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AN1048D Schematic ( PDF Datasheet ) - ON Semiconductor

Teilenummer AN1048D
Beschreibung RC Snubber Networks
Hersteller ON Semiconductor
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Gesamt 22 Seiten
AN1048D Datasheet, Funktion
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
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AN1048D Datasheet, Funktion
AN1048/D
ǒ ǓIMPROVING
dV
dt c
ǒ Ǔ ǒ ǓThe same steps that improve
dV
dt
s
aid
dV
dt
c
except
when stored charge dominates turn-off. Steps that reduce
the stored charge or soften the commutation are necessary
then.
Larger TRIACs have better turn-off capability than
smaller ones with a given load. The current density is lower
in the larger device allowing recombination to claim a
greater proportion of the internal charge. Also junction
temperatures are lower.
TRIACs with high gate trigger currents have greater
turn-off ability because of lower spreading resistance in the
gate layer, reduced Miller effect, or shorter lifetime.
The rate of current crossing can be adjusted by adding a
commutation softening inductor in series with the load.
Small high permeability “square loop” inductors saturate
causing no significant disturbance to the load current. The
inductor resets as the current crosses zero introducing a
large inductance into the snubber circuit at that time. This
slows the current crossing and delays the reapplication of
blocking voltage aiding turn-off.
The commutation inductor is a circuit element that
introduces time delay, as opposed to inductance, into the
circuit.
It
will
have
little
influence
on
observed
dV
dt
at
the
device. The following example illustrates the improvement
resulting from the addition of an inductor constructed by
winding 33 turns of number 18 wire on a tape wound core
(52000-1A). This core is very small having an outside
diameter of 3/4 inch and a thickness of 1/8 inch. The delay
time can be calculated from:
ts +
(N A B 10*8)
E
where:
ts = time delay to saturation in seconds.
B = saturating flux density in Gauss
A = effective core cross sectional area in cm2
N = number of turns.
For the described inductor:
ts + (33 turns) (0.076 cm2 ) (28000 Gauss)
(1 108 ) ń (175 V) + 4.0 ms.
The saturation current of the inductor does not need to be
much larger than the TRIAC trigger current. Turn-off fail-
ure will result before recovery currents become greater than
this value. This criterion allows sizing the inductor with the
following equation:
Is +
Hs
0.4
ML
pN
where :
Hs = MMF to saturate = 0.5 Oersted
ML = mean magnetic path length = 4.99 cm.
Is
+
(.5)
.4
(4.99)
p 33
+
60
mA.
SNUBBER PHYSICS
UNDAMPED NATURAL RESONANCE
w0
+
I
ǸLC
Radiansńsecond
Resonance
determines
dV
dt
and
boosts
the
peak
capacitor
voltage when the snubber resistor is small. C and L are
related
to
one
another
by
ω02.
dV
dt
scales
linearly
with
ω0
when the damping factor is held constant. A ten to one
reduction
in
dV
dt
requires
a
100
to
1
increase
in
either
component.
DAMPING FACTOR
Ǹρ
+
R
2
C
L
The damping factor is proportional to the ratio of the
circuit loss and its surge impedance. It determines the trade
off
between
dV
dt
and
peak
voltage.
Damping
factors
between
0.01 and 1.0 are recommended.
The Snubber Resistor
Damping
and
dV
dt
When
ρ
t
0.5,
the
snubber
resistor
is
small,
and
dV
dt
depends mostly on resonance. There is little improvement
in
dV
dt
for
damping
factors
less
than
0.3,
but
peak
voltage
and snubber discharge current increase. The voltage wave
has a 1-COS (θ) shape with overshoot and ringing. Maxi-
mum
dV
dt
occurs
at
a
time
later
than
t
=
0.
There
is
a
time
delay before the voltage rise, and the peak voltage almost
doubles.
When ρ u 0.5, the voltage wave is nearly exponential in
shape.
The
maximum
instantaneous
dV
dt
occurs
at
t
=
0.
There is little time delay and moderate voltage overshoot.
When
ρ
u
1.0,
the
snubber
resistor
is
large
and
dV
dt
depends mostly on its value. There is some overshoot even
through the circuit is overdamped.
High load inductance requires large snubber resistors and
small snubber capacitors. Low inductances imply small
resistors and large capacitors.
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AN1048D pdf, datenblatt
AN1048/D
Highly overdamped snubber circuits are not practical
designs. The example illustrates several properties:
1. The initial voltage appears completely across the circuit
inductance. Thus, it determines the rate of change of
current through the snubber resistor and the initial ddVt .
This result does not change when there is resistance in
the load and holds true for all damping factors.
2. The snubber works because the inductor controls the
rate of current change through the resistor and the rate
of capacitor charging. Snubber design cannot ignore
the inductance. This approach suggests that the snubber
capacitance is not important but that is only true for
this hypothetical condition. The snubber resistor shunts
the thyristor causing unacceptable leakage when the
capacitor is not present. If the power loss is tolerable,
dV
dt
can
be
controlled
without
the
capacitor.
An
example is the soft-start circuit used to limit inrush
current in switching power supplies (Figure 25).
Snubber With No C
E
AC LINE SNUBBER
L
E
AC LINE SNUBBER
L
RS
RECTIFIER
BRIDGE
G
ǒ ǓdV
dt
f
+
ERS
L
RS
RECTIFIER
G BRIDGE
C1
C1
Figure 6.25. Surge Current Limiting For
a Switching Power Supply
ǒ ǓTRIAC DESIGN PROCEDURE
dV
dt c
1. Refer to Figure 18 and select a particular damping
factor (ρ) giving a suitable trade-off between VPK and ddVt .
Determine
the
normalized
dV
dt
corresponding
to
the
chosen
damping factor.
The voltage E depends on the load phase angle:
ǒ ǓE + Ǹ2 VRMS Sin (f) where f + tan*1
XL
RL
where
φ = measured phase angle between line V and load I
RL = measured dc resistance of the load.
Then
Ǹ ǸZ
+
VRMS
IRMS
RL2 ) XL2 XL +
Z2 * RL2 and
L
+
2
XL
p fLine
.
If only the load current is known, assume a pure inductance.
This gives a conservative design. Then:
L+
2p
VRMS
fLine IRMS
where E +
Ǹ2
VRMS.
For example:
E+
Ǹ2
120
+ 170 V;
L
+
(8
A)
120
(377
rps)
+ 39.8 mH.
Read from the graph at ρ = 0.6, VPK = (1.25) 170 = 213 V.
Use
400
V
TRIAC.
Read
dV
dt (ρ+0.6)
+
1.0.
2. Apply the resonance criterion:
ǒ Ǔ ǒ Ǔw0 +
spec
dV
dt
ń
ddVt(P)E .
w0
+
5
(1)
106 VńS
(170 V)
+
29.4
103 r ps.
C
+
1
w02
L
+
0.029
mF
3. Apply the damping criterion:
Ǹ ǸRS + 2ρ
L
C
+
2 (0.6)
39.8
0.029
10*3
10*6
+
1400 ohms.
ǒ ǓdV
dt
SAFE AREA CURVE
c
Figure 26 shows a MAC15 TRIAC turn-off safe
operating area curve. Turn-off occurs without problem
under
the
curve.
The
region
is
bounded
by
static
dV
dt
at
low
ǒ Ǔvalues of
dI
dt
and delay time at high currents. Reduction
c
of the peak current permits operation at higher line
frequency. This TRIAC operated at f = 400 Hz, TJ = 125°C,
and ITM = 6.0 amperes using a 30 ohm and 0.068 μF
snubber. Low damping factors extend operation to higher
ǒ ǓdI
dt
, but capacitor sizes increase. The addition of a small,
c
saturable commutation inductor extends the allowed
current rate by introducing recovery delay time.
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