Effects of ZnO varistor degradation on the overvoltage protection mechanism of electronic boards By...

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


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.


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


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


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


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)


Finding thresholdfor 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


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


0 50 100 150 200 250 300 350495

500

505

510

515

520

525

530

535

540



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


0 50 100 150 200 250 300 350512

514

516

518

520

522

524

526

528

530

532



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

23

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

THANKS YOU

QUESTIONS?

Info: [email protected]

Effects of ZnO varistor degradation on the overvoltage protection mechanism of electronic boards By...

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