Lec 09: Single-phase Boost PFC Converter - I

Introduction to Single-Phase Boost Power Factor Correction Converter

Overview of Previous Lecture

• The lecture begins with a welcome and an introduction to the topic of single-phase boost power factor correction converters, building on concepts from the previous class.

• In the last session, the focus was on single-phase full-bridge diode rectifiers, which are uncontrolled rectifiers, and their operation with capacitive filters.

Key Understandings from Previous Discussions

• It was established that unity power factor current can be drawn from a source if a resistive load is used at the output; however, this does not yield a constant DC voltage.

• Adding a capacitive filter allows for varying output voltage but results in peaky currents that deviate from sinusoidal characteristics and require oversized components.

Power Factor Correction (PFC) Converters

Concept of PFC Converters

• The discussion transitions to using different types of power converters to achieve unity power factor current draw, introducing the concept of PFC converters.

• A typical configuration includes a full-bridge rectifier followed by various types of converters like boost or flyback converters to maintain both unity power factor and constant output voltage.

Functionality of Boost PFC Converter

Structure and Components

• The boost PFC converter consists of a full bridge diode rectifier at its front end followed by a boost converter with inductors and capacitors.

• The average current through the load is defined as IL while some current flows into Ic; understanding these currents is crucial for analyzing converter performance.

Operation Mechanism

• A generic switch (like MOSFET or IGBT) can replace diodes in the circuit design for flexibility in controlling current flow within the converter.

Voltage Waveforms in Boost Converter

Input Voltage Characteristics

• During positive half cycles, specific diodes become forward biased resulting in an output voltage that mirrors input AC characteristics.

Output Voltage Analysis

• The output voltage waveform reflects modulus values based on input AC signals during both positive and negative cycles.

Control Mechanisms for Varying Output Voltage

Controlling Vconverter Voltage

• The Vconverter voltage can be varied by adjusting turn-on times for switches S1 and S2 within the half bridge configuration.

Achieving Desired Current Patterns

• By manipulating switch timings, it’s possible to ensure that average inductor currents follow patterns similar to those dictated by vd voltages.

Ensuring Unity Power Factor Operation

Synchronization Between Currents

• To achieve unity power factor operation, it’s essential that average currents align with input AC waveforms without phase lag or lead between them.

Considerations for Switching Frequency

Importance of Switching Frequency

• Emphasis is placed on ensuring that switching frequency (fsw), must be significantly greater than supply frequency (fs), allowing effective control over voltage variations during switching periods.

Implications of Constant Voltage Assumption

Benefits of Constant Modulus Vs

Assuming |Vs| remains constant during switching periods simplifies calculations related to maintaining desired average current levels across switching cycles.

Duty Ratio Dynamics During Operation

Positive Half Cycle Operations

• In positive half cycle operations where D1 and D4 are forward biased, duty ratio D indicates how long switch S1 remains active within each cycle.