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Auxiliary Supply

2017-08-23 07:10  
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Many power-factor-correction circuits use a boost converter to generate a regulated dc output voltage from the ac line inp
Auxiliary Supply

ut while forcing the load to draw sinusoidal current, which maximizes the power factor. This circuit`s full-wave rectifier the auxiliary winding`s output to completely cancel out line variations and provide a regulated output voltage. The circuit essentially sums the two phases of the boost inductor`s voltage to eliminate the 120-Hz components. The regulated output tracks the power-factor-controlled pre-regulator output voltage and it can be used in the corrected output voltage`s feedback loop. An isolated auxiliary winding consists of the desired number of turns wound on the boost inductor. You can vary the exact value of the auxiliary supply`s output voltage by adjusting or scaling the auxiliary winding`s number of turns. Figure 63-4 (b)`s rectifier develops two separate, but individually unregulated voltages, across capacitors CI and C2. Each of these voltages varies in amplitude at twice the ac-line frequency. When switch Ql is on, the boost inductor connects directly across the input supply, and a voltage proportional to the instantaneous input voltage develops across capacitor CI. Once the switch turns off, the inductor voltage reverses and clamps to a voltage equal to Vout - V^. During this interval, a voltage proportional to V0ut - Vin develops across C2. The sum of these two capacitor voltages produces a regulated auxiliary voltage that is proportional to V0ut?· The voltage across the output capacitor equals V^ ( Vout - ViN ), which cancels the input-line variations.

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