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Table I. Pulse Technology Comparison Type of Power Supply SCR SCR with filter Linear Switcher Switcher with filter Ripple Efficiency Bandwidth Size Cost High Highest Low Mid Lowest Low High Lowest Largest Low Lowest Lowest Highest Largest Highest Highest High Mid Lowest Mid Low High Mid Small Mid SCR, silicon-controlled rectifier. pulsed outputs. The most common design consists of a standard SCR phasecontrolled rectifier with a semiconductor switch on the output. Although this system can be successfully employed in almost all pulsing applications, there are some drawbacks, mainly the inherent limitations associated with pulse rise and fall times. When faster pulsing speeds or square waves are required, linear power supplies are a viable technology. A linear design consists of a fixed output power supply, followed by a parallel combination of field-effect transistor (FET) or bipolar transistors, with the exact configuration determined by the output voltage levels required. This bank of transistors determines the final output by pulsing the fixed DC supplied to it. The efficiency of a linear supply is generally less than that of a SCR phasecontrol design, due to the fact that the rectification section always provides full power to the regulator, which must then dissipate the energy difference between full power and the desired output voltage. On the other hand, linear designs are capable of providing virtually perfect square wave pulses, due to the ability of the transistors to cycle on and off rapidly. A reversing linear system can also provide transition through zero output with no dead time. A relatively new configuration, when compared with SCR and linear designs, is the switch mode power supply, more commonly known as a switcher. Although an SCR phase-controlled power supply technically is a switcher, practical considerations usually limit pulse repetition rates to 12 times line frequency. Functionally, a switcher will typically start by rectifying the incoming line directly. This raw DC will then be chopped by a variable pulse width modulator, feeding the primary of a high-frequency transformer. The high-frequency transformer performs the desired voltage/current transformation. The output from the secondary of the transformer is then rectified and filtered. Switchers have a number of advantages over the other designs. Because of the higher frequencies, both transformer and filter inductor sizes and weights can be reduced, resulting in a more compact unit. Additionally, switchers have efficiencies comparable to that of phase-control systems. This is due to the fact that the semiconductors are either fully on (saturated) or off, as opposed to the linear supplies, where the semiconductors are biased in the active region. Table I illustrates the relative merits of each design when considering ripple efficiency, bandwidth, physical size, and initial cost. The configuration that is most suited to your application will depend on factors such as those. Contact your power-supply manufacturer for additional information. 741

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