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
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
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
di
+
V
d
-
L
Rs
Io
Df
Cs
Io
Df
=
d t
V
d
L
t
i Df(t)
I rr
Sw
t
v (t)
Df
Vd
• L = stray inductance
• Sw closes at t = 0
• Rs - C s = snubber circuit
•
Copyright © by John Wiley & Sons 2003
• Diode voltage
without snubber
diL
Diode breakdown if V d + L 
dt
> BVBD
Snubbers - 20
L
di L
d t
Effect of Adding Snubber Resistance
Snubber Equivalent Circuit
•
Governing equat ion L 
•
Boundary 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 R b
 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 2 , L 3 = str ay i nductances
Switching trajectory of switch
•
L = L1 + L 2 + L 3
ideal ized
s wi tchi ng
lo ci
i sw
t6
t
t 5
turn-of f
o
t1
turn-on
• Over cur rent at tur n-on due to
diode rever se recovery
t
4
t3
V
Copyright © by John Wiley & Sons 2003
• Over voltage at tur n-off
due to str ay inductance
d
Snubbers - 22
vsw
Turn-off Snubber for Controlled Switches
Step-down converter with turn-off snubber
Equivalent circuit during switch turn-off.
• Simplifying a ssumptions
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
•
Capacito r v olt age and current for 0 < t <
•
Iot
t f i i Cs(t ) =
t fi
and v (t) =
Cs
Iot f i
For Cs = Cs1 , v Cs = V d at t = t f i yiel ding C s1 =
2V d
Circuit wavef orms for var yin g va lues of Cs
Copyright © by John Wiley & Sons 2003
Snubbers - 24
Iot
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 rms lin e-t o-lin e volt age V LL =
Copyright © by John Wiley & Sons 2003
3
Vs
2
v LL = v
Snubbers - 26
bn
v
v
an = ba
 t1
bn
Trajectory comparision
with and without capacitor
Turn off snubber circuit