Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Fig. 3. Common pulse waveforms. pulse reverse, and pulse-on-pulse reverse. Fig. 3 illustrates a few of the many dif- ferent pulse waveforms available. The pulsing profile you use will be determined by the type of plating finish desired, the makeup of the plating bath, and the type of power supply available. There are three basic types of power supply technologies employed to achieve pulsed outputs. The most common design consists of a standard SCR phase-con- trolled 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 tran- sistors, 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 phase-con- trol design, due to the fact that the rectification section always provides full pow- er 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 out- put 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 637

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