Auxiliary Supply

Description: Many power factor correction circuits utilize a boost converter to produce a regulated DC output voltage from the AC line input while ensuring that the load draws a sinusoidal current, thus maximizing the power factor. This circuit employs a full-wave rectifier that uses the output from an auxiliary winding to completely eliminate line variations and provide a stable output voltage. Essentially, the circuit sums the voltages from the two phases of the boost inductor to remove the 120 Hz components. The regulated output follows the voltage of the power factor-controlled pre-regulator and can be integrated into the feedback loop of the corrected output voltage. An isolated auxiliary winding consists of a specified number of turns wound on the boost inductor, and the output voltage of the auxiliary supply can be adjusted by modifying the number of turns on the auxiliary winding. The rectifier in Figure 63-4(b) generates two separate, unregulated voltages across capacitors C1 and C2, with each voltage varying in amplitude at twice the AC line frequency. When switch Q1 is closed, the boost inductor connects directly to the input supply, causing a voltage proportional to the instantaneous input voltage to develop across capacitor C1. When the switch opens, the inductor voltage reverses, clamping to a voltage equal to Vout - Vin. During this time, a voltage proportional to Vout - Vin develops across C2. The combined voltages from these two capacitors yield a regulated auxiliary voltage that is proportional to Vout. The voltage across the output capacitor is equal to Vin + (Vout - Vin), effectively canceling out the input line variations.

In power factor correction applications, the boost converter circuit plays a critical role in converting the AC input into a stable DC output while maintaining a high power factor. The circuit architecture typically includes a boost inductor, a full-wave rectifier, and an auxiliary winding. The boost inductor is crucial as it stores energy during the on-phase of the switch (Q1) and releases it during the off-phase, thereby regulating the output voltage.

The full-wave rectifier, which is connected to the auxiliary winding, serves to rectify the AC input to DC while also providing feedback to stabilize the output voltage. The auxiliary winding, which is wound around the boost inductor, generates a voltage that is used to cancel out fluctuations in the input line voltage, ensuring that the output remains stable regardless of input variations.

The feedback mechanism is essential for maintaining the output voltage at the desired level. By tracking the pre-regulator output voltage, the circuit can dynamically adjust to changes in load and input conditions, thus optimizing performance. The design allows for flexibility in the tuning of the auxiliary voltage by varying the number of turns in the auxiliary winding, which can be critical for applications demanding specific voltage levels.

Overall, this power factor correction circuit effectively combines energy storage, voltage regulation, and feedback control to achieve optimal performance, making it suitable for various applications where power quality is essential. Many power-factor-correction circuits use a boost converter to generate a regulated dc output voltage from the ac line inp 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.