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EEVblog 1486 - The REAL Truth Why Film Capacitors FAIL!

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EEVblog:
Think you know how film capacitors fail and degrade in capacitance over time? - self-healing due to surges, right? WRONG!

Capacitor expert and AVX Fellow Ron Demcko confirms what's really going on after a teardown of some failed and one good polypropylene X class capacitor.

00:00 - Teardown of a some failed film capacitors
00:52 - Self Healing and drop in capacitance
01:44 - Capacitance Measurements
02:15 - Teardown of a new Suntan brand polypropylene X2 film capacitor
03:03 - Different failure modes based on size and winding pressure
05:52 - Unwrapping the film
07:51 - The film inside a NEW film capacitor
09:19 - Teardown of the FAILED uTx brand heater capacitor with half capacitance
10:06 - How Schoopage and the end pin terminations work
12:23 - Teardown continues...
12:45 - WOW! What on earth is this?
14:54 - Elecami Wolf also did a teardown
16:05 - Teardown continues...
17:01 - Separating the film showing both slef-healing and extensive metal film corrosion
18:29 - Another uTx brand failed film capacitor from a Corsair PSU, with 90% loss in capacitance!
Elecami Wolf Youtube Channel: https://www.youtube.com/c/ElecamiWolf
19:55 - Let's call an expert! Ron Demcko from AVX tells us the REAL REASON for the failure!

PartialDischarge:
When he talks about corona and “noise” he is actually referring to partial discharges, high frequency currents that flow thru the insulation.
This is highly dependant on temperature and voltage and also construction. Right now partial discharges are not a required type test on capacitors but the thing is that some of them may have these even below the max rated AC voltage, so the industry is happy not talking about it. Designers that choose capacitors for the long term on expensive installations (power inverters...) do test these partial discharges before selecting.

Here’s an old but interesting doc on this
https://ntrs.nasa.gov/api/citations/19940028538/downloads/19940028538.pdf

Huluvu:
Based on the time to failure most likely the PP foil was already suffering from to high moisture during the manufacturing process. Manufacturers very often use only the 85/85 test which is useless when the moisture was already trapped within the foil.
A proper method would require a well done THB (temperature + humidity + bias) test which basically costs more time = money.

EEVblog:

--- Quote from: Huluvu on July 16, 2022, 05:43:37 am ---Based on the time to failure most likely the PP foil was already suffering from to high moisture during the manufacturing process. Manufacturers very often use only the 85/85 test which is useless when the moisture was already trapped within the foil.
A proper method would require a well done THB (temperature + humidity + bias) test which basically costs more time = money.

--- End quote ---

Yep, I suspect so too.

floobydust:
Nice followup on these film cap failures - but missing what an EE should do about it. I've had dropper caps last 1 year, others last 20 years.
I'm not a believer in the AVX fellow's moisture hypothesis as the main failure mechanism, sorry. It seemed like mostly speculation that didn't make sense enough to design something that lasts.

Vishay response: "It {failures} is because of Corona effect. Corona effect is electrical discharge (partial breakdown) which results from ionization of air on the surface or between the capacitor plates in AC voltage applications or in rapid changing DC voltages (pulses). However, film capacitor has the feature called self-healing. After one discharge, one self-healing. As a result, loss increases and capacitance decreases.
When there is too many corona effects accumulated, the capacitor will be failed.
Therefore, if we make internal 'series construction', we can make the voltage between capacitor plates half. It will be much lower than corona starting voltage, in order to avoid corona effect.
F1772 is internal series construction. At last, I suggest you also read our attached application note. Please note two tables in it.
We recommend that for continuous across line application and in series with the mains application, internal series construction capacitor should be used."

Vishay has specialty film caps designed for "series impedance" {dropper} applications that have a third-plate? to half the E-field I believe.
Dave's failure mechanisms chart is from CERN Metallized Film Capacitor Lifetime Evaluation and Failure Mode Analysis 

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