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When should OSCON caps be used?

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    When should OSCON caps be used?

    When is it profitable to use OSCON over conventional aluminium capacitors?



    An intensive search to answer this question resulted in two examples that encapsulate the issues. Finally conclusions can be drawn.



    Example 1.

    A reference design defined: www.onsemi.com/pub/Collateral/NCP5332A-D.PDF

    The design recommends either eight OSCON capacitors or 10 Rubycon.

    Evaluation of this design and calculation reveals the pros and cons of the OSCONS applied in this case.



    The Pros.

    OSCONS are smaller (often the same diameter), a lower component count.



    The Cons.
    • The OSCONS cost about 300% more per component.
    • Using the formula supplied there is 17% less transient performance.
    • The bulk capacitance is down 56% conversely the Rubicon caps are better by 229%.
    • The loss of bulk capacitance will further degrade transient performance (this parameter is not covered by the formula, it only considers ESR). To clarify this point, consider ESR without any bulk capacitance. Transient voltage drop and ripple both increase as capacitance decreases. (Note that future VRM circuits operating at Mhz frequencies will not need bulk capacitance).


    In summary the choice of eight OSCON caps degrades performance in this example. Apparently not enough to require the adding of one more cap to improve matters.



    Example 2.

    An Epox 8RDA+ motherboard has six Teapo 3300uF 6.3V making a hefty 20,000uF bulk capacitance at the VRM output. The temperatures measured on the mother board at the VRM input caps is 45C, at the output caps 40C, the ambient temperature being 25C.



    The simplest and lowest cost replacement of the Teapo caps is to use like value ultra low ESR caps from Rubicon, Nichicon and Samxon. This will ensure quality and improved performance related to a decrease in ESR and no loss of bulk capacitance.



    Endurance .

    Endurance is often cited as the reason to install OSCON. I have seen an unconfirmed quote that OSCON caps have an endurance of 200,000hrs at 65C (does this apply to all series?). However is this a realistic benefit?



    Taking the temperatures in example 2 above it can be estimated that a quality aluminium cap rated for 2000 hrs at 105C will have an endurance of 128,000hrs at 45C (32 years for 10hrs every day). This is more than sufficient; OSCON is not needed in this case and many other motherboards.



    Where is OSCON profitable?
    • Where small size is required.
    • Where higher frequencies are applied
    • At temperature extremes. Perhaps new hotter motherboards or cold conditions.


    Conclusions

    The evidence is quite clear that OSCON caps are not necessary for replacement of bad aluminium caps. Quality aluminium caps have equivalent low ESR, maintain bulk capacitance, cost less, endurance is good and there is minimum change to an existing design. The VRM circuit can produce voltage overshoot (spikes) if radical changes are made, so it is best to stick close to the original design. OSCON caps are best applied where their unique properties can be applied. In the future designers may use more if small size, higher temperature and higher frequency characteristics are required.
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    #2
    Re: When should OSCON caps be used?

    If space is a problem or the ripple current will be much higher than the original vrm unit is desinged for (e.g overcloking), than oscon caps can realy be a mutch better solution than elekttrolytics. This aplplies espescially for input capacitors (those placed bevore the inductor an mosfet). For example, the Asrock k7s8xe+ and all other k7s8x boards dos use a 2 phase vrm desined for about 36 to 42a (as to follow AMD spezifikations). Tehy used 2 x 1500uF KZE caps per phase. Now if you follow the st L6917 desing guide, if you want to increase outputcurrent, the input ripple will increase. If you consider a real ocing (of course, the board was`t desined for) the input caps would be opposed to an mutch higher ripple current than rated. There you come even close to the limmits of some 2200uF kcz one, and no more space is left to increase the numbers.. Ok, this is belongs on the amd side only for bad or cheap desined boards, usuall 3 phase or more vrm`s arent a problem, due to the inpurt ripple current will decrease with each phase substantial.
    But if you are loking to the amps figures of some Intel processors, even with many phases there are substantiall ripple currents, and oscon caps will be obviously the better solution. As the lytics needs to be acap to the cpu, heat an space would be the next problems, which an intel vrm unit desing hase to face with. OScons are therfore well suited.

    Comment


      #3
      Re: When should OSCON caps be used?

      How much smaller are the OSCON caps? Let's say there's a heatsink interference with the six Teapo 3300uF 6.3V on the Epox board due to their height. Would OSCONs cut their height in half? One-third? Hardly at all?

      Comment


        #4
        Re: When should OSCON caps be used?

        I have bought some 2700uF, 10mOhm ESR,2.4v, 10x13mm. think that is small enought compared to a normal caps. if i had more space, i had be used some 16mm oscons. They are even shorter. But if thats short enought i don`t know.
        gernerally the outputfilter caps could be replaced by 2,4v ones, the inputfilter not.

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