Flyback Converter Auxiliary Switch On-Time Control
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Solution Overview
Problem
Soft switching flyback converters face efficiency degradation and loss of zero voltage switching (ZVS) in wide range output applications due to excessive circulation current, particularly in variable output scenarios like USB PD adapters, where fixed on-time control methods fail to maintain optimal switching conditions across varying output voltages.
Innovation Solution
A control device and method that adjusts the on-time threshold of an auxiliary switch based on the output voltage and a reference value, ensuring zero voltage switching of the primary-side switch by controlling the on-time of the auxiliary switch, thereby maintaining efficient operation across different output voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If fixed on-time control method is used for auxiliary switch, then simple control structure is maintained, but zero voltage switching is lost and efficiency degrades in wide range output applications
Solution Approach 1:
The patent applies dynamics principle by making the on-time threshold of the auxiliary switch variable rather than fixed. The control device dynamically adjusts the on-time threshold according to the output voltage to ensure zero voltage switching is achieved across wide range output applications. This resolves the contradiction by enabling adaptability through dynamic parameter adjustment while maintaining a relatively simple control structure.
Solution Approach 2:
The patent changes the parameter of on-time threshold from a fixed value to a variable that depends on output voltage. The control device calculates and adjusts the on-time threshold based on the detected output voltage, allowing the system to adapt to different output conditions. This parameter change enables the system to maintain zero voltage switching across wide range outputs without requiring complex control architecture.
2Reliability
If on-time of auxiliary switch is increased to ensure ZVS, then zero voltage switching is achieved, but excessive circulation current occurs and efficiency degrades
Solution Approach 1:
The patent adjusts the on-time threshold parameter of the auxiliary switch based on the output voltage. By calculating the appropriate on-time threshold according to the detected output voltage, the system ensures zero voltage switching is achieved without excessive circulation current. This dynamic parameter adjustment resolves the contradiction by optimizing the on-time to be sufficient for ZVS but not excessive, thereby minimizing energy loss.
Solution Approach 2:
The patent implements feedback control by detecting the output voltage and using it to adjust the on-time threshold of the auxiliary switch. The control device continuously monitors the output voltage and modifies the on-time threshold accordingly, creating a closed-loop control system. This feedback mechanism ensures that the on-time is precisely controlled to achieve ZVS while avoiding excessive circulation current and associated energy losses.
3Power
If switching frequency is increased to improve power density, then power density increases, but switching loss becomes dominant and efficiency decreases
Solution Approach 1:
The patent applies dynamics principle by enabling the flyback converter to operate at higher switching frequencies while maintaining efficiency through dynamic adjustment of the auxiliary switch on-time threshold. By ensuring zero voltage switching across the operating range, the system can increase switching frequency to improve power density without suffering from excessive switching losses. This resolves the contradiction by making the system adaptable to high-frequency operation.
Solution Approach 2:
The patent converts the potential harm of high switching frequency (increased switching loss) into a benefit by ensuring zero voltage switching is achieved. By properly controlling the auxiliary switch on-time to enable ZVS, the system can operate at high frequencies where the switching loss would normally be problematic, but instead the soft switching condition minimizes the loss. This allows high power density to be achieved without the usual penalty of dominant switching losses.
Data Source
AI summary
A control device, which is applied to a flyback converter including an auxiliary switch, includes: an on-time setter, configured to set an on-time threshold according to a reference value and an output voltage of the flyback converter; and an on-time controller, configured to output a control signal to turn on the auxiliary switch, and turn off the auxiliary switch when on-time of the auxiliary switch reaches the on-time threshold. According to the present disclosure, it is able to achieve zero voltage switching of a primary-side switch of the flyback converter with variable outputs.