
AN1008
Application Notes
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AN1008 - 2
2002 Teccor Electronics
Thyristor Product Catalog
I
T
: Current Rating
SCR
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.
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 (P
D
) may be determined using the 
following equation for temperature rise:
where T
(max)
 is the maximum rated junction temperature (at 
zero rated current), T
 is the actual operating case temperature, 
and R
θ
JC
 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.
I
TSM
: 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.
Figure AN1008.3
Peak Surge Current versus Surge Current Duration
I
TM
: Peak Repetitive On-state Current — SCR and Triac
The I
TM
 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. 
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.
As an example, surge current of 400 A at 60 Hz has a di/dt of 
π
400/8.3 or 151.4 A/ms.
P
D
T
-----------------------------------
T
C
–
R
θ
JC
=
1
10
100
1000
10
20
30
40
50
60
80
100
250
300
400
1000
Surge Current Duration – Full Cycles
P
O
T
)
40 A TO-218
25 A T0-220
15 A TO-220
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.
SUPPLY FREQUENCY: 60 Hz Sinusoidal
LOAD: Resistive
Maximum Rated Value at Specified
Case Temperature
Notes:
di
dt
----
π
 I
-----------------
(
)
t
=