Primary-Side State Estimator for LED Driver Current Sensing
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Solution Overview
Problem
Existing LED driver and lighting power circuits face inefficiencies and cost issues due to the need for bulky and expensive current sensing components like current transformers and optical isolation for closed-loop control, especially in low power converters.
Innovation Solution
The implementation of a primary-side current mode control sense resistor to estimate output current using a state estimator circuit, which charges a capacitor based on the sensed current and provides a signal to an error amplifier for control, eliminating the need for direct sensing and bulky isolation components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If current transformers and optical isolation components are used for output current sensing, then measurement precision is improved, but device complexity and cost increase
Solution Approach 1:
The patent creates an electrical copy of the secondary output current by using the transformer primary current (which is magnetically coupled to the secondary) as a proxy measurement. The sense resistor measures the primary current, and through the known transformer turns ratio, this provides an accurate representation of the secondary output current without requiring direct sensing components on the secondary side.
Solution Approach 2:
The patent introduces the transformer magnetic coupling as an intermediary mechanism to transfer current information from the isolated secondary side to the primary side. By measuring the primary current that is magnetically linked to the secondary, the system obtains output current information without breaking the isolation barrier or requiring complex sensing components.
2Measurement precision
If direct output sensing is implemented, then measurement precision is improved, but loss of energy increases due to lower driver efficiency
Solution Approach 1:
Instead of directly sensing the output current (which would require inserting sensing components into the high-current output path and cause power losses), the patent creates an electrical copy of the current information through magnetic coupling on the primary side, where sensing occurs at much lower current levels, minimizing energy loss.
Solution Approach 2:
The patent replaces direct electrical contact sensing (which would be in the current path) with magnetic field-based sensing through the transformer coupling. This substitution allows current information to be obtained without physical insertion into the current path, eliminating the direct energy loss associated with sense resistors in the output circuit.
3Measurement precision
If current transformers are used for sensing, then measurement precision is improved, but device complexity and cost increase
Solution Approach 1:
The patent uses the existing transformer in the converter circuit to create a copy of the output current information on the primary side, eliminating the need for separate current transformer components. This approach leverages the already-present magnetic coupling infrastructure to achieve accurate current sensing.
Solution Approach 2:
The patent makes the existing transformer serve multiple functions: power transfer isolation and current sensing intermediary. By utilizing the transformer's magnetic coupling for both its primary power function and as a sensing intermediary, the system eliminates the need for dedicated current transformer components, reducing overall system cost.
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 output current estimation without sacrificing efficiency or increasing costs, enabling compact and cost-effective closed-loop control in power converters for LED drivers and other light sources.
Implementation Method 1
A sense resistor is connected between the transformer primary winding and a circuit ground
Implementation Method 2
The transformer has primary and secondary windings, with the secondary coupled to provide power to a light source
Implementation Method 3
The estimator includes a capacitance with a first terminal coupled with the PWM controller comparator input, as well as a second terminal coupled with the error input of the error amplifier
Data Source
AI summary
An LED driver circuit and an isolated DC-DC converter therefor are presented, in which a primary side state estimator circuit is provided for inferred output current sensing for closed loop control of pulse width modulated flyback or buck converters. Specifically, the estimator circuit includes a capacitance having a first terminal coupled to a comparator input of a pulse width modulation controller, and a second terminal coupled to an error input of an error amplifier, and a switching device having a first power terminal coupled to the second terminal of the capacitance, a second power terminal coupled to the circuit ground, and a control input terminal coupled to a drive output of the pulse width modulation controller.