AN1008
Application Notes
IT: Current Rating
SCR
SUPPLY FREQUENCY: 60 Hz Sinusoidal
LOAD: Resistive
RMS ON-STATE CURRENT [ T(RMS)]:
Maximum Rated Value at Specified
Case Temperature
Notes:
1000
400
1) Gate control may be lost
during and immediately
following surge current interval.
2) Overload may not be repeated
until junction temperature has
returned to steady-state
rated value.
I
For RMS and average currents, the restricting factor is usually
confined so that the power dissipated during the on state and as
a result of the junction-to-case thermal resistance will not pro-
duce a junction temperature in excess of the maximum junction
temperature rating. Power dissipation is changed to RMS and
average current ratings for a 60 Hz sine wave with a 180° con-
duction angle. The average current for conduction angles less
than 180° is derated because of the higher RMS current con-
nected with high peak currents. The DC current rating is higher
than the average value for 180° conduction since no RMS com-
ponent is present.
300
250
150
120
100
80
60
50
40
30
20
10
The dissipation for non-sinusoidal waveshapes can be deter-
mined in several ways. Graphically plotting instantaneous dissi-
pation as a function of time is one method. The total maximum
allowable power dissipation (PD) may be determined using the
following equation for temperature rise:
1
10
100
1000
Surge Current Duration – Full Cycles
Figure AN1008.3
Peak Surge Current versus Surge Current Duration
ITM: Peak Repetitive On-state Current — SCR and Triac
T
– T
C
J(MAX)
R
P
= -----------------------------------
The ITM rating specifies the maximum peak current that may be
applied to the device during brief pulses. When the device oper-
ates under these circumstances, blocking capability is main-
tained. The minimum pulse duration and shape are defined and
control the applied di/dt. The operating voltage, the duty factor,
the case temperature, and the gate waveform are also defined.
This rating must be followed when high repetitive peak currents
are employed, such as in pulse modulators, capacitive-discharge
circuits, and other applications where snubbers are required.
D
θJC
where TJ(max) is the maximum rated junction temperature (at
zero rated current), TC is the actual operating case temperature,
and RθJC is the published junction-to-case thermal resistance.
Transient thermal resistance curves are required for short inter-
val pulses.
Triac
The limiting factor for RMS current is determined by multiplying
power dissipation by thermal resistance. The resulting current
value will ensure an operating junction temperature within maxi-
mum value. For convenience, dissipation is converted to RMS
current at a 360° conduction angle. The same RMS current can
be used at a conduction angle of less than 360°. For information
on non-sinusoidal waveshapes and a discussion of dissipation,
refer to the preceding description of SCR current rating.
di/dt: Rate-of-change of On-state Current — SCR and Triac
The di/dt rating specifies the maximum rate-of-rise of current
through a thyristor device during turn-on. The value of principal
voltage prior to turn-on and the magnitude and rise time of the
gate trigger waveform during turn-on are among the conditions
under which the rating applies. If the rate-of-change of current
(di/dt) exceeds this maximum value, or if turn-on with high di/dt
during minimum gate drive occurs (such as dv/dt or overvoltage
events), then localized heating may cause device degradation.
During the first few microseconds of initial turn-on, the effect of
di/dt is more pronounced. The di/dt capability of the thyristor is
greatly increased as soon as the total area of the pellet is in full
conduction.
The di/dt effects that can occur as a result of voltage or transient
turn-on (non-gated) is not related to this rating. The di/dt rating is
specified for maximum junction temperature.
As shown in Figure AN1008.4, the di/dt of a surge current can be
calculated by means of the following equation.
ITSM: Peak Surge (Non-repetitive) On-state
Current — SCR and Triac
The peak surge current is the maximum peak current that may be
applied to the device for one full cycle of conduction without
device degradation. The maximum peak current is usually speci-
fied as sinusoidal at 50 Hz or 60 Hz. This rating applies when the
device is conducting rated current before the surge and, thus,
with the junction temperature at rated values before the surge.
The junction temperature will surpass the rated operating tem-
perature during the surge, and the blocking capacity may be
decreased until the device reverts to thermal equilibrium.
The surge-current curve in Figure AN1008.3 illustrates the peak
current that may be applied as a function of surge duration. This
surge curve is not intended to depict an exponential current
decay as a function of applied overload. Instead, the peak current
shown for a given number of cycles is the maximum peak surge
permitted for that time period. The current must be derated so
that the peak junction temperature during the surge overload
does not exceed maximum rated junction temperature if blocking
is to be retained after a surge.
π(I
)
di
TM
t
---- = -----------------
dt
As an example, surge current of 400 A at 60 Hz has a di/dt of
π400/8.3 or 151.4 A/ms.
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AN1008 - 2
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Thyristor Product Catalog
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