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Are electrolytic caps necessary?

Discussion in 'Circuit Simulation & PCB Design' started by Pommie, Jun 24, 2017.

  1. Pommie

    Pommie Well-Known Member Most Helpful Member

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    I'm currently designing a project using smt parts. Looking for 1206 size capacitors produces 22uF 10V ceramic capacitors. Can these be used instead of the normally called for electrolytics? Will they also be suitable as bypass capacitors or is there something inherently better about the 100nF ceramics normally used? Having not come across these small high value ceramics before I'm hoping someone here has experience of them.

    Thanks,

    Mike.
     
  2. JLNY

    JLNY Active Member

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    I suppose that it depends on the application, but certainly there is no hard rule stating that one material has to be used over another as long as the values, voltage ratings, etc. are suitable for the application and won't be a significantly higher cost.

    In some ways ceramic capacitors may even have advantages over electrolytics because they are non-polar, and ceramic caps often have low ESR at high frequency. I think that 100nF ceramic caps are often used in conjunction with higher-value electrolytics for their low ESR at RF frequencies where a high-value electrolytic would be too inductive at RF to provide good suppression.

    If your only reason for using ceramic caps is to use SMD components, though, there are certainly SMD electrolytic caps:

    http://au.element14.com/c/passive-c...apacitors?capacitance=22uf&voltage-rating=10v
     
    Last edited: Jun 24, 2017
  3. Colin

    Colin Member

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    Bypass generally only needs to be 100n You can get tant 10u and 22u and higher.
     
  4. dave

    Dave New Member

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  5. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    Here are two references I use:
    TI
    http://www.ti.com/lit/an/sloa069/sloa069.pdf
    See particularly pages: 3 and 8
    Good reading like many TI application notes, IMHO.

    Murata
    http://www.murata.com/~/media/webre...atalog/products/emc/emifil/c39e.ashx?la=en-us
    Heavy reading, but some ideas, e.g., use a variety of different capacitances.

    In brief, electrolytic capacitors turn into inductors at a lower frequency than do MLCC capacitors. If you are working at low frequency with a switching power supply, electrolytic capacitors may give higher capacitance in a smaller package. There are few disadvantages (other than size) to ceramic capacitors. Less important, there may be an advantage to using decoupling capacitors of different values around the board instead of slavishly using just 100 nF versions.

    John
     
  6. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Typically a 100nF ceramic cap is used across power and ground for each IC on a PCB to decouple the high frequency noise, with one or more large(10-100μF) electrolytic caps on each board to suppress the lower frequency ripple and noise.
     
  7. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Some times I need to build a board that is only 0.25 inches thick. Ele. Caps are too tall.

    Many times I build switching power supplies running at 1mhz. I need capacitors very close to the IC+MOSFETs. At 1 and 2mhz the lead/trace length to get to a ele. cap is too much. Most of the ceramic capacitors are good at high current. (can't use "tant")
     
  8. Mosaic

    Mosaic Well-Known Member

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  9. Pommie

    Pommie Well-Known Member Most Helpful Member

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    Thanks all for the info. The circuit in question is a micro USB to a Lipo charger followed by a stepup regulator to 5V. Here's the schematic, charger.png
    The battery connects to the central connector. Looking through the datasheets suggests that ceramic caps are to be preferred. Can anyone see any problems with this circuit?

    ronsimpson Why can't tantalums be used?

    Thanks,

    Mike.
    Datasheets attached for anyone interested.
     

    Attached Files:

    Last edited: Jun 25, 2017
  10. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Tantalums can not handle high current. (like on the input/output of a switching power supply)
    I use ceramic near the PWM and then might use tantalums for "bulk" energy storage.
    >0.1uF ceramic + 10uF ceramic at the PWM then 220uF tantalums or Ele then at the load(s) 0.1uF on each IC.
     
  11. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    I think there is some disagreement about use of tantalum for filtering power supplies. It used to be that ripple current was a major disadvantage. Today, it appears that view is changing (particularly with low ESR caps) to just staying within design limits.

    See: http://www.vishay.com/docs/40031/apprippl.pdf
    John
     
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  12. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    As I've mentioned before, historically tantalum capacitors were EXTREMELY unreliable - far more so than electrolitics.

    And every time I've mentioned it previously people pop up and say that was because their voltage rating were highly imaginary - and that the claimed voltage specification of the capacitor needed to be much higher than the rail they were used on.

    I can't comment on modern ones, as TV manufacturers stopped using them due to the huge problems.

    Still it made repairs fairly easy - just look for a tantalum capacitor and check if it's S/C. Only problem was large numbers of them on the same voltage rail, spread all over a large PCB.
     
  13. Mosaic

    Mosaic Well-Known Member

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  14. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    It may be worthy to note that the application note I cited is titled:
    "Application Notes AC Ripple Current
    Calculations Solid Tantalum Capacitors"

    The thrust of that note, which is dated April, 2006, is about calculating power dissipation by tantalum capacitors and the need to stay within manufacturer's specifications. It does provide a table for maximum RMS voltage compared to maximum DC voltage:
    upload_2017-6-26_12-47-21.png
    But that is not about imaginary ratings, but rather power dissipation. In fact, it does not use the term "derating," as according to the note, that is not what the limitation is; although, some people like to use that term for simplicity of communication.

    John
     
  15. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    Bypass capacitors nowadays are almost exclusively a ceramic type due to their low ESR and ESL. Electrolytics tend to have higher parasitic inductance and capacitance and are not suitable for bypass, but instead are often used across the bus (not across the IC supply) to help filter out any additional noise from the power supply. It entirely depends on your application and the requirements set forth by the upstream and downstream components' datasheets.

    Many device datasheets specify what type of capacitor is recommended for a certain application. For example, I recently designed a basic power supply with a number of regulators, some passives, some control logic, and a micro. The datasheets for the regulators recommended tantalum capacitors specifically for the output. This was because the regulators relied on a certain range of ESR for stability (don't ask my why the manufacturer designs around a relatively uncontrolled number -- I have no idea), and electrolytic capacitors tend to have a much higher ESR than tantalums (too high to keep the output properly stable). You also often can't find high capacitance values in ceramic packages, and this is where tantalums can be useful.
     
  16. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Please state our goal here.
    How much current will you pull from the USB source? (500mA)
    How much current do you want to charge the LIPO with? (500mA)
    How much current for the load? What are you powering? (100mA)
    What size of LIPO?
    What voltage will you put on the LIPO? (3.7v)
    How long will the lipo support the load?
    Will you be "charging" and powering the load at the same time?
     
  17. Pommie

    Pommie Well-Known Member Most Helpful Member

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    The LIPO is a 1000mAh single cell and will be charged at 1/2C (500mA). See here.
    The chip above stops charging at 4.2
    The load is a pic driving one or more vibrators - normally 30mA - occasionally (once per hour max) around 300mA for 0.5 Sec.
    It may be accidentally left on whilst charging but I don't think this is a problem.
    The off state will be the chip in sleep mode drawing a few 10s of nA.
    I'm hoping the LIPO will last at least 1 day with constant use - 12 hours would suffice.
    If a problem with battery life I can switch to the 2Ah one stocked by Sparkfun.

    Mike.
     
  18. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    I don't think you can get 300mA out of the 61220. Probably more like 150 to 200mA. See page 6.
    Note both the battery charging current and the 61220 input current come from the input regulator.

    The first IC is a linear regulator. 500mA in 500mA out.


    The last IC is PWM. Power in = power out - some loss. So input current and output current have a non 1:1 relationship.
    The last IC is a boost power supply. If the battery was at 2.5V and the output is at 5V and 100mA; then it will bake 25omA of input current.
    Output 5V 0.1A 0.5watts
    Input 2.5V 0.2A 0.5watts + 10 to 15% more current for efficiency.
     
  19. Pommie

    Pommie Well-Known Member Most Helpful Member

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    Sorry Ron, for some reason I had it in my head I had 500mA available. I can sort that so it's never more than 100mA and then only for less than 1 second.
    When (accidentally) powered whilst charging it will never take more than 30mA. Do you (or anyone else) have experience with LIPOs and capacity. In theory I have 3.7Wh and I'm using ~150mW - is it reasonable to expect at least 12 hours (1.8Wh) from this setup?

    Thanks all,

    Edit, is it showing that I'm a software guy? :)

    Mike.
     
  20. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    The datasheet for most boost PWM might say "500mA switch" or transistor but that is the peak and only for a small percent of time. It is easy to mis-read the data sheet.

    You understand now. I think you have moved events in time so only one buzzer is on at one time. I think 100mA peak and 30mA all the time is good.
    3.7Wh is like 370mWh for 10 hours. or 185mWh for 20 hours.

    Look for the minimum discharge voltage. (3.0V??) It is the voltage where you should stop pulling power from the battery. Your current is low so there is no problem there. Watch for max charge voltage and min discharge voltage.
     

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