs
Vbc( )
t1T (on)3
T afterrecovery1
iT1
P
A
2 L
iL
+
-
• Assumptions
• Trigger angle α = 90° so that vLL(t) = maximum = 2 VLL
• Reverse recovery time trr << period of ac waveform so that
vLL(t) equals a constant value of vbc(ωt1) = 2 VLL
• Worst case stray inductance Lσ gives rise to reactance equal
to or less than 5% of line impedance.
• Line impedance = Vs
2Ia1 =
2VLL
6Ia1 =
VLL
3Ia1
where Ia1 = rms value of fundamental component of the
line current.
• ωLσ = 0.05 VLL
3Ia1
Snubbers - 25W.P. Robbins © 1997
Component Values for Thyristor Snubber
• Use same design as for diode snubber but adapt the formulas to the thyristor circuit notation
• Snubber capacitor Cs = Cbase = Lσ
I r r
Vd
2
• From snubber equivalent circuit 2 Lσ diLσdt
= 2 VLL
• I rr = diLσdt
trr = 2VLL2Lσ
trr = 2VLL
2 0.05 VLL
3 I a1ω
trr = 25 ωI a1trr
• Vd = 2 VLL
• Cs = Cbase = 0.05 VLL
3 I a1ω
25 ωI a1trr
2VLL 2
= 8.7 ωI a1trr
VLL
• Snubber resistance Rs = 1.3 Rbase = 1.3 VdI rr
• Rs = 1.3 2VLL
25ωI a1trr =
0.07 VLL ωI a1trr
• Energy dissipated per cycle in snubber resistance = WR
• WR = LσI r r
2
2 +
CsVd2
2 = 18 ω Ia1 VLL(trr)2