Electrolytic capacitor lifetime....SMPS in totally enclosed plastic case.

[treez]

As you know, The equation (5) of page 13 of this…
https://www.cde.com/resources/catalogs/AEappGUIDE.pdf
..is what governs wet electrolytic capacitor lifetime...

Lop = Mv × Lb × 2^((Tm – Tc)/10[°C]) (Equation (5))

Where
Lop is the expected operating life in h,
Mv is a unitless voltage multiplier for voltage derating,
Lb is the expected operating life in h for full rated voltage
and temperature,
Tm is the maximum permitted internal operating temperature
in degC, and
Ta is the actual capacitor internal operating temperature
in degC.

Its not clear in many wet electrolytic capacitor datasheets, when they say “2000hrs at 105degC”. Do they mean “105degC ambient”, or “105degC case temperature”, or “105degC internal core temperature”?

The other problem with wet electrolytics is of course related to the potential for the electrolyte to simply leak out. This exerpt from cornell dubilier shows the problem with short term high temperature transients in wet electrolytics….

However, as a capacitor heats up toward its maximum permitted
core temperature, the rules change. At temperatures above the
maximum core temperature and by 125 degC for most types the
electrolyte can be driven from capacitor element and the ESR
can increase as much as 10 times. By this mechanism, transient
over-temperature or over-current can permanently increase the
ESR and make the capacitor unusable.

….so this is the problem, can we be sure that our customer won’t ever temporarily operate the product in such a hot ambient that this temporary overheating of the Electrolytic capacitor occurs?
Another point, is, in Equation (5) above…all those temperature parameters depend on knowing what is the internal ambient temperature in the power supply enclosure. Do we really know where the customer is going to use it, and how hot it may become (even for only short intervals).

In any case, the internal ambient temperature in a 60W , “fully enclosed plastic case” offline power supply can easily get above 105degC when the external ambient is just 30degC.....and 105degC is the max temperature of the majority of electrolytic capacitors.

Then there’s the storage history of the electrolytic capacitor… before being sold, it may have already been stored for years in a hot warehouse, but they pretend they are new capacitors, and fiddle the date code, if there is even a date code.

 

[BigBoss]

How the internal temperature exceeds 105 degree in a 60W SMPS ?? Is there no air ventilation ?? Not reliable at all at least for other semiconductor components..
Anyway, they are average values for elco capacitors and they may have more or less working hours.
My CD player is working 2-3 hours per day and it's brilliant since 1998.
There are also very high quality Electrolytic capacitors which have 12000 working hours..Not bad huh ??

 

[treez]

I mean, if you search digikey for “Aluminium electrolytic capacitor, 450v, 47uF”…..there are virtually none above 105degc.

If one operates an El cap above quoted temp, then it can suffer serious damage, significantly affecting its lifetime. I have a Cornell Dubilier doc which states this. The “10 dgec” law doesn’t work when you go above the quoted temp.

A 60w offline SMPS, totally enclosed in a plastic enclosure, will easily get above 105degC (internal ambient) at 30degc external ambient.

Here is such a power supply...

https://www.mouser.co.uk/datasheet/2/260/LPC-60-spec-1109552.pdf

This is why an alternative to el caps is well needed.

 

[Easy peasy]

105 deg, max hot spot temp anywhere in the cap, obviously in a 65 deg C environment, the min temp of any part of the cap is 65 deg C, in a 105 deg C ambient you can't really allow any power dissipation in the cap as this will push some part over 105degC ....

 

[Easy peasy]

there are some 125 deg C electro's ...

 

[treez]

Yes you are right, though in the 47uF, 450V bracket, there is only one (El Cap) on digikey at 125degc..this one... Epcos B43866C5476M

I am sure you would agree that the lack of second sources in this bracket would mean that even this one would be unuseable.
...Its not much better with films, just that Films dont fail so dramatically especially after short overtemperature periods......films recover from this far better than El Caps.

Basically it appears that their cannot be any such thing as a long life offline PSU in a totally enclosed plastic case (for use at 30degc+ ambient and >6 hours per day on full load) if it has wet El Caps in it

 

[d123]

Hi,

Break and replace culture, keeps the world turning and consuming.

Fish tank 12V/?A LED strip SMPS plug-in supply (a tightly-packed/stuffed to the edges sealed enclosure 'wall wart') failed/went faulty after 3 years of ~12 hours every day - it was only the 1,000uF electrolytic capacitor that needed replacing. In a sense, quite impressive for something so small with no ventilation.

 

[treez]

Thanks, i wonder if it (the 1mF) went short or gradually opened?...if you can know they always go open, then you can put say a 5*10uF , 0805 ceramic at the output and that can take up the reigns when the el cap goes open.

 

[d123]

Hi treez,

My impression is that it failed short because on flicking the DC side light switch, it hiccuped/the LED strip flickered on for a split second then went off for a longer second or three over and over, until the capacitor was replaced. I assume(d) it had over-current protection due to that behaviour.

 

[treez]

Thanks, yes, it almost certainly went short and was hiccuping as you say.

 

[Easy peasy]

The trick is to design for min heat losses and have the warm bits connected to the plastic case as well as can be manged - with fins on the plastic case and a high degree of thermal conductivity on the ( black) plastic ... also feet to stand it off the ground / carpet / linoleum to allow more air circulation ... and a power turn down when 95 deg C is reached ...

 

[dick_freebird]

Simple equations tend to depend on assumptions
and those assumptions may be obscure.

Such as an assumption of uniform temp rise, as
some of the discussion touches upon, across
time and internal position.

Have you ever instrumented up the cap in question
to see its case temp in the product under worst
case recommended product-case-temp?

Potting might keep electrolyte in place even
in cases where the seal itself would fail? Might
also improve the capacitor's thermal path.

 

[treez]

..Sorry, cap in #7 must have gone open not short....otherwise leds wouldnt have lit at all....it probably trips out on OV.

Anway....
Offline 60W+ SMPS in totally enclosed plastic would probably have internal ambient >105degC?........

I’ve sometimes done 60W+ Offline SMPS that went into Totally enclosed plastic enclosures with no internal fans. I was not allowed to examine the thermal results, and thought this was because…

1…it was the companies IP relating to the product …and/or….
2…The thermal results may have been higher than would have been liked

___________************_______________**************

At some places, I have noticed a thermal test being done on a (normally) totally plastic enclosed 60w+ offline SMPS, and there was ventilation getting in to the enclosure as they had not plugged up the dremelled holes which were allowing the thermocouple wires to get in. Also, the plastic enclosure halves were obviously not as one (otherwise it would have been impossible to mount the thermocouples). As such there was a tiny gap opening all round the enclosure, which had been sealed up with a few layers of kapton tape. –A poor test setup.

____________****************__________________******************

CASE A: However, I did a 60W, 240VAC Offline Flyback LED driver that went into a plastic enclosure that had overall gaps in the plastic of some 30mm by 5mm at each end. This plastic enclosure itself was in a totally enclosed (sealed) metal enclosure. (the plastic enclosure was ~ 16cm x 6 cm x 5cm height) This metal enclosure was part of a single casting to which elsewhere was fixed a LED bank (on MCPCB) with 60W of electrical power going into it. The internal ambient of this plastic enclosure was recorded as 71degc (60W,240VAC input) when the ambient outside the containing metal enclosure was recorded as 25degc. The metal casting by the leds (a bit away from the power supply part) was recorded as 37degc.
…..since the internal ambient of this was 71degc, then I believe its obvious that the internal ambient of a totally plastic enclosed 60W+ offline SMPS would have an internal ambient of well above 71degC.
Would you agree?

____________________*********************______________________*************

CASE B: I once examined a Test Report for a Totally plastic enclosed Flyback SMPS which was 60W, 240VAC. This SMPS is sold in the 100’000s. (dims = ~11cm by 5cm by 2.5cm height)
(this power supply would always be on max power when run, but , in most use cases, was not usually run for very long each day)
The report said external ambient was 40degc. It recorded “Top of Case” as being 60degC. When I enquired as to whether this was internal or external they replied only to say that it was “near the main MOSFET, Q1”. Some of the lower power handling components in this enclosure were recorded as having temperatures in the mid 50’s (degc), down to 54degc. As such, I assume that the “top of case” temperature reading must have been on the outside of the case. The bulk input 100uF electrolytic was measured as “69degC”.

I believe that the “internal ambient” measurement was not done because it looked too high and may have worried the customer. A very hot component on the thermal cam was the RCD clamp resistor, but the temperature of this was not in the report, and not in the updated report, even after we asked for it.
The test report was obviously in error, because its difficult to see how some components (obviously inside the case) had temperatures which were colder than the outside of the plastic case.

___________**************________________***************_____________

All of this does point to the fact, that the internal ambient for an 60W+ offline SMPS in a totally plastic enclosed enclosure, is quite likely to have an internal ambient above 105degc. Would you agree?
As Cornell Dubilier says, operation of an el cap above datasheet quoted temp, even for small periods, can result in significant lifetime reduction of the El Cap.

 

[treez]

Hello,

In DER-601 (below), there is a 60W innoswitch SMPS Flyback design.
The Innoswitch is massively far ahead of any other Flyback controller in its feature set.

There is a metal heat spreader placed inside the enclosure in DER-601. How much reduction in component temperatures occur due to this heat spreader do you think?

Also, are there any design reports showing SMPS’s totally enclosed in plastic, but the electrolytic capacitors in separate “chambers” to the main SMPS components? (and joined by wires, but the wires holes through the “chamber” walls are sealed). This being done to keep the electrolytic capacitors cooler. I notice that Innoswitch is stated as being good for operating temperatures up to 150degC, so suspect the heat spreader method isn’t for the innoswitch benefit?


DER-601
**broken link removed**

 

[FvM]

Heat spreaders are used in most hermetically enclosed SMPS, may be except very small ones. The idea is to dissipate heat by conduction rather than internal convection to the enclosure. Enclosure surface has to be still cooled by convection.

 

[treez]

Thanks, i wonder if it would be cheaper to just put the electrolytic caps in a separate chamber, away from the heat from the other components...because FETs and diodes and tranformers etc can go well above 110degc and be ok