Flyback Input AC Voltage Sensing for Primary-High Topology
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Solution Overview
Problem
In conventional flyback circuit topologies, particularly the primary-high configuration, sensing the input AC voltage is challenging when the primary switch is turned off, hindering functions like Brown-in/out and X-cap discharge due to the switch's position between the rectifier bridge and primary winding.
Innovation Solution
A voltage sensing circuit and method that involve a processing circuit and sensing resistors to subtract and sample differential voltages across the primary winding, allowing for accurate sensing of the input AC voltage even when the primary switch is off, by coupling the processing circuit to the AC source through rectifying circuits and using a control ground reference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the primary switch is positioned between the rectifier bridge and primary winding in a primary-high flyback topology, then EMI performance is improved, but the ability to sense input AC voltage when the primary switch is turned off is lost
Solution Approach 1:
The patent introduces an intermediary sensing circuit that includes a sensing resistor connected to the control ground reference and a processing circuit that subtracts voltages to obtain the AC voltage sensing signal. This intermediary mechanism allows voltage sensing without requiring direct access to the input AC voltage terminals, thus resolving the contradiction between EMI performance and voltage sensing capability.
Solution Approach 2:
The patent segments the voltage sensing function into separate components: a sensing resistor for voltage division and a processing circuit for differential voltage subtraction. This segmentation allows the sensing circuit to operate independently from the main power switching circuit, enabling AC voltage sensing while maintaining the primary-high topology's EMI performance.
2Device complexity
If a conventional voltage sensing method is used in primary-high flyback circuit, then the circuit structure remains simple, but accurate sensing of input AC voltage cannot be achieved when primary switch is off
Solution Approach 1:
The patent employs an intermediary processing circuit that subtracts the voltage at the second terminal from the voltage at the first terminal to obtain the differential voltage representing the AC voltage sensing signal. This intermediary processing enables accurate voltage sensing without complex circuit reconfiguration, maintaining relative simplicity while achieving measurement precision.
3Reliability
If the primary switch is turned off for Brown-in/out and X-cap discharge functions, then safety functions are enabled, but input AC voltage sensing becomes impossible
Solution Approach 1:
The patent implements preliminary action by continuously monitoring the differential voltage across the sensing resistor even when the primary switch is off. The processing circuit is designed to capture and hold the AC voltage sensing signal during the off-state, enabling safety functions like Brown-in/out and X-cap discharge to operate reliably while maintaining voltage sensing capability throughout the switching cycle.
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
Enables effective sensing of the input AC voltage in primary-high flyback circuits, facilitating functions such as Brown-in/out and X-cap discharge, with the sensed signal accurately reflecting the input AC voltage waveform and amplitude.
Implementation Method 1
a first rectifying circuit, having a first input terminal, a second input terminal, a first output terminal and a second output terminal, wherein the first input terminal and the second input terminal are coupled to an AC source
Implementation Method 2
the processing circuit subtracts a voltage at the second terminal of the processing circuit from a voltage at the first terminal of the processing circuit, to obtain a differential voltage
Implementation Method 3
a primary winding, coupled in series between two output terminals of the first rectifying circuit... a secondary winding, magnetically coupled to the primary winding
Data Source
AI summary
Input AC voltage sensing for a flyback circuit. The flyback circuit is configured as “primary-high”, namely a primary switch of the flyback circuit is positioned “high” to receive an input AC voltage through a first rectifying circuit. A primary winding of the flyback circuit is coupled to a primary ground reference. A voltage sensing circuit has a processing circuit and a first sensing resistor. The processing circuit has a first terminal coupled to the input AC source through a second rectifying circuit, a second terminal coupled to the primary ground reference and an output terminal. The processing circuit subtracts a voltage at the second terminal from a voltage at the first terminal to obtain a differential voltage, which is then sampled and held as an input AC voltage sensing signal.