Auxiliary Supply

Posted on May 3, 2012

Many power-factor-correction circuits use a boost converter to generate a regulated dc output voltage from the ac line inp

Auxiliary Supply
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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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