Series String LED Driver with FET Bypass Control
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Solution Overview
Problem
Existing LED driver technologies dissipate significant power and are inefficient in controlling the brightness of individual LEDs in series-connected strings, particularly in applications requiring high brightness and color mixing, leading to increased power dissipation and heating.
Innovation Solution
A method and apparatus that utilize a current source providing constant current over a wide range of output voltages, combined with field effect transistors (FETs) in parallel with each LED to control current flow, allowing for efficient modulation of LED brightness through duty cycle adjustment and minimizing power dissipation by bypassing LEDs when not in use.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If linear drivers are used to control the current and brightness of each LED, then individual LED brightness control is achieved, but power dissipation increases significantly
Solution Approach 1:
The patent replaces linear drivers with pulse-width modulation (PWM) based switching control. Instead of using linear voltage regulation to control LED brightness, the invention uses high-frequency switching of LEDs in series strings with duty cycle modulation to achieve individual LED brightness control. This substitution of control mechanism dramatically reduces power dissipation in the driver circuitry while maintaining precise brightness control capability.
Solution Approach 2:
The patent employs periodic switching of LED strings at high frequencies (e.g., 100Hz or higher) to achieve dimming and brightness control. By rapidly switching LED strings on and off with varying duty cycles, the invention controls perceived brightness without continuous power dissipation. This periodic action allows multiple LED strings to share a common power supply while maintaining individual brightness control through temporal multiplexing.
2Ease of operation
If a large number of LEDs are controlled by multiple drivers, then individual LED control is achieved, but device complexity increases
Solution Approach 1:
The patent merges multiple LED strings into series connections, allowing a single power supply to drive multiple LEDs simultaneously. By connecting LED strings in series and using a common power supply with centralized PWM control, the invention reduces the number of independent drivers needed. The control system generates phase-shifted PWM signals to independently control each LED or LED group within the series string, achieving individual control without requiring separate power supplies for each LED.
Solution Approach 2:
The patent creates a universal control architecture where a single power supply and control circuit can manage multiple LED strings through PWM modulation. The control system generates multiple PWM signals with different duty cycles and phase relationships to control different LEDs within the same power supply domain. This multi-functional approach allows one power supply to replace what would traditionally require multiple separate driver circuits.
3Device complexity
If time division multiplexing is used to reduce the number of drivers, then device complexity is reduced, but overall brightness decreases
Solution Approach 1:
The patent dynamically adjusts the duty cycles of PWM signals controlling different LED strings to maintain optimal brightness. By using high-frequency switching and appropriate duty cycle modulation (e.g., 50% or higher for each string), the invention ensures that sufficient average current flows through each LED to maintain high brightness. The dynamic control allows compensation for the time-division aspect, ensuring perceived brightness remains high while reducing driver complexity.
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 reduces LED driver complexity and power dissipation, enabling efficient control of LED brightness and color mixing while minimizing heating, thus improving the overall efficiency of LED-based lighting and display systems.
Implementation Method 1
field effect transistors (FETs) in parallel with each LED to control current flow, allowing for efficient modulation of LED brightness through duty cycle adjustment
Implementation Method 2
driving light emitting diodes (LEDs), and more specifically to efficient duty cycle control of multiple LEDs in a series connected string of such LEDs
Implementation Method 3
One type of white LED uses a blue or ultraviolet emitting die with a phosphor deposited on or in proximity to the die. The mix of direct light from the die and light emitted by the excited phosphor blends to generate the color of such LEDs.
Data Source
AI summary
A current source generates, with high efficiency, a current that is substantially constant over a wide range of output voltages. This current is injected into the first end of a series-connected string of LEDs, with the second end of the string connected through a resistor to ground. The voltage developed across this resistor, which is a measure of current flow in the series string, is fed back to the current source, wherein feedback maintains nearly constant current output over a wide range of output voltages. A field effect transistor (FET) is placed in parallel with each LED in the string. A level shift gate driver couples a pulse width modulated control signal to the gate of each FET. With the FET being coupled across a particular LED, the LED can be bypassed when the FET is actuated or receive current when the FET is deactuated. By modulating the duty cycle of each FET, the brightness of each associated LED may be varied smoothly over its full range.