Primary-Side Constant-Current Control for Accurate Overcurrent Protection
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Solution Overview
Problem
Conventional switch mode power supplies (SMPS) with primary side control lack flexibility due to hardware limitations and are influenced by transformer inductance errors, requiring complex circuitry for overcurrent protection and output voltage adjustment.
Innovation Solution
A constant-current control device with a voltage waveform detector and a constant-current controller that generates a discharging period signal to adjust the output current, using a two-stage control mechanism with integrators and current sources to manage overcurrent and overpower protection, allowing selective switching between different current sources based on integration trends and boundary voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If primary side control SMPS is used to dynamically adjust output voltage according to interior IC voltage for overcurrent protection, then overcurrent protection capability is improved, but hardware flexibility is reduced and transformer inductance error affects accuracy
Solution Approach 1:
The patent implements dynamic control by switching between two current sources (first and second current sources) based on real-time detection of integration results and boundary conditions. The controller dynamically adjusts which current source is activated, enabling adaptive overcurrent protection that responds to changing operating conditions rather than relying on fixed hardware characteristics.
Solution Approach 2:
The patent changes control parameters by using an integrator to accumulate current detection values over time and compare against boundary values. This parameter transformation approach converts instantaneous current measurements into time-integrated results, enabling more accurate overcurrent detection that is independent of transformer inductance variations.
2Ease of operation
If peak value of current detecting voltage is adjusted to control output current, then output current adjustment capability is improved, but circuit complexity increases due to discharging time detection requirement
Solution Approach 1:
The patent extracts the essential control function by separating the current detection and integration process from complex timing circuits. Instead of requiring full discharging time detection, the system extracts only the necessary information through the integrator and comparator, simplifying the circuit while maintaining control capability.
Solution Approach 2:
The integrator serves as an intermediary element that mediates between the current detection terminal and the control output. It accumulates current detection values and provides a smoothed control signal, eliminating the need for complex discharging time detection circuits while achieving effective current control.
3Reliability
If conventional primary winding control is used, then overcurrent protection is achieved, but accuracy is reduced due to transformer inductance error
Solution Approach 1:
The patent implements feedback control by continuously detecting current through the current detection terminal, integrating these values, and using the integration results to control the primary side switch. This closed-loop feedback mechanism compensates for transformer inductance errors by continuously adjusting control based on actual measured current rather than relying on theoretical inductance values.
Solution Approach 2:
The integrator performs preliminary accumulation of current detection values before final control decisions are made. This preliminary action allows the system to average out measurement noise and compensate for transient inductance variations, improving measurement precision before the control output is generated.
Data Source
AI summary
A constant-current control device for a power supply including a primary side switch and a secondary winding includes a voltage waveform detector, configured to generate a discharging period signal within a time length when the secondary winding is discharging according to a first feedback voltage and a control voltage; and a constant-current controller, wherein an integrator of the constant-current controller is configured to receive one of a first current source and a second current source according to the discharging period signal and a current detecting voltage to generate an integrator result voltage, wherein the current detecting voltage is related to a secondary winding current value flowing through the secondary winding and the secondary winding current value is positively related to a secondary output current value of the power supply.