X-capacitor failure modes?

[Joerg]

[...]

Still, watch those X-caps in them boat anchors where you now have so
many of. I have seen these golden-clear plastic caps in many of them. My
lab has pretty good line spike filtering and the cap ran at less than
50% of its rated voltage yet it decided to blow. Problem is, things such
as this impedance analyzer can be unattended for long periods of time
when you run an experiment. Same with a spectrum analyzer when it runs a
baseline scan over a 100MHz swath with 1Hz BW. You don't want to come
back from lunch only to see trucks with flashing blue lights surrounding
your building because a fire alarm went off.

I turn it off at the power bar for that sort of reason. Means that the
ovenized XOs take awhile to warm up, but oh well.

Doesn't always work. Another incident I had was a 6h endurance test of a
new design for a customer. I sat in the office next door doing something
else. Then ... sniff .. sniff ... what on earth ... ran into the lab and
sure enough a transistor in a linear bench supply had shorted. Instead
of a regulated 9V my prototype now got close to 40V at several amps. It
was not a pretty sight and took out several other pieces of equipment.

 

[Joerg]

X rated capacitors are generally metallised film, such that where the
film breaks down the metal is vaporised destroying and preventing any
further current path. As a result the capacitance will generally
degrade over time, so find the apparent 0.28uF a little hard to explain.

That 0.28uF is the main reason I posted my question. I expected half or
more of the capacitance to be gone up in smoke, literally.

 

[Phil Allison]

"Mike Perkins"

X rated capacitors are generally metallised film, such that where the film
breaks down the metal is vaporised destroying and preventing any further
current path. As a result the capacitance will generally degrade over
time, so find the apparent 0.28uF a little hard to explain.

** Class X caps are specially wound so as to not suffer from internal corona
discharge at 250VAC - the technique is called "two in series" which
creates a floating electrode inside the cap that sits at half the applied
voltage.

Corona is caused by tiny trapped air pocket inside the cap during winding,
only avoided by winding in vacuo. Discharge initiates at about 150VAC (
depending on the air pressure) - so having series pair solves the problem.

A damaged X cap might lose one of the pair and so the capacitance goes up.


.... Phil

 

[Spehro Pefhany]

"Mike Perkins"


** Class X caps are specially wound so as to not suffer from internal corona
discharge at 250VAC - the technique is called "two in series" which
creates a floating electrode inside the cap that sits at half the applied
voltage.

Corona is caused by tiny trapped air pocket inside the cap during winding,
only avoided by winding in vacuo. Discharge initiates at about 150VAC (
depending on the air pressure) - so having series pair solves the problem.

A damaged X cap might lose one of the pair and so the capacitance goes up.


... Phil

Interesting information, Phil!



Best regards,
Spehro Pefhany

 

[John K]

Doesn't always work. Another incident I had was a 6h endurance test of
a new design for a customer. I sat in the office next door doing
something else. Then ... sniff .. sniff ... what on earth ... ran into
the lab and sure enough a transistor in a linear bench supply had
shorted. Instead of a regulated 9V my prototype now got close to 40V
at several amps. It was not a pretty sight and took out several other
pieces of equipment.

I just lost a brand new motherboard to a defective power supply. I'm
thinking of putting 50 amp SCR's across all the critical voltages for the
pc and lab supplies. Trigger the SCR with a TL431 set to 10% above nominal.
Real cheap - may only cost a few pennies. Better to blow a fuse than lose
an important piece of equipment.

JK

 

[Phil Allison]

"Spehro Pefhany"

"Phil Allison"



Interesting information, Phil!

** You mean you did not already know that ?

FYI:

There are also " three in series " X caps for higher voltages like 440VAC.

It is possible to vacuum impregnate some types of X caps ( eg film, foil,
paper ) and eliminate all trapped air.

But few cap makers will attempt winding caps in vacuo.

Staff hate wearing those clumsy space suits ...



..... Phil

 

[Spehro Pefhany]

I just lost a brand new motherboard to a defective power supply. I'm
thinking of putting 50 amp SCR's across all the critical voltages for the
pc and lab supplies. Trigger the SCR with a TL431 set to 10% above nominal.
Real cheap - may only cost a few pennies. Better to blow a fuse than lose
an important piece of equipment.

JK

Decent power supplies claim to have OVP built in. Eg.

http://www.seasonicusa.com/G-series-450-550-650.htm



Best regards,
Spehro Pefhany

 

[Spehro Pefhany]

"Spehro Pefhany"

** You mean you did not already know that ?

Nope. Maybe it's common knowledge, but not for me. I've been involved
in specifying such caps (griping about the size), and even in the test
equipment to verify the long-term reliability at 85°C, but not the
internal construction.

FYI:

There are also " three in series " X caps for higher voltages like 440VAC.

It is possible to vacuum impregnate some types of X caps ( eg film, foil,
paper ) and eliminate all trapped air.

But few cap makers will attempt winding caps in vacuo.

Staff hate wearing those clumsy space suits ...

I've only been in one film cap plant, and I don't remember seeing much
of the winding stuff. They were powder coated rather than the Euro
potted-molded "box" type.

.... Phil

Best regards,
Spehro Pefhany

 

[Phil Allison]

"Spehro Pefhany"

Nope. Maybe it's common knowledge, but not for me. I've been involved
in specifying such caps (griping about the size), and even in the test
equipment to verify the long-term reliability at 85°C, but not the
internal construction.

** If you read Wireless World magazine - you might have come across a
couple of articles by " Cyril Bateman" published in the late 1980s IIRC.
Cyril worked as an applications engineer for Hunts Capacitors in the UK.

In the article, the story of the development of class X caps is told.

In the days of ( fully impregnated ) paper/foil caps, it had been common to
use 600VDC examples as suppression caps at 240VAC with little problem. When
metallised polyester film caps arrived and folk tried the same thing - BIG
problem !!.

Cyril's investigated, found it was internal corona damage and advised all
customers to cease using metallised film caps in this way.

The first solution was to make a component that was simply two 600V caps
wired in series - then someone devised the cunning trick of winding a
single cap with a floating metal layer inside.

In the days before class X caps became readily available, some dealers here
in Sydney advised the use of 1000V or 1600V metallised polycarbonate caps
( Philips 341 series) for the job - which rapidly caught fire when exposed
to 240VAC.

I like to think I had a little to do with changing that situation....



..... Phil

 

[John K]

Decent power supplies claim to have OVP built in. Eg.

http://www.seasonicusa.com/G-series-450-550-650.htm

Yes, and short-circuit protection. Unfortunately, I think the short-
circuit protection relies on the 12V. If it is crowbarred the protection
is lost.

I'm starting to think of putting overvoltage protection at the input to
the circuit. The IPB075N04L is p-channel, 40V, 50A(?) and 7.5 mOhm. It
costs $0.2018 at Arrow:

"http://octopart.com/partsearch#search/requestData&q=IPB075N04LGATMA1"

A simple TL431 driving a 2N4403 could short the gate to the source and
turn the mosfet off. This could work for input voltages from 5V to
probably 30V or so.

Now the problem is how long would it take to shut down given an
instantaneous jump to a higher voltage, for example +5V shorted to +12V?

Would it be fast enough to protect sensitive logic?

JK

 

[Klaus Kragelund]

I just lost a brand new motherboard to a defective power supply. I'm

thinking of putting 50 amp SCR's across all the critical voltages for the

pc and lab supplies. Trigger the SCR with a TL431 set to 10% above nominal.

Real cheap - may only cost a few pennies. Better to blow a fuse than lose

an important piece of equipment.

When I worked with a design for space flight, we did that. Added a giant SCR and a trigger circuit, to protect the DUT in case of a PSU malfunction (or a programming error since it was a programable PSU)

Cheers

Klaus

 

[John K]

When I worked with a design for space flight, we did that. Added a
giant SCR and a trigger circuit, to protect the DUT in case of a PSU
malfunction (or a programming error since it was a programable PSU)

Klaus

I'd hate to think of triggering an SCR in a spacecraft. How do you turn
it off?

How do you protect the PSU against a short on the output? You can't
simply blow a fuse.

Why use a programmable power supply? Why have the power supply deliver
enough voltage to damage a load? Power is hard to generate in space. Any
excess would be wasted as heat, which is difficult to get rid of in
space.

Why not design the power supply so it cannot deliver enough overvoltage
to damage a circuit. For example, spacecraft need batteries to provide
power when the solar arrays are in the Earth's shadow. Batteries are
capable of some known maximum voltage. They also have some minimum
voltage when discharged. Have the power supply deliver some fixed ratio
of the battery voltage, and design the load to operate between those
limits. For example, a simple forward converter output is determined by
the transformer turns ratio:

http://en.wikipedia.org/wiki/Forward_converter

Then the only problem you need to worry about is what happens if the
power supply fails and doesn't deliver enough voltage.

I'd think you would have backup power supplies and simply switch to a
known good one.

JK

 

[Spehro Pefhany]

I'd hate to think of triggering an SCR in a spacecraft. How do you turn
it off?

Remotely cycle the power to the affected module.


Best regards,
Spehro Pefhany

 

[Adrian Tuddenham]

Ah, ok, so it could indeed be the X-cap then. I was impressed how much
smoke it let off. The whole room (size of a bedroom) was full of smoke.

My local garage had a similar capacitor failure in their gas analyser
abut a month ago, it filled the whole workshop with smoke and left a
dirty streak up the wall behind the machine. They could hardly believe
that the cause was so trivial.

 

[Klaus Kragelund]

I'd hate to think of triggering an SCR in a spacecraft. How do you turn

it off?

Well, this PSU was on ground, testing the modules to be used for space flight. The SCR was to protect the space modules, not the PSU.

Cheers

Klaus

 

[John K]

Well, this PSU was on ground, testing the modules to be used for space
flight. The SCR was to protect the space modules, not the PSU.

Klaus

Thanks. That makes a lot more sense. Much easier to change the fuse!

JK

 

[Anthony Stewart]

Check out Panasonic's design of X class PU and PE caps.

http://www.digikey.com/product-sear...e=1&chp=0&quantity=0&ptm=0&fid=0&pageSize=500

Also see the video of the segmented film being shorted by arcs in action to see how shorting is localized to minimize healing required (the isolated segments connected with tiny fuse links are not shown)

 

[Joerg]

Sounds like a ping-pong failure mechanism to me!

Mine sounded more like a Thompson sub machine gun. Rat-tat-tat ..
rat-tat ... rat-tat-tat-tat ... Got me out of my chair prontissimo.

 

[Wim Ton]

There is an other failure mode: total loss of capacity due to internal cracks. Not noticeable when used in a filter, but when used as a voltage dropper in a power supply.

 

[Phil Allison]

"Wim Ton"

There is an other failure mode:
total loss of capacity due to internal cracks.

** ????

Not noticeable when used in a filter, but
when used as a voltage dropper in a power supply.

** I have seen that kind of failure and attributed it to one of the pigtail
terminations going open.

If you look at how it is done, it's a wonder it ever works.


.... Phil