WCCA

Question 1

Q1. Monte-Carlo vs. EVA vs. RSS
Scenario: 5 V divider → 3.3 V, 1 % resistors, –55 °C to +125 °C, 5 V ±5 %.

Answer 1

Q1. Monte-Carlo vs. EVA vs. RSS
Scenario: 5 V divider → 3.3 V, 1 % resistors, –55 °C to +125 °C, 5 V ±5 %.
Step 1 – EVA (extreme-value)
Take every part at min or max
Vmin = 5.25 × (R2_min / (R1_max + R2_min)) = 3.20 V
Vmax = 4.75 × (R2_max / (R1_min + R2_max)) = 3.41 V
Range = 210 mV (worst-case box)
Step 2 – RSS (root-sum-square)
Treat tolerances as independent Gaussians
σ_temp = 50 ppm × 180 °C = 0.9 %
σ_tol = 1 % / √3 = 0.577 %
σ_total = √(0.577² + 0.577² + 0.9²) = 1.23 %
3σ span = 3 × 1.23 % × 3.3 V = 122 mV (tighter box)
Step 3 – Monte-Carlo (sample)
LTspice line:
.step param tol 0.9 1.1 0.001
Run 5 000 cases → histogram 3.3 V ± 60 mV (3σ) → matches RSS ✔

Question 2

Q2. NASA-STD-5012 derating table walk-through
Part: 0603 1 kΩ, ¼ W, ±100 ppm/°C, used in 5 V divider, 85 °C ambient.

Answer 2

Q2. NASA-STD-5012 derating table walk-through
Part: 0603 1 kΩ, ¼ W, ±100 ppm/°C, used in 5 V divider, 85 °C ambient.
Step 1 – voltage derating
Max applied = 5 V
Rated working = 75 V
Derate limit = 50 % × 75 V = 37.5 V → 5 V < 37.5 V ✔
Step 2 – power derating
P_actual = V² / R = 25 V² / 1 kΩ = 25 mW
Rated power = 250 mW @ 70 °C, derate 50 % @ 85 °C → 125 mW
25 mW < 125 mW ✔
Step 3 – temp derating
Tmax part = 155 °C
Tambient = 85 °C
ΔT allowed = 155 – 85 = 70 °C → pass (no further action)