Switching Power Supply Service Manual
6. RELIABILITY
6.1 Life Cycle and Breakdown
Switching power supply have proven to be highly reliable when used in household appliances and industrial products and are highly rated throughout industry.
The following figure shows the failure rate curve (bathtub curve) during a typical product life cycle
6.1.1 Early Failure Period
To prevent early failure, each manufacturer screens out parts or performs aging tests on the finished product. When switching power supplies are delivered to users, the power supplies have already entered the random failure period.
6.1.2 Random Failure Period
The stability of each switching power supply depends on its own reliability (Mean Time Between Failure:MTBF).
Basically, the failure rate is very low. However, the failure rate in the random failure period differs depending on installation and operating conditions (ambient temperature, installation method, derating, ventilation, vibration and shock) which are determined by the user.
6.1.3 Fatigue Failure Period
At some time, the switching power supply will enter the fatigue failure period.
6.2 Ambient Temperature and Service Life
High efficiency switching at a high frequency, improvement in parts, and integration technology have greatly reduced the size of switching power supplies. Integration density has caused the internal parts to be packed more closely together.
Each part of a switching power supply distinctly differs in its service life depending on the ambient temperature.
An electrolytic capacitor used as a smoothing filter is more sensitive to variations in the ambient temperature because of the chemical reactions which occur inside it.
Typically, the service life of electrolytic capacitors by half if the ambient temperature increases by 10ºC . This characteristic determines the service life of the switching power supply.
Figure 20 shows the relationship between the service life of the switching power supply and ambient temperature. If the switching power supply is used at high temperatures, the electrolytic capacitor may enter the fatigue failure period while the other parts are still in the random failure period. To increase the service life of the switching power supply, it may be necessary to replace the electrolytic capacitor.
6.3 Overhaul
As electronic technology improves, the number of continuous operating systems increases. And though the service life of switching power supplies is also increasing, they cannot be used forever.
Therefore, periodic overhauls are required to ensure the reliability of the power supply. How often an overhaul needs to be performed depends on operating conditions and temperature. Service life is most seriously affected when a.power supply is operated continuously. In this case, the frequency of overhauls to the power supply should be as follows.
Ta is between 40ºC and 45ºC:once every three years
Ta is between 35ºC and 40ºC:once every four years
Ta is between 30ºC and 35ºC:once every five years
(Ta indicates the ambient temperature of the power supply.)
The above temperature value differ according to each switching power supply. For details on overhauls and service life, contact the manufacturer.
6.4 Loading Capacity and Environmental Temperature
Power supply operating temperatures depend on output load, ambient temperature, and cooling. The most ideal way to increase the service life of a power supply is to NOT continuously operate it at full capacity. This becomes even more critical in warmer climates.
Example: Two identical power supplies (subjected to the same environmental conditions) were operated using different loads. One had a 40% load condition, the other had an 80% load condition. The power supply with the 40% load lasted four times longer that the power supply with the 80% load condition.
Every model of the WENZHOU MINGWEI Power Supply line is subjected to an environmental test during the prototype phase.
Fig. 21 indicates the method
Fig. 22 & Fig. 23 shows typical results of these tests.
TA:Temperature inside chamber
TA1:Temperature inside power supply
TPC:Temperature of power supply case
TCAP:Temperature of output capacitor
IMPORTANT NOTE
All information is based on the WENZHOU MINGWEI Switching Power Supply User Manual - Version 1 and is provided in good faith, but without any warranty or guarantee as to fitness to purpose. It is solely left to the discretion of the user as to the suitability of the information for their application.