# What is MTBF? What is DMTBF? How is this different from the Life Cycle?

MTBF (Mean Time Between Failure) and Life Cycle are both indicators of reliability. MTBF can be calculated by two different methodologies, which are “part count” and “stress analysis”. The regulations, MIL-HDBK-217F Notice 2 and TELCORDIA SR/TR-332(Bellcore) are commonly used to calculate MTBF. MIL-HDBK-217F is a United States military standard, and TELCORDIA SR/TR-332(Bellcore) is a commercial regulation. MEAN WELL utilize MIL-HDBK-217F(Stress Analysis) as the core of MTBF. The exact meaning of MTBF is, after continuously using the power supply for a certain amount of time, the average time that the probability of proper operation is down to 36.8%（e-1=0.368）. Currently MEAN WELL is adopting MIL-HDBK-217F, to predict the expected reliability through Stress Analysis (excluding fans); this MTBF means the probability of the product can continue the normal work after working continuously up to the calculated MTBF time is 36.8% (e-1=0.368). If the power supply is continuously used at double the MTBF time, the probability of proper operation becomes 13.5%（e-2=0.135.

DMTBF (Demonstrated Mean Time Between Failures) is a way of evaluating MTBF in a relatively short period of time based on accelerated deterioration (high stress, high temperature) tests under specific parameters and conditions. Please refer to the following equation for MTBF calculation.
This method compares to the previous methods, this calculation uses real test data to demonstrate the reliability of the power supply.

Where

MTBF： Mean Time Between Failure

• X2：Can be found in chi-square distribution
• N：Number of sampling
• AF：Acceleration factor, which can be derived from acceleration factor equation.
• Ae=0.6
• K(Boltzmann Constant)=(eV/k)
• T1：Rated temperature of specification. Note: Kelvin will be the unit use for calculation
• T2：The temperature that is used in the meaning of acceleration, and the chosen temperature could not result in physical change in materials.

Note: Kelvin will be the unit use for calculation.

Life Cycle (Capacitor Life Cycle) is found by using the temperature rise of electrolytic capacitors under maximum operating temperature to estimate the approximate life of the power supply. For example, RSP-750-12 MTBF=109.1K hours(25°C); electrolytic capacitor C110 Life Cycle=213K hours (Ta=50℃).

MEAN WELL considers the capacitor lifecycle calculation as the most important indicator for the estimated lifetime. The (D)MTBF is the main indicator for the reliability of the power supply. For more information please see:  Investigation of The Lifetime & Reliability of Power Supply -MEAN WELL EUROPE Switching Power Supply

Categories: Charging, Industrial, Medical, Reliability
Tags: Bellcore, Boltzmann, Capacitor Life Cycle, Capacitor Lifecycle Calculation, Demonstrated Mean Time Between Failures, DMTBF, Life Cycle, Mean Time Between Failure, MIL-HDBK-217F, MTBF, TELCORDIA SR/TR-332