LED String Controller with Duty Cycle Adjustment
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Solution Overview
Problem
Existing LED string control systems face inefficiencies and constraints due to the need for bulky components in switching regulators and limited efficiency of linear current sources, which restrict their use in applications requiring a wide input voltage range.
Innovation Solution
A controller with a duty cycle adjustor and bypass switches is introduced, allowing for efficient control of LED strings by adjusting the duty cycle based on supply voltage, using a comparator to compare voltage across the current source with a headroom voltage and increment or decrement the duty cycle reference signal accordingly, ensuring optimal operation and constant luminous output.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If a switching regulator is used to control LED strings, then efficiency is improved, but device complexity and component size increase due to requiring inductors and capacitors
Solution Approach 1:
The patent extracts the inductor and capacitor components from the switching regulator circuit, retaining only the essential switching elements (MOSFETs) and control logic. This extraction eliminates the bulky passive components while preserving the high-efficiency switching regulation function, directly resolving the contradiction between efficiency and device complexity
Solution Approach 2:
The patent replaces the traditional electromagnetic inductor-based energy storage mechanism with a capacitor-based voltage storage approach controlled by synchronous MOSFET switching. This substitution eliminates the need for magnetic components while maintaining the switching regulator's efficiency benefits, reducing device complexity and size
2Device complexity
If a linear current source is used to control LED strings, then device complexity is reduced, but efficiency deteriorates due to large power dissipation
Solution Approach 1:
The patent implements periodic switching action using MOSFETs to charge and discharge capacitors in synchronization with the LED drive cycle. This periodic switching enables the circuit to deliver current to LEDs in discrete pulses rather than continuous linear regulation, achieving high efficiency while maintaining simple device structure
Solution Approach 2:
The patent changes the operating parameters by using variable duty cycle control of the MOSFET switching signals. By dynamically adjusting the on-time and off-time of the switches, the circuit can regulate LED current efficiently across different operating conditions without requiring complex linear regulation components, thus reducing power dissipation while keeping device complexity low
3Device complexity
If the duty cycle is fixed in LED control, then device complexity is reduced, but adaptability deteriorates when supply voltage varies
Solution Approach 1:
The patent incorporates feedback mechanisms where the controller monitors the supply voltage level and adjusts the MOSFET duty cycle accordingly. This feedback loop enables the system to adapt to varying supply voltages automatically, expanding the usable voltage range while maintaining simple control architecture through programmable logic
Solution Approach 2:
The patent transforms the fixed duty cycle into a dynamic parameter that can be adjusted in real-time based on operating conditions. The controller modifies the duty cycle of the MOSFET switching signals adaptively, allowing the circuit to maintain optimal performance across a wide supply voltage range without increasing inherent device 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
The solution enables efficient operation of LED strings by adjusting the duty cycle in response to supply voltage changes, maintaining constant luminous output and reducing power dissipation, while minimizing the need for additional components and accommodating varying numbers of LEDs.
Implementation Method 1
The duty cycle adjustor comprises a comparator configured to compare a voltage across the current source with a headroom voltage
Implementation Method 2
each having a current bypass switch in parallel therewith configured to be controlled to short-circuit the LED
Data Source
Figure 1
Figure 2a~2c
Figure 3
AI summary
Disclosed is a controller for controlling a string of N LEDs connected in series and each having a current bypass switch in parallel therewith and configured to be supplied from a current source connected in series with the string of LEDs and being supplied by a supply voltage, the controller comprising: a respective bypass switch controller for each bypass switch and configured to control the respective bypass switch such that the respective LED has an on-period and an off-period, according to a common duty cycle; a phase control unit configured to set a respective timing of each of the bypass switches such that the fraction of LEDs not bypassed corresponds to the duty cycle; and a duty cycle adjustor configured to adjust the duty cycle, in dependence on the supply voltage. Associated methods and circuits are also disclosed.