Download Snubber Circuits

Lecture Notes
Snubber Circuits
Outline
A.
Overview of Snubber Circuits
B.
Diode Snubbers
C.
Turn-off Snubbers
D.
Overvoltage Snubbers
E.
Turn-on Snubbers
F.
Thyristor Snubbers
Copyright © by John Wiley & Sons 2003
Snubbers - 1
Overview of Snubber Circuits for Hard-Switched Converters
Function: Protect semiconductor devices by:
• Limiting device voltages during turn-off transients
• Limiting device currents during turn-on transients
• Limiting the rate-of-rise (di/dt) of currents through
the semiconductor device at device turn-on
• Limiting the rate-of-rise (dv/dt) of voltages across
the semiconductor device at device turn-off
• Shaping the switching trajectory of the device as it
turns on/off
Copyright © by John Wiley & Sons 2003
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 dv/dt during device turn-off
3. Polarized L-R snubbers
• Used as turn-on snubbers to shape the turn-off
switching trajectory of controlled switches.
• Limit di/dt during device turn-on
Snubbers - 2
Need for Diode Snubber Circuit
d i Df
+
V
d
-
L
Rs
Io
Df
Cs
Io
d t
Vd
=
L
t
i Df(t)
I rr
Sw
v
t
(t)
Df
Vd
• L = stray inductance
• Sw closes at t = 0
• Rs - Cs = snubber circuit
•
• Diode voltage
without snubber
diL
Diode breakdown i f V d + L 
dt
Copyright © by John Wiley & Sons 2003
L
di L
d t
> BV BD
Snubbers - 3
Equivalent Circuits for Diode Snubber
L
• Simplified snubber the capacitive snubber
+
Vd
Rs
cathode
Diode
snap-off
L
anode
+
-
+
v
Cs
-
V
d
i Df
-
t
• Rs = 0
• Worst case assumptiondiode snaps off instantaneously
at end of diode recovery
•v
d 2 v Cs
v Cs
= -v
Cs
Df
Vd
•
Governing equat ion -
•
Boundary cond it ions - v Cs(0 + ) = 0 and iL(0 + ) = I rr
Copyright © by John Wiley & Sons 2003
Cs
dt 2
+
LCs
=
LCs
Snubbers - 4
Performance of Capacitive Snubber
•
v Cs( t ) = V d - V d cos( o t ) + V d
•
•
Cbase
sin( o t )
Cs
o =
1
;
L Cs
V cs,max = V d

1

 I 2
 rr 
C base = L  

V d 
Cbase
1 +
Cs
+



5
4
V
Cs,max
3
Vd
2
1
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
C base
Cs
Copyright © by John Wiley & Sons 2003
Snubbers - 5
Effect of Adding Snubber Resistance
Snubber Equivalent Circuit
•
Governing equat ion L 
•
Boun dary con dition s
i( 0 + ) = I rr
and
d2 i
di
i
+ Rs
+
= 0
2
dt
C
dt
s
V d - I rr Rs
di(0 + )
=
dt
L
Diode voltage as a function of time
Vdf
Vd (t) = - 1 a = o
e-t
sin(at -  + ) ; Rs Š 2 Rb
 cos()
Rs
2
1- (/ o) ;  = 2 L ; o =



1
 (2-x)  
-1
;  = tan 

LCs
 4 - x2
L [Irr]2
Cs
Rs
Vd
 = C ; x = R ; Rb = I
; Cb =
;  = tan-1(/a)
2
b
b
rr
Vd
Copyright © by John Wiley & Sons 2003
Snubbers - 6
Performance of R-C Snubber
3
•
•
•
•
At t = t m v Df ( t ) = V max
t an-1 ( a/ )
 - 
tm =
+ 
•0
a
a
V max
Vd
 =
= 1 +
Cs
Cbase
1+
and
-1 - x
x=
R s,opt
C s= C base
R
= 1.3
base
2
V max
exp( -t m )
V
d
1
Rs
Rbase
R sI rr
•
Ls I rr 2
Cbase =
Vd2
and
Rbase =
Copyright © by John Wiley & Sons 2003
V
Vd
Irr
d
0
0
1
Rs
R base
2
Snubbers - 7
Diode Snubber Design Nomogram
Wtot
2
L s I rr/2
3
WR
2
L s I rr/2
0
2
0
V max
Vd
0
0
0
0 0
0 0
0 0
0 0 0 0
0 0 0 0 0
1
0
0
for R = R
s
s,opt
0 0
R s,op
R base
0
0
Copyright © by John Wiley & Sons 2003
1
C s / C base
2
3
Snubbers - 8
Need for Snubbers with Controlled Switches
Step-down converter
L
L1
V
d
L2
i
L3
Switch current and voltage waveforms
sw
Sw
Io
i
I rr
sw
I
+
vsw
-
di
dt
L
o
I
V
d
o
vsw
to
•
di
dt
t
t
1
t
3
4
t5
t
6
L1 , L , L = stray inductances
2
3
Switching trajectory of switch
•
L = L1 + L + L
2
3
idealized
s witching
loci
i sw
t 5
t6
turn-off
to
t1
turn-on
• Overvoltage at turn-off
due to stray inductance
• Overcurrent at turn-on due to
diode reverse recovery
t
4
t3
vsw
Vd
Copyright © by John Wiley & Sons 2003
Snubbers - 9
Turn-off Snubber for Controlled Switches
Step-down converter with turn-off snubber
Equivalent circuit during switch turn-off.
• Simplifying assumptions
1. No stray inductance.
2. isw(t) = Io(1 - t/tfi)
3. isw(t) uneffected by snubber circuit.
Copyright © by John Wiley & Sons 2003
Snubbers - 10
Turn-off Snubber Operation
•
Capac ito r v olt age and current for 0 < t <
•
Io t
t f i i Cs( t ) =
t fi
and v (t) =
Cs
Io t
2
2C t
s fi
Io t f i
For Cs = Cs1 , v Cs = V d at t = t f i yiel ding C s1 =
2 Vd
Circuit waveforms for var yin g values of Cs
Copyright © by John Wiley & Sons 2003
Snubbers - 11
Benefits of Snubber Resistance at Switch Turn-on
v sw
• Ds shorts out Rs
during Sw turn-off.
• During Sw turn-on,
Ds reverse-biased and
Cs discharges thru Rs.
i
D
t rr
I rr
f
i sw
Io
Vd
Rs
I rr
• Turn-on with Rs > 0
• Turn-on with Rs = 0
• Energy stored on Cs dissipated
in Sw.
• Energy stored on Cs dissipated
in Rs rather than in Sw.
• Voltage fall time kept quite
short.
• Extra energy dissipation in Sw
because of lengthened voltage
fall time.
Copyright © by John Wiley & Sons 2003
Snubbers - 12
Effect of Turn-off Snubber Capacitance
Energy dissipation
WR = d issipat ion in
resistor
WT = d issipat ion in
switc h Sw
Io t f i
Cs1 =
2 Vd
Wtota l = W R + W T
i sw
Io
Wbase = 0 .5 V d Io t f i
Cs < Cs1
RBSOA
Switching trajectory
Cs = Cs1
Cs > Cs1
V
Copyright © by John Wiley & Sons 2003
d
v sw
Snubbers - 13
Turn-off Snubber Design Procedure
Selection of Cs
• Minimize energy dissipation (WT) in BJT at turn-on
• Minimize WR + WT
• Keep switching lo cus within RBSOA
• Reasonable value is Cs = Cs1
Snubber recovery time (BJT in on-state)
• Capacitor voltage = Vd exp(-t/RsCs)
• Time for vCs to drop to 0.1V d is 2.3 RsCs
Selection of Rs
• BJT must remain on for a time of 2.3 R sCs
Vd
• Limit icap(0+) = R < Irr
s
Vd
• Usually designer specifies Irr < 0.2 I o so R = 0.2 I o
s
Copyright © by John Wiley & Sons 2003
Snubbers - 14
Overvoltage Snubber
• Step-down converter with
overvoltage snubber comprised
of Dov, Cov, and Rov.
• Overvoltage snubber limits
overvoltage (due to stray I
nductance) across Sw as it
turns off.
•
Swit ch Sw wavefor ms w it hout overvolta ge snubb er
•
t f i = s wit ch cur rent fall t ime ; kV d = overvo lta ge o n Sw
diL
Io
• kV d = L 
= L
dt
t fi
• L =
Copyright © by John Wiley & Sons 2003
kV d t f i
Io
Snubbers - 15
Operation of Overvoltage Snubber
š
•
Dov on f or 0 < t <
š
•
t f i <<
LCov
2
LCov
2
• Dov,Cov provide alternate path
for inductor current as Sw turns
off.
•
Switch current can fall to zero
much faster than Ls current.
•
Df forced to be on
(approximating a short ckt) by Io
after Swis off.
•
Equivalent circuit after turn-off
of Sw.
Copyright © by John Wiley & Sons 2003
Snubbers - 16
Overvoltage Snubber Design
• Cov =
•
•
( v sw,max ) 2
Limi t v sw,max t o 0 .1 V d
kV d t f i
Using L s =
Io
• Cov =
•
L s Io 2
kV d t f iIo 2
Io (0 .1 V d ) 2
in equat ion for Cov yiel ds
=
1 0 0k t f i Io
Vd
Cov = 20 0 k Cs1 wh ere Cs1 =
t f iIo
2 Vd
which is u sed
in t ur n-off snubbe r
• Recovery t ime of Cov ( 2. 3 Rov Cov ) must be le ss
th an of f -t ime d ura t ion, t off , o f th e swit ch Sw.
•
Rov -
t off
2 .3 C ov
Copyright © by John Wiley & Sons 2003
Snubbers - 17
Turn-on Snubber
Step-down converter
with turn-on snubber
• Snubber reduces Vsw at switch
turn-on due drop across
inductor Ls.
• Will limit rate-of-rise of switch
current if Ls is sufficiently
large.
Switching trajectory with and without turn-on snubber.
Copyright © by John Wiley & Sons 2003
Snubbers - 18
Turn-on Snubber Operating Waveforms
Small v alues o f snu bber in ductan ce (L s < L s1 )
•
•
•
•
•
disw
con t rol led b y s wit ch S w
dt
and dr ive circui t .
v sw =
LsIo
t ri
Large values o f snu bber induc t anc e (L s > L s1 )
Vd
Io
limit ed by c ircu it to
<
Ls
t ri
disw
dt
Ls1 =
V d t ri
Io
Irr reduc ed wh en L s > L s1 b ecause Irr proport iona l to
Copyright © by John Wiley & Sons 2003
disw
dt
Snubbers - 19
Turn-on Snubber Recovery at Switch Turn-off
I o RLs exp(-R Ls t/L s )
Io R
Ls
is
w
v
s
w
V
d
Io
t rv
• Switch waveforms at turn-off with turn-on snubber in circuit.
• Assume switch current fall time
tri = 0.
• Inductor current must discharge
thru DLs- RLs series segment.
• Overvoltage smaller if tfi smaller.
• Time of 2.3 L s/RLs required for inductor current to decay to 0.1 I o
• Off-time of switch must be > 2.3 Ls/RLs
Copyright © by John Wiley & Sons 2003
Snubbers - 20
Turn-on Snubber Design Trade-offs
Selection of indu cto r
• Larger L s decreases energy dissipation in switch at turn-on
• Wsw = WB (1 + Irr/Io)2 [1 - Ls/Ls1]
• WB = VdIotfi/2 and Ls1 = Vdtfi/Io
• Ls > Ls1 Wsw = 0
• Larger L s increases energy dissipation in R Ls
• WR = WB Ls / Ls1
• Ls > Ls1 reduces magnitude of reverse recovery current Irr
• Inductor must carry current Io when switch is on - makes
inductor expensive and hence turn-on snubber seldom used
Selection o f resistor RLs
• Smaller values of RLs reduce switch overvoltage Io RLs at turn-off
• Limiting overvoltage to 0.1V d yields RLs = 0.1 V d/Io
• Larger values of RLs shortens minimum switch off-time of 2.3 L s/RLs
Copyright © by John Wiley & Sons 2003
Snubbers - 21
Thyristor Snubber Circuit
3-phase thyristor circuit with snubbers
•
v an( t ) = V ssin( t ) , v bn ( t ) = V ssin( t - 120 ) ,
v cn ( t ) = V ssin( t - 240 )
Phase-to-neutral waveforms
v an

v
bn
3 V ssin( t - 60 )
•
v LL ( t ) =
•
Maximum r ms lin e-t o-lin e vol t age V LL =
Copyright © by John Wiley & Sons 2003
3
Vs
2
v LL = v
bn
v an = v
ba
 t1
Snubbers - 22
Equivalent Circuit for SCR Snubber Calculations
Assumptions
•
Trigger angl e  = 90  so tha t v LL ( t ) = ma ximum =
2 V LL
Equivalent circuit after T1 reverse recovery
•
Reve rse r ecove ry t ime t rr << perio d of a c w avefo rm so t hat
v LL ( t ) equals a cons t ant valu e of v ba( t 1 ) =
•
•
•
i
2 V LL
Worst c ase st ray induc t anc e L g ive s rise to react anc e equal
to o r less th an 5 % o f line imped ance.
Line imped anc e =
Vs
2 Ia1
=
2 V LL
6 Ia1
=
V LL
2L 
+
V (  t)1
ba
3 Ia1
L
P
T1 after
recovery
Cs
i
-
T 3 (on)
T1
Rs
A
wh ere Ia1 = rms value of f undame nt al co mp onent o f th e
line cu rrent .
V LL
L = 0.05
3 Ia1
Copyright © by John Wiley & Sons 2003
Snubbers - 23
Component Values for Thyristor Snubber
•
•
Use s ame d esign as f or diode snubb er bu t adapt th e f or m ulas to th e
th y risto r cir cui t not ation
Snub b er capacit or Cs = Cbase = L 
•
Fro m s nubb er eq uiv alent ci r cui t 2
•
Ir r =
diL
dt
t rr
2 V LL
=
L
diL
0.05
2
3 I a1 
Vd =
•
Cs = Cbase =
•
Snub b er resist anc e Rs = 1.3 Rbase = 1.3
2 V LL
0.05
•
2 V LL
t r r = 25 Ia1 t r r
V LL
•
•
=
dt
2 V LL
t rr =
2 L
 I 2
 rr 


V d 
Rs
=
V LL
3 I a1 
1.3
25



2 V LL
25 Ia1 t r r
2
Ia1 t r r 

=

2 V LL 
0.07
=
8.7
Ia1 t r r
V LL
Vd
Irr
V LL
Ia1 t rr
Ene r gy dissipat ed pe r c y cle in snubb er re sist ance = W R
•
WR =
L Ir r 2
2
Copyright © by John Wiley & Sons 2003
+
CsV d 2
2
=
18  Ia1 V LL ( t rr ) 2
Snubbers - 24