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II
SECTION 2 - THERMAL CONSIDERATIONS FOR STD SYSTEMS
Pro-Log Series 7000 logi.c cards are designed with components rated at +70
0
C oi
higher.
We recommend a maximum ambient free-air temperature of +55°C for this
system.
This provides for a +30
0
C temperature rise over normal room temperatur~
Card placement and card rack orientation are
imp~rtant
considerations in
configuring a system.
Distribution of power throughout the card rack can
contribute to optimizing system performance and lifetime.
Even power dis-
tribution can be achieved by spacing high power-dissipating cards between
cards of lower power.
In
an STD/Series 7000 system a sequence of six cards such as 7803 CPU,
7702 ROM, 7602 I/O, 7701 RAM, 7502 I/O, 7604 I/O is a good example of even
power distribution.
This sequence was used in Example 2.
Nominal pO\'Jer
dissipation for these and other Series 7000 cards is listed in Table 1.
Further temperature reduction can be achieved by leaving a card slot empty
adjacent to the component side of a high power dissipating card, or by· providing
forced air cooling to improve air
circ~lation
in the card rack.
This application note
includes
sample
calculations for two system situations.
Example 1 is for three fully loaded 7701 RAM cards on 1/21: and 1" centers.
Example 2 is for six cards sequenced as listed above, for 1/2" centers and again
for 1" spacing on component side of 7701 RAM card.
(See Configuring a Typical system)O
To determine whether a particular card's temperature parameters will
be exceeded, the user can
calculate the maximum acceptable ambient temperature
with the method outlined below.
1.
2.
3.
4.
Measure the case temperature (TC) of the hottest device or devices
on the card(s) in question.
This can be accompl ished by placing a
thermocouple probe imbedded in silicone grease in the sockets under
.selected IC's.
Typically this Te will result in the worst case
junction temperature (T J ).
Use the data in Table 2 to determine the maximum acceptable device
junction temperature (T
J
).
This table shows different failure
~cceleration
factors for different temperatures (T J ).
Use Table 3 to find the approximate thermal resistance junction-
to-case for the device or devices being considered.
For more
accurate caku'lations use the ReJC from the specific device manufacturer.
Using the manufacturer's data sheets, determine the maximum power
the device dissipates.
5.
Calculate the maximum TA acceptable.
See sample calculations.
6.
If the card cage is enclosed, measure the exhaust temperature (T
)
'after the temperature has stabilized.
If this temperature is
mu~h
O·
above TA, subtract TA from T A , and lO\A/er the acceptable TA' calculated
for the system by this amount.
18
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