Flyback Converter Valley-Sensing Control for Low-Loss Turn-On
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Solution Overview
Problem
Quasi-resonant flyback converters face challenges in reducing turn-on losses in switching transistors, particularly when operating in discontinuous conduction mode or boundary conduction mode, due to the lack of precise control over switching times relative to the oscillatory voltage valleys.
Innovation Solution
A control circuit that includes a valley sensing circuit to detect valleys in the oscillating voltage on the switch node, ensuring the switching transistor is turned on only at a valley point, thereby minimizing switching losses. This circuit utilizes a combination of current mirrors, low-pass filters, comparators, and timers to infer the secondary coil current crossing zero and synchronize switching with the valley of the switch node voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If valley sensing circuit and synchronized control are implemented, then switching losses are reduced, but device complexity increases
Solution Approach 1:
The valley sensing circuit detects and marks valley points in advance during the switching cycle. The control circuit uses these pre-detected valley signals to synchronize the turn-on of switching transistors, ensuring switching occurs at optimal moments before the harmful resonant current reaches its peak, thereby reducing switching losses while maintaining controlled complexity through advance preparation
Solution Approach 2:
A dedicated valley sensing circuit acts as an intermediary between the oscillating voltage and the control logic. This intermediary component processes the complex oscillatory waveform to extract valley point information and provides simplified control signals to the switching transistor gates, reducing switching losses while containing complexity within the intermediary sensing circuit rather than the overall control system
2Productivity
If switching transistor turns on at valley point, then efficiency is improved, but control precision requirements increase
Solution Approach 1:
The control circuit continuously monitors the switch node voltage to detect valley points in real-time and uses this feedback information to adjust the timing of switching transistor turn-on. This closed-loop feedback mechanism ensures high precision valley detection by comparing the actual voltage waveform against reference levels and dynamically adjusting control signals to achieve optimal switching timing, thereby improving efficiency while managing precision requirements through active correction
Solution Approach 2:
The patent replaces complex analog timing circuits with a microcontroller-based digital control system that uses software algorithms to detect valley points. The microcontroller samples the voltage waveform digitally and uses computational methods to identify valley moments with high precision, substituting mechanical/analog timing mechanisms with programmable logic that achieves precise valley detection while improving converter efficiency
Data Source
AI summary
A converter control circuit includes a terminal to be coupled to a switch node. The control circuit includes a valley sensing circuit coupled to the terminal and detects valleys in an oscillating voltage on the switch node. The valley sensing circuit has a first output and asserts a first control signal on the first output indicative of occurrence of a valley. A logic gate has a second output and asserts a second control signal on the second output to turn on a switching transistor. A switch-on control circuit has a first input and a second input. The first input couples to the first output. The second input couples to the second output. The switch-on control circuit asserts a third control signal to turn on the switching transistor responsive to the second control signal indicating that the switching transistor is to be on while the first control indicates a valley.