RF Mica Modeler

Cornell Dubilier RF Mica Modeler

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Overview

Cornell Dubilier’s RF Mica Modeler provides an interactive console to assist the electrical design engineer in selecting the best mica capacitor within a capacitance range of 0.5 to 91,000 pF and voltage range of 100 volts to 4,000 volts, targeting applications in the frequency range of 1 kHz to 4 GHz.

Capacitor selection

The modeler charts estimates of the high-frequency electrical performance for three major classes of our circuit board mounted mica capacitors: (1) Leaded mica capacitors used in through-hole circuit board designs, (2) surface-mounted metal case mica / PTFE capacitors style MIN02 (5 mm square) and MCM01 (10 mm × 12 mm) for high current and high power handling, and (3) SMT mica chip capacitors in sizes 0805, 1210, 1812, 2220 and 3838.

Once one of the above three package styles is selected from the “Package Style” drop-down box, a complete list of standard catalog part numbers is provided in order of increasing capacitance in the “CDE Part Number” selection box. As soon as a part number is selected, the outline drawing is shown and the physical dimensions are shown.

Connection to S-parameter test circuit: Series/Shunt control

The S-parameters can be modeled for the capacitor mounted on a test circuit transmission line, in either Series configuration or in Shunt (parallel) configuration, as chosen in the dropdown box in the lower left of the modeler. For the S-parameter plots, the capacitor’s scattering parameters are plotted, de-embedded to the capacitor terminals as two reference planes as near as practicable to the locations at which the capacitor terminals would typically be soldered to circuit board traces on a length of transmission line with 50Ω characteristic impedance. When configured as the test circuit for the capacitor, such a line would be driven from a first port at one end of the line by a sinusoidal energy source with a purely resistive source impedance of 50 ohms, with the capacitor mounted in the physical center of the line. The other, unexcited end of the line is a second test port that is terminated with a purely resistive load impedance of 50 ohms to prevent reflections. The capacitor on such a transmission line can be mounted in series with one interrupted conductor on the lines, known as Series Connection, or it can be mounted across the line, which would be Shunt Connection. In both connection schemes, since the capacitor is assumed to be mounted symmetrically and equidistant with respect to the two ports (known as a “reciprocal” mounting scheme), S21 is precisely equal to S12, and S11 is exactly equal to S22.

Circuit inductance (nH): Inductance Slider Control

If there is additional circuit inductance in series with the capacitor connection, such as additional lead or trace length that you want to include in the model for the capacitor, you include its effects on the displayed and modeled parametric curves by using the “Circuit Inductance” slider control. This additional inductance is added to the capacitor’s internal inductance (ESL) and affects the resonant frequency as well as the impedance and S-parameter plots. It also affects the output files, if generated (discussed below in the Generate Model Output section) and this inductance value— if nonzero— is listed in the header field of the output file for documentation purposes. It does not affect the Maximum RMS Current plot.

Heatsink Theta slider control

For the metal-case style only, we provide a slider tool immediately above the current ratings chart so that a value of the heatsink’s thermal resistance to ambient can be specified, as these capacitors are intended to be attached to a heatsink or soldered to a copper heat spreader. This is discussed in the next section on the charts which are plotted by the modeler.

Interactive charts

The modeler creates six charts for the selected capacitor. Exact frequency and parameter value can be observed by hovering your mouse over a curve. When two or more curves are in close proximity to the point of interest, note that the rectangular border around the displayed coordinates takes on the color of the legend of the series to which you are pointing.

In the left column the top chart plots the estimated, typical capacitance versus frequency for three temperatures. The nominal capacitance at the applicable test frequency is used as the base capacitance value; obviously the device tolerance would be applicable to actual capacitors. The temperature and frequency coefficients assumed by the modeler are typical values, but are not warranted.

Below the capacitance chart are plotted the typical impedance magnitude and series resistance. At the device’s resonant frequency, the apex of actual impedance magnitude curve will always touch the ESR curve. Note that due to the limited number of data points in the modeler plot, for such high-Q capacitors as mica, this contact will sometimes not be precisely resolved in the impedance and ESR chart. 

In the bottom-left chart we plot the Q of the capacitor, which is the ratio of the reactance to the resistance. Q is the multiplicative inverse of D, the dissipation factor, which can be visualized from the logarithmic Q chart by mentally reflecting it about the horizontal line with the ordinate value of Q=D=1.

In the right column, the top plot graphs an estimate of the maximum expected rms current handling at three ambient temperatures, assuming the capacitor is mounted to a typical circuit board that is also at the ambient temperature in natural convection. In actual applications, we recommend that the current be derated and that the capacitor be tested under worst-case conditions for qualification purposes.

At lower frequencies the rated current plot will generally have a linear portion  that is directly proportional to frequency, arising from the rms voltage limit of the capacitor, which at low frequencies is an electric field intensity limit and not a joule heating limit.

For package styles other than the SMT Metal Case, these RMS current ratings are calculated based upon the estimated ESR, and at a maximum core heat rise of 60 °C, limited to a 125 °C maximum core temperature when mounted to a circuit board in natural, free convection.

For the SMT Metal Case styles, the maximum rms current handling calculation is based upon an assumed maximum allowable heat rise above ambient of 120 °C, also limited to a 185 °C maximum core temperature. The default thermal resistance of 50 °C/W is for natural convection without a heatsink or a large heat spreader. The Heatsink Thermal Resistance to Ambient Slider Control can be dragged leftward to select a much lower heatsink-to-ambient thermal resistance, even as low as 1 °C/W which would imply an exotic heatsink such as liquid-cooled copper. An aggressive heatsink such as less than 10 °C/W will greatly affect the joule-heating-limited current handling capabilities at higher frequencies. Because of limitations of the current carrying capacity of the device tabs, the current in all cases is limited to 25 amps rms.

The middle chart in the right column is a plot of the real and imaginary components of S-parameter S11, which is identical to S22 due to the assumed symmetric mounting scheme of the capacitor. The final chart in the lower right is a plot of the real and imaginary components of S-parameter S21, which is identical to S12.

Frequency span limitations

The lower frequency span is chosen to cover the frequency at which the capacitance and ESR limits are tested, which is 1 kHz for capacitors rated 1,000 pF and above and 1 MHz for capacitors rated less than 1,000 pF. Because sometimes capacitors rated as low as 10 pF are used at frequencies below 1 MHz, in the range of 10 to 999 pF the lowest plotted frequency is 100 kHz instead of 1 MHz.

The upper frequency range is chosen to cover the smaller of 6 GHz and a small integer multiple of the first resonant frequency. The multiple is limited by the electrical size of the capacitor as well as the possibility of circulating currents due to imbalance of the conductor geometry within the capacitor. Be aware that above the maximum plotted frequency, there may be notches in the impedance magnitude and upward spikes in the series resistance due to internally circulating currents. For our leaded mica capacitors, part numbers beginning with CMR will generally have better performance in this regard. If you are unable to find a capacitor covering your frequency range, please contact Cornell Dubilier’s mica design engineering department, as we may be able to create a higher performance capacitor to more effectively address the necessary frequency span.

Generate Model Output button

You may generate an output of the model as a frequency listing with Z-parameter or S-parameter values by selecting the desired output format in the “Output Format” drop-down box, including the capacitor core temperature, then clicking the Generate Model Output button. Note that the output format in the popup window will be in space-delimited Touchstone format per the version 1.0 standard. The S-parameter output is 2-port .s2p with a reference impedance of 50Ω. For the Z-parameters, the format is still compliant to the Touchstone 1.0 standard, but is single-port with the reference impedance set to 1 ohm, so that the resistance and reactance in ohms are listed without the need to apply a factor of 50.

The data listing appears in a popup window, so popup blockers need to be disabled. The window itself is not by default stored to your hard drive or cloud storage path, and we have found that the best way to store it as a text or .s2p file is simply to click anywhere in the popup window, Ctrl-A to select all the text, then Ctrl-C to copy, and Ctrl-V to paste it into Notepad or another text editor. Then you may save that file in your text editor in text format with the desired name and extension that will allow import into your RF modeler of choice, or even plotted in a spreadsheet, etc. The data is the same as what you see in the corresponding modeler charts, reflecting a logarithmic frequency spacing of 100 points per decade, which is a 2.4% point-to-point frequency increment. 

To paste the output data into a spreadsheet, you may select all the text in the output popup window using Ctrl-A, copy it with Ctrl-C, and paste into spreadsheet using Ctrl-V. In Excel to parse the data into separate columns, you may select the cells in the single column containing the frequency and data, then go to Excel’s Data tab, and under Data Tools click “Text to Columns” and indicate this range is Delimited (Next) with (checkbox) Space, and the data will be parsed into separate columns.

JavaScript

Per Atwood’s Law, the Cornell Dubilier RF Mica Modeler is implemented in JavaScript. Therefore JavaScript needs to be enabled in your browser. The implementation of JavaScript varies slightly from browser to browser, but the RF Mica Modeler applet has been tested in recent releases of the most popular browsers: Chrome, Firefox, Opera and Edge. 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 performance curves or data. Also any pop-up blocker may need to be turned off in order for the model output popup text result window to open.

Hints: 

Using the applet effectively   

Use Ctrl-mouse-wheel to zoom as necessary. Graphics quality is automatically scaled, allowing screen caps of outstanding quality.

ToolTips display much useful information as the cursor moves over relevant fields on the form. ToolTips also display coordinates on the various chart series, and will display any metric dimensions as English.

Modeler Limitations and Cautionary Notes:  This RF modeler is based on mathematical models of the physical construction of the capacitors, cross-checked with a limited amount of room-temperature vector network analyzer data. Also note that there is little or no conservatism built into the applet, and the typical ESR is not a maximum ESR limit, and the maximum rms current is not a warranted life test condition. We would encourage you to verify performance and to discuss your application requirements such as minimum lifetime with our RF Mica application engineers.

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

Legal Disclaimer:  The CDE RF Mica Capacitor Modeler is 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.