Rev. 01 Jan 96
AMPSS
®
Reference Manual
AMPSS
®
Accessories
11
Thermal Design Process and Example
Temperature & MTBF
APMSS
modules are designed to be able to run at
baseplate temperatures of 100°C in the case of the
AM80 series and 85°C for other series. However, for
normal operation the modules should not be run at
the maximum allowable temperature since the Mean
Time Between Failures (MTBF) will reduce sharply
as temperature increases. For example, an
AM80-300L-050F40 operating at 5V@40A output,
with a baseplate temperature of 50°C, has an MTBF
of over one million hours. If the temperature is
doubled to 100°C this figure drops to 155,000 hours.
The following rules should be followed to ensure
reliable operation -
• At the maximum system ambient tempera-
ture the APMSS
baseplate temperature
rating should not be exceeded.
• At the normal system ambient operating
temperature the APMSS
baseplate tem-
perature must be low enough to meet MTBF
requirements.
The Thermal Design Process
1. Determine heat generated by module from its
losses. The minimum efficiency at relevant line
and load conditions should be used in calculat-
ing the losses.
2. Determine maximum baseplate temperature
rise to stay within module temperature rating
at maximum system ambient.
3. Define maximum system baseplate tempera-
ture to meet MTBF in normal system operating
conditions or at the temperature at which the
MTBF is specified.
4. Select/design heatsink and airflow require-
ment.
5. Test using the
APMSS
imbedded TEMP-MON
feature which allows direct and convenient
monitoring of baseplate temperature. (BM/AM/
AL/AK Series)
Thermal Design Example
This example is for the following parameters:
• Single 5V AM80 module used in a distributed
power system.
• Average load 30A (150 Watts)
• Normal operating ambient temperature 25°C
• Maximum ambient temperature 60°C
• MTBF required - 800,000 hours (from system
requirements)
• Efficiency measured at 83% (Efficiency =
Output power/ Input power)
1. Heat generated = Power Out x [(1/Efficiency)-1]
= 150 x [(1/0.83)-1]
= 31 Watt
2. Maximum baseplate temperature 100°C (from
AM80 specifications).
⇒ At 60 °C (max. ambient temp.) the maximum
baseplate temperature rise is 40°C.
3. To achieve 800,000 hours MTBF, baseplate
temperature must not exceed 61°C.
⇒ Maximum baseplate temperature rise (from 25°C
operating ambient) is 36°C.
4. Choose the lowest temperature rise of ➁ or ➂ i.e.
36°C.
The cooling system must dissipate 31 Watts with a
maximum baseplate temperature rise of 36°C.
⇒ Thermal resistance = 36/31 = 1.16°C/Watt.
To ensure good thermal contact Astec recommends
the use of Thermstrate
®
thermal mounting pads.
Thermal resistance of the Thermstrate
®
interface
between baseplate and heatsink is 0.1°C/W.
For this example (overall thermal resistance 1.16 °C/
W) the heatsink thermal resistance should be a
maximum of 1.06°C/W. A 10% safety margin is
desirable so a heatsink achieving 0.95°C/W is
chosen.
To achieve this level of cooling using natural convec-
tion would require an very large heatsink. It would
therefore be better to employ forced air cooling. A
thermal resistance vs air flow characteristic should
be referenced to determine the required airflow for
the heatsink you are using.
