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PSUs 101: A Detailed Look Into Power Supplies

Current Ripple And Cap Life Calculation

Current Ripple Measurement On A Cap

Current ripple is one of the most important factors for measuring a cap's lifetime. Below, we will show you an example of how you can measure the current ripple on a bulk cap used in the APFC converter. We'll assume that the voltage ripple of a bulk cap is 5Vpp and that the ESR of this cap is 0.5Ω. To keep things simple, we will also make the assumption that ESR is kept constant (because ESR is frequency-dependent). We have to first convert the Vpp value to Vrms (Vrms = Vpp × 1/  √2= Vpp × 0.7071), so in our case we have:

        (3)             Vrms = 5V × 0.7071 = 3.5355 Vrms

All we have to do now is use Ohm's law (I = V/R) and find the rms ripple current through the cap since we already know its ESR.

        (4)             3.5355 Vrms / 0.5Ω = 7.071A

The above ripple current value is pretty high and will apply huge stress to the bulk cap. By using the equation P = I2 × R, we find that 25W (7.0712 × 0.5Ω) of power has to be dissipated, so the bulk cap will get very hot after a while. In this case, we need a very powerful cooling fan (which means increased noise levels) to help the bulk cap keep its temperature at acceptable levels. The most effective solution to restrict ripple current is to reduce ripple voltage and select a bulk capacitor with the proper ESR value.

Electrolytic Cap Life Calculation

Having a way to easily calculate a cap's lifetime is very important. Here is the basic equation that we will use.

        (5)                 L2 = L1 x (Vr/Vo) x 2x   

In the above equation where:    x = (Tm-(Ta+ΔT))/10

    Ta  represents the ambient temperature.
    Tis the max rated temperature of the capacitor.
    ΔT  is the temperature rise of the capacitor due to ripple current.
    Vr  is the maximum voltage rating of the capacitor.
    Vis the operating voltage of the capacitor.
    L1  indicates the load life rating of the capacitor (provided by its manufacturer).
    L2  is the calculated lifetime of the capacitor under the current operating conditions.

What is APFC?

APFC stands for active power correction factor and an APFC converter shapes the current waveform to make it match the voltage waveform. It also makes the PSU compatible with a wide range of voltage inputs.

Now, let's say that we have an electrolytic bulk cap with a 2000-hour load life rating, 420V max voltage rating and 105 °C max temperature rating. If we use this capacitor inside an APFC converter where the DC bus is around 380VDC, assuming that its internal temperature will be kept at 50 °C while the temperature rise of the cap due to ripple current will be 10 °C, its estimated lifetime will be calculated by the following equation:

        (6)            L2 = 2000 x (420/380) x 24.5 ≈ 50,019 hours     

If we assume that the ambient temperature is 40 °C, then we have the following:

        (7)            L2 = 2000 x (420/380) x 25.5 ≈ 100,037 hours      

The above example clearly shows that 10 °C difference in the operating temperature of an electrolytic capacitor can double or cut in half its estimated lifetime. In addition, caps with 105 °C rating have four times the life compared with similar specification caps that have an 85 °C temperature rating.

  • Alexis Shaw
    In your list of top-tier capacitor manufacturers you missed out on some of the better american and european manufacturers, while these may not be used on many consumer-grade power supplies they are definitely top-tier and if you were to find them you would be happy. I suggest the addition of at least:
    Cornell Dubilier (USA)
    Illinois Capacitor (Now owned my Cornell Dubilier)
    Kemet Corporation (USA)
    ELNA (Japan)
    EPCOS (TDK company) (Germany)
    Vishay (USA)
    Würth Elektronik (Germany)
    Reply
  • Aris_Mp
    Thank you very much for the list you provided. I am aware of almost all cap brands that you mentioned but unfortunately so far I found none of them inside a desktop/consumer grade PSU. I will think about it however (and also make a research on these cap brands), if I should include them as well inside my list.

    Reply
  • InvalidError
    16585466 said:
    Thank you very much for the list you provided. I am aware of almost all cap brands that you mentioned but unfortunately so far I found none of them inside a desktop/consumer grade PSU.
    There is a very high probability you have seen PSUs with several Kemet capacitors in them. You never noticed them simply because SMD capacitors are too small to carry logos, brand name or even value designations.

    The other brands are mostly found in specialty applications such as lab instruments, industrial machines and high-end audio.
    Reply
  • Math Geek
    very interesting read. more in depth than i need to know yet for the most part understandable and with careful reading it did not leave me confused.

    nice article.
    Reply
  • TallestJon96
    I only read 2/3 of it, but it's a good article.

    I basically have committed PC heresy with my cx600m. However I think that I'm in the clear with my 65w CPU and 145w CPU. I'd bet my total power draw is actually below 300w, the supposed highest efficiency point of a PSU.

    As a gamer, not a professional, I think it is better to get low power parts, and get a higher rating than you need, rather than get high power parts and high quality PSUs.

    Additionally, if you compare power consumption of a typical system from today to one from 5 years ago, power draw is considerably lower, with the exception of certain graphics cards. *cough* 390x *cough*
    Reply
  • powernod
    I decided to sign up at Tom's forum, and the only reason was to state how excellent is Aris's article!!!
    Thanks Aris for this very useful article on behalf of us all who want to learn the basic knowledge for PSUs.
    Haven't finished it yet, but i'm very anxious for it !!!
    Reply
  • GoZFast
    Very nice article!!! You made me remember my college physics courses lol
    Reply
  • traumadisaster
    I'm glad there are people dedicated to this but I'm not. I can't even read all of the chapter titles in this article. I disagree with the importance you place on this and all of the references you made to this being crucial knowledge.

    PSU and MB are insignificant to me and I can blindly pick one by reviewing user comments from newegg in about 5 min, and it will last for years. For less than $100 each I'm set for nearly a decade.

    CPU and gfx card now that affects fps and is over $1000, actually the most important part to me.
    Reply
  • Alexis Shaw
    16589602 said:
    I'm glad there are people dedicated to this but I'm not. I can't even read all of the chapter titles in this article. I disagree with the importance you place on this and all of the references you made to this being crucial knowledge.

    PSU and MB are insignificant to me and I can blindly pick one by reviewing user comments from newegg in about 5 min, and it will last for years. For less than $100 each I'm set for nearly a decade.

    CPU and gfx card now that affects fps and is over $1000, actually the most important part to me.

    I heartily dissagree, user are not the best way to judge reliability, and a bad powersupply is at fult most of the time there is a hardware issue. Further a power supply should last more than one system build, and in general I keep mine for a decade at a time at least. So an investment in a good power supply is not a waste, and a bad one will kill that precious $1000 GPU or CPU. The demo dart power supply on the motherboard is a similar story, however in general they are of higher quality than a cheap mains supply.

    Reply
  • Alexis Shaw
    16585679 said:
    16585466 said:
    Thank you very much for the list you provided. I am aware of almost all cap brands that you mentioned but unfortunately so far I found none of them inside a desktop/consumer grade PSU.
    There is a very high probability you have seen PSUs with several Kemet capacitors in them. You never noticed them simply because SMD capacitors are too small to carry logos, brand name or even value designations.

    The other brands are mostly found in specialty applications such as lab instruments, industrial machines and high-end audio.

    As well as SMT ceramic capacitors, Kemet makes through hole aluminium electrolytic capacitors. These are of high quality, though not as well known as their SMT capacitors. They also make high quality polymer SMT capacitors that are used as bulk capacitors on the power distribution circuitry on laptops and other devices.
    Reply