Medical Power Supply Reliability Demonstration: MTBF and DMTBF

MTBF (Mean Time Between Failures) is a well-accepted method for measuring power supply reliability. The MTBF factor is calculated and tested based on assumptions such as an electrical component’s stress factor under known circumstances.  The calculated MTBF is used to identify potential design flaws and to improve power supply design via reliability and field life. In order to achieve a high MTBF rating and a long service life, all Phasium medical power supplies are specifically designed with high-quality components. These include resistors and electrolytic capacitors from known, qualified suppliers in Japan and the United States as well as silicon from similar known, trusted vendors. Using trusted component suppliers ensures that the MTBF is the minimum expectation for all Phasium products. Because of this dedicated supply chain management, many products will work well beyond the hours for which they are rated.

Currently, the most common calculation method for MTBF is Part Count plus Stress Analysis following guidelines such as MIL-HDBK-217F Notice 2 and TELCORDIA SR/TR-332(Bellcore). MIL-HDBK-217F Notice 2 is the Reliability Prediction of Electronic Equipment in the US Military Handbook, and TELCORDIA SR/TR-332(Bellcore) is the generally accepted Reliability Prediction for commercial electronics.

Proving Reliability with DMTBF

DMTBF (Demonstrated Mean Time Between Failures) is a method used to test reliability. Under a controlled high-stress environment (including temperature, humidity, electrical, mechanical, etc.) extended life testing is used to accelerate deterioration to predict MTBF and validate normal power supply operation in a short period of time. DMTBF is an effective way to demonstrate and verify a MTBF calculation.

The Bathtub Curve (Figure 1) is a well-known failure rate over time curve for power supplies. There is a high failure rate during the beginning phase and final phase of the power supply’s service life. DMTBF’s verification is at the beginning stage of the bathtub curve.

Uncovering the Blind Spots

Early failure is mainly caused by poor workmanship, shoddy components and even design flaws. This makes DMTBF the most strict and effective verification for a product’s MTBF prediction. These issues—poor workmanship, shoddy components and design flaws—are the overlooked blind spots in the calculation of  MTBF.  To provide the accurate verification of the calculated MTBF, DMTBF must be used. Through DMTBF verification, the reliability of medical power supplies can be predicted, verified  and even improved, which ensures customer satisfaction through improved competitiveness of a medical device as well as a reduction in overall quality risk.

Figure 1: The Bathtub Curve.

Megmeet’s DMTBF Calculation Equation:

Note:

a- Trust Level
r- Failure rate
A- Credibility Factor
T1- Normal Temperature
T2- Actual Testing Temperature
Ea- Activation Energy, Electronics, Ea=0.3-1.0eV. Most common choice: Ea=0.6eV
K- Boltzmann Constant, equals to 8.617E-05 eV/K
Afp-High temperature acceleration factor Arrhenius model
Aft-Power acceleration factor =60min/(Ton+Toff), Common pick: 2
DC-Duty Cycle
n- Number of Units Under Testing
Tt-Total Experiment Time
MTBFspec- Calculated MTBF from Datasheet

Test Case: Verifying the MTBF calculation for the MP120 Series with DMTBF

The MP120 series is a 2” x 4” x 1.35” 120W/150W convection/forced air cooled open frame medical power supply has been in production since 2015. The input voltage is 85-264Vac, and main output voltage is 12Vdc, 19Vdc, 24Vdc, 28Vdc and 48Vdc respectively. 5V STDBY can supply 0.5A with remote control connection. The MP120’s specification shows an MTBF spec of 200k Hours at 25°C under full load.

The MP120-24 was implemented in a medical monitoring system in 2015. During the manufacturing process, 45 samples of MP120 were selected for a 50-day deterioration test with a resulting failure rate (r) of 0. From the above DMTBF Calculation Equation, we get a DMTBF of 238k Hours (see table below). DMTBF testing verifies the power supply’s design truly matches and exceeds the 200k Hours MTBF requirement provided in the design specification.