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Power Electronics
Lecture(9)
Prof. Mohammed Zeki Khedher
Department of Electrical Engineering
University of Jordan
1
Short pulse
Long pulse
Pulse train generator
Pulse train with timer
and AND gate
Gate Triggering Methods
- Efficient & reliable method for turning on SCR.
• Types
 R - Triggering.
 RC - Triggering.
 UJT - Triggering.
6
R-Triggering
vO
a
b
LOAD
i
R1
R2
vS=Vmsint
VT
D
R
Vg
Resistance firing circuit
7
RC Triggering
vO
LOAD
+
R
D2
vS=Vmsint
VC
C
D1
VT
-
RC half-wave trigger circuit
8
Gate triggering characteristics
Gate input characteristics
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IGM w
IG
rp
o
n
s
x
q
y
z
1
3
4Load line
m
VG
VGD
FIGURE 18.2 Typical gate characteristics of an SCR.
Gate protection circuit
High temperature due to:
Lecture Notes
Snubber Circuits
Outline
Copyright © by John Wiley & Sons 2003
A.
Overview of Snubber Circuits
B.
Diode Snubbers
C.
Turn-off Snubbers
D.
Overvoltage Snubbers
E.
Turn-on Snubbers
F.
Thyristor Snubbers
Snubbers - 18
Overview of Snubber Circuits for Hard-Switched Converters
Function: Protect semiconductor devices by:
Types of Snubber Circuits
• 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
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 - 19
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
•
Copyright © by John Wiley & Sons 2003
• Diode voltage
without snubber
diL
Diode breakdown i f V d + L 
dt
> BV BD
Snubbers - 20
L
di L
d t
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 - 21
Need for Snubbers with Controlled Switches
Step-down converter
L1
V
d
i
L3
•
L2
sw
Io
+
vsw
-
Sw
L1 , L , L = stray inductances
2
3
Switching trajectory of switch
•
L = L1 + L + L
2
3
idealized
s witching
loci
i sw
t6
t 5
turn-off
to
t1
turn-on
• Overcurrent at turn-on due to
diode reverse recovery
t
4
t3
Vd
Copyright © by John Wiley & Sons 2003
• Overvoltage at turn-off
due to stray inductance
Snubbers - 22
vsw
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 - 23
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 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 - 24
Io t
2
2C t
s fi
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 - 25
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
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
Snubbers - 26
bn
v an = v
ba
 t1
bn
Trajectory comparision
with and without capacitor
Turn off snubber circuit