Active Clamp Flyback Feedback Sensing for Light-Load Regulation
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Solution Overview
Problem
Active clamp flyback converters face challenges in load regulation, especially at light loads, due to the influence of leakage inductance, and existing feedback methods are costly and have a limited lifespan.
Innovation Solution
An active clamp flyback converter design that includes a transformer with a primary and secondary winding and leakage inductance, a main power switch, an auxiliary switch, a feedback voltage generator, and a controller. The feedback voltage generator uses sample & hold circuits and an operational circuit to generate a feedback voltage by sampling current and voltage signals, accounting for leakage inductance and time differences to control the switches effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an opotocoupler, isolated capacitor or auxiliary winding is used to feed back the output voltage, then galvanic isolation is achieved, but lifespan is reduced and cost increases
Solution Approach 1:
The patent extracts the feedback function from traditional isolated feedback components (op-amp, auxiliary winding) and implements it through a simplified voltage sampling network that measures the switch node voltage and derives output voltage information without requiring galvanically isolated feedback components
Solution Approach 2:
The patent introduces a virtual intermediary - the switch node voltage - that serves as a proxy for the output voltage. By measuring the switch node voltage during specific switching intervals and using the known transformer turn ratio, the system indirectly obtains output voltage information without direct isolated feedback
2Device complexity
If the primary voltage is directly sampled to obtain the output voltage, then the feedback circuit is simplified, but load regulation deteriorates due to leakage inductance influence
Solution Approach 1:
The patent performs preliminary action by sampling the switch node voltage at specific moments during the switching cycle (when the auxiliary switch is ON and the primary current has discharged the leakage inductance). This timing-based sampling ensures that the measured voltage accurately reflects the reflected output voltage without contamination from leakage inductance effects
Solution Approach 2:
The patent employs dynamic sampling that adapts to the switching state. The feedback voltage generator dynamically selects sampling intervals based on the auxiliary switch state and timing signals, ensuring accurate measurement during light load conditions while maintaining simplicity of the feedback circuit
3Loss of energy
If light load operation is supported, then efficiency is improved, but load regulation deteriorates due to leakage inductance effects
Solution Approach 1:
The patent applies partial action by using the auxiliary switch for a controlled duration during light load conditions. The auxiliary switch is turned on for a specific time interval that is sufficient to discharge the leakage inductance but not excessively long, optimizing both efficiency and measurement accuracy for light load operation
Solution Approach 2:
The system dynamically adjusts its operation based on load conditions. During light load, the auxiliary switch is activated and the feedback voltage generator uses the switched-capacitor-based sampling network to accurately measure the reflected output voltage, compensating for leakage inductance effects and maintaining precise load regulation across varying load conditions
Data Source
AI summary
An active clamp flyback converter with accurate sense of an output voltage is discussed. The active clamp flyback converter adopts a feedback voltage generator which generates a feedback voltage in response to an input voltage, a voltage at a switch node, an auxiliary control signal, and a current signal indicative of a current flowing through a primary winding.