2

As above.

I am deciding between 3 capacitors. I need 6 capacitors arranged in 2 banks with 3 capacitors in parallel each for a V+/V- unregulated power supply for an audio amplifier.

  • Capacitor 1: 10000 μF, 85°C, 5.5 A ripple current, 15k hours: $9.50 USD
  • Capacitor 2: 15000 μF, 105°C, 3.8 A, 5k hours: $7.50
  • Capacitor 3: 22000 μF, 85°C, 7.6 A, 3k hours: $11.80

All capacitors are 50 V. Capacitors 1 and 3 are around 28 mΩ ESR. No ESR info/graph for capacitor 2 (I checked the datasheet).

I am tempted to pick capacitor 1 for its longer life.

Thoughts?

ocrdu
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mrjayviper
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    It really depends upon how much use you want to get out of your circuit before it needs repair or replacing. – Math Keeps Me Busy Sep 21 '22 at 13:15
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    What is your maximum operating temperature? the 105 C / 5k hrs part, if you use it at 85 C and lower, can be expected to last longer than 5k hrs. The 85 C / 15k hrs part, if you use it at 105 C, can't be expected to last longer than 100 ms. It's probably worth asking both vendors for a lifetime estimate at the maximum operating temperature for your application. – The Photon Sep 21 '22 at 13:19
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    Note that the life of cap 2 will be measured at 105 °C, while the other two will be measured at 85 °C. At the same 85 °C, cap 2's lifespan will be longer than the 5k hours rated. I forget the rule of thumb for how much longer. – Hearth Sep 21 '22 at 13:20
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    In fact, cap 2 has better lifetime at 85°C than cap 1. It is only rated lower due to its higher temperature specification. The typical rule of thumb for capacitor lifetime ratings is that for each 10°C decrease in temperature, you double the lifetime of the capacitor. – Klas-Kenny Sep 21 '22 at 13:20
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    You need to look at the graphs at the temperature and ripple current you intend to use them, and buy one that is cheapest that fills your longevity specs at the temperature and current you need. – Justme Sep 21 '22 at 13:20
  • @Justme no graph for cap 2. Need to check 1 and 3 – mrjayviper Sep 21 '22 at 13:25
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    https://electronics.stackexchange.com/questions/490969/replacing-these-capacitors – Andy aka Sep 21 '22 at 14:07
  • Does cap number 2 list the "DF" of the capacitor at 100 or 120 Hz? You can calculate ESR from that: ESR = DF / (2 * Pi * F * C). DF is from the datasheet, F is the frequency at which DF is specified (probably 120 Hz) and C is the nominal capacitance. Everything about number 2 is probably specified at 105 C, including the ripple current rating. At 85 C it will probably have more ripple allowed as well as longer life (as others have said). Ripple is probably a thermal limit. – user57037 Sep 22 '22 at 01:07

2 Answers2

3

For a reliable product it is very important, but it's a complicated topic.

You need to read up and understand the application of Arrhenius Law (aka Arrhenius Equation) to this.

Note that this is an empirical formula and each manufacturer interprets it differently, with varying coefficients for each parameter in their aging estimations.

Capacitor lifespan is related to the fact that Aluminium 'wet' capacitors are all of mechanical, electrical and chemical in nature.

The failure modes are mostly caused by chemical material degradation over time, through effects such as migration, oxidation and leakage.

This paper by Dr. Arne Albertsen of JIANGHAI EUROPE GmbH is one I have referred to many times.

An 'hours' rating of 1000hr @ 70C does not mean that at 1001 hours it will blow up. It just means that at 70C together with a specified voltage and ripple current for 100% of that duration it will have a much higher mean time before failure, MTBF.

If you push all the parameters to their limits a capacitor can fail much sooner.

The manufactures get these numbers by testing a large sample of product for a long time with elevated parameters and extrapolating to where the failure rate is acceptable.

The fundamental issue is the careful selection of materials for their physical properties and chemical interactions combined with the care during the design and production life cycles of the product.

A cheap, short lived capacitor will have less time and money spent on it's design and manufacture.

An expensive, long lived capacitor will have gone through much more design, test, material selection and quality control during manufacture.

Jay M
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According to the Arrhenius law, capacitor 2 will be the longest-lived with about 20 khrs at 85°C or 1280 khrs at 25°C.

tobalt
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  • What I've found in the past that it's the operational parameter closest to the device limit that most affects the lifespan. Operate it at <50% of the limit of all possible parameters and the declared MTBF takes over. Interestingly I have a 40 year old Panasonic microwave oven with a keypad and digital display, so it will include wet electolytics, and it has never failed - even the light bulb is 20 years old. I would argue they got something very right with their cap design (or if you believe in planned obsolescence, very wrong). – Jay M Sep 22 '22 at 12:55