Flyback Converter Overload Detection via Auxiliary Winding
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Solution Overview
Problem
Conventional power conversion systems face challenges in accurately evaluating output current and determining overload conditions in flyback voltage converters, particularly in Continuous Conduction Mode (CCM) and Discontinuous Conduction Mode (DCM), due to difficulties in measuring peak and trailing edge currents.
Innovation Solution
The proposed solution involves a calculation circuit that uses a detection module and sample-and-hold latch to capture and calculate the average output current by sampling the primary current through a sensing resistor, employing leading edge blanking signals to prevent false triggers, and using specific current relationships to derive equations for output current calculation in both CCM and DCM modes, which do not rely on peak or trailing edge current values.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional current measurement methods are used in flyback converters, then the system can operate in CCM and DCM modes, but the output current evaluation becomes inaccurate especially in CCM and DCM modes
Solution Approach 1:
The patent introduces an auxiliary winding on the transformer as an intermediary element to indirectly measure the primary current. Instead of directly measuring the difficult-to-access peak and trailing edge currents on the primary side, the auxiliary winding provides a proportional voltage signal that serves as a mediator for current detection. This resolves the measurement difficulty while maintaining accuracy in both CCM and DCM modes.
Solution Approach 2:
The patent replaces direct electrical current measurement with voltage measurement through the auxiliary winding. By substituting the mechanical/electrical current sensing approach with a voltage-based detection method, the system achieves accurate current evaluation without the complexity of directly measuring peak and trailing edge currents.
2Device complexity
If peak or trailing edge current values are used for output current calculation, then the calculation can be simplified, but the measurement becomes unreliable in CCM and DCM modes
Solution Approach 1:
The patent creates a copy of the primary current information through the auxiliary winding voltage signal. Instead of directly using peak or trailing edge current values that are difficult to measure accurately, the system copies the current waveform information via the auxiliary winding and uses this copied signal for calculation, achieving both simplicity and precision.
3Reliability
If conventional overload detection methods are used, then the system can protect against overcurrent, but false triggers occur due to leading edge spikes
Solution Approach 1:
The patent applies preliminary action by introducing a leading edge blanking signal that suppresses the harmful leading edge spike before it can cause false overload triggers. This preventive measure is applied in advance during the switching transition, eliminating the harmful effect at its source while maintaining reliable overload detection capability.
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 allows for accurate determination of output current without relying on peak or trailing edge current measurements, effectively addressing the challenge of overload detection and ensuring precise current evaluation in both conduction modes.
Implementation Method 1
a sensing resistor RS, having one end connected to the source terminal of the main switch QM and the other end connected to the ground, for sensing and detecting a primary current IP flowing through the primary winding 130A of the transformer 130 and providing a feedback voltage that equals a product of the primary current IP and the resistance value of the sensing resistor RS
Data Source
AI summary
A circuit and a method for evaluating a load condition in a flyback converter are disclosed. A first current source is used for providing a preset current ISUM equal to a sum of the off current value IOFF and the blanking current value ILEB to charge a first capacitor, and a second current source is used for providing a reference current IREF to charge a second capacitor. A comparator receives a voltage applied on the first capacitor at its positive input end and a voltage applied on the second capacitor at its negative input end. The output current transmitted to the load by the flyback converter is varied to the change of the preset current ISUM, as such the load condition is detected by the comparison result generated by the comparator.