Circuit part FMEA example

What is FMEA?

Failure Mode and Effects Analysis – procedure that is standardized by IEC 60812 and required by most of functional safety, quality and reliability standards, part of any system or process analysis. FMEA helps to check if all safety mechanisms are in place, device or process works correctly and all important causes and effects are taken into account.

Circuit Part Under Analysis

Fig. 1 – Circuit under analysis

D1 – The diode is reversed polarity such that if a negative battery side is connected to the item, this negative voltage would short to ground, protecting the item from damage.

C1 – The capacitor is for the EMI filtering.

L1 – The coil primarily for filtering

R91 – The resistor carries the battery voltage to the switching transistors

FMEA Brainstorming

1. If D1 or C9 should short to ground, the battery would also short to ground which could lead to the draining of the vehicle battery. Such failure is considered as hazardous. Therefore, for the failure modes of both parts “short”, the Severity rank is 10. Occurrences are 3 were calculated from the parts failure rates under their respective stresses for the vehicle life, and then matched to the Occurrence scale of the automotive FMEA. Detection is very low 1, as shorting of any of the parts would be immediately noticed in test – item not operational.

2. Opening of D1 or C9 would not cause any damage to the item, except if the diode opens, then there would be no reverse battery protection, while with the capacitor open, there would be no EMI filtering – possible noise for the other equipment in the vehicle.

3. If L1 opens, the item would not be operational as the battery would be disconnected and the warning display would not be lit – Severity 9. Coils do have a very low failure rate, so that the Occurrence is 2. Detection is 1, since the item would not be operational.

4. If R91 failed as open, it would render the item inoperable, which also would be Severity 9. Since resistors have a very low failure rate, the Occurrence is 2. Detection is 1, since the item would not be operational.

FMEA Results

Fig. 2 – FMEA results

Knowing Failure Modes For Circuits

One of the most popular questions is where to get failure modes for the components that are within circuit under analysis? There several standards containing such kind of failure modes with the coefficients that allow to distribute overall component failure rate over failure modes defined.

One of the well known standards is MIL-HDBK-338B back from 1998. Below there is a chapter from this standard with the failure modes for the components that can be used for your FMEA.

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