Flyback Converter CCM Control for Stable Duty Cycle and Current Sensing
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Solution Overview
Problem
Existing flyback converters in continuous current mode face challenges with subharmonic oscillation, limited duty cycle, and inaccurate average output current calculations due to noise and sampling delays, especially when using smaller filter capacitors, which affect power density and stability.
Innovation Solution
A control method that adjusts turn-off time and peak current values to stabilize the entry into continuous current mode, calculates average output current without distinguishing between discontinuous and continuous modes, and uses proportional coefficients to adapt to varying conditions, eliminating the need for slope compensation and complete transformer current signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If a smaller filter capacitor C1 is used, then the volume of the power supply is reduced, but the voltage fluctuation on the filter capacitor increases significantly
Solution Approach 1:
The patent implements dynamic control of the flyback converter by adjusting the duty cycle based on detected voltage fluctuations. The control circuit monitors the filter capacitor voltage and dynamically modifies the switching duty cycle to compensate for voltage drops, enabling the system to maintain stable output even with smaller filter capacitors that would otherwise cause excessive voltage variation.
Solution Approach 2:
The control system automatically detects voltage fluctuations on the filter capacitor and self-adjusts the duty cycle without external intervention. The system monitors its own operating conditions and performs real-time compensation, allowing the power supply to maintain stability autonomously while using reduced-capacitance filter capacitors.
2Stability of the object's composition
If the duty cycle is increased to compensate for voltage fluctuation, then the output voltage stability is improved, but subharmonic oscillation occurs when duty cycle exceeds 50%
Solution Approach 1:
The patent implements a feedback control mechanism that detects voltage fluctuations on the filter capacitor and uses this information to adjust the duty cycle. By monitoring the actual voltage condition and providing feedback to the control circuit, the system can increase duty cycle when needed while detecting the approach to instability and making corrective adjustments, thereby preventing subharmonic oscillation.
Solution Approach 2:
The control system applies partial duty cycle adjustments rather than maximum increases, only compensating for the detected voltage fluctuation magnitude. This partial action approach provides sufficient compensation to maintain output stability while avoiding excessive duty cycle increases that would trigger subharmonic oscillation and system instability.
3Device complexity
If peak current control technique is used, then the implementation is simplified and loop stability is improved, but the duty cycle is limited to within 80% and output voltage cannot be maintained at lower Vbus voltage
Solution Approach 1:
The patent merges peak current control with voltage fluctuation detection and compensation functions into a unified control system. The control circuit simultaneously performs peak current limiting and duty cycle adjustment based on filter capacitor voltage monitoring, combining multiple control objectives into one integrated approach that maintains simplicity while expanding duty cycle capability.
Solution Approach 2:
The control circuit is designed to perform multiple functions: peak current control, voltage fluctuation detection, duty cycle adjustment, and prevention of subharmonic oscillation. This multi-functional universal controller maintains the simplicity of peak current control while adding adaptability to operate across a wider duty cycle range and maintain output voltage stability under varying Vbus conditions.
4Measurement precision
If average output current calculation is performed under CCM, then the current control accuracy is improved, but the calculation is prone to errors from turn-on noise and sampling delay
Solution Approach 1:
The control system performs preliminary detection of voltage fluctuations on the filter capacitor before executing the average current calculation. By detecting the voltage condition first and using this information to guide the subsequent current sampling and calculation timing, the system prepares the measurement process in advance, reducing the impact of turn-on noise and sampling delays on the final current measurement accuracy.
Data Source
AI summary
Disclosed is a control method for a flyback converter in continuous current mode (CCM). When the flyback is operated in critical current mode, a current peak value is recorded as ipk1 and corresponding turn-off time as Toff1, which is recorded as a first period; if a current peak value ipk of a certain period is not higher than ipk1, the switch is turned on after its original logic, which is recorded as a second period; if ipk is higher than ipk1 with difference Δi, the turn-off time is controlled to Toff1−k·Δi, thereby preventing the current from returning to zero at the end, such that entering CCM, which is recorded as a third period; and then, the flyback is switched between DCM and CCM based on ipk. The present invention can stably control and regulate a duty cycle of the flyback entering CCM, and the calculation of the average output current.