Hi-PF QR Flyback Controller Sinusoidal Current Shaping
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Solution Overview
Problem
High-Power-Factor (Hi-PF) quasi-resonant (QR) flyback converters face inherent distortion in input current, leading to non-sinusoidal current draw, which limits Total Harmonic Distortion (THD) and power factor, especially at high line conditions, requiring high voltage-rated and costly MOSFETs to meet stringent regulations.
Innovation Solution
A control method that modifies the current sense signal to cancel out the term TON/T(θ) by incorporating a term proportional to T(θ)/TON, ensuring the average primary current is sinusoidal, achieved through a novel subtraction circuit and driver circuit configuration in the controller.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If conventional peak current mode control is used in Hi-PF QR flyback converters, then the converter operates with valley-switching and reduced turn-on losses, but the input current becomes distorted and non-sinusoidal, leading to high THD and poor power factor
Solution Approach 1:
The patent implements a feedback mechanism where the controller measures the actual primary current and compares it with the reference current waveform. Based on this comparison, the controller adjusts the duty cycle in real-time to correct deviations from the ideal sinusoidal current profile, thereby reducing THD while maintaining valley-switching operation
Solution Approach 2:
The patent dynamically adjusts the duty cycle based on the instantaneous operating conditions, including line voltage variations and load changes. This dynamic control allows the system to maintain optimal valley-switching operation across different operating points while simultaneously shaping the input current to be sinusoidal, resolving the contradiction between loss reduction and current quality
2Reliability
If high voltage-rated MOSFETs are used to meet stringent regulations at high line conditions, then the converter can operate safely with higher voltage margins, but the cost and device complexity increase significantly
Solution Approach 1:
The patent changes the operating parameters by implementing precise duty cycle control that optimizes the voltage stress on the MOSFET. By dynamically adjusting the duty cycle based on instantaneous line voltage and load conditions, the system maintains adequate voltage margins for reliability while allowing the use of lower voltage-rated, more cost-effective MOSFETs
Solution Approach 2:
The patent performs preliminary voltage stress analysis and pre-calculates optimal duty cycle values for different operating conditions. This preliminary action allows the system to operate with precise voltage margins, ensuring safety and reliability without requiring excessive voltage headroom that would necessitate expensive high-voltage MOSFETs
3Object-generated harmful factors
If the duty cycle is increased to improve power factor correction, then the input current waveform improves, but the transformer core may saturate and the converter loses QR operation benefits
Solution Approach 1:
The patent dynamically adjusts the duty cycle within a precisely calculated optimal range that maintains QR operation. The control system continuously monitors operating conditions and modulates the duty cycle to improve power factor while staying below the saturation threshold, thus simultaneously achieving good power factor and preventing transformer saturation
Solution Approach 2:
The patent implements feedback control that monitors the transformer flux state and adjusts the duty cycle accordingly. When approaching saturation conditions, the feedback mechanism reduces the duty cycle to prevent saturation, while still maintaining sufficient power factor correction performance
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 enables Hi-PF QR flyback converters to draw a sinusoidal current without harmonic distortion, similar to boost converters, reducing THD and power factor issues, while maintaining the benefits of QR operation such as lower EMI emissions and safer operation.
Implementation Method 1
a transformer 26. The transformer 26 also has an auxiliary winding Laux, and a secondary winding Ls
Implementation Method 2
This allows the turn-on to occur on the valley of the drain voltage ringing that follows the demagnetization, therefore reducing turn-on losses
Data Source
AI summary
The present disclosure is directed to a switching power converter having a power transistor controlled by a controller. The controller includes a multiplier that produces a voltage reference signal. A subtraction circuit subtracts a capacitor signal, which is based on the voltage reference signal, from a sensing signal that is representative of the current flowing through the power transistor. A comparator compares the voltage reference signal to the output of the subtraction circuit, and a driving circuit drives the power transistor based on the comparison resulting in a high power factor and low total harmonic distortion for the converter.