Synchronous Rectifier NSN Detection in Primary-Side AC-DC Converters
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Solution Overview
Problem
AC-DC converters face issues with false negative sense (NSN) detection and failure to detect real NSN events, particularly at high and low input line voltages, leading to efficiency loss and potential transistor breakdown due to cross-conduction.
Innovation Solution
A method for sensing a negative voltage threshold on the drain node of a synchronous rectifier in the secondary-side of the AC-DC converter, using a SR-controller to integrate voltage and time, compare with a reference voltage, and combine signals to generate a real NSN detect signal, independent of primary-side control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If primary-side control is used to simplify the converter structure, then device complexity is reduced, but measurement precision of NSN detection deteriorates leading to false detections
Solution Approach 1:
The patent introduces an intermediary NSN detection circuit on the primary side that mediates between the primary-side control and secondary-side synchronous rectifier. This circuit includes a comparator that compares the primary-side voltage with a reference voltage to generate an accurate NSN detection signal, resolving the measurement precision issue while maintaining primary-side control simplicity.
Solution Approach 2:
The patent replaces the traditional secondary-side voltage sensing method with an electrical field-based primary-side voltage sensing approach. By using a comparator circuit to electrically compare voltages and generate detection signals, it achieves precise NSN detection without the complexity of secondary-side sensing circuits.
2Device complexity
If traditional NSN detection method is used, then device complexity is low, but reliability of power transfer deteriorates due to false detections and missed detections
Solution Approach 1:
The patent implements a feedback mechanism where the NSN detection circuit continuously monitors the primary-side voltage and provides real-time detection signals to the controller. The controller uses this feedback to accurately determine when to activate the synchronous rectifier, preventing both false detections and missed detections, thereby improving power transfer reliability.
Solution Approach 2:
The patent performs preliminary voltage comparison and detection before the actual power transfer occurs. The NSN detection circuit prepares the detection signal in advance based on primary-side voltage conditions, allowing the controller to proactively prepare for synchronous rectifier activation and avoid unreliable reactive detection during power transfer.
3Ease of operation
If simple voltage threshold detection is used, then ease of operation is high, but measurement precision deteriorates at varying input line voltages
Solution Approach 1:
The patent employs a dynamic detection approach where the comparator continuously compares the primary-side voltage with a reference voltage. This dynamic comparison adapts to varying input line voltages automatically, maintaining measurement precision across different operating conditions without requiring manual threshold adjustments.
Solution Approach 2:
The patent changes the detection parameter from a fixed secondary-side voltage threshold to a variable primary-side voltage comparison. By monitoring changes in primary-side voltage relative to a reference, the system maintains accurate NSN detection across varying input line voltages while keeping the operation simple through automatic adaptation.
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
Improves efficiency by accurately detecting real NSN events and preventing false detections, especially at higher output powers.
Implementation Method 1
a transformer 102 having a primary-side electrically coupled to an AC input through an electromagnetic interference filter (EMI) filter 104 and a rectifying circuit
Implementation Method 2
a primary controller 112 controls a power switch (PS), such as PS field effect transistor (PS_FET) 114 to switch power to the primary-side on or off
Data Source
AI summary
An AC-DC converter and method of operating the same is provided to sense negative voltage (NSN) on a synchronous rectifier (SR_DRAIN) on a secondary-side of the converter. The SR_DRAIN voltage is sensed and a first integration signal (volt-sec) generated based on a time and voltage for which the SR_DRAIN voltage is greater than a bus voltage (VBUS_IN) output from the secondary. When Volt-sec is greater than a reference voltage a volt-sec based NSN detect signal is generated. A second integration signal (integ_resetb) is generated based on the time for which the SR_DRAIN voltage is greater than VBUS_IN. A pulse width of integ_resetb is determined using a counter, and, when it exceeds a reference by a predetermined percentage, a counter-expiry signal is generated. The volt-sec based NSN detect signal and the counter-expiry signal are logically combined to generate a real NSN detect signal when one or both are present.