Electrolytic capacitors not always suited for 60w offline SMPS in totally enclosed plastic cases....

[Flyback]

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.

------_________--------------____________------------______________-----------------______________________
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 degc” 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.

 

[gophert]

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

or some good

….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?

No. You cannot control what anyone does nor can you prevent them from doing it.

the best you can do is cover your ass by recording the temp vs time in the lamp.
105°C is really hot - are you expecting someone will put your device on an oven or boiling water? Oh, wait, boiling water is slightly less than 100°C in most parts of the world so that's not even a concern.
What do you expect your customers are going to do with your device?

 

[Mickster]

You cannot control what anyone does nor can you prevent them from doing it.

And this is why we have to have so many rules/regulations and safety guidelines...to try to protect people from themselves.
Never under-estimate the ingenuity of idiots:

 

[gophert]

And this is why we have to have so many rules/regulations and safety guidelines...to try to protect people from themselves.
Never under-estimate the ingenuity of idiots:

the benefit of regulations is that, at some point, one only has to design a device to meet regulations. In other words, a toaster doesn't have to be designed safely enough to drop into a swimming pool and not electrocute anyone. It simply has to have a cord less than 6-feet long. That way, any outlet close to a sink or pool will have a GFCI outlet to protect the user. If the user adds an extension cord and plugs into a distant outlet without GFCI protection and kills themselve, no blame on the manufacturer - the user did it on purpose or, if not, they at least did our gene pool a favor.

 

[alec_t]

Do they mean “105degC ambient”, or “105degC case temperature”, or “105degC internal core temperature”?

Ask the manufacturer. This data may be manufacturer-dependent.

 

[unclejed613]

i am sure the "105C" is ambient temperature since there's really no way to control the internal temperature of the cap. ESR easily becomes a "runaway" problem because as the ESR increases, so does the internal temperature of the cap, accelerating the increase in ESR.

 

[Flyback]

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

...I just wish i had a power resistor , and a small plastic box that was totally encloseable...then i could make it (the resistor) disippate 6w, and run a thermocouple in and measure the internal ambient with the box totally sealed up.
I believe it would be above 105degc when external ambient was 30degc.
...Just feel how hot a 6w light bulb gets, and thats in open air.

 

[Visitor]

They don't have power resistors where you are? How about one of those fancy ones that light up and a variation to adjust it to 6 watts. Or one or two of those fancy 12 volt light up resistors and a bench supply?

(Sorry, the sarcasm has been strong recently)

 

[gophert]

...Just feel how hot a 6w light bulb gets, and thats in open air.

And yet, I can put my hand on my car's headlight lens cover when a 55W incandescent headlight glows within and feel only a cozy level of warm, not >105°C.

I think you can fairly easily estimate whether a 6w heater will bring a box of a given size, wall thickness and material to 105°c.

 

[kubeek]

Also you can have a look at notebook power supplies. They use metal sheets for heatsinking and place them right at the wall of the power supply, effectively removing the heat through the plastic wall.

 

[gophert]

They use metal sheets for heatsinking and place them right at the wall of the power supply, ...

and greatly reducing EM emissions.

also, as kubeek pointed out, the fairly fast moving air* inside the box and a reasonably sized box will allow significant heat to transfer to surroundings.

- do the calculation for yourself. A typical 60w (12VDC x 5 amp) smps has a box size of 10cm x 6cm x 3cm for 180cm2 surface area (0.018m2). Polycarbonate shell @ 2.5mm thickness (0.2W/(m*K)).

- with the above, you'll quickly see that the box, even though closed and plastic, will pass more than 100W when the outer box is still at room temperature (25°C) to make a 80°C temp gradient vs 105°C box interior. Even when the exterior of the box and surrounding air reaches 50°C, it the box walls will still pass >50W.

you'll have to do the experiment yourself with your conditions. Also, remember that the ESR of the cap can lead to system heating itself - heat comes from all components.