Rules for over ripple currenting electrolytic capacitors?

[cupoftea]

Hi,
Customer has sent us an 8W Flyback (24vout, 132khz, 100-265vac). They want the flyback increased to 17W. We are limited to keep using the EFD15/8/5 Former and core set. The input electrolytics cannot change. They are two 10uF, 400V Electro's , each with ripple current rating of 90mA at 100Hz.

Capacitor 400AX10MEF:

https://www.rubycon.co.jp/wp-content/uploads/catalog-aluminum/AX.pdf

The new ripple current in each of them will be Approx 462mA total.....
(343mA at 100Hz; 305mA at switching frequency)

The device is only used for 20 minutes each day. But it will be at full power for all that time.
The caps will have reasonable blown air over them whenever the product is ON.
Air temperature around the caps will be around 40degC max, and as discussed, its blown air.
The caps are "2000hrs at 105degC"

I presume we will be ok to violate the ripple current rating?

What are the rules for "allowable violation" of electrolyic capacitor ripple current?

 

[cupoftea]

Presumbaly you just revert to the "2000hrs at 105degc" and Arrhenius equation?

 

[cupoftea]

Must admit its hard to get "junction" to case thermal resistance figures for all electro caps.
And we cant afford the thermocouple equipped test caps that you can get.

 

[crutschow]

Can you attach temperature sensors to the caps and monitor their temperature while they are powering the load with a 40°C ambient?

 

[D.A.(Tony)Stewart]

Arhennius Laws may prevail but forced air alone will not determine the internal temperature rise. Yet this will not guarantee their product will survive the warranty period. E-caps are good thermal and electrical insulators and knowing the thermal resistance is key as well as the rate of change in ESR leading to thermal runaway failure. I would not presume to exceed max ratings.

Double Snapmount Life / Temperature Calculator | CDE

Visit this page for CDE's Snapmount Life / Temperature Calculator double snapplet.

www.cde.com

--- Updated Apr 13, 2023 ---

each with ripple current rating of 90mA at 100Hz.

Caps are always rated at 120 Hz and/or 100 kHz. industry. std.

 

[cupoftea]

Thanks, i must admit, you can get good temp calcs for snap-in or screw in el caps.
Not for radial.
Power engineers are so unusual.......
Show them a fet datasheet with no Tj-c figure and they go mad....show them a radial el cap datasheet with utterly no thermal resistance figure from core to case, and they couldnt give a dam!

I have a 10Meg file of ell cap app notes...took hours to read today.......but still doesnt give any idea about thermal resistance figure from core to case...i would post the docs here....but its disallowed to post such big files, understandably.

 

[crutschow]

still doesnt give any idea about thermal resistance figure from core to case..

Since the dissipation is relatively low and spread throughout the capacitor body volume I would expect there is not a significant difference between the core temperature and the case temperature, which is why that data is not published.

 

[mtwieg]

If you dig around enough you can find some rough guidelines on estimating thermal performance for electrolytic caps, both case-to-ambient and core-to-case.
The most comprehensive app note I've found is this one, with the tables starting on page 17. Maybe you've already seen it:

https://www.cde.com/resources/technical-papers/AEappGuide.pdf

Thanks, i must admit, you can get good temp calcs for snap-in or screw in el caps.
Not for radial.

I'm guessing by "radial" you mean radial caps with simple straight leads (the larger snap-in and screw terminal caps are still radial). I also have noticed that most descriptions of core-to-case thermal resistance only covers larger case sizes. This is likely because as case size decreases, it becomes irrelevant (at least for natural convection).

 

[Calm down]

450V10UF or electrolytic capacitor with 105 ℃ for 3000 hours can be selected, or the transformer can be modified to reduce IPK current. It would be better to achieve the above three points simultaneously.

 

[cupoftea]

The most comprehensive app note I've found is this one, with the tables starting on page 17.

Thanks, yes, unfortunately, those tables are only for screw-in or snap-in versions. As you say, we are using radial leaded type.