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Filters they are similar to those used in linear

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FiltersThey are similar to those used in linear regulators are utilized both for the input and output. Inputfilters involve capacitors between 1000 and 2200 μF (sometimes up to 5000 μF). Output filters mayhave capacitance up to 470 μF. Working DC voltage rating (WVDC) of theinput filter capacitors mustbe about 150% of the peak voltage that may appear at the output of the input rectifier.Capacitors have been designed to have higher capacitance to volume ratio, small equivalentseries resistance (ESR) and series inductance for more effective operation at high frequencies.Aluminum electrolytic capacitors are used at the input filtering. It is preferable to place a tantalum orother low value capacitor with much smaller ESR in parallel. This second capacitor is generally placedclose to the collector of the switching transistor. Multi-layer ceramic capacitors are used for outputRectifier diodesAlternatingsignalChoppedDC inputPulsatingDC outputStep-downtransformerFigure 6.24 The full-wave with bridge rectifier type output rectifier
Sources of Electrical Energy / 286filtering at high frequencies. Electrolytic capacitors can also be used if the frequency of operation islow. High frequency operation requires smaller capacitor size.Elements of the RF Regulator/Switching NetworkThe heart of every switching regulator is the RF regulator network shown in Figure 6.25. It chops theDC voltage from the input filter at 20 kHz or higher (up to 1 MHz is considered in recent designs).Pulse-Width-Modulation (PWM) shown in the figure is mostly used to drive the switching transistorfor chopping. Pulse width varies according to the load (closed-loop control system). Basic compo-nents of the system involves the switching element, high frequency step-down transformer, outputrectifier and filter discussed above, and sense amplifier and modulator.The Switching ElementPower MOSFETs are mostly preferred over bipolar junction transistors. Power MOSFETs have thefollowing major advantages:xCan be driven directly by control ICs without a need for a drive circuitry.xThey don't store charge during saturation. Hence, they have very low transition time thatallows them to work at high switching frequencies.Figure 6.25 Block diagram of the switching network
Sources of Electrical Energy / 287xThey don't have destructive secondary break-down reducing or even eliminating the need fora speed limiting snubber network.However, they have some disadvantages as:xLarge on resistance (4-5 Ω versus 0.1 Ω in bipolar).xSensitivity to reverse voltage spikes and,xlarge die size.In recent years, bipolar transistors have been developed that can switch amperes of currents in 2 μsor less and withstand voltage over 1000 volts.The High-Frequency TransformerA transformer is used to convert high-voltage, chopped DC into a lower voltage secondary AC signal.

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Term
Spring
Professor
NoProfessor
Tags
Alternating Current, The Bible, The Land, power supply, Measuring instrument

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