ACPL-38JT-000E View Datasheet(PDF) - Avago Technologies
Part Name
Description
MFG CO.
'ACPL-38JT-000E' PDF : 33 Pages View PDF
Step 3: Compare the calculated power dissipation with the absolute
maximum values for the ACPL-38JT:
For the example,
PI = 90.8 mW < 150 mW (abs. max.) OK
PO = 187.01 mW < 600 mW (abs. max.) OK
Therefore, the power dissipation absolute maximum
rating has not been exceeded for the example.
Please refer to the following Thermal Model section for an
explanation on how to calculate the maximum junction
temperature of the ACPL-38JT for a given PC board layout
configuration.
SWITCHING ENERGY vs. GATE RESISTANCE
9
(VCC2 / VEE2 = 25 V / 5 V)
8
7
6
5
4
Ess (Qg = 650 nC)
3
2
1
0
0
50
100
150
200
Rg (Ω)
Figure 76. Switching energy plot for calculating average Pswitch
(for ACPL-38JT output driving an IGBT rated at 600 V/100 A).
Thermal Model
The ACPL-38JT is designed to dissipate the majority of the
heat through pins 1 for the input IC and pins 9 and 12 for
the output IC. (There are two VEE pins on the output side,
pins 9 and 12, for this purpose.) Heat flow through other
pins or through the package directly into ambient are
considered negligible and not modeled here.
In order to achieve the power dissipation specified in the
absolute maximum specification, it is imperative that
pins 1, 9, and 12 have ground planes connected to them.
As long as the maximum power specification is not
exceeded, the only other limitation to the amount of
power one can dissipate is the absolute maximum junction
temperature specification of 140°C. The junction tempera-
tures can be calculated with the following equations:
Tji = Pi (i1 + 1A) + TA
Tjo = Po (o9,12 + 9,12A) + TA
where Pi = power into input IC and Po = power into output
IC.
Since 1A and θ9,12A are dependent on PCB layout
and airflow, their exact number may not be available.
Therefore, a more accurate method of calculating the
junction temperature is with the following equations:
Tji = Pii1 + TP1
Tjo = Poo9,12 + TP9,12
These equations, however, require that the pin 1 and pins
9, 12 temperatures be measured with a thermal couple on
the pin at the ACPL-38JT package edge.
Pi = 90.8 mW, Po = 314 mW, TA = 125°C, and assuming the
thermal model shown in Figure 77 below.
Tji = (90.8 mW)(60°C/W + 50°C/W) + 125°C = 135°C
Tjo = (187.01 mW)(30°C/W + 50°C/W) + 125°C = 140°C
If we, however, assume a worst case PCB layout and no
air flow where the estimated q1A and q9,12A are 100°C/W.
Then the junction temperatures become
Tji = (90.8 mW)(60°C/W + 100°C/W) + 125°C = 140°C
Tjo = (187.01 mW)(30°C/W + 100°C/W) + 125°C = 149°C
both of which are within the absolute maximum specifi-
cation of 150°C.
If the calculated junction temperatures for the thermal
model in Figure 77 is higher than 150°C, the pin tempera-
ture for pins 9 and 12 should be measured (at the package
edge) under worst case operating environment for a more
accurate estimate of the junction temperatures.
From the earlier power dissipation calculation example:
Tji
I1 = 60°C/W
TP1
1A = 50°C/W*
Tjo
l9, 12 = 30°C/W
TP9, 12
9, 12A = 50°C/W*
TA
Tji = junction temperature of input side IC
Tjo = junction temperature of output side IC
TP1 = pin 1 temperature at package edge
TP9,12 = pin 9 and 12 temperature at package edge
I1 = input side IC to pin 1 thermal resistance
I9,12 = output side IC to pin 9 and 12 thermal resistance
1A = pin 1 to ambient thermal resistance
9,12A = pin 9 and 12 to ambient thermal resistance
* The 1A and 9,12A values shown here are for PCB layouts shown in Figure 77
with reasonable air flow. This value may increase or decrease by a factor of
2 depending on PCB layout and/or airflow.
Figure 77. Thermal Model for ACPL-38JT
31
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