By Hadi Yadavari (Presenter) B. Sal, M. Altun, E.N. Erturk, B. Ocak Emerging Circuits and Computation (ECC) Group Effects of ZnO varistor degradation on the overvoltage protection mechanism of electronic boards By Hadi Yadavari (Presenter) B. Sal, M. Altun, E.N. Erturk, B. Ocak

OUTLINE INTRODUCTION FIELD RETURN DATA ANALYSIS Degradation Mechanism Heavy Overload Moderate Overload Degradation Mechanism 8/20 us surge current derating test 2 ms surge current test Accelerated Ac voltages (AC ageing) Finding threshold 1mA test DEGRADATION AND SYSTEM RELIABILITY Simulation of Varistor Model and Degradation Factor Results of simulations DC Stress Analysis Surged Pulse Voltage Analysis Time Domain Conclusion

What is Varistor? ZnO varistors are variable resistors appropriate varistor ZnO varistors are variable resistors

Degradation of Varistor long-term AC or DC voltage stresses and surges degrade the varistor (with decreasing in Vv) Vv Increasing In some cases FIELD RETURN DATA ANALYSIS How we understand? What are the proofs? DEGRADATION MECHANISM What are the effects of Degradation on system? DEGRADATION AND SYSTEM RELIABILITY

FIELD RETURN DATA ANALYSIS well-maintained field data with over 1000 board failures To evaluate Degradation *a lot of failures Power supply block. *geographical regions with poor electrical grid quality.

FIELD RETURN DATA ANALYSIS Investigating the causes of failures The Causes of failures Count Percentage Electrical grid and overvoltage 373 30% Quality 141 11% Humidity 51 4% Triac Relay 124 10% The microcontroller 23 1.80% Shock, Broken PCB Production Errors 143 11.50% other 21 1.70% TOTAL 1249 100%

FIELD RETURN DATA ANALYSIS Data shows the main overvoltage failures are Crucial components to analysis Varistor 142 Machine Stopped U1 + R23 213 U1 (Integrated circuit) 77

FIELD RETURN DATA ANALYSIS Overload Response Heavy Overload Surge currents far beyond the specified ratings. In extreme cases the varistor will burst. approximately 33% of overvoltage failures Varistor faults =142 as open circuited and punctured.

FIELD RETURN DATA ANALYSIS Overload Response Moderate Overload Surge currents or continuous overload of up to approx. one and a half times the specified figures. more than 50% of overvoltage failures U1 + R23 Faults = 213 couldn’t protect, system fails Moderate Overload => Degradation

Varistor Degradation Tests

Varistor Degradation Tests Reliability tests for choosing an appropriate varistor for electronic boards by Arcelik company, Istanbul, Vv parameter of varistors can change (Increasing trend)

Varistor Degradation Tests 8/20 us surge current derrating test 100 unipolar surge currents (8/20 μs) with 30s in-terval are applied to a varistor . Series V peak (V) 10mm/275V 1350 10mm/300V 1400 10mm/320V 1500 10mm/350V 1600 14mm/300V 1900 14mm/320V 2 000 20mm/275V 3450

Varistor Degradation Tests 8/20 us (surge current derrating) test results In total 276 varistor were tested . Results -> increasing trend Vv Parameter Vv parameter change percentages

Varistor Degradation Tests 2 ms surge current derating 100 unipolar surge currents (2 ms), with 120s interval are applied to a varistor In total 230 varistor were tested . Results show hat large number of changes are between 2% and 10%. Series V peak (V) 10mm/275V 630 10mm/300V 690 10mm/320V 765 10mm/350V 825 14mm/300V 685 14mm/320V 740 20mm/275V Vv parameter change percentages

Varistor Degradation Tests Accelerated Ac voltages (AC ageing) Finding threshold we started current level from 10 mA - Voltage plunged dramatically - Burned up

Varistor Degradation Tests Accelerated Ac voltages (AC ageing) Finding threshold - Decreased to 5mA and 3mA - Still dramatic decline

Varistor Degradation Tests Accelerated Ac voltages (AC ageing) Finding threshold For 2mA tests,

Varistor Degradation Tests Accelerated Ac voltages (AC ageing) Finding threshold for 1.5mA test. very close to the threshold.

Varistor Degradation Tests Accelerated Ac voltages (AC ageing) Finding threshold - repeated for 0.7 mA and 1 mA increasing trend in Vv

Varistor Degradation Tests Accelerated Ac voltages (AC ageing) Finding threshold 0,7 mA-> increasing trend in Vv

Varistor Degradation Tests Accelerated Ac voltages (AC ageing) In conclusion for this family of varistor samples 1.5 => threshold value above 1.5 mA, heavy overload less than the threshold moderate overload Degradation in varistor

DEGRADATION AND SYSTEM RELIABILITY worst-case design criteria for the protected circuit   Maximum Operating Voltage for U1 VDS(sw) Switching Drain Source Voltage(Tj=25 ... 125°C) -0.3 ... 730 V VDS(st) Start Up Drain Source Voltage (Tj=25 ... 125°C) -0.3 ... 400 V ID Continuous Drain Current Internally limited VDD Supply Voltage 0 ... 50 V IFB Feedback Current 3 mA Type (untapped) VRMS VDC imax (8/20 μs Wmax (2 ms) Pmax S10K320 320 V 420 V 2500 A 50.0 J 0.40 W Vv (1 mA) ΔVv   vc, max (ic) ic Ctyp (1 kHz) 510 V ±10 % 840 V 25.0 A 170 pF a proper ZnO varistor In case of Varistor Degradation??

Simulation of Varistor Model and Degradation Factor 𝑙𝑜𝑔𝑉=𝑏1+𝑏2. log 𝐼 +𝑏3. 𝑒 −lo g ( 𝐼 +𝑏4. 𝑒 lo g ( 𝐼 𝐼>0 degradation factor 𝑙𝑜𝑔𝑉−𝐿𝑜𝑔𝐷𝑒𝑔=𝑏1+𝑏2. log 𝐼 +𝑏3. 𝑒 −lo g ( 𝐼 +𝑏4. 𝑒 lo g ( 𝐼

Results of simulations DC Stress Analysis Maximum possible overvoltage value Circuitry does not pass a voltage above 730V Deg Factor Vv Vsis.max 1 503 1230 1.01 505.2 1137.5 1.02 508.1 1058 1.03 511.84 994 1.04 516.6 942.8 1.05 522.6 899.8 1.06 530.1 866.75 1.07 539.2 839.3 1.08 550.08 817.5 1.09 551.68 798.59 1.1 553.7 786.15 1.11 556.3 775.4 1.12 559.55 766.2 1.13 563.6 758.38 1.14 568.6 751.8 1.15 574.8 747.55 1.16 582.4 744.5 VU1 <730v => Max Voltage input?

Results of simulations DC Stress Analysis

Results of simulations Surged Pulse Voltage Analysis circuit has capacitive and inductive features. DC analysis can neglect the effect of these features MAx Output 730 Input :1230 Deg Factor=1

Results of simulations

Results of simulations Time Domain analysis More Realistic

Results of simulations

CONCLUSION moderate overload => degrade the ZnO varistor characteristic => increasing of (Vv). several reliability and AC aging tests results =>increasing of Vv for moderate overvoltages threshold value => the border of moderate and heavy overvoltages. we simulated the related circuit in SPICE software for different analysis and different overvoltage types. As ZnO varistor degrading, the related system capability to handle the overvoltage is decreasing

So don’t TRUST the ZnO varistor!  CONCLUSION If we consider a typical overvoltage type for a specific environment, the designed circuit maybe stand against the overvoltages, So don’t TRUST the ZnO varistor!  But in case of the degradation the failing is possible

THANKS YOU QUESTIONS?