Flyback Power Supply Load Detection Circuit
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Solution Overview
Problem
In flyback system switching power source apparatuses, accurately judging the output load on the secondary side of a transformer without adding complex circuit configurations or incurring significant loss is challenging, particularly due to issues with synchronous rectifying element control and ringing effects.
Innovation Solution
A flyback system switching power source apparatus with a load detection circuit that compares output voltage with a voltage at a node between the synchronous rectifying element and the transformer, using voltage dividing circuits and an analog comparator to generate a signal indicating the output load magnitude, allowing for efficient load judgment and optimized control of the synchronous rectifying element.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If voltage detection method is used to detect synchronous rectifying element off timing, then circuit complexity is reduced, but detection precision deteriorates due to ringing causing erroneous off timing detection
Solution Approach 1:
The detection process is segmented into two independent stages: first detecting the off timing of the synchronous rectifying element using voltage detection, then detecting the magnitude of output load using current detection. This segmentation allows each detection function to use the most appropriate method without interference, resolving the contradiction between circuit simplicity and detection accuracy.
Solution Approach 2:
The patent introduces an intermediary current detection circuit that measures the magnitude of output load current. This intermediary detection mechanism provides additional information that helps distinguish between genuine off timing (when load current is zero) and false off timing caused by ringing (when load current is still present), thereby improving detection precision without significantly increasing overall circuit complexity.
2Loss of energy
If synchronous rectifying element is turned on during primary switching on-period, then power transfer efficiency is improved, but harmful factors increase due to output terminal short-circuiting
Solution Approach 1:
The patent implements feedback control by continuously monitoring the magnitude of output load current and using this information to control the turning on and off timing of the synchronous rectifying element. The feedback mechanism ensures that the synchronous rectifying element is only turned on when appropriate (when it contributes to power transfer) and turned off when it would cause harmful short-circuiting, thus achieving both high efficiency and safety.
3Reliability
If minimum on-time is set for synchronous rectifying element, then reliability is improved by preventing erroneous off timing, but loss of energy increases due to reverse current flow during light load
Solution Approach 1:
The patent makes the on-time control of the synchronous rectifying element dynamic by adjusting it based on the detected magnitude of output load current. Under heavy load conditions, a longer minimum on-time is applied to ensure reliable operation and prevent erroneous off timing. Under light load conditions, the on-time is shortened or the synchronous rectifying element is turned off entirely, preventing reverse current flow and reducing energy loss. This dynamic adjustment resolves the contradiction between reliability and energy efficiency.
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 approach enables accurate detection of output load magnitude with minimal loss, optimizing the control of the synchronous rectifying element and improving power efficiency by ensuring proper timing and reducing reverse current flows during low load conditions.
Implementation Method 1
a synchronous rectifying element to rectify a current in the secondary winding
Implementation Method 2
voltage dividing circuits and an analog comparator to generate a signal indicating the output load magnitude
Data Source
AI summary
Disclosed a switching power source apparatus including: a transformer including a primary winding and a secondary winding; a switching element to intermittently apply a voltage to the primary winding by an on-off operation; and a synchronous rectifying element to rectify a current in the secondary winding, wherein the switching power source apparatus is a flyback system switching power source apparatus to receive input of electric power from a primary winding side to perform voltage output to a secondary wiring side, and the switching power source apparatus further comprises a load detection circuit to compare an output voltage to be output onto the secondary winding side with a voltage at a node between the synchronous rectifying element and the transformer by adding predetermined weighting to the voltages to be compared so as to generate a signal indicating a magnitude of an output load.