Why do some companies love putting capacitors close to heatsinks? - Page 1
Forum section: electronics / general

author: Razor512
author_rank: Regular Contributor
timestamp: August 30, 2013, 04:01:34 pm
content: Is it some sort of insane tradition, or do they just feel that customers will prefer their capacitors to be hot (since they like so many other things hot (eg fried eggs, chicken, soup, etc)?I see it from a range of products but most consistently with AcerIs there any benefit to putting cheap capacitors next to heatsinks that get too hot to touch?eg the acer x191w monitor

author: ju1ce
author_rank: Regular Contributor
timestamp: August 30, 2013, 04:25:32 pm
content: The caps will fail faster when they run hot. To maximize the planned obsolescence they also used CapXon capacitors, which are the worst!

author: DavidDLC
author_rank: Frequent Contributor
timestamp: August 30, 2013, 04:47:12 pm
content: Quote from: Razor512 on August 30, 2013, 04:01:34 pmI think you already know the answer, then why to ask ?

author: Razor512
author_rank: Regular Contributor
timestamp: August 30, 2013, 04:58:26 pm
content: It just seems weird for companies to do it especially when cheap capacitors are used. (a while back when the US switched to digital over the air TV, we got one of the free converter boxed made by RCA and it failed within a year from a swollen and durst capacitor made by capxon and it was not even close to a heat producing component, it just decided to burst for no reason)For the heatsink, it may well be the case of design limitations, but how much extra work is it really to just work in a few extra mm of space in between the heatsink capacitors? it seems that it would extend the life with very little additional cost.

author: ignator
author_rank: Regular Contributor
timestamp: August 30, 2013, 05:27:09 pm
content: The mindset was to compact it as small as possible, and use the heat sink for some additional function, like board stiffening (or reuse an old existing one to save on tooling), and the ME and mechanical designer on the project didn't have an EE that knew the art, at least in this area. Note that the big (400volt) electrolytic is laid down flat, so it is a manual operation to mount, was well the heat sink hardware, and many other parts on the photoed assembly. Remember the goal is to all ways make it as small as possible, using the fewest parts, fewest circuit card layers (1layer is what I see in most consumer junk), and it's a SMPS, which is supposed to run super cool, it comes down to how many reviews are done with the right people who have the experience of seeing field failures, and learning what not to do, as well other mentors that have their gray beard experience, and hopefully one or two clever people on the team with real good pattern recognition skills that see bad design as they are that gifted.My critique of this design is for high volume, it has too much manual labor. It looks like they built it with 100% all human pick and place assembly. I guess that saves on all that expensive equipment.AND the vendor's only needs it to last 4-5 years, who the heck keeps any hardware longer then that (I do, but I'm frugal)

author: mariush
author_rank: Super Contributor
timestamp: August 30, 2013, 06:23:57 pm
content: Sometimes it's unavoidable... capacitors have to be as close as possible to the IC and the leads have to be as short as possible so they won't act as inductors and cause instability.However, manufacturers don't really have an excuse for continuing to use cheap capacitors - i would think they got reports from sellers and warranties that these brands of capacitors fail)... I think it's just planned obsolescence.

author: eman12
author_rank: Contributor
timestamp: August 30, 2013, 08:31:30 pm
content: Quote from: mariush on August 30, 2013, 06:23:57 pmYes and another reason can be to "prevent" so called "ground loops".

author: eman12
author_rank: Contributor
timestamp: August 30, 2013, 08:39:40 pm
content: In practice, when you are designing a circuit you SHOULD put a capacitor (say a 100pF or even an electrolyte one if needed) "as near as" possible to the "power rails" of EACH chip in the circuit. Some times a given circuit has more than 10-20 of those kinds of caps. In one of my designs I used more than 24 of them as near as possible to each IC so that smooth the current going to every chip.....

author: ddavidebor
author_rank: Super Contributor
timestamp: August 30, 2013, 09:45:15 pm
content: Quote from: Razor512 on August 30, 2013, 04:01:34 pmBecause with a few simple formula you can calc. The life of the cap, and with it of your product

author: EEVblog
author_rank: Unknown
timestamp: August 31, 2013, 12:06:16 am
content: It's a trade off.a) Good electrical design practice dictates that you must put the caps as close as possible the device (more so for switching regs and LDO's) (to minimise trace inductance)Butb) Good thermal design practice dictates you have to have to have them as far away from heat sources as possible.Generally, the datasheets and app notes will usually always stress a) as being important, whilst almost always ignoring b)So you end with a generation of designers who only really think about a)

author: c4757p
author_rank: Super Contributor
timestamp: August 31, 2013, 12:26:44 am
content: In my experience they stress a) as beingmuchmore important than it truly is, at least in the case of linear regs. And the traditional ones really don't give a shit at all - like with those poor, poor capacitors riding the LM317's heat sink in the Rigol PSU. Those could have been an inch or more farther away with no problem at all, as long as the traces between them are fat enough... And there was plenty of space to do that, too...

author: Fsck
author_rank: Super Contributor
timestamp: August 31, 2013, 12:46:36 am
content: it keeps them warm on the cold winter nights.all they need to do is make sure the overwhelming majority survive past the warranty period..I've got a Dell 17" LCD that's 9 years old, works fine and I still use it..

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 31, 2013, 04:14:46 am
content: The stray inductance issue was already mentioned, but there's also a point in reducing the size of the board in some applications.I have seen a lot of bad capacitors in PC power supplies, with basically all of them having a "smart" fan. Rewire the fan direct to 12V and the machine would become obsolete long before anything inside fails.

author: SeanB
author_rank: Super Contributor
timestamp: August 31, 2013, 04:49:00 am
content: Smart fan is a good mini oven, just low noise as it warms up to cooking point.Often the simple way is to have some ceramic capacitors right next to the heatsink and the hot power device and then a small gap to the bulk capacitor, so that the heatsink is both cooler and the capacitor is cooler and longer lasting. As you can get ceramic capacitors in quite high values and high voltage ( even old disc ones are available in 0.22uF 100V with leads and are not too big) and they will tolerate a high temperature there is no reason to not do the separation and still have adequate decoupling. Just takes a little attention to design, some broad traces and a good local ground plane.

author: Unknown
author_rank: Unknown
timestamp: August 31, 2013, 04:06:33 pm
content: The large value ceramic caps probably use a dielectric like Z5U. The capacitance of Z5U is strongly dependent on DC bias, which might be present in decoupling applications. See the attached figure (source) for some example data.

author: SeanB
author_rank: Super Contributor
timestamp: August 31, 2013, 04:48:09 pm
content: You do not need much capacitance close to the device if you have a bulk capacitor close by and a low inductance track to it. You just need a little, and it does not matter if it's capacitance is lower than the zero bias value as long as it does have some.

author: ddavidebor
author_rank: Super Contributor
timestamp: August 31, 2013, 09:03:22 pm
content: Quote from: EEVblog on August 31, 2013, 12:06:16 amI'm thinking... Why no one use a little piece lf isolation foam? Is super cheap!

author: c4757p
author_rank: Super Contributor
timestamp: August 31, 2013, 09:06:38 pm
content: A lot of heat will be coupled through the PCB, traces and leads.

author: Neilm
author_rank: Super Contributor
timestamp: August 31, 2013, 10:05:52 pm
content: Quote from: c4757p on August 31, 2013, 12:26:44 amStuff stray inductance. One time you will need to have electrolytics near a heatsink hasn't been mentioned - if you are dealing with very high pulse currents. I had to design a supply that under worst case load conditions would have 60A pulses. I ended up with a solid copper pour on 2 layers from the caps to the switcher which was mounted on a very big heatsink. As it was I can see the effects of the resistance of the copper. I just had to select decent caps (105 degree Panasonics) with the lowest ESR I could find. I'll cross fingers and hope that we don't get any back with them bust.Neil

author: peter.mitchell
author_rank: Super Contributor
timestamp: September 07, 2013, 07:02:50 am
content: I saw this while browsing ali express;http://www.aliexpress.com/item/None-noise-walkway-high-power-toroidal-tube-amplifier-220v-amplifier-board-fan-speaker-computer-amplifier/833654292.htmltakes the cake.

author: mcinque
author_rank: Unknown
timestamp: September 07, 2013, 08:21:33 am
content: Quote from: peter.mitchell on September 07, 2013, 07:02:50 amOutstanding.