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4.
TEMPERATURE
4.1
Service life
A product’s service life, at a given
temperature, is the period of time during
which this product is expected to operate
delivering all features for which it has
been designed. The service life of any product
is determined by the components of this
product with a shorter service life. In
the case of power supplies, these components
are usually the electrolytic capacitors
which also have a service life completely
dependent on the temperature; this is shown
in figure 21.
4.2 Reliability
A power supply’s failure rate, as a
function of time, is similar to that of
any electronic device. Three stages can
be distinguished , as shown in figure 22:
(A) Premature failure or “child mortality”.
During this stage of the device’s service
life, the failure probability is higher.
This is due to the fact that some imperfections
may remain undetected in the components
used. Their influence is minimized by carrying
out a burn-in of all units before being
marketed.
(B) Random failure. This is the stage of
the service life after the premature failure
period has elapsed. During this time period,
the failure rate is minimum.
(C) Ageing failure. Once the device’s
service life is ended, the components featuring
faster ageing start to degrade causing the
failure rate to increase.
A power supply’s failure rate, as a
function of the ambient temperature, also
has a similar behaviour to that of any electronic
device. The common way to refer to a reliability
forecast is by using the term MTBF (Mean
Time Between Failure) expressed in hours.
As shown in figure 23, the temperature effect
upon MTBF is dramatic, since it features
an exponential dependency on temperature,
which is obtained from the Arrhenius formula.
As a consequence of this, a noticeable increase
of MTBF and service life may well reward
investments in resources to decrease, even
by just a few degrees, the power supply
ambient temperature, such as, for example,
improving ventilation, etc.
4.3
Derating
As explained in previous sections, temperature
plays a significant role in a power supply.
These devices, since they dissipate heat,
change their own ambient temperature as
a function of their output power. Then,
a certain flexibility in respect to the
upper temperature range as a function of
the output load is possible. A typical temperature
derating curve as a function of the load
can be seen in figure 24.
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