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| Descripción | RC Snubber Networks | |
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AN1048/D
RC Snubber Networks
For Thyristor
Power Control and
Transient Suppression
By George Templeton
Thyristor Applications Engineer
http://onsemi.com
APPLICATION NOTE
INTRODUCTION
Edited and Updated
RC networks are used to control voltage transients that
could falsely turn-on a thyristor. These networks are called
snubbers.
The simple snubber consists of a series resistor and
capacitor placed around the thyristor. These components
along with the load inductance form a series CRL circuit.
Snubber theory follows from the solution of the circuit’s
differential equation.
Many RC combinations are capable of providing accept-
able performance. However, improperly used snubbers can
cause unreliable circuit operation and damage to the semi-
conductor device.
Both turn-on and turn-off protection may be necessary
for reliability. Sometimes the thyristor must function with a
range of load values. The type of thyristors used, circuit
configuration, and load characteristics are influential.
Snubber design involves compromises. They include
cost, voltage rate, peak voltage, and turn-on stress. Practi-
cal solutions depend on device and circuit physics.
STATIC
dV
dt
WHAT
IS
STATIC
dV
dt
?
Static
dV
dt
is
a
measure
of
the
ability
of
a
thyristor
to
retain a blocking state under the influence of a voltage
transient.
ǒ ǓdV
dt
DEVICE PHYSICS
s
Static
dV
dt
turn-on
is
a
consequence
of
the
Miller
effect
and regeneration (Figure 1). A change in voltage across the
junction capacitance induces a current through it. This cur-
ǒ Ǔrent is proportional to the rate of voltage change
dV
dt
. It
triggers the device on when it becomes large enough to
raise the sum of the NPN and PNP transistor alphas to unity.
A
I1 CJP
CJN
ICN IJ
NPN
IBP IA
PNP
IJ ICP
V
I2
G
dv CJ
dt G
t
IBN
IK
K
IA
+
1
*
CJ
dV
dt
(aN )
ap)
TWO TRANSISTOR MODEL
OF
SCR
CEFF + 1*(aCNJ)ap)
A
PE
NB
C
PB
NE
K
INTEGRATED
STRUCTURE
ǒ ǓFigure 6.1.
dV
dt
Model
s
© Semiconductor Components Industries, LLC, 2008
June, 2008 − Rev. 3
1
Publication Order Number:
AN1048/D
Free Datasheet http://www.datasheet-pdf.com/
1 page  
AN1048/D
ǒ Ǔdi
dt c
0
ǒ ǓdV
dt c
TIME
VMT2‐1
VOLUME
STORAGE
CHARGE
IRRM
CHARGE
DUE TO
dV/dt
Figure 6.10. TRIAC Current and Voltage
at Commutation
E
V
E
ǒ ǓCONDITIONS INFLUENCING
dV
dt c
Commutating
dV
dt
depends
on
charge
storage
and
recov-
ery dynamics in addition to the variables influencing static
ddVt . High temperatures increase minority carrier life-time
and the size of recovery currents, making turn-off more dif-
ficult. Loads that slow the rate of current zero-crossing aid
turn-off. Those with harmonic content hinder turn-off.
Circuit Examples
Figure 13 shows a TRIAC controlling an inductive load
in a bridge. The inductive load has a time constant longer
than the line period. This causes the load current to remain
constant and the TRIAC current to switch rapidly as the line
voltage reverses. This application is notorious for causing
ǒ ǓTRIAC turn-off difficulty because of high
dI
dt
.
c
RS C
LS
60 Hz i
i
ǒ ǓdI
dt c
-
t
DC MOTOR
RL
+
ǒ ǓL
R
u 8.3
ms
VT
0
td
TIME
Figure 6.11. Snubber Delay Time
0.5
0.2
0.2 0.1
0.1 0.05
0.05
0.03
RL = 0
M=1
0.02 IRRM = 0
0.02
0.01
VT
E
0.005
0.001 0.002 0.005 0.01 0.02 0.05 0.1 0.2 0.3 0.5
DAMPING FACTOR
Figure 6.12. Delay Time To Normalized Voltage
1
Figure 6.13. Phase Controlling a Motor in a Bridge
High currents lead to high junction temperatures and
rates of current crossing. Motors can have 5 to 6 times the
normal current amplitude at start-up. This increases both
junction temperature and the rate of current crossing, lead-
ing to turn-off problems.
The line frequency causes high rates of current crossing
in 400 Hz applications. Resonant transformer circuits are
doubly periodic and have current harmonics at both the pri-
mary and secondary resonance. Non-sinusoidal currents
can lead to turn-off difficulty even if the current amplitude
is low before zero-crossing.
ǒ ǓdV
dt
FAILURE MODE
c
ǒ ǓdV
dt
c
failure
causes a
loss
of phase
control. Temporary
turn-on or total turn-off failure is possible. This can be
destructive if the TRIAC conducts asymmetrically causing a
dc current component and magnetic saturation. The winding
resistance limits the current. Failure results because of
excessive surge current and junction temperature.
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AN1048/D
The optocoupler conducts current only long enough to
trigger the power device. When it turns on, the voltage
between MT2 and the gate drops below the forward thresh-
old voltage of the opto-TRIAC causing turn-off. The opto-
ǒ Ǔcoupler sees
dV
dt
s
when the power TRIAC turns off later
in the conduction cycle at zero current crossing. Therefore,
it is not necessary to design for the lower optocoupler
ǒ ǓdV
dt
c
rating.
In
this
example,
a
single
snubber
designed
for the optocoupler protects both devices.
VCC
1
2
3
100
4 1N4001
5
6 51
MCR265-4
MCR265-4
100 1N4001
(50 V/μs SNUBBER, ρ = 1.0)
1 MHY
430 120 V
400 Hz
0.022
μF
Figure 6.23. Anti-Parallel SCR Driver
Optocouplers with SCRs
Anti-parallel
SCR
circuits
result
in
the
same
dV
dt
across
the optocoupler and SCR (Figure 23). Phase controllable
opto-couplers require the SCRs to be snubbed to their lower
dV
dt
rating.
Anti-parallel
SCR
circuits
are
free
from
the
charge storage behaviors that reduce the turn-off capability
of TRIACs. Each SCR conducts for a half-cycle and has the
next half cycle of the ac line in which to recover. The turn-
off
dV
dt
of
the
conducting
SCR
becomes
a
static
forward
blocking
dV
dt
for
the
other
device.
Use
the
SCR
data
sheet
ǒ ǓdV
dt
s
rating in the snubber design.
A SCR used inside a rectifier bridge to control an ac load
will not have a half cycle in which to recover. The available
time decreases with increasing line voltage. This makes the
circuit less attractive. Inductive transients can be sup-
pressed by a snubber at the input to the bridge or across the
SCR. However, the time limitation still applies.
ǒ ǓOPTO
dV
dt c
Zero-crossing optocouplers can be used to switch
inductive loads at currents less than 100 mA (Figure 24).
However a power TRIAC along with the optocoupler
should be used for higher load currents.
80
70
60
CS = 0.01
50
40
30 CS = 0.001
20
NO SNUBBER
10
0
20 30
40 50 60 70 80
TA, AMBIENT TEMPERATURE (°C)
90 100
(RS = 100 Ω, VRMS = 220 V, POWER FACTOR = 0.5)
Figure 6.24. MOC3062 Inductive Load Current versus TA
A phase controllable optocoupler is recommended with a
power device. When the load current is small, a MAC97A
TRIAC is suitable.
Unusual circuit conditions sometimes lead to unwanted
ǒ Ǔoperation of an optocoupler in
dV
dt
mode. Very large cur-
c
rents in the power device cause increased voltages between
MT2 and the gate that hold the optocoupler on. Use of a
larger TRIAC or other measures that limit inrush current
solve this problem.
Very short conduction times leave residual charge in the
optocoupler. A minimum conduction angle allows recovery
before voltage reapplication.
THE SNUBBER WITH INDUCTANCE
Consider an overdamped snubber using a large capacitor
whose voltage changes insignificantly during the time
under consideration. The circuit reduces to an equivalent
L/R series charging circuit.
The current through the snubber resistor is:
ǒ Ǔi
+
V
Rt
1 * e*tt
,
and the voltage across the TRIAC is:
e + i RS.
The voltage wave across the TRIAC has an exponential
rise with maximum rate at t = 0. Taking its derivative gives
its value as:
ǒ ǓdV
dt
0
+
V
RS
L
.
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| PDF Descargar | [ Datasheet AN1048D.PDF ] | |
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