It should be noted that reliability is a factor between 0 and 1 and does not have a dimension. The failure rate though, is measured particularly in the electronics industry. The FIT (failures in time) of a device is the number of failures that can be expected in one billion (109) hours of operation. Considering a power MOSFET suitable for a 250W power supply (basic failure rate is 12 FIT), working in a temperature approaching 100°C (thermal coefficient is 3.7), with quality factor JANTX (according to MIL-S-19500 is 8) and an environmental factor GF (Ground Fixed is 1.6), the total failure rate will be 2.312 Failures/106h. Once environmental factors are considered, the basic
failure rate of 12 FIT has increased to 2,131.2 FIT. As MTTF is the inverse to failure rate, this results in a MTTF for the power MOSFET of about 470,000 hours. The initial MTTF however, without calculating the environmental and thermal factors, was about 83,000,000 hours. To improve the reliability of the MOSFET and hence
the power supply, the designer could choose to increase the thermal derating of the part. By significantly cooling the part down and setting the working temperature of the MOSFET at 80°C rather than 100°C, the thermal coefficient would go from 3.7 to 2.7, making the MTTF 643,000 hours – over a 36% improvement. The reduction in the failure rate, under pre-set operating
and environmental conditions, is possible only through careful analysis of the Electrical Stress and Thermal Stress factors for correct dimensioning of the power MOSFET. This is possible by means of a check carried out through the derating plans, illustrated in Figures 2 and 3, suitably constructed on the basis of the application and type of component. The Technical Reference TR-332 standard was
developed, originally by Bellcore and now known as Telcordia, for the telecommunication industry where manufacturers deemed that MIL-HDBK-217 was a little too harsh. Its international popularity is growing, but typically the MIL-HDBK-217 calculated results will show a higher failure rate than Bellcore/Telcordia standard for the same system. This difference between the standards stems from the original intended use of the MIL standard for aerospace and military, or mission critical applications.
The gathering and analysis of data It is possible to calculate the reliability figures, with a good degree of accuracy, using data gathered from HALT (Highly Accelerated Life Test) trials. The number of samples to test though is usually limited, the number of test stations fixed
January 2017
www.electronicscomponentsworld.com /
and the amount of time available to run tests restricted. This impacts the “confidence gap” within which the designer wants the calculated parameter to fall. This analysis requires the use of a statistical function called chi squared 2 (a,b) – a function of two variables, tabulated in the main statistical trials, as well as in the main electronic calculation sheets. For the sake of simplicity, calculation functions of a
“monolateral lower confidence gap” are reproduced in two special cases: “Trials broken off with substitution” and “Trials broken off without substitution”. The first case mainly concerns calculation of the true MTTF in the case of returns from the field, though this method is unsatisfactory as the manufacturer doesn’t know how long the units have been operating, nor the operating conditions. The second case on the other hand concerns the HALT trials. Once the HALT process has been completed, and the
appropriate corrective actions have been taken, a custom production screening process to can be created to identify process defects. Highly Accelerated Stress Screening, also known as HASS, applies all stresses simultaneously. Based on HALT limits, HASS stress levels evaluate the power supply near its operating limits.
What to look for in a power supply Start with the desired useful life of the power supply, this will depend upon the application and often the cost of downtime or repair. Review any reliability data that the manufacturer offers. If only an MTBF figure is given, ask for the detailed calculations, particularly if the number seems abnormally high. In Japan, the JEITA RCR-9102 standard is often used.
Originally based on MIL-HDBK-217F, the standard has several revisions and calculated MTBF numbers in RCR- 9102B are half those in the original standard. As a note of caution, electrolytic capacitor life should
also be considered, particularly at higher ambient temperatures. Several power supply manufacturers state capacitor life in their reliability data. Ensure that if the power supply has both convection and forced air ratings, the life is stated for the desired cooling method. Reputable manufacturers will be able to provide
detailed calculations – including the effect of ripple current that can shorten capacitor life due to excessive internal heating if not properly considered at the design stage.
TDK-Lambda UK
www.uk.tdk-lambda.com
www.electronicsproductionworld.com EIU 23
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30
