One of the most overlooked, but important performance characteristics of a capacitor used in power circuits is its equivalent series resistance (ESR). ESR determines the capacitor's I2R heating losses, which in turn impact the efficiency, pulse handling capability, and reliability of its associated circuit.
Meeting the need for low ESR capacitors are multilayer polymer (MLP) capacitors. This technology takes the concept of metallized polymer film capacitors to its ultimate level in terms of size efficiency, ease of manufacture, and overall performance. As opposed to aluminum polymer or “organic polymer” capacitors, which are basically electrolytic capacitors, these pure polymer capacitors are electrostatic types consisting of thin film layers of polymer-based dielectric material with vapor-deposited metal that acts as electrode plates.
To understand the significance of the MLP capacitor, let's first look at the effects of ESR. By definition, ESR is a performance characteristic representing the energy losses of the “equivalent” series resistance of a capacitor. It takes into account all of the losses — lead resistance, termination resistance and “plate” resistance, as well as the inherent losses of the dielectric material. It assumes that a single resistance in series with an “idealized” capacitor can represent all the losses. Fig. 1 plots ESR vs. frequency for various capacitor types. Equations related to ESR are:
RIR=Losses resulting from leakage currents and resistance (IR) in the capacitor
RS=Series losses due to leads, terminations, plates, etc. (metal losses)
DFDie=Inherent losses in the dielectric caused by molecular polarization dielectric absorption factors of the particular material.
Having higher than expected ESR values can produce detrimental results. For example, whereas ceramic and tantalum capacitors are appropriate when applied in small signal or low-to-moderate ac duty cycle applications, you must take care to avoid catastrophic failures due to increasing bias voltages and ambient temperature changes. In high current ac applications both X7R MLCs (multilayer ceramics) and tantalum electrolytics suffer from ESR increases. This application-induced, parasitic property of MLC and tantalum capacitors causes their impedance and ESR to increase significantly from any published catalog specifications. Under dc bias voltage these capacitor types roll off in capacitance value, causing their impedance and ESR to increase. Under increasing ac voltage, the dissipation factor of MLC types increases, causing ESR shifts.
The MLP capacitor employs metallized electrode construction as shown in Fig. 2. These electrode plates are composed of 100 to 300 of vapor deposited base metal resulting in 1Ω to 5Ω/sq of plate resistance (100=0.0000003937 in.). MLP construction overcomes the “high” surface resistivity of the deposited metal electrodes by stacking thousands of layers of plate resistance in parallel; producing an extremely low total effective resistance (Rt) for the capacitor (typically under 10mΩ). A capacitor could have 1Ω per layer of dc resistance, however, because there can easily be 2000 paralleled stacks within the capacitor, the total dc resistance of the electrode plates would be 5mΩ.
The stacking of thousands of layers of very thin film dielectric also allows for the production of relatively high capacitance values and associated high current ratings in small package sizes. Depending on the capacitor's rated voltage, the dielectric thickness of each layer can range from 0.6 to 8 microns, with a micron being about 1/10,000 the thickness of a human hair (1 micron=0.000040 in.). The “thin” layers of film stacked 2000 layers of 1 micron thick dielectric results in a basic capacitor block thickness of only 2.03 mm.
Because of its low ESR, within the family of electrostatic pure polymer film capacitors, MLP types offer the highest power density and dv/dt pulse ratings. Today, chip and lead framed chip type multilayer polymer (linear stacked film) capacitors are available from 50Vdc to 500+Vdc for through-hole and surface mount applications. The film chip units are thermally treated under pressure so that the capacitors may be surface mounted at high reflow temperatures (typically 220°C).
MLP surface mount products are even equipped with a patented (U.S. Patent 6,278,601), integrated IR shield to protect the capacitors from infrared reflow soldering. The high frequency ESR and ripple current carrying capability of these capacitors is remarkable given their small size. Fig. 3 illustrates the maximum rms current vs. capacitance for MLP capacitors at 500 kHz.
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