AUXILIARY_SUPPLY

Published:2009/7/10 21:54:00 Author:May | From:SeekIC

Many power-factor-correction circuits use a boost converter to generate a regulated dc output voltage from the ac line input while forcing the load to draw sinusoidal current, which maximizes the power factor. This circuit's full-wave rectifter 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 voldage 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 wind-ing's number of tums. Figure 63-4(b)'s rectifter develops two separate, but individually unregulated volt-ages, across capacitors C1 and C2. Each of these voltages varies in amplitude at twice the ac-line frequency. When switch Q1 is on, the boost inductor connects directly across the input supply, and a volt-age proportional to the instantaneous input voltage develops across capacitor C1. Once the switch tums off, the inductor voltage reverses and clamps to a voltage equal to VOUT -VIN. During this interval, a voltage proportional to VOUT -VIN develops across C2. The sum of these two capac-itor voltages produces a regulated auxiliary voltage that is proportional to VOUT. The voltage across the output capacitor equals VIN+ (VOUT- VIN), which cancels the input-line variations.



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