Tech Tutorial: Choose the correct capacitors for reliable automotive applications

[PeteS in CA]

Tech Tutorial: Choose the correct capacitors for reliable automotive applications
While not as glamorous as microprocessors or DSPs, passives, such as capacitors, must still have reliability for auto use
By Dave Richardson, Vishay Intertechnology
Automotive DesignLine
(07/07/2006 4:09 PM EDT)

Selecting the most reliable capacitors for today's automotive electronics requires the design engineer to examine a number of different device parameters and performance characteristics. The next step is to consider the automotive environment in which the electronics will be used and the specific applications for which they are intended. Here is a look at the characteristics of the four major dielectric types of capacitors-tantalum, aluminum electrolytics, poly-films, and ceramic; an explanation of the concepts of temperature and voltage coefficients of capacitance; and how these and other factors affect the choice of capacitor for a given application.

[Super Nade]

I didn't know Vishay Dale made caps? I know they make other passive devices like resistors, chokes, inductors etc...

Thanks mate. That was quite informative.

[PeteS in CA]

Vishay is a gigantic conglomerate of bought-up companies or parts of companies. Most divisions do passives; those that make caps are BCcomponents, Cera-Mite, Roederstein, (part of) Sprague, Vitramon. Not sure whether the piece of the original Sprague does tantalums, films, or some Al Els.

[linuxguru]

* 14-year bump *

Vaguely related: I found a small stash of Elna RKB automotive electrolytics with nice specs: 1000 uF / 35V / 12.5 mm / 1880 mA ripple current / 135 c/ 3000 h.

http://www.elna.co.jp/en/capacitor/a.../pdf/rkb_e.pdf

It's the combination of high ripple current, temperature and endurance that makes it interesting. Measured ESR was not too shabby either at 30 mohm - these would be great for SMPS recaps, and would probably survive fan failures.

[momaka]

Very interesting find, linuxguru.

Indeed 135C -rated caps should probably soldier through a fan failure. However, if the temperatures inside the PSU start going above 100C, I suspect many of ferrite core components will probably start loosing their properties, so the PSU will probably fail again. But at least it's good to know you may not have to do a full recap after a fan failure.

[Per Hansson]

When the fans fail in cheap power supplies what happens is that the primary side transistors desolder themselves as they get too hot and the PSU explodes, ask me how I know
And yes, this may save both the caps and fuse, not sure much else survives though
I think the proper solution is to have the damn PSU shutdown when the heat sinks become 100°C but what do I know since I'm not a PSU designer?

[PeteS in CA]

Originally Posted by linuxguru

* 14-year bump *
...

As soon as I saw the thread title and didn't remember I'd posted it I knew it had to be quite a while ago.

[PeteS in CA]

Originally Posted by Per Hansson

When the fans fail in cheap power supplies what happens is that the primary side transistors desolder themselves as they get too hot and the PSU explodes, ask me how I know
...

Long, long ago when I worked at Boschert we built up a prototype of a 750W P/S. We didn't have the inrush thermistors we needed, so we installed what was used in Boschert's OL400 series, 400W. We ran the proto at load overnight and when we came in the next morning we found that the thermistors had run so hot that they desoldered themselves and, being positioned upside down, had fallen out of the PCB.

[momaka]

Hey, that sounds like a new method of circuit protection. Just install a thermistor or whatever else with leaded (or maybe even low-melt temperature solder) and if it desolders itself, the device ran too hot. Sounds like a good way to prevent parts from blowing IMO.

[Per Hansson]

Well if the PSU manufacturers are unable to come up with a logic based solution I have an idea for them:
Use a mechanical temperature based switch on the live wire, automatically switches PSU off when temperature gets up to its rating

[PeteS in CA]

Originally Posted by Per Hansson

Well if the PSU manufacturers are unable to come up with a logic based solution I have an idea for them:
Use a mechanical temperature based switch on the live wire, automatically switches PSU off when temperature gets up to its rating

Boschert used to have thermal switches on the L-brackets of many of their OL series power supplies. Back in ancient times ....

[momaka]

Originally Posted by Per Hansson

Well if the PSU manufacturers are unable to come up with a logic based solution I have an idea for them:
Use a mechanical temperature based switch on the live wire, automatically switches PSU off when temperature gets up to its rating

That's a pretty good idea.

Actually, a lot of appliances with heaters inside (like your clothes iron, for example) already have thermal switches like that. The idea is that in case the bi-metallic switch fails, the thermal switch will open and disconnect power, thus preventing a possible fire.