Primary Side Constant Current Control Using Delayed Feedback
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Solution Overview
Problem
In switched mode power supplies with primary side control, the difference between estimated and real discharge times leads to misjudgment of average output current, causing failure in regulation due to the time lag in feedback voltage signal dropping to 0V after the power switch turns off.
Innovation Solution
A constant current control unit is introduced, utilizing a voltage-waveform detector and a delayed feedback signal to accurately determine the discharge time of the inductive device, with a constant current controller stabilizing the maximum average output current by integrating current-sense signals and adjusting the power switch duty cycle.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the feedback voltage signal is used to determine discharge time, then the control circuit can detect the discharge status, but the time lag causes misjudgment of discharge time and failure in current regulation
Solution Approach 1:
The patent applies preliminary action by generating a delayed feedback voltage signal in advance to compensate for the inherent time lag. The delay circuit pre-adjusts the timing of the feedback signal so that when it reaches the discharge time detection circuit, the timing aligns with the actual discharge moment, enabling accurate discharge time measurement without being affected by the original signal delay.
2Device complexity
If the discharge time is misjudged due to time lag, then the control can be simplified, but the average output current regulation fails
Solution Approach 1:
The patent introduces a delay circuit as an intermediary element between the feedback voltage signal source and the discharge time detection circuit. This intermediary component processes the feedback signal by introducing a controlled delay, transforming the timing characteristics to match the actual discharge timing, thereby enabling reliable current regulation without significantly increasing overall system complexity.
3Ease of manufacture
If primary side control is used to reduce cost and improve efficiency, then photo coupler and secondary detection circuit are eliminated, but discharge time detection accuracy deteriorates
Solution Approach 1:
The patent maintains the cost and efficiency advantages of primary side control by implementing a delay circuit that pre-adjusts the feedback voltage signal timing. This preliminary timing adjustment enables the primary side control circuit to accurately detect discharge time using only primary side components, eliminating the need for secondary side detection circuits and photo couplers while achieving accurate measurement.
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 solution accurately determines the discharge time, enabling precise stabilization of the average output current, improving regulation accuracy and preventing overcurrent conditions.
Implementation Method 1
PSC directly senses a reflective voltage across an auxiliary winding to indirectly know the output voltage over the secondary winding
Implementation Method 2
When the power switch 34 is ON, the primary winding PRM of the transformer energizes. When it is OFF, the transformer de-energizes via the secondary winding SEC
Data Source
AI summary
Herein is disclosed a constant current control unit and a control method, for a switched mode power supply with primary side control. The switched mode power supply comprises a power switch and an inductive device. A reflective voltage of the inductive device is detected to generate a feedback voltage signal. By delaying the feedback voltage signal, a delayed signal is generated. According to the feedback voltage and the delayed signal, a discharge time of the inductive device is determined when the power switch is OFF. According to the discharge time and a current-sense signal, a maximum average output current of the switched mode power supply is stabilized. The current-sense signal represents a current flowing through the inductive device.