LED Current Scaling via PWM Signal Coupling
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Solution Overview
Problem
Existing methods for controlling the average current to LEDs with varying forward voltages in parallel configurations are inefficient, leading to reduced dimming resolution and complex firmware requirements, as well as potential LED damage due to uneven voltage distribution.
Innovation Solution
A method and apparatus that generate scaling signals to modify original control signals, allowing for independent control of the average current to each string of light-emitting elements, using coupling means such as AND logic gates to produce effective control signals for switching means, thereby enabling efficient current scaling without the need for complex firmware or additional components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a common voltage source is used to drive multiple parallel LED strings with different forward voltages, then all LEDs can be powered from a single source, but LEDs with lower forward voltage requirements will receive excess current and may be overdriven or damaged
Solution Approach 1:
The patent divides the control of each LED string into independent segments by providing individual current scaling circuits for each parallel string. Each string receives a scaled version of the master PWM signal, allowing independent current adjustment. This segmentation enables each LED string to be controlled according to its specific forward voltage requirements while maintaining a common power source architecture.
Solution Approach 2:
The patent changes the current parameter for each LED string by introducing scaling factors (e.g., 50%, 25%, 12.5%) that modify the amplitude or duty cycle of the PWM signal delivered to each string. This parameter adjustment ensures that LEDs with lower forward voltage receive proportionally less current, preventing overdrive while maintaining efficient PWM-based dimming control.
2Reliability
If current-limiting resistors are used for each LED in parallel configuration, then current can be limited to prevent overdrive, but power losses increase and efficiency decreases
Solution Approach 1:
The patent replaces the passive mechanical/resistive current limiting approach with an active electronic control system. Instead of using resistors to drop excess voltage and dissipate power as heat, the system uses scaled PWM signals to actively control the average current delivered to each LED string. This substitution eliminates the continuous power loss associated with resistive current limiting while maintaining reliable current control.
3Reliability
If transistor current mirrors or independent current sources are used for each parallel LED string, then current equalization can be achieved, but the number of components increases and efficiency decreases
Solution Approach 1:
The patent creates a universal current control architecture where a single master PWM signal serves all LED strings, and scaling circuits provide multi-functional current adjustment. The scaling factors can be configured through simple resistive dividers or digital control, allowing the same basic circuit topology to serve multiple LED strings with different current requirements. This universal approach reduces component count compared to dedicated current mirrors for each string.
Solution Approach 2:
The patent implements a nested control structure where scaled PWM signals are generated by nesting additional control stages within the existing PWM framework. The master PWM signal is nested with scaling circuits that generate proportionally reduced versions for individual strings. This nested approach allows current scaling to be integrated within the existing control architecture rather than requiring separate parallel control systems.
4Ease of operation
If PWM duty cycle is reduced to control average current, then LED current can be dimmed from 0 to 100%, but LEDs with different forward voltages cannot all be dimmed to 0% when using a common voltage source
Solution Approach 1:
The patent introduces asymmetric control where different LED strings receive different scaling factors of the master PWM signal. Strings with lower forward voltage receive proportionally smaller scaling factors (e.g., 50% or 25% of the master signal), while strings with higher forward voltage receive the full or near-full signal. This asymmetric distribution of control authority allows each string to achieve its full 0-100% dimming range independently, with the overall system dimming controlled by the master PWM signal.
Data Source
AI summary
The present invention provides a method and apparatus for scaling the average drive current supplied to a light-emitting element or string thereof by coupling a scaling signal to an original control signal thereby generating an effective control signal for control of the light-emitting element(s). The scaling signal can be a modulated signal, for example a Pulse Width Modulation (PWM) signal, Pulse Code Modulation (PCM) signal, or other signal and modifies the original control signal to produce an effective control signal. The effective control signal is subsequently used to control the supply of power to the light-emitting element(s) from a power source via a switching device. The effective control signal essentially modifies the ON time of the light-emitting element(s), thereby modifying the average drive current passing through the light-emitting element(s). The scaling signal is coupled to the original control signal by a coupling mechanism, thereby enabling the modification of the original control signal by the scaling signal forming the effective control signal.