Primary Regulated Offline LED Driver With Power Factor Correction
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Solution Overview
Problem
Existing offline LED driver circuits for low power bulbs face challenges due to the size and cost constraints of the necessary components, particularly in achieving accurate and constant LED current regulation with minimal external components and space, especially when dealing with varying LED forward voltage and requiring power factor correction.
Innovation Solution
A highly integrated driver circuit with a transformer, power transistor, differential circuit, capacitor, and injection circuit that senses current, compares it to a variable reference, and modulates the current flow using an AC signal derived from the rectified DC signal, allowing for compact and cost-effective power factor correction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If secondary-side feedback with opto-coupler is used to achieve accurate LED current regulation, then LED current accuracy is improved, but device size and component count increase significantly
Solution Approach 1:
The invention extracts and eliminates the opto-coupler and secondary-side feedback components from the circuit. Instead of using secondary-side current sensing with opto-coupler isolation, the patent implements primary-side current sensing that directly measures the current through the primary winding, removing the need for complex isolation barriers and secondary-side circuitry while maintaining accurate current regulation.
Solution Approach 2:
The primary-side current sensing circuit performs multiple functions: it senses the primary current, regulates the LED output current, and provides power factor correction. This multi-functional approach eliminates the need for separate secondary-side feedback circuits and opto-couplers, reducing component count while achieving accurate LED current regulation.
2Device complexity
If primary control topology is used to reduce component size, then device complexity is reduced, but LED current constancy deteriorates when LED forward voltage varies
Solution Approach 1:
The invention implements a feedback mechanism where the sensed primary current is compared against a reference, and the control circuit adjusts the switching duty cycle to maintain constant LED current despite variations in LED forward voltage. The feedback loop continuously monitors and corrects current deviations, ensuring stable LED operation without requiring secondary-side feedback components.
Solution Approach 2:
The control circuit dynamically adjusts the switching parameters based on the sensed primary current and LED voltage conditions. By continuously adapting the duty cycle and switching frequency in response to changing LED forward voltage, the system maintains constant LED current output while using only primary-side components.
3Measurement precision
If sample and hold circuit is used to measure peak primary current, then measurement precision is improved, but device complexity and response time delay increase
Solution Approach 1:
The invention replaces complex high-speed sample and hold circuits with a simpler current sensing approach that uses the inherent properties of the primary winding and switching circuitry. The system uses straightforward current sampling during the switching cycle without requiring expensive high-speed analog components, achieving adequate measurement precision with much simpler circuitry.
4Use of energy by moving object
If power factor correction is implemented in offline LED driver, then energy efficiency is improved, but device size and cost increase
Solution Approach 1:
The invention merges the power factor correction function with the primary-side current regulation circuitry. By implementing power factor correction through the same primary-side sensing and control mechanism used for LED current regulation, the system achieves both functions simultaneously without adding separate correction components or increasing device size.
Data Source
AI summary
A fly-back type switched current regulator includes a primary transformer winding coupled to receive a rectified DC signal derived from an AC signal. The drain of a power transistor is coupled to the primary winding, with the source of the power transistor coupled to an input of a comparison circuit and a primary transformer winding sense resistor. A control terminal of the power transistor is coupled to an output of the comparison circuit. A capacitor stores a variable reference signal for application at a first capacitor terminal to another input of the differential circuit. The variable reference signal is compared to a winding current signal generated by the sense resistor by the comparison circuit. An injection circuit applies an AC signal derived from the rectified DC signal to a second terminal of the capacitor so as to modulate the stored variable reference signal. The regulator is coupled to drive LEDs.