1. Form the Team
Date Started: Sep. 22, 2002
Internal team member: A, B, C
External team member: N/A
2. Describe the Problem
One field failed unit of XXX model was returned from the customer on Sep.
20. The failure occurred at the end-user’s site with the symptom described as ‘The unit burned when powered on with line and could not be powered on again.’
3. Describe the Root Cause
3.1 What Found
After carefully investigation it is found that components listed below in the XXX circuit are damaged:
Part Spec. Description
D5,D6,D7,D8 XX A/XX V Rectifier Diode
3.2 Circuit Schematic
2.3 Fishbone Diagram
3.4 Experiments to Reproduce the Failure
3.4.1 D5-D8 temperature measurement
Condition: 1) The unit operates at ambient temperature; 2) Connecting a xx Ohm resistor between positive and negative terminals of the battery to stimulate very low battery voltage.
Result: Case temperature of the diodes stabilizes at about 60 degree Celsius which is much lower than the maximum rating with adequate margin.
3.4.2 Battery short circuit
Condition: Short circuit the battery and measure the current passes through D5-D8 with current probe.
Result: The current is zero
3.4.3 Low battery voltage
The unit operates with the DC power provided by a xx V(which is much lower than the nominal xx V) battery. Measure the current passes through D5-D8 and no over-current is found.
3.4.4 Over-current through D5-D8 when power on with nominal I/P AC voltage
The DC power is provided by a battery with nominal voltage. Measure the current passes through D5-D8 with current probe at the moment when power on the unit with nominal AC input voltage. Over-current is found.
3.4.5 High input AC voltage and low battery voltage when power on
Connect a xx Ohm resistor between positive and negative terminals of the battery to stimulate very low battery voltage. Measure the current passes through D5-D8 with current probe at the moment when power on the unit with xx AC. The current reaches xx A which is much larger than the maximum rating xx A and that can easily destroy the diodes.
3.5 Conclusion
Based on the above experiments, the root cause is that at the moment when power on the unit, especially with low battery voltage and high input AC voltage, out-of-spec current through the diodes presents.
4. Containment Plan
Fix the unit, replace damaged components
5. Permanent C/A Plan
1) Change Cxx from xx μF/xx V electrolytic capacitor to xx uF/xx V X-Cap to reduce the current through the rectifier when power on the unit.
2) Use an integrated rectifier to replace D5-D8.
3) RD release ECR for customer approval of the above engineering changes after the effectiveness is verified.
4) RD release ECN to validate the changes after the ECR is approved.
6. Verify the effectiveness of C/A
1) Measure the current through the integrated rectifier at the same condition of 2.4.5, no over-current is found.
2) Perform power cycling on 10 engineering changed units under the condition described at 2.4.5 for 48 hours, no failure was found.
7. Prevent Recurrence
Update the Standard Circuit Design Procedure (Doc. #: xxxxxxxx). Add an item in Section 11 – xx Circuit Design Standard, to require: 1) using X-Cap rather than electrolytic capacitor parallel with the rectifier, and, 2) using integrated rectifier rather than individual diodes in xx circuit design.
8. Congratulate the Team
The ECR has been sent to XXX (the customer name) for approval on Sep. 25. and the updated procedure will be released formally by DCC immediately when receive the approval. Thanks for the excellent work of the team.
Date Closed: Sep. 26, 2002
[为不引起公司可能的不快，删去一些信息及名称]