Snubber Networks. AN1048D Datasheet

AN1048D Datasheet PDF, Equivalent


Part Number

AN1048D

Description

RC Snubber Networks

Manufacture

ON Semiconductor

Total Page 22 Pages
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AN1048D Datasheet
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/

AN1048D Datasheet
AN1048/D
ǒ ǓCONDITIONS INFLUENCING
dV
dt s
Transients occurring at line crossing or when there is no
initial voltage across the thyristor are worst case. The col-
lector junction capacitance is greatest then because the
depletion layer widens at higher voltage.
Small transients are incapable of charging the self-
capacitance of the gate layer to its forward biased threshold
voltage (Figure 2). Capacitance voltage divider action
between the collector and gate-cathode junctions and built-
in resistors that shunt current away from the cathode emit-
ter are responsible for this effect.
180
160
140
MAC 228A10 TRIAC
TJ = 110°C
120
100
80
60
40
20
0 100 200 300 400 500 600 700 800
PEAK MAIN TERMINAL VOLTAGE (VOLTS)
ǒ ǓFigure 6.2. Exponential
dV
dt
versus Peak Voltage
s
Static
dV
dt
does
not
depend
strongly
on
voltage
for
opera-
tion below the maximum voltage and temperature rating.
Avalanche multiplication will increase leakage current and
reduce
dV
dt
capability
if
a
transient
is
within
roughly
50
volts
of the actual device breakover voltage.
A
higher
rated
voltage
device
guarantees
increased
dV
dt
at
lower voltage. This is a consequence of the exponential rat-
ing method where a 400 V device rated at 50 V/μs has a
higher
dV
dt
to
200
V
than
a
200
V
device
with
an
identical
rating. However, the same diffusion recipe usually applies
for all voltages. So actual capabilities of the product are not
much different.
Heat increases current gain and leakage, lowering
ǒ ǓdV
dt
,
s
the
gate
trigger
voltage
and
noise
immunity
(Figure 3).
170
150
130
MAC 228A10
VPK = 800 V
110
90
70
50
30
10
25 40 55 70 85 100 115 130 145
TJ, JUNCTION TEMPERATURE (°C)
ǒ ǓFigure 6.3. Exponential
dV
dt
versus Temperature
s
ǒ ǓdV
dt
FAILURE MODE
s
Occasional unwanted turn-on by a transient may be
acceptable in a heater circuit but isn’t in a fire prevention
sprinkler system or for the control of a large motor. Turn-on
is destructive when the follow-on current amplitude or rate
is excessive. If the thyristor shorts the power line or a
charged capacitor, it will be damaged.
Static
dV
dt
turn-on
is
non-destructive
when
series
imped-
ance limits the surge. The thyristor turns off after a half-
cycle
of
conduction.
High
dV
dt
aids
current
spreading
in
the
thyristor, improving its ability to withstand ddIt. Breakdown
turn-on does not have this benefit and should be prevented.
140
120
MAC 228A10
100 800 V 110°C
80
60
40 RINTERNAL = 600 Ω
20
0
10 100 1000
GATE‐MT1 RESISTANCE (OHMS)
ǒ ǓdV
Figure G6.a4t.eEtxopMonTe1nRtieasl istdatncseversus
10,000
http://onsemi.com
2
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Features Datasheet pdf AN1048/D RC Snubber Networks For Thyrist or Power Control and Transient Suppress ion By George Templeton Thyristor Appli cations Engineer http://onsemi.com AP PLICATION NOTE INTRODUCTION Edited and Updated RC networks are used to contr ol voltage transients that could falsel y turn-on a thyristor. These networks a re called snubbers. The simple snubber consists of a series resistor and capac itor placed around the thyristor. These components along with the load inducta nce form a series CRL circuit. Snubber theory follows from the solution of the circuit’s differential equation. Man y RC combinations are capable of provid ing acceptable performance. However, im properly used snubbers can cause unreli able circuit operation and damage to th e semiconductor device. Both turn-on an d turn-off protection may be necessary for reliability. Sometimes the thyristo r must function with a range of load va lues. The type of thyristors used, circ uit configuration, and load characteristics are influential. Snub.
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