1. 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

2. 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

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

4. 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

5. 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.

6. 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%

7. 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

8. 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.

9. 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

10. Varistor Degradation Tests

11. 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)

12. 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

13. 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

14. 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

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

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

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

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

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

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

21. 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

22. DEGRADATION AND SYSTEM RELIABILITY
worst-case design criteria for the protected circuit
Maximum Operating Voltage for U1
VDS(sw)
Switching Drain Source Voltage(Tj= °C) V
VDS(st)
Start Up Drain Source Voltage (Tj= °C) V ID
Continuous Drain Current
Internally limited
VDD
Supply Voltage 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??

23. 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

24. 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?

25. Results of simulations
DC Stress Analysis

26. 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

27. Results of simulations

28. Results of simulations
Time Domain analysis
More Realistic

29. Results of simulations

30. 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

31. 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

32. THANKS YOU QUESTIONS?