Quasi-Resonant Flyback Converter Shaping Circuit for Low THD
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Solution Overview
Problem
High-power-factor quasi-resonant flyback converters fail to achieve low total harmonic distortion (THD) and unity power factor due to distorted sinusoidal input current, which is exacerbated by the need for high-voltage-rated and costly MOSFETs to manage reflected voltages, making it challenging to meet stringent market specifications.
Innovation Solution
A novel control method for Hi-PF QR flyback converters that generates a sinusoidal input current by modifying the current reference signal to cancel out the term introduced by the chopping of the primary current, using a multiplier and current generator to produce a sinusoidal voltage reference, thereby achieving sinusoidal input current and low THD.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If peak current mode control is used with valley-switching in a quasi-resonant flyback converter, then the converter achieves high power factor and simple control, but the input current becomes distorted and THD increases
Solution Approach 1:
The patent introduces an intermediary shaping circuit between the rectifier output and the flyback converter input. This circuit includes a capacitor connected in parallel with the rectifier output and a resistor connected in series with the capacitor, forming an RC network that shapes the input voltage waveform. The shaping circuit acts as a mediator that modifies the voltage applied to the flyback converter, enabling sinusoidal input current without changing the basic peak current mode control architecture.
Solution Approach 2:
The patent changes the operating parameters of the flyback converter by introducing the shaping circuit that modifies the input voltage waveform characteristics. The RC network parameters (resistor and capacitor values) are specifically selected to achieve the desired voltage shaping effect, transforming the rectified sinusoidal voltage into a waveform that produces sinusoidal current when processed by the valley-switching flyback converter.
2Reliability
If high-voltage-rated MOSFETs are used to manage reflected voltages, then the converter can handle voltage spikes, but the cost and device complexity increase
Solution Approach 1:
The patent applies beforehand cushioning by introducing a clamp circuit with a capacitor and resistor that absorbs and dissipates voltage spikes before they can damage the MOSFET. The clamp circuit is positioned between the rectifier output and the flyback converter, providing a protective path for voltage transients. This cushioning mechanism allows the use of lower-voltage-rated, less expensive MOSFETs while maintaining reliable operation under voltage stress conditions.
3Productivity
If the primary current is chopped during OFF-time in a flyback converter, then the converter operates in discontinuous mode, but the average primary current becomes lower and input current distortion increases
Solution Approach 1:
The shaping circuit acts as an intermediary that compensates for the current chopping effect. By modifying the input voltage waveform through the RC network, the circuit ensures that when the chopped primary current is averaged over the switching cycle, the result is a sinusoidal waveform that tracks the input voltage, thereby maintaining high power factor despite discontinuous operation.
Data Source
AI summary
The present disclosure is directed to a high power factor quasi resonant converter. The converter converts an AC power line input to a DC output to power a load, generally a string of LEDs. The power input is fed into a transformer being controlled by a power switch. The power switch is driven by a controller having a shaping circuit. The shaping circuit uses a current generator, switched resistor and capacitor to produce a sinusoidal reference voltage signal. The controller drives the power switch based on the voltage reference signal, resulting in a sinusoidal input current in a primary winding of the transformer, resulting in high power factor and low total harmonic distortion for the converter.