Flyback Converter Primary-Side Feedback Control Circuit
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Solution Overview
Problem
Flyback converters with primary-side feedback control face challenges in accurately sampling voltages across auxiliary windings when input voltages are at valleys, leading to errors in output voltage detection and high deviations in secondary-side current measurements due to shortened demagnetization times.
Innovation Solution
A control circuit that includes current and voltage sampling circuits, with a control signal generating circuit that adjusts the switching mode based on current thresholds, allowing for variable blanking times and adaptive control between constant on time and peak current modes to maintain accurate sampling and prevent errors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the converter operates in constant on time mode to achieve higher power factor, then the input current follows the input voltage well, but when input voltage is at valley the current peak becomes very low leading to shortened demagnetization time and high deviation in output voltage detection
Solution Approach 1:
The patent applies dynamics by making the on-time period dynamic rather than fixed. The control circuit adjusts the on-time period based on the detected secondary-side current characteristics (valley detection or zero-crossing detection). When the secondary-side current valley occurs before the predetermined time after the power switch is turned off, the on-time period is extended to ensure adequate demagnetization time, thereby maintaining accurate voltage sampling and output voltage detection accuracy across varying input voltage conditions.
2Measurement precision
If a fixed blanking time is set to avoid sampling during voltage oscillation, then sampling accuracy is improved during normal operation, but when input voltage is at valley the demagnetization time is insufficient causing error voltage sampling
Solution Approach 1:
The patent implements feedback by detecting the valley or zero-crossing point of the secondary-side current and using this information to adjust the on-time period. The control circuit continuously monitors the secondary-side current characteristics and provides feedback to the on-time period determination unit, which adjusts the primary-side current on-time period accordingly. This feedback mechanism ensures that the demagnetization time is sufficient even when input voltage is at valley, preventing error voltage sampling while maintaining accurate detection.
3Productivity
If the on time period is fixed to maintain constant on time mode operation, then power factor is improved, but the peak value of secondary-side current decreases when input voltage is at valley resulting in shortened demagnetization time
Solution Approach 1:
The patent applies parameter changes by dynamically adjusting the on-time period parameter based on operating conditions. Instead of maintaining a fixed on-time period, the control circuit modifies this parameter in response to detected secondary-side current characteristics. When the valley or zero-crossing point indicates insufficient demagnetization time, the on-time period is extended. This parameter adjustment maintains adequate demagnetization time while preserving high power factor operation through continuous adaptation to input voltage variations.
Data Source
AI summary
The disclosure relates to a control circuit, a control method and a flyback converter of primary-side feedback control including the control circuit. When an input voltage is greater than a predetermined threshold, a peak value of an input current of the flyback converter is controlled to vary with the input voltage by the switching control signal. When the input voltage is less than the predetermined threshold, the peak value of the input current is controlled to be increased by the switching control signal to make demagnetization time of a secondary winding of the flyback converter be greater than a minimum time. Thus, the peak value of the primary-side current may not become too small because of a decreased input voltage, further avoiding occurrence of an error sampling after a blanking time due to excessive variations in demagnetization time.