Power Supply Capacitor Q and A

By Jonny Gerow, on December 30, 2013

Q: What purpose do capacitors serve in a power supply?

A: There are two different locations for capacitors in a power supply:  The “primary” side and the “secondary” side.  The primary side is where the AC comes into the power supply.  The secondary side is after the DC output voltages are regulated.  The large capacitors on the primary side take the relatively unregulated voltage that’s been converted from the AC input to DC and attempts to maintain a constant DC voltage for the rest of the power supply.  The capacitors on the DC side are part of the filtering process that helps eliminate any residual AC ripple from the DC output.

The above diagram shows the division of the primary and secondary side of an Corsair RM850 PSU.

Q:  How does Corsair calculate the life expectancy of a capacitor when deciding what to use in a particular power supply?

A:  Capacitors have a few specifications and ratings.  Of course, voltage and capacitance are the two best known specifications.  But for calculating the life of a capacitor, there’s the temperature rating which is usually 85° or 105°C.  There’s also the maximum ripple current allowed.  If all of the aforementioned are run to their limits, 24/7, that’s when you run into the capacitor’s lifetime rating.  This rating is typically in the range of 2000 to 6000 hours; this is equivalent to only 83 to 250 days.  Fortunately, the relationship between life and temperature follows a chemical reaction formula called “Arrhenius' Law of Chemical Activity”. The law put simply says that life of a capacitor doubles for every 10 degree Celsius decrease in temperature.  So if a 105°C rated capacitor is operating at 85°C, for example, your 2000 hour life has increased to 8000 hours and that’s still assuming it is filtering the maximum amount of ripple current it is rated for.  The complete formula used to calculate the capacitor life are as follows:

 

Let’s look at an RM850 as an example.  There are a set of six Ltec LXY series capacitors used to filter the +12V.  Three are 3300uF, 16V while the other three are 2200uF, 16V.

The Ltec capacitors on the secondary side of a Corsair RM850 PSU.

The first are rated for 3.4A RMS of ripple current while the latter are rated for 2.375A. The first have a lifetime rating of 3000 hours while the latter have a lifetime rating of 4000 hours.  All are rated at 105°C. But because they only have to handle less than 1A of ripple current and are operating at temperatures around half of what they’re rated at (44° to 53°C versus 105°C), they have a calculated life expectancy of greater than 15 years.   

Q:  Why does it seem that fewer Japanese capacitors are being used in better power supplies than in the past?

A:  The answer here is simple.  Because the power supplies are better!  More efficient components within the PSU are the reason why the PSU is more efficient.  Better efficiency means less heat.  Also, modern day switching technologies allow for less ripple for the secondary caps to deal with.  These two things combined mean that capacitors can last much longer, so Japanese capacitors are not always required.

Q: Are Japanese capacitors actually better than Chinese capacitors?

A: Japanese capacitors have a reputation for excellent quality control.  So for extreme conditions, it is more desirable to use Japanese brand capacitors.  On paper, there are often Chinese capacitors with the same specifications as an equivalent Japanese capacitor, including low ESR (equivalent series resistance) models.  Japanese capacitors are also said to use a superior electrolyte that is more resilient to higher temperatures. Japanese capacitors are also known to use some of the purest aluminum available.  That said, many of the Chinese manufacturers are buying Japanese electrolyte formulas and the companies that refine the aluminum for the Japanese capacitor manufacturers are opening facilities in China so they can be closer to their Chinese capacitor customer.

Showa Denko completes high-purity aluminum foil plant in China:  http://www.sdk.co.jp/english/news/13382/13769.html

Q: Why the distrust for Chinese capacitors?

A: In addition to the attitude that Chinese manufacturing is inferior, much of the distrust towards Chinese capacitors started in 2002, when a electrolyte formula was stolen from a Japanese capacitor company and brought over to a capacitor company in Taiwan.  The formula was written down incorrectly and this caused many premature failures:

http://spectrum.ieee.org/computing/hardware/leaking-capacitors-muck-up-motherboards

Aside from this incident, there have been reports of premature failures in components using Chinese capacitors, but most of these failures were the result of poor design.  Capacitors were either subjected to high temperatures or were required to handle too much ripple current… or worse; both.

Q:  So Japanese capacitors never fail prematurely?

A: Absolutely not.  Between 2003 and 2005, Dell, HP and Apple, among other manufacturers, encountered a problem with faulty Japanese capacitors that affected millions of computers:

http://bits.blogs.nytimes.com/2010/07/01/dell-speaks-about-its-struggles-with-faulty-pc-components/

Q: Are Japanese brand capacitors always made in Japan?

A: Due to high labor costs in Japan, typically no.  They’re usually made throughout Asia.  Unfortunately, many people assume because a capacitor is a Japanese brand, it’s made in Japan.  One does not usually see the country of origin unless they buy the capacitors for themselves from a retail outlet.

United Chemi-Con capacitors manufactured in Indonesia. 

Panasonic capacitors manufactured in Malaysia.

Q:  When does Corsair decide to use Japanese capacitors?

In higher end units, Japanese capacitors are used for overall increased reliability even if there is no increase in calculated lifetime for doing so.  Corsair does tend to use Japanese primary capacitors whenever possible because of the extreme conditions the primary capacitor is subject to.  These capacitors are large and therefore have a larger surface area to dissipate heat, but they still get rather hot due to the higher temperatures of the primary side’s heat sink located nearby.  Also, the unregulated DC voltage charging the primary capacitor on the primary side can potentially have a great deal of ripple.  

The primary side capacitor in the Corsair RM850 is mostly surrounded by heatsinks.

Once again using the RM850 as an example, we can see that a Nichicon brand, GL Series 560uF, 420V capacitor is used on the primary side.  It’s rated at 105°C and can handle 1.5A of ripple current for 2000 hours of 24/7 operation.  But being as close to the primary heat sink, where components are dissipating temperatures as high as 76°C under full load, the surface temperature of this capacitor can get as hot as 44°C.  Furthermore, this capacitor could potentially encounter ripple current as high as 3.2A.  That’s more than double what it is rated for.  Even with these conditions taken into consideration, the life of this particular capacitor is still calculated as greater than 15 years.  But since the conditions can potentially be so harsh, Corsair made the decision to use a Japanese capacitor here to prevent any potential for premature failures.

Q:  What are “solid” capacitors and why are so few used in power supplies?

A:  All of the capacitors shown in the photos above are, more specifically, “aluminum electrolytic” capacitors.  These capacitors use a paper soaked with a liquid electrolyte.  Solid capacitors are sometimes used, but not exclusively and only on the secondary side.

The above photo shows some of the solid capacitors used in an AX1200i.

“Solid capacitors” still use aluminum foil inside, but use a solid polymer as an electrolyte instead of a liquid.  This makes the capacitor less susceptible to environmental changes such as heat and humidity.  Solid capacitors also have a lower ESR (equivalent series resistance) which makes them more efficient.    Sounds great, right?  The problem is, solid capacitors are very small and come in limited varieties.  For example:  I can get a 2700uF solid cap… but it will only be 2.5V!  I can get a 16V solid cap… but only as high as 1000uF.  We do find solid capacitors here and there inside a computer power supply, but they simply do not come in large enough capacity (high enough voltage or great enough capacitance) to use in any great volume within a computer power supply.  

Other capacitors used in computer power supplies are “metalized polypropylene” capacitors, or “film capacitors”.  These are generally used for EMI filtration on the AC input of a power supply.

Conclusion

Recent improvements in power supply technologies that help reduce ripple and improve overall efficiency have greatly increased the lifespan of computer power supplies.  While Corsair appreciates the higher quality standards of Japanese brand capacitors, and will continue to use them for enthusiast level products (HX and up) and as primary capacitors in the majority of Corsair’s entry level power supply series, we want to ensure our customers that we perform very rigorous testing and continuously work on improving power supply technologies and that component selection is a very important part of the process of engineering a Corsair computer power supply.


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