PFC Converter Phase Offset Optimization Under Light Load

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Solution Overview

Problem

Current Power Factor Correction (PFC) converter designs fail to maintain optimal power factor under light loading conditions, leading to increased power distribution system root mean square (RMS) current/power loss and power-line voltage distortion due to inherent current distortion and reactive current from EMI filter capacitors.

Innovation Solution

A control system for PFC converters that determines when power output falls below a light loading threshold and iteratively adjusts the phase offset between AC reference voltage and current to optimize the power factor, using analog or digital circuitry to identify the phase offset resulting in the best power factor.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If a PFC Boost converter operates in Continuous Conduction Mode (CCM) under moderate or heavy loading conditions, then the power factor is near unity and EMI compliance requirements are met, but the performance under light loading conditions (typically 15% below maximum rated output power) is insufficient

Engineering Contradiction:

Improvepower factor performanceVSAvoidlight loading condition performance

Core Design Contradiction:

ReliabilityVSAdaptability or versatility

Solution Approach 1:

The patent applies dynamics by enabling the PFC converter to switch between Continuous Conduction Mode (CCM) and Discontinuous Conduction Mode (DCM) based on loading conditions. The converter dynamically adjusts its operating mode to maintain optimal power factor performance across the full load range, particularly improving light loading performance while preserving heavy loading performance.

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The patent changes the operating parameters of the PFC converter by adjusting the switching frequency and duty cycle when transitioning from CCM to DCM under light loading conditions. These parameter changes enable the converter to maintain near-unity power factor even at light loads where traditional CCM operation would result in degraded performance.

Inventive Principle:

Principle #35Parameter changes

2Object-affected harmful factors

If EMI filter capacitors are used to meet Electromagnetic Interference (EMI) compliance requirements, then EMI compliance is achieved, but current distortion and reactive current degrade the power supply power factor under light loading conditions

Engineering Contradiction:

ImproveEMI complianceVSAvoidpower distribution system RMS current/power loss

Core Design Contradiction:

Object-affected harmful factorsVSLoss of energy

Solution Approach 1:

The patent converts the harmful effect of EMI filter capacitors into a beneficial outcome by using Discontinuous Conduction Mode (DCM) operation to counteract their reactive current. The DCM operation creates a current waveform that compensates for the capacitive reactive current, thereby improving power factor despite the presence of EMI filter capacitors.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

Solution Approach 2:

The patent employs feedback control to monitor the power factor and loading conditions, then adjusts the PFC converter operation accordingly. When light loading is detected, the control system switches to DCM and iteratively optimizes the phase offset to achieve the best power factor, providing continuous feedback-based adjustment to counteract EMI filter capacitor effects.

Inventive Principle:

Principle #23Feedback

3Reliability

If the phase offset between AC reference voltage and AC reference current is adjusted to optimize power factor under light loading conditions, then power factor improves, but additional control complexity is introduced

Engineering Contradiction:

Improvepower factorVSAvoidcontrol system complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The patent applies preliminary action by pre-calculating and storing optimal phase offset values for different loading conditions in lookup tables. When light loading is detected, the controller retrieves the pre-computed phase offset from memory, avoiding complex real-time calculations and reducing control system complexity while still achieving optimal power factor.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The control system performs self-service by automatically detecting loading conditions and adjusting the phase offset without external intervention. The system monitors its own operating state and autonomously switches between CCM and DCM while optimizing phase offset, eliminating the need for manual configuration or external control.

Inventive Principle:

Principle #25Self-service

Data Source

PatentUS11522442B1Power factor of AC-DC off-line power supplies under light load condition

Publication Date: 2022.12.06 CISCO TECHNOLOGY INC

AI summary

Embodiments herein describe control circuitry for operating a PFC converter in an AC-DC power supply under light loading conditions. The embodiments herein improve the power factor by identifying an optimized phase offset (γ) between an AC reference voltage and an AC reference current used to control the PFC converter. In one embodiment, the control circuitry iteratively changes the phase offset between the reference voltage and current and measures its impact on the power factor. The control circuitry then selects the phase offset that results in the best power factor to use when operating the PFC converter under light loading conditions.