Effects of ZnO varistor degradation on the overvoltage protection mechanism of
electronic boardsBy Hadi Yadavari (Presenter)
B. Sal, M. Altun, E.N. Erturk, B. Ocak
Emerging Circuits and Computation (ECC) Group
OUTLINE INTRODUCTION FIELD RETURN DATA ANALYSIS
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
1
What is Varistor?
2
appropriate varistor
ZnO varistors are variable resistors
Degradation of Varistor
3
long-term AC or DC voltage stresses and surges degrade the varistor
(with decreasing in Vv)
How we understand?
What are the proofs?
What are the effects of Degradation on system?
FIELD RETURN DATA ANALYSIS
DEGRADATION MECHANISM
DEGRADATION AND SYSTEM RELIABILITY
In some cases Vv Increasing
FIELD RETURN DATA ANALYSIS
4
well-maintained field data with over 1000 board failures
To evaluate Degradation
*a lot of failures
*geographical regions with poor electrical grid quality.
Power supply block.
FIELD RETURN DATA ANALYSIS
5
The Causes of failures Count Percentage
Electrical grid and overvoltage 373 30%
Quality 141 11%
Humidity 51 4%
Triac 373 30%
Relay 124 10%
The microcontroller 23 1.80%
Shock, Broken PCB Production Errors 143 11.50%
other 21 1.70%
TOTAL 1249 100%
Investigating the causes of failures
FIELD RETURN DATA ANALYSIS
Varistor 142 Machine Stopped
U1 + R23 213 Machine Stopped
U1 (Integrated circuit) 77 Machine Stopped
6
Data shows the main overvoltage failures are
Crucial components to analysis
FIELD RETURN DATA ANALYSIS Overload Response
Heavy Overload Surge currents far beyond the specified ratings. In extreme cases the varistor will burst.
7
approximately 33% of overvoltage failures
as open circuited and punctured.
Varistor faults =142
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.
8
U1 + R23 Faults = 213 more than 50% of overvoltage failures
couldn’t protect, system fails
Moderate Overload => Degradation
Varistor Degradation Tests
9
Varistor Degradation Tests
10
- 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 test100 unipolar surge currents (8/20 μs) with 30s in-terval are applied to a varistor .
11
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 34500 5 10 15 20 25
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
8/20 us testi
Varistor Degradation Tests 8/20 us (surge current derrating) test results
12
%0-%2 %2-%6 %6-%100
20
40
60
80
100
120
140
160
180
94
168
14
In total 276 varistor were tested . Results -> increasing trend Vv Parameter
Vv parameter change percentages
Varistor Degradation Tests 2 ms surge current derating100 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%.
13
Series V peak (V)
10mm/275V 630
10mm/300V 690
10mm/320V 765
10mm/350V 825
14mm/300V 685
14mm/320V 740
20mm/275V 630%0- %0-%2 %2-%10 %10+
0
20
40
60
80
100
120
43 45
100
42
2 ms test results
Vv parameter change percentages
Varistor Degradation Tests Accelerated Ac voltages (AC ageing)
Finding threshold
we started current level from 10 mA
14
- Voltage plunged dramatically - Burned up
Varistor Degradation Tests Accelerated Ac voltages (AC ageing)
Finding threshold- Decreased to 5mA and 3mA - Still dramatic decline
15
0 2 4 6 8 10 120
100
200
300
400
500
600
N:28, 5 mA test
0 10 20 30 40 50 60 70 80 900
100
200
300
400
500
600
N:20, 3 mA test
Varistor Degradation Tests Accelerated Ac voltages (AC ageing)
Finding thresholdFor 2mA tests,
16
0 20 40 60 80 100 120 140420440460480500520540
No:15, 2 mA test
0 10 20 30 40 50 60 70480
490
500
510
520
530
No:72, 2 mA test
0 20 40 60 80 100 120 140420
440
460
480
500
520
540
f(x) = − 3.45282392153926 ln(x) + 520.020706967727f(x) = − 9.79936671506677 ln(x) + 519.065633674919
2 mA test (Series 1 No:15, Series 2 No:72)
Series1 Logarithmic (Series1) Series3 Logarithmic (Series3)
Varistor Degradation Tests Accelerated Ac voltages (AC ageing)
Finding threshold- for 1.5mA test. - very close to the threshold.
17
0 50 100 150 200 250 300 350460
465
470
475
480
485
490
495
No:18, 1.5 mA test
0 50 100 150 200 250 300 350495
500
505
510
515
520
No:12, 1.5 mA test
Varistor Degradation Tests Accelerated Ac voltages (AC ageing)
Finding threshold- repeated for 0.7 mA and 1 mA increasing trend in Vv
18
0 50 100 150 200 250 300 350495
500
505
510
515
520
525
530
535
540
No:68, 1 mA testi (topraksız)
0 50 100 150 200 250 300 350500
505
510
515
520
525
530
535
540
No:13, 1 mA testi (topraksız)
0 50 100 150 200 250 300 350495
500
505
510
515
520
525
530
535
540
No:37, 1 mA testi (topraksız)
Varistor Degradation Tests Accelerated Ac voltages (AC ageing)
Finding threshold0,7 mA-> increasing trend in Vv
19
0 50 100 150 200 250 300 350495
500
505
510
515
520
525
No:59, 0.7 mA testi (1 saat topraklı)
0 50 100 150 200 250 300 350510
515
520
525
530
535
540
545
550
No:90, 0.7 mA testi (1 saat topraklı)
0 50 100 150 200 250 300 350512
514
516
518
520
522
524
526
528
530
532
No:36, 0.7 mA testi (24 saat topraklı)
Varistor Degradation Tests Accelerated Ac voltages (AC ageing)
20
In conclusion for this family of varistor samples 1.5 => threshold value
heavy overload above 1.5 mA,
less than the threshold moderate overload
Degradation in varistor
DEGRADATION AND SYSTEM RELIABILITY worst-case design criteria for the protected circuit
21
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(1 mA)
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 Varistor Model
22
𝑙𝑜𝑔𝑉=𝑏1+𝑏2. log ( 𝐼 )+𝑏3.𝑒¿ ¿
𝑙𝑜𝑔𝑉 − 𝐿𝑜𝑔𝐷𝑒𝑔=𝑏1+𝑏2. log ( 𝐼 )+𝑏3.𝑒¿ ¿degradation factor
Results of simulations DC Stress Analysis Maximum possible overvoltage value Circuitry does not pass a voltage above 730V
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Deg Factor Vv
Vsis.max
1 503 12301.01 505.2 1137.51.02 508.1 10581.03 511.84 9941.04 516.6 942.81.05 522.6 899.81.06 530.1 866.751.07 539.2 839.31.08 550.08 817.51.09 551.68 798.59
1.1 553.7 786.151.11 556.3 775.41.12 559.55 766.21.13 563.6 758.381.14 568.6 751.81.15 574.8 747.551.16 582.4 744.5
VU1 <730v => Max Voltage input?
Results of simulations DC Stress Analysis
24
503508.1
516.6530.1
550.08553.7
559.55568.6
582.4
600.38604.6
611.1620.5
634.72650.8
655.40
200
400
600
800
1000
1200
1400
Results of simulations Surged Pulse Voltage Analysis
25
MAx Output 730
Input :1230
Deg Factor=1
- circuit has capacitive and inductive features.
- DC analysis can neglect the effect of these features
Results of simulations
26
503505.2
508.1
511.84516.6
522.6530.1
539.2
550.08
551.68553.7
556.3
559.55563.6
568.6574.8
582.4591.8
600.38602.2
604.6607.4
611.1615.4
620.5 627
634.72644
650.8652.8
655.40
1000
2000
3000
4000
5000
6000
7000
Vsis.max dc Logarithmic (Vsis.max dc) Pulse duration 1msLogarithmic (Pulse duration 1ms) Pulse Duration 0.75 ms Logarithmic (Pulse Duration 0.75 ms)Pulse Duration 0.5 ms Logarithmic (Pulse Duration 0.5 ms)
Results of simulations Time Domain analysis More Realistic
27
Time
0s 5ms 10ms 15ms 20ms 25ms 30ms 35ms 40msV(V2:+) V(D2:2) V(V2:-)
-1.0KV
0V
1.0KV
2.0KV
Results of simulations
28
503505.2
508.1
511.84516.6
522.6530.1
539.2
550.08
551.68553.7
556.3
559.55563.6
568.6574.8
582.4591.8
600.38602.2
604.6607.4
611.1615.4
620.5 627
634.72644
650.8652.8
655.40
500
1000
1500
2000
2500
3000
3500
Time Domain (Worst-case) Pulse Duration:0.5 msLogarithmic (Time Domain (Worst-case) Pulse Duration:0.5 ms)Time Domain (Worst-case) Pulse Duration:0.25 msLogarithmic (Time Domain (Worst-case) Pulse Duration:0.25 ms)Time Domain (Worst-case) Pulse Duration:0.1 msLogarithmic (Time Domain (Worst-case) Pulse Duration:0.1 ms)
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
29
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!
30
But in case of the degradation the failing is possible