Sinrace Power Supply Technology (Shenzhen) Co., Ltd.

The normal measure of reliability of power converters is given as Mean Time between Failure (MTBF). The MTBF is normally calculated based on the predicted failure rate of the components utilized within the product, a so called parts count method. When comparing the MTBF of various supplies there are a number of key parameters to check to ensure that the specifications are indeed comparable. Firstly the methodology needs to be identical; typical methods are MIL217 at its various issue levels and Bellcore RPP (now managed by Telcordia Technologies). These two methodologies will give very different results and cannot be compared to one another. Where the MTBF is given to the same specification then in order to be compared it must also be stated under the same environmental conditions to prove a useful tool to the system designer.

The most influential factor in terms of reliability and lifetime is the ambient temperature and effective cooling of the power supply. Convection cooled products need adequate space to cool effectively and forced cooling requirements need to be carefully considered to ensure that the product is adequately cooled in the specific application. Manufacturers are increasingly providing key measurement points within the sub assembly to ensure that the product will be both safe and reliable and to ensure adequate lifetime. Thermal de-rating data is normally provided up to ambient temperatures around 60 to 70 degrees C. Careful consideration of this data is required since some products de-rate from as low as 40 degrees C. For example, most power supplies from XP Power de-rate from a minimum of 50 degrees C, some from 60 degrees C. Also, be aware that while a unit might be specified to operate at an ambient temperate of, say, 40 degrees C, when the unit is enclosed within end-user equipment the internal temperature can be much higher than that.

Airflow is another important consideration. Be aware that some products may specify an airflow rate that might be difficult to achieve in practice e.g. 20 – 30 CFM. As a guide, look for a required airflow less than 15 CFM. Typically, XP’s products require 10 – 13 CFM