General Description Basic Construction – A multilayer ceramic (MLC) capacitor is a monolithic block of ceramic containing two sets of offset, interleaved planar electrodes that extend to two opposite surfaces of the ceramic dielectric. This simple Ceramic Layer structure requires a considerable amount of sophistication, both in material and manufacture, to produce it in the quality and quantities needed in today’s electronic equipment. Electrode End Terminations Terminated Edge Terminated Edge Margin Electrodes Multilayer Ceramic Capacitor Figure 1 Formulations – Multilayer ceramic capacitors are available in both Class 1 and Class 2 formulations. Temperature compensating formulation are Class 1 and temperature stable and general application formulations are classified as Class 2. Class 1 – Class 1 capacitors or temperature compensating capacitors are usually made from mixtures of titanates where barium titanate is normally not a major part of the mix. They have predictable temperature coefficients and in general, do not have an aging characteristic. Thus they are the most stable capacitor available. The most popular Class 1 multilayer ceramic capacitors are C0G (NP0) temperature compensating capacitors (negative-positive 0 ppm/°C). 88 Class 2 – EIA Class 2 capacitors typically are based on the chemistry of barium titanate and provide a wide range of capacitance values and temperature stability. The most commonly used Class 2 dielectrics are X7R and Y5V. The X7R provides intermediate capacitance values which vary only ±15% over the temperature range of -55°C to 125°C. It finds applications where stability over a wide temperature range is required. The Y5V provides the highest capacitance values and is used in applications where limited temperature changes are expected. The capacitance value for Y5V can vary from 22% to -82% over the -30°C to 85°C temperature range. All Class 2 capacitors vary in capacitance value under the influence of temperature, operating voltage (both AC and DC), and frequency. For additional information on performance changes with operating conditions, consult AVX’s software, SpiCap. General Description EIA CODE Percent Capacity Change Over Temperature Range RS198 Temperature Range X7 X6 X5 Y5 Z5 -55°C to +125°C -55°C to +105°C -55°C to +85°C -30°C to +85°C +10°C to +85°C Code Percent Capacity Change D E F P R S T U V ±3.3% ±4.7% ±7.5% ±10% ±15% ±22% +22%, -33% +22%, - 56% +22%, -82% Effects of Voltage – Variations in voltage have little effect on Class 1 dielectric but does affect the capacitance and dissipation factor of Class 2 dielectrics. The application of DC voltage reduces both the capacitance and dissipation factor while the application of an AC voltage within a reasonable range tends to increase both capacitance and dissipation |factor readings. If a high enough AC voltage is applied, eventually it will reduce capacitance just as a DC voltage will. Figure 2 shows the effects of AC voltage. Cap. Change vs. A.C. Volts X7R Capacitance Change Percent Table 1: EIA and MIL Temperature Stable and General Application Codes 50 40 30 20 10 0 12.5 EXAMPLE – A capacitor is desired with the capacitance value at 25°C to increase no more than 7.5% or decrease no more than 7.5% from -30°C to +85°C. EIA Code will be Y5F. Symbol Temperature Range A B C -55°C to +85°C -55°C to +125°C -55°C to +150°C Symbol R S W X Y Z Cap. Change Zero Volts Cap. Change Rated Volts +15%, -15% +22%, -22% +22%, -56% +15%, -15% +30%, -70% +20%, -20% +15%, -40% +22%, -56% +22%, -66% +15%, -25% +30%, -80% +20%, -30% Temperature characteristic is specified by combining range and change symbols, for example BR or AW. Specification slash sheets indicate the characteristic applicable to a given style of capacitor. 50 Figure 2 Capacitor specifications specify the AC voltage at which to measure (normally 0.5 or 1 VAC) and application of the wrong voltage can cause spurious readings. Figure 3 gives the voltage coefficient of dissipation factor for various AC voltages at 1 kilohertz. Applications of different frequencies will affect the percentage changes versus voltages. D.F. vs. A.C. Measurement Volts X7R 10.0 Dissipation Factor Percent MIL CODE 25 37.5 Volts AC at 1.0 KHz Curve 1 - 100 VDC Rated Capacitor 8.0 Curve 2 - 50 VDC Rated Capacitor Curve 3 - 25 VDC Rated Capacitor 6.0 Curve 3 Curve 2 4.0 Curve 1 2.0 0 .5 In specifying capacitance change with temperature for Class 2 materials, EIA expresses the capacitance change over an operating temperature range by a 3 symbol code. The first symbol represents the cold temperature end of the temperature range, the second represents the upper limit of the operating temperature range and the third symbol represents the capacitance change allowed over the operating temperature range. Table 1 provides a detailed explanation of the EIA system. 1.0 1.5 2.0 2.5 AC Measurement Volts at 1.0 KHz Figure 3 Typical effect of the application of DC voltage is shown in Figure 4. The voltage coefficient is more pronounced for higher K dielectrics. These figures are shown for room temperature conditions. The combination characteristic known as voltage temperature limits which shows the effects of rated voltage over the operating temperature range is shown in Figure 5 for the military BX characteristic. 89 General Description capacitors and is why re-reading of capacitance after 12 or 24 hours is allowed in military specifications after dielectric strength tests have been performed. 5 Typical Curve of Aging Rate X7R 0 +1.5 -5 -10 0 -15 -20 25% 50% 75% Percent Rated Volts 100% Figure 4 Capacitance Change Percent Typical Cap. Change vs. Temperature X7R -1.5 -3.0 -4.5 -6.0 -7.5 +20 1 10 100 +10 0VDC 0 -10 -30 -55 -35 Characteristic C0G (NP0) X7R, X5R Y5V 1000 10,000 100,000 Hours Max. Aging Rate %/Decade None 2 7 Figure 6 -20 -15 +5 +25 +45 +65 +85 +105 +125 Temperature Degrees Centigrade Figure 5 Effects of Time – Class 2 ceramic capacitors change capacitance and dissipation factor with time as well as temperature, voltage and frequency. This change with time is known as aging. Aging is caused by a gradual re-alignment of the crystalline structure of the ceramic and produces an exponential loss in capacitance and decrease in dissipation factor versus time. A typical curve of aging rate for semistable ceramics is shown in Figure 6. If a Class 2 ceramic capacitor that has been sitting on the shelf for a period of time, is heated above its curie point, (125°C for 4 hours or 150°C for 1⁄2 hour will suffice) the part will de-age and return to its initial capacitance and dissi-pation factor readings. Because the capacitance changes rapidly, immediately after de-aging, the basic capacitance measurements are normally referred to a time period sometime after the de-aging process. Various manufacturers use different time bases but the most popular one is one day or twenty-four hours after “last heat.” Change in the aging curve can be caused by the application of voltage and other stresses. The possible changes in capacitance due to de-aging by heating the unit explain why capacitance changes are allowed after test, such as temperature cycling, moisture resistance, etc., in MIL specs. The application of high voltages such as dielectric withstanding voltages also tends to de-age 90 Capacitance Change Percent Capacitance Change Percent Typical Cap. Change vs. D.C. Volts X7R Effects of Frequency – Frequency affects capacitance and impedance characteristics of capacitors. This effect is much more pronounced in high dielectric constant ceramic formulation than in low K formulations. AVX’s SpiCap software generates impedance, ESR, series inductance, series resonant frequency and capacitance all as functions of frequency, temperature and DC bias for standard chip sizes and styles. It is available free from AVX and can be downloaded for free from AVX website: www.avx.com. General Description Effects of Mechanical Stress – High “K” dielectric ceramic capacitors exhibit some low level piezoelectric reactions under mechanical stress. As a general statement, the piezoelectric output is higher, the higher the dielectric constant of the ceramic. It is desirable to investigate this effect before using high “K” dielectrics as coupling capacitors in extremely low level applications. Reliability – Historically ceramic capacitors have been one of the most reliable types of capacitors in use today. The approximate formula for the reliability of a ceramic capacitor is: Lo = Lt 共共共共 Vt Vo where Lo = operating life Lt = test life Vt = test voltage Vo = operating voltage X Tt To Y Tt = test temperature and To = operating temperature in °C X,Y = see text Historically for ceramic capacitors exponent X has been considered as 3. The exponent Y for temperature effects typically tends to run about 8. A capacitor is a component which is capable of storing electrical energy. It consists of two conductive plates (electrodes) separated by insulating material which is called the dielectric. A typical formula for determining capacitance is: Energy Stored – The energy which can be stored in a capacitor is given by the formula: E = 1⁄2CV2 E = energy in joules (watts-sec) V = applied voltage C = capacitance in farads Potential Change – A capacitor is a reactive component which reacts against a change in potential across it. This is shown by the equation for the linear charge of a capacitor: I ideal = C dV dt where I = Current C = Capacitance dV/dt = Slope of voltage transition across capacitor Thus an infinite current would be required to instantly change the potential across a capacitor. The amount of current a capacitor can “sink” is determined by the above equation. Equivalent Circuit – A capacitor, as a practical device, exhibits not only capacitance but also resistance and inductance. A simplified schematic for the equivalent circuit is: C = Capacitance L = Inductance Rp = Parallel Resistance Rs = Series Resistance C = .224 KA t C = capacitance (picofarads) K = dielectric constant (Vacuum = 1) A = area in square inches t = separation between the plates in inches (thickness of dielectric) .224 = conversion constant (.0884 for metric system in cm) Capacitance – The standard unit of capacitance is the farad. A capacitor has a capacitance of 1 farad when 1 coulomb charges it to 1 volt. One farad is a very large unit and most capacitors have values in the micro (10-6), nano (10-9) or pico (10-12) farad level. Dielectric Constant – In the formula for capacitance given above the dielectric constant of a vacuum is arbitrarily chosen as the number 1. Dielectric constants of other materials are then compared to the dielectric constant of a vacuum. Dielectric Thickness – Capacitance is indirectly proportional to the separation between electrodes. Lower voltage requirements mean thinner dielectrics and greater capacitance per volume. Area – Capacitance is directly proportional to the area of the electrodes. Since the other variables in the equation are usually set by the performance desired, area is the easiest parameter to modify to obtain a specific capacitance within a material group. RP L RS C Reactance – Since the insulation resistance (Rp) is normally very high, the total impedance of a capacitor is: Z= where 冑 RS2 + (XC - XL )2 Z = Total Impedance Rs = Series Resistance XC = Capacitive Reactance = XL = Inductive Reactance 1 2 π fC = 2 π fL The variation of a capacitor’s impedance with frequency determines its effectiveness in many applications. Phase Angle – Power Factor and Dissipation Factor are often confused since they are both measures of the loss in a capacitor under AC application and are often almost identical in value. In a “perfect” capacitor the current in the capacitor will lead the voltage by 90°. 91 General Description di I (Ideal) I (Actual) Loss Angle Phase Angle ␦ f V IR s In practice the current leads the voltage by some other phase angle due to the series resistance RS. The complement of this angle is called the loss angle and: Power Factor (P.F.) = Cos f or Sine ␦ Dissipation Factor (D.F.) = tan ␦ for small values of ␦ the tan and sine are essentially equal which has led to the common interchangeability of the two terms in the industry. Equivalent Series Resistance – The term E.S.R. or Equivalent Series Resistance combines all losses both series and parallel in a capacitor at a given frequency so that the equivalent circuit is reduced to a simple R-C series connection. E.S.R. C Dissipation Factor – The DF/PF of a capacitor tells what percent of the apparent power input will turn to heat in the capacitor. Dissipation Factor = E.S.R. = (2 π fC) (E.S.R.) XC The watts loss are: Watts loss = (2 π fCV2 ) (D.F.) Very low values of dissipation factor are expressed as their reciprocal for convenience. These are called the “Q” or Quality factor of capacitors. Parasitic Inductance – The parasitic inductance of capacitors is becoming more and more important in the decoupling of today’s high speed digital systems. The relationship between the inductance and the ripple voltage induced on the DC voltage line can be seen from the simple inductance equation: V = L di dt 92 The dt seen in current microprocessors can be as high as 0.3 A/ns, and up to 10A/ns. At 0.3 A/ns, 100pH of parasitic inductance can cause a voltage spike of 30mV. While this does not sound very drastic, with the Vcc for microprocessors decreasing at the current rate, this can be a fairly large percentage. Another important, often overlooked, reason for knowing the parasitic inductance is the calculation of the resonant frequency. This can be important for high frequency, bypass capacitors, as the resonant point will give the most signal attenuation. The resonant frequency is calculated from the simple equation: 1 fres = 2冑 LC Insulation Resistance – Insulation Resistance is the resistance measured across the terminals of a capacitor and consists principally of the parallel resistance R P shown in the equivalent circuit. As capacitance values and hence the area of dielectric increases, the I.R. decreases and hence the product (C x IR or RC) is often specified in ohm farads or more commonly megohm-microfarads. Leakage current is determined by dividing the rated voltage by IR (Ohm’s Law). Dielectric Strength – Dielectric Strength is an expression of the ability of a material to withstand an electrical stress. Although dielectric strength is ordinarily expressed in volts, it is actually dependent on the thickness of the dielectric and thus is also more generically a function of volts/mil. Dielectric Absorption – A capacitor does not discharge instantaneously upon application of a short circuit, but drains gradually after the capacitance proper has been discharged. It is common practice to measure the dielectric absorption by determining the “reappearing voltage” which appears across a capacitor at some point in time after it has been fully discharged under short circuit conditions. Corona – Corona is the ionization of air or other vapors which causes them to conduct current. It is especially prevalent in high voltage units but can occur with low voltages as well where high voltage gradients occur. The energy discharged degrades the performance of the capacitor and can in time cause catastrophic failures.