AN587 Datasheet, PDF (2/3 Pages) STMicroelectronics – TRANSISTOR PROTECTION BY TRANSIL

APPLICATION NOTE
3. TRANSIL POWER DISSIPATION
We can consider two cases, single pulse
operation and repetitive pulse operation.
3.1. Single pulse operation
In this case, in order to define a TRANSIL we
need the peak power P_P and the pulse current
standard duration t_P.
Pp is given by: P_P = V_{BR min} × I _P
If we approximate the pulse current with a
triangle, the standard exponential equivalent
pulse duration tp is calculated by the formula:
t_P
=
−
_1.4
L
2r


×
In


V_BR
V_{BR min}
min ^{− V}cc
− V_cc + rI _P


The energy in the TRANSIL can be expressed
by:
W
=


V_BR
min
r
×
L

×
_{I P}

+


V
BR
min
r
− V_cc


In


V
V_BR
BR min
min ^{− V}cc
− V_cc + rI _P




When r tends to zero we find:
W
=
1
2
LI
P
2



V
V_BR
BR min
min
− V_cc



3.2. Repetitive pulse operation
In repetitive pulse operation the power
dissipation can be calculated by the following
formula:
P
=
F
×


V
BR
min
r
×L

×
_{I P}

+


V_BR
min
r
−
V_cc


In
_VBRV_mBiRn
min ^{− V}cc
− V_cc + rI
P





When r tends to zero we find:
P
=
1
2
FLI
P
2


V
V_BR
BR min
min
− V_cc


Where F is the switching frequency.
4. EXAMPLE OF APPLICATION
A typical application would be a switched coil
supplied by a battery. The different parameters
of the application are:
V_CC = 14V, L = 10mH, r = 3, I_P = 4A
TRANSIL: SM6HT36A, V_{BR min} = 34.2V
4.1. Single pulse
We find
P_P = 32.4 × 4 = 136.8W
t_P
=
−1.4
× 10
2×
×
3
10
−3
In


34.2 − 14
34.2 − 14 + 3
×
4


t _P = 1.08ms
The datasheet gives P_P ≈ 450W for t_P = 2ms, so
the SM6HT36A can be used in this application.
4.2. Repetitive pulse operation
The switching frequency is equal to 10Hz so
P = 10 × ^{34.2 × 10 × 10 −3} ×
3
₄
+


34.2 −
3
14 _In
34.2 − 14
34.2 − 14 + 3
×

4  _
P = 980mW
We have:
T_j = P.R_th(j-a) + T_{amb max}
With T_{amb max} = 50°C, R_th(j-a) = 75°C/W (with
copper surface under each lead equal to
2.5cm²) we find:
T_j = 0.98 x 75 + 50 = 123.5°C
Tj max beeing equal to 175°C we can also use
this TRANSIL in repetitive pulse operation.
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