Active Power Factor Correction

APFC is a complete electronic system that corrects both the current phase angle φ and the harmonic distortions present in the current waveform by increasing ܭ_௣.

This is usually achieved by the use of a DC-DC converter following the bridge rectifier, and the switching of a reactive component into and out of the circuit. The reactive component switched is generally inductive as capacitors exhibit ringing of the voltage waveform upon switching; this requires the reactance of an inductive biased circuit to be passively overbalanced to capacitive.

One of the key drawbacks to an active power factor correction approach is the generation of high-frequency EMI; this requires passive filtering before the bridge rectifier.

Figure 4-14 shows illustrates the basic accepted topology of a power supply incorporating active power factor correction:

Figure 4-14: Functional Diagram Power supply Including Active Power Factor Correction

The presence of the PFC stage reduces the harmonics present in the current waveform, hence greatly improving the purity factor ܭ௣Ǥ The switching element employed is usually an inductor, and this can be used to reduce the current phase angle φ if the circuit reactance has been overbalanced to capacitive resulting in excellent power factor correction.

The drawback is that the PFC stage is complex and incorporates high frequency switching which increases generation of EMI, as well as lowering overall efficiency due to increased switching losses; this lowering of efficiency results in greater heat production and therefore the PFC stage tends to consist of bulky components increasing the size, weight, and lowering the reliability of the electronics.

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The DC-DC converter stage can be implemented in three various base configurations, which are as follows:

x Buck x Boost x Buck-Boost

4.4.3.1Buck Converter

The buck converter, which is illustrated in Figure 4-15, is an implementation of the DC-DC Converter stage in a typical APFC:

Figure 4-15: Active Power Factor Correction Buck DC-DC Converter

The buck converter has a step-down conversion, which allows an output voltage lower than that of the input voltage. However, the converter can only operate when the line voltage exceeds the output voltage; this behaviour results in discontinuous conduction of the current within the inductor which results in higher EMI generation.

The step-down conversion of the buck converter is described by the equation:

ܸ_௢௨௧ൌ ܦܸ_௜௡ (4.23)

Where ܸ௢௨௧ is the output voltage, ܸ௜௡is the input voltage, and ܦ is the duty cycle of the transistor; this equation ignores the semi-conductor voltage drops.

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4.4.3.2Boost Converter

The boost converter, which is illustrated in Figure 4-16, is an implementation of the DC-DC Converter stage in a typical active power factor correction circuit:

Figure 4-16: Active Power Factor Correction Boost DC-DC Converter

The boost converter has a step-up conversion relationship with the input voltage, and therefore the output voltage always exceeds the line voltage; this results in continuous current flow through the inductor as operation throughout the entire cycle of the line voltage is possible which reduces EMI generation.

As the boost converter is of a step-up relationship with the input voltage, it is a typical choice in electronics that must operate from a wide range of input voltages; typically 115Vrms or 230Vrms supplies.

The step-up conversion of the boost converter is described by the equation:

ܸ_௢௨௧ ൌ ܸ௜௡

ͳ െ ܦ (4.24)

Where ܸ௢௨௧ is the output voltage, ܸ௜௡is the input voltage, and ܦ is the duty cycle of the transistor; this equation ignores the semi-conductor voltage drops and assumes continuous current flow in the inductor.

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4.4.3.3Buck-Boost Converter

The buck-boost converter, which is illustrated in Figure 4-17, is an implementation of the DC-DC Converter stage in a typical APFC:

Figure 4-17: Active Power Factor Correction Buck-Boost DC-DC Converter

The buck-boost converter has both a step-up and step-down conversion relationship with the line voltage; this allows an output voltage which is higher or lower than the input voltage allowing greater freedom in design. This behaviour allows conversion throughout the entire cycle of the line voltage.

The major drawback is that the output voltage is inverted with respect to the input voltage which results in greater stress for the switching components, and the inductor current is discontinuous which results in a greater generation of EMI. The buck-boost can be implemented in a two-switch design which alleviates the stress of the inverted output voltage, at the cost of increased conduction loss and component cost [49, 50].

The voltage conversion of the buck-boost converter is described by the equation:

ܸ௢௨௧ൌ

ܦ

ͳ െ ܦܸ௜௡ (4.25)

Where ܸ௢௨௧ is the output voltage, ܸ௜௡is the input voltage, and ܦ is the duty cycle of the transistor; this equation ignores the semi-conductor voltage drops and assumes continuous current flow in the inductor.

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