PFC Flyback SMPS THD Reduction via Voltage-Dependent Non-Linear Resistance
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Solution Overview
Problem
Power factor corrected flyback switch mode power supplies experience increased total harmonic distortion (THD) due to distorted AC input current waveforms, which degrade power factor and are not effectively addressed by existing control circuits.
Innovation Solution
An additional current injection is implemented using a voltage-dependent non-linear resistance coupled with the current injection resistor in the PFC controller, injecting more current during peak AC line voltage periods to shape the input current waveform closer to a sinusoidal pattern, thereby reducing THD.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a PFC flyback SMPS uses a standard current injection method with a fixed resistor, then the circuit is simple and low cost, but the AC input current waveform becomes distorted (flattened) at peak voltage, increasing THD
Solution Approach 1:
The patent changes the resistance parameter dynamically by introducing a voltage-dependent non-linear resistance (zener diode) in parallel with the current injection resistor. This causes the injection current to vary with input voltage, reducing the flattened waveform effect at peak voltage and lowering THD from 15% to 9%
Solution Approach 2:
The zener diode acts as an intermediary element that modifies the current injection characteristic. It introduces a voltage-dependent conductance path that compensates for the waveform distortion without requiring complex control circuitry
2Object-generated harmful factors
If a PFC flyback SMPS uses a standard current injection method, then the power factor is degraded, but adding complex control circuits to improve waveform shaping increases device complexity and cost
Solution Approach 1:
The zener diode creates a self-regulating current injection system where the injection current automatically adjusts based on input voltage level. The circuit uses the input voltage itself to control the injection current, eliminating the need for external control signals or complex feedback circuits
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach decreases THD from approximately 15% to 9% and improves the power factor by making the input current waveform more sinusoidal, effectively addressing the distortion issues in flyback SMPS.
Implementation Method 1
A voltage dependent non-linear resistance is coupled in parallel with the normal current injection resistor that supplies current to the PFC flyback controller multiplier input to inject appropriate values of additional current into the PFC flyback controller based upon the AC input voltage value(s)
Data Source
AI summary
Total harmonic distortion (THD) at the AC line input of a power factor corrected (PFC) flyback switch mode power supply (SMPS) is reduced by adding additional current injection to the current programming signal of a PFC controller during the period of the line cycle where the AC input current wave shape is most flattened, i.e., around the peak portion of the AC line voltage. The input current drawn by the PFC flyback SMPS will then more closely resemble a desired sinusoidal waveform. A voltage dependent non-linear resistance is coupled in parallel with the normal current injection resistor that supplies current to the PFC flyback controller multiplier input to inject appropriate values of additional current into the PFC flyback controller based upon the AC input voltage value(s) so as to better shape the AC input current to the desired sinusoidal waveform.