Cornell Dubilier Thermal / Life Calculators


Introduction

Links to the Screw Terminal applet:

 Double Applet (recommended for large screens such as desktop and laptop computers)

 Single Applet (recommended for smaller screens such as tablet computers)


Sections in the Introduction below:

  • New Calculators! New Features
  • Overview
  • How the Applet Works
  • About JavaScript 
  • Browser Requirements
  • Using the Applet Effectively
  • Special Notes for the Screw-Terminal Applet
  • Applet Limitations and Cautionary Notes
  • Legal Disclaimer

  

New Calculators! New Features!

 

Cornell Dubilier is in the process of developing new life modeling tools with increased functionality and ease of use compared to the Java life-modeling applets we first deployed 15 years ago.  

These new calculators offer new capabilities without requiring browser plug-in's such as Java or Flash, while preserving the look and feel of the legacy calculators.  We have avoided the Java security and compatibility issues altogether by re-writing the code purely in HTML and JavaScript.  Of course, you must keep JavaScript enabled in your browser, which is generally the default setting. 

 While our old calculators only supported Internet Explorer, these new calculators have been developed for and tested in Chrome, Firefox, Opera, Safari and Internet Explorer.  They even work on smart phones and tablets, including iPhones and iPads.

 The first life modeling applet to be re-coded is one of our most popular tools, which models the lifetime of our screw terminal electrolytic capacitors, from the economical DCMC series to intermediate series 500C and 520C to our high-performance 105 ºC types 550C and 101C.

 We are planning to offer revised calculators for other types such as our plug-in's (4CMC, 400C, 420C, 450C and 401C) and snap-in's (380L/LX and 381L/LX/LR) later in 2015.

 Tool Tips explain the purpose of each button or field in the applets as they are pointed to with the mouse. Once the user gains familiarity, this feature may be turned off by unchecking the Tool Tips checkbox in the applets. Message boxes are more informative and easier to read and to close. There's a new feature in these Tool Tips that displays the rated 85 ºC ripple current when the mouse hovers over the CDE part number.

 Another feature is that reliability (MTBF) and failure rate (FIT) as well as core-to-case and case-to-ambient thermal resistances are displayed right on the calculator output field.

New sizes and voltages extend our screw-terminal line-up. We now offer 550V rated capacitors (600V surge) in types DCMC and 500C. We also now offer 500V/105 ºC rated type 550C. In addition, we have added a new length, 7.625", to all of our 3" and 3.5" diameter screw-terminal capacitors. This length is the most economical for many high-volume applications for large screw-terminal capacitors.

There is a feature in the double applets that allows you to copy the information from the left pane to the right or vice-versa; just click the arrow button corresponding to the desired copy direction. This is great for playing "what-if" scenario analysis to home in on the best solution to your application. In fact, we recommend always using the "double applets" if you are using a desktop monitor. The "single applets" are best only for small screens in portrait format, such as tablet computers.

For the screw-terminal life calculator applets, the sequence for modeling the capacitor is fairly straightforward. You may first choose a capacitor type such as from one of our competitors for cross-reference, and the applet will find the CDE equivalent type. Or you can choose the CDE type yourself. Then choose the diameter, length and voltage, then click Search Catalog to look up our standard, full-can design. The default capacitance may be reduced by up to 50%, as this is an editable field.

A new part number and estimated ESR and ripple current rating will be generated automatically. You may also even enter the ESR and full-can capacitance, but this will display the part number as "Contact Us" because it may or may not be achievable.

The database search occurs automatically in the background and is much faster than the previous Java applets, generally milliseconds instead of seconds.

You can type over and replace the calculated ESR at either or both frequencies at the calculated core temperature if you uncheck the "Calc ESR?" box. This feature can enhance the applet's accuracy if you have measured the actual ESR of our capacitor at the temperature and frequency. Otherwise the applet displays its automatically-calculated ESR's so you can see the effects of frequency and temperature.

 

Overview:

 

This is a Javascript applet that calculates temperature rise above ambient from the capacitor's thermal resistances and the ripple-current power. Then it calculates the expected operating life for the core temperature, adjusted for ESR increase over life.

 

How the applet works:

 

The applet calculates core temperature based on CDE's 7-R Thermal Model. This is a lumped-parameter model based on extensive thermal tests and finite element analysis thermal models and is developed and discussed in our  technical paper at:

 

http://www.cde.com/resources/technical-papers/Predicting-Operating-Temperature-and-Expected-Lifetime.pdf

 

First, the applet calculates axial and radial thermal resistances from the core of the capacitor element using the capacitor element size, can size and type of construction. It also calculates the thermal resistance from the can wall and bottom to the ambient air and to an attached heatsink, if any. It expresses the thermal loop equations in terms of these resistances, the generated power, and the air and heat sink temperatures to obtain the core temperature.

To calculate the dissipated power caused by the ripple current, the applet first calculates the ESRs at Frequency 1 and Frequency 2 at room temperature. Then it calculates the power and ESRs at actual core temperature as an iterative loop including both the electrical and thermal circuits because the ESR depends on the core temperature, the core temperature depends on the power and the power depends on the ESR. The total power is the sum of the two ripple-current powers.

Finally, the applet calculates average core temperature over life by bumping the temperature rise up 50% to adjust for possible ESR increase during life.

While room-temperature ESR can more than double during a capacitor's life, the hot ESR increases more slowly and we believe that increasing the delta-T by 50% is a reasonable approximation to the expected average increase in the hot ESR over its lifetime.

The applet calculates expected life as Lb x Mv x 2^((Tmax-Tcore)/10) where Lb is the base life, Tmax is the maximum permitted core temperature, Mv is a voltage-derating multiplier and Tcore is the average core temperature over life. See the Application Guide in our Aluminum Electrolytic Capacitors catalog and on our website for a full discussion of this approach:

http://www.cde.com/resources/catalogs/AEappGUIDE.pdf

 

About JavaScript

JavaScript is a scripting language with C-like syntax and is ubiquitous to web browsers.  The applet itself is downloaded to your computer and the execution and modeling takes place locally on your computer.

The implementation of JavaScript varies slightly from browser to browser, but the life calculator applets have been tested in recent releases of the five most popular browsers as of 2015. The slight variations from one browser to another are generally in applet appearance (e.g., sizes of fonts and textboxes) and should not yield different life predictions. In Opera we have observed that the text fields do not highlight when the user enters an invalid entry.

 

Browser requirements

 

This applet requires a recent browser for full performance. We recommend using Chrome, Firefox, Internet Explorer browsers of 2013 or later vintage. Opera and Safari  also appear to work satisfactorily.

The 'Printable Form' command button below the applet(s) open a new browser windows with the applet results displayed as a text summary which can be saved as a text file or readily printed or cut/paste into an e-mail, etc.

If your browser allows enabling and disabling of JavaScript, obviously this setting must be enabled.


Using the applets effectively

 

The applets are useful not only in comparing different capacitor types such as the DCMC vs 520C, but also in determining what ESR and life characteristics are needed. For example, if the typical ESR is 18 milliohms and the life of a DCMC in your conditions is too low, you may play what-if and evaluate a higher-grade

capacitor such as a 520C or our highest grade, the 550C. If even the 550C cannot handle your ripple current load, you can experiment with case size and airflow, and you can even lower the hot ESR's by manually entering them, and advise us that you need the lower ESR. You can fax to (864) 843-3800 or e-mail http://www.cde.com/contact  your design to us using copy/paste from the Printable Form. Be sure to include your name, company name, address, phone and fax number in your

inquiry, along with your specific questions. We'll promptly propose a capacitor for your requirements.

Air is assumed to contact the entire can. Adjust air speed if a significant portion of the can is insulated. Use 50 lfm for free convection cooling. This is 0.25 meters per second.

The valid airflow velocity range is 50 to 5000.

Our capacitors are most often used in DC link applications where there are usually two groups of frequency harmonics,  those associated with the rectified mains and those drawn from the inverter. The frequency variation of the ESR of a capacitor is discussed in our paper at:

http://www.cde.com/resources/technical-papers/impedance.pdf

and has two predominant terms, one fairly constant with frequency and the other proportional to 1/f. Therefore the ESR typically decreases with increasing frequency. The applets allow for the application of two frequencies, so the best approach is to enter an rms current value for Ripple 1 that accounts for all low-frequency harmonics and enter an rms value for Ripple 2 that accounts for all high-frequency harmonics. Note that entering the fundamental frequency values of the rectified mains ripple (i.e. twice the mains frequency for single-phase and six times the mains frequency for three-phase) and of the inverter switching rate will generally be a good approach, as it will be slightly conservative.

Note: The single applet displays one instance of the applet in the browser window and is better for small or low-resolution monitors, while the double applet displays two instances of the applet for side-by-side comparison of results from different capacitor and thermal scenarios. If you are on an extremely low-bandwidth connection you may notice that the load time of the double applet is longer than for the single applet, as the database information for each capacitor type is downloaded separately for each applet instance.

This applet is only valid for Cornell Dubilier capacitors, as our construction and characteristics are unique.

Bookmark this page and return as often as you like for your capacitor life calculations. We'll be adding features and refinements throughout the coming months.

Special Notes for the Screw-Terminal Capacitor Applet

For assistance in filling out the 'Full Can Capacitance' and 'ESR' fields, you can use our latest catalog ratings, as they are full-can ratings. You can look up these ratings using the applet's new 'Search Catalog' button for our current capacitor types. This feature will populate the 'Capacitance' and 'Full Capacitance' fields with the nominal capacitance, and the 'ESR' field with 70% of the ESR limit, which is generally close to a capacitor's typical initial ESR.

Our screw-terminal capacitor types are listed in order of increasing performance:  DCMC, 500C,  520C, 101C and 550C. These types incorporate our Thermal-Pak construction and run much cooler in high ripple current applications, especially when attached to a heatsink.

'Heatsink Characteristics' in the applet means: 1. If 'ºC/W' is checked (the default), a heatsink with this thermal resistance coupled to the air at the specified air temperature. 2. If 'ºC' is checked: Infinite heatsink (zero thermal resistance) at this heatsink temperature coupled to the capacitor through its bottom insulation, allowing for the mounting hole if it's a stud mount. Use option 2 with care, as it is only appropriate for a massive or liquid-cooled heatsink.

 

Applet Limitations and Cautionary Notes: 

The applet comprises three models: impedance, thermal, and life. None of these models is perfect or exact. Since life is an exponential function of temperature, the error in predicting the life will be an exponential function of the error in predicting the core temperature. The ESR and thermal models (core heat rise above the ambient temperature) are generally each within 10% but have sometimes erred as much as 20%. For most applications where the ripple current is modest, this error does not cause an appreciable reduction in accuracy, but when the initial core rise is over 20 ºC, the possible error in calculating the life can be significant. Another source of error may occur when multiple capacitors are used in a bank or when the capacitors are placed in proximity to other hot components. These effects should be taken into account when entering the ambient air temperature for the capacitor. We encourage you to use the applet as a tool of modeling effects of heatsink, airflow, and capacitor characteristics, but we strongly recommend that you follow up with evaluating actual capacitors with thermocouples, especially if you are designing close to the performance limits.

Also, note that there is little or no conservatism built into the applet, and the typical ESR is not a maximum ESR limit (the screw-terminal applet uses 70% of the limit as the typical ESR while the snapmount applet calculates the ESR from the foil surface area and electrolyte-paper properties), so remember to look at what the performance would be if the ESR were 40% higher than typical. Even though today's aluminum electrolytic capacitors are far advanced compared to the glycol-borate capacitors of the 1970's, the physics are essentially the same, and therefore the same old rules of thumb about 'lytics still apply:

Derate the voltage and don't run them really hot if you want long life, good reliability, and robustness. Derating the DC voltage allows capacitors to handle line surges in modern systems with poor power quality, even when the capacitors are hot. Ensuring the capacitors run below 85 ºC will not only make them last longer by extending the wearout period, but will also keep your system out of trouble resulting from random capacitor failures. Our aluminum electrolytic >capacitors will generally run at failures rates in the 10-30 FIT (failures per billion unit hours) range at rated voltage, 45 ºC, but like wearout (life), the reliability is an exponential function of temperature, and a large bank (say 32 capacitors) running near rated voltage at 100 ºC is a recipe for a high rate of field failure, like 10% per year system failure rate. A discussion of our lifetime and reliability models may be found at:

http://www.cde.com/resources/technical-papers/reliability.pdf

Legal Disclaimer: 

The CDE Capacitor Thermal/Life Calculator applets are not a contract, license, or authorization of any kind. Specifications and model are subject to change without notice. Cornell Dubilier assumes no liability on accuracy, completeness or suitability for any application. The only warranty is the one-year, application express warranty (copy available upon request).