Failures over useful life are random but have an average rate: Poisson Process

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Failures over useful life are random but have an average rate: Poisson Process

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Failures over useful life are random but have an average rate: Poisson Process

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Result #3:

# Failures/yr = #Units * Sli = 7.20

Reliability Analysis of a Low Voltage Power Supply Design

for the Front-End Electronics of the ATLAS Tile Calorimeter

Gary Drake, Member IEEE,

James Proudfoot

Argonne National Laboratory, Lemont, IL USA

Abhirami Senthilkumaran, Bruce Mellado,

Anusha Gopalakrishnan, Sanish Mahadik

University of Wisconsin-Madison, Madison, WI USA

On Behalf of the ATLAS Tile Calorimeter System

The TileCAL Low Voltage System

LVPS Brick

- Power for TileCAL Front-End Electronics
- Novel Switching DC-DC Power Supply
- Custom, Compact, High-Efficiency, 250 Watt
- 8 Different Voltages Customized Bricks
- Water Cooled; System Interface & Monitoring
- Environment: Magnetic Field, Radiation Tolerant

- 256 boxes on detector, 2048 bricks, + spares
- Reliability is Important Infrequent Access

LVPS Box

8 bricks per Box

LVPS

Access

on

Detector

Detector Section

End of Long Barrel

Drawer Electronics

We have performed a reliability analysis on the new upgraded supplies 2048 Bricks in the detector system

Reliability Analysis Methodology

- Mean Time Between Failures MTBF
- Expected time between failures
- MTBF = 1 / l
- This is not “useful lifetime”

- Probability of Failure-Free Operation
- Probability of no failures at time t
- R(t) = e-lt
- Must calculate l for the entire unit

- Failures over useful life are random but have an
average rate: Poisson Process

- Probability of k failures between time t and t+t:
Poisson Distribution

- Failures in Electronics
- Failures generally
described by the

Bathtub Curve

- Interested in region of
Constant Failure Rate

- Failures generally

- l = Average number of units failing per unit time
- Measured in Failures In Time FITS (# / 109 hrs)

Calculations

- Comparison with Previous Design
- Rated voltage of capacitor is 20V
- Calculated failure rate: 12.4 bricks/year
- Observed failure rate: 5.2 bricks/year
- From 3 years of operation

- Series-Parallel Model and
- Voltage De-rating for Capacitor
- Tantalum capacitors most critical
- Rated for 35V, used at <= 15V
- Higher voltage rating reduces failure

- When a capacitor fails
- Probability of short = 0.75; Probability of open = 0.25
- Also include 4 caps in parallel + diodes + LC filter

- Series Reliability Model
- Any single failure can cause brick to fail
- Use only critical parts in model

- Assume
- Part failures are independent & random
- Start with single tantalum cap

- Parts Count Method
- Use FITS values for each part

- Statistical Analysis
- Poisson distribution; Neyman procedure

- Use observations as a correction

# Failures/yr = # Units * S [li * wi] X x

# Failures/yr = #Units * S [li * wi] = 5.03

Result #1:

Result #2:

- Expect 2.11 failures per year in the system

- Still Dominated by Tantalum Reliability

- Dominated by Tantalum Reliability