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SM045C156KHN650 Scheda tecnica(PDF) 8 Page - AVX Corporation |
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SM045C156KHN650 Scheda tecnica(HTML) 8 Page - AVX Corporation |
8 / 112 page 7 SMPS Capacitors Application Information on SupraCap ® High speed switch mode power supplies require extremely low equivalent series resistance (ESR) and equivalent series inductance (ESL) capacitors for input and output filtering. These requirements are beyond the practical limits of electrolytic capacitors, both aluminum and tantalums, but are readily met by multilayer ceramic (MLCs) capacitors (Figure 1). Theoretical SMPS’s output filter capacitor values are in the range of 6-10 μF/amp at 40KHz and drop to less than 1 μF/amp at 1MHz. Most electrolytic applications use 10 to 100 times the theoretical value in order to obtain lower ESR from paralleling many capacitors. This is not necessary with SupraCap ® MLC capacitors which inherently have ESRs in the range of milliohms. These extremely low values of ESR mean low ripple voltage and less self-heating of the capacitor. Output noise spikes are reduced by lowering the filter capac- itance self-inductance. The ripple current is a triangle wave form with constant di/dt except when it changes polarity, then the di/dt is very high. The noise voltage generated by the filter capacitor is VNoise = L Capacitor di/dt AVX SupraCap ® devices have inductance value less than 3nH. Figure 2 compares a 5.6 μF MLC to a 5.6 μF tantalum which was specially designed for low ESR and ESL. When subjected to a di/dt of 200 mA/ns the tantalum shows an ESR of 165 mΩ and an ESL of 18nH versus the MLC’s 4 mΩ and 0.3 nH. These performance differences allow considerable reduction in size and weight of the filter capacitor. Additionally, MLCs are compatible with surface mount technology reflow and assembly techniques which is the desirable assembly for conversion frequencies exceeding 1 MHz. Electrolytic capacitors (both aluminum and tantalum) are not compatible with normal vapor phase (VPS) or infrared (IR) reflow temperatures (205-215°C) due to electrolyte and structural problems. AVX SupraCap ® devices are supplied with lead frames for either thru-hole or surface mount assembly. The lead frames act as stress relief for differences in coefficients of expansion between the large ceramic chip ( 10 ppm/°C) and the PC boards. DSW 16 50mV 50nS TPOS-7 Ta MLC CSW 1 50nS 50mV VZR-0.2 V=2.0mV T=25.5nS Figure 2 ESR Comparison of Different Capacitor Technologies ESR -vs- Frequency 100μF Filter Capacitors 1.E-03 1.E-02 1.E-01 1.E+00 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 Frequency (Hz) Aluminum Electrolytic 100μF / 50V Low ESR Solid Tantalum 100μF / 10V Solid Aluminum Electrolytic 100μF / 16V MLCC 100μF / 50V Figure 1 SUPRACAP® - LARGE CAPACITANCE VALUE MLCs |
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