Flyback Converter Synchronous Rectification Shoot-Through Prevention
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Solution Overview
Problem
Flyback power converters face challenges in synchronous rectification, particularly under continuous conduction mode, where shoot-through occurs due to increased voltage and current stress on the synchronous rectification switch, leading to reduced switching efficiency and higher costs.
Innovation Solution
The flyback power converter design includes a transformer, main switch, synchronous rectification switch, control circuit, sampling circuit, and operation circuit, which samples the drain-to-source voltage to generate logic signals for timing and discharging, allowing the synchronous rectification switch to be cut off earlier, preventing shoot-through by operating independently on the secondary side without the need for signal isolating elements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If synchronous rectification control chip is used to detect drain-to-source voltage and control switch timing, then synchronous rectification can be achieved under discontinuous conduction mode, but under continuous conduction mode the switch cannot be cut off early enough causing shoot-through
Solution Approach 1:
The patent introduces an intermediary timing circuit that processes the drain-to-source voltage signal and generates a delayed cut-off control signal. This intermediary circuit acts as a mediator between the voltage detection and the switch control, adding the necessary time delay to prevent shoot-through while maintaining synchronous rectification functionality.
Solution Approach 2:
The patent implements preliminary timing action by using a timing circuit that anticipates the required cut-off time. The circuit performs preliminary processing of the voltage signal to determine the optimal cut-off moment before the actual switch cut-off occurs, ensuring the switch is turned off early enough to prevent shoot-through in continuous conduction mode.
2Productivity
If synchronous rectification switch is kept on longer to maintain current flow, then continuous conduction mode operation is possible, but voltage and current stress on the switch increases
Solution Approach 1:
The patent implements dynamic control of the synchronous rectification switch by adjusting the cut-off timing based on real-time voltage detection. The timing circuit dynamically determines the optimal cut-off moment, allowing the switch to remain on long enough for continuous conduction mode operation while minimizing voltage and current stress through precise timing control.
Solution Approach 2:
The patent uses feedback from the drain-to-source voltage detection to control the switch cut-off timing. The voltage signal is fed back to the timing circuit, which adjusts the cut-off moment based on the actual voltage conditions, thereby optimizing the balance between maintaining continuous conduction mode operation and reducing voltage and current stress on the switch.
3Reliability
If signal isolating elements like pulse transformers or photocouplers are used for primary-side switching signal input, then galvanic isolation is achieved, but device complexity and cost increase
Solution Approach 1:
The patent implements self-service by using the secondary-side synchronous rectification switch and its inherent drain-to-source voltage to generate the timing control signal. The circuit serves itself by using its own operating parameters (voltage and timing) to control the switch, eliminating the need for external signal isolating elements and primary-side control inputs.
Solution Approach 2:
The patent extracts and eliminates the unnecessary signal isolating elements from the circuit. By realizing that the secondary-side switch and voltage can autonomously generate the required timing control, the patent removes the primary-side switching signal input path and associated isolating components, thereby simplifying the device while maintaining galvanic isolation through the inherent transformer isolation.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This design effectively reduces voltage and current stress on the synchronous rectification switch, enhancing switching efficiency and achieving higher power density with lower costs by eliminating the need for signal isolating components like pulse transformers or photocouplers.
Implementation Method 1
The transformer has a primary side and a secondary side, wherein both ends of a magnetizing inductance are coupled to a winding on the primary side of the transformer
Implementation Method 2
the body diode is switched on to provide a current path for the magnetizing inductance to release energy
Data Source
AI summary
A flyback power converter is disclosed. The flyback power converter includes a voltage transformer, a main switch, a synchronous rectification switch, a synchronous rectification control circuit, a sampling circuit and an operation circuit. A control end of the main switch receives a main switch signal so as to control the main switch. The synchronous rectification control circuit transmits control signal to control end of the synchronous rectification switch according to sensing signal received. The sampling circuit samples the state of the synchronous rectification switch so as to generate first logic signal and second logic signal. The operation circuit executes timing for charging/discharging according to the first and the second logic-signal, so as to output switch cut-off pulse signal to a voltage-dividing circuit. If voltage of the sensing signal is lower than predetermined threshold voltage, the synchronous rectification switch enters into cut-off state according to the control signal.