Sub-harmonic Elimination in Flyback Power Converters
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Solution Overview
Problem
Flyback power converters experience sub-harmonic issues due to acute variations in the on-time of the power switch caused by reduced capacitor sizes, leading to startup failures and overcurrent protection discrepancies at varying input voltages.
Innovation Solution
A circuit and method that utilize a current limit signal controller and selector to adjust the final current limit signal based on the duty ratio of the power switch, switching between different current limit signals to stabilize the maximum current value and prevent acute duty ratio changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If the capacitor Cbulk is downsized to lower costs, then the product cost is reduced, but the hold up time is shortened and voltage Vbulk varies significantly causing sub-harmonic problems
Solution Approach 1:
The patent applies dynamics by making the current limit signal adaptive rather than fixed. The circuit dynamically adjusts the current limit signal based on the duty ratio detected from the power switch, allowing the system to respond to changing operating conditions and prevent sub-harmonic oscillations while using a smaller capacitor.
Solution Approach 2:
The patent changes the parameter of the current limit signal based on the duty ratio. When the duty ratio exceeds a threshold, the circuit switches to a different current limit signal level, effectively changing the operating parameters to maintain stability and eliminate sub-harmonic problems.
2Use of energy by moving object
If the power switch M1 has a longer on time to obtain adequate energy at low input voltage, then the energy transfer is improved, but the off time is shortened causing incomplete energy release and acute duty ratio variation
Solution Approach 1:
The patent uses feedback by detecting the duty ratio of the power switch and using this information to adjust the current limit signal. The detection circuit monitors the actual duty ratio and feeds this information back to the control circuit, which then modifies the current limit signal to prevent acute duty ratio variations and ensure complete energy release.
Solution Approach 2:
The circuit performs self-service by automatically adjusting its own control parameters based on its operating state. The control circuit monitors the duty ratio and autonomously modifies the current limit signal without external intervention, enabling the system to self-correct sub-harmonic tendencies.
Data Source
AI summary
A circuit and method are provided for a power converter to select one from a plurality of current limit signals as a final current limit signal according to the present duty ratio of a power switch for the pulse width modulation of the next cycle, so that the duty ratio of the power switch in the next cycle is prevented from acute variation to eliminate sub-harmonic which otherwise may happen.