Phase-Shifted PWM Clock Synchronization for LED String Brightness Stability
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Solution Overview
Problem
Conventional circuit arrangements for operating multiple semiconductor light source strings in video projectors experience visible fluctuations in brightness due to asynchronous switching frequencies of PWM controllers, leading to low-frequency beats and ripple in luminous flux.
Innovation Solution
The circuit arrangement synchronizes the switching frequencies and phases of PWM controllers across semiconductor light source units by providing phase-shifted clock signals, reducing low-frequency beats and allowing smaller capacitors to smooth residual ripple, thus minimizing brightness fluctuations and enhancing dynamic range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If PWM controllers are used to drive multiple semiconductor light source strings independently, then each string can be controlled separately, but visible brightness fluctuations occur due to asynchronous switching frequencies
Solution Approach 1:
The patent merges the clock signal generation of multiple independent PWM controllers into a single synchronized clock source. By combining the clock signals and distributing them with phase shifts to each PWM controller, the system maintains independent control capability while eliminating brightness fluctuations caused by asynchronous switching. This resolves the contradiction by merging the conflicting independence and stability requirements into a unified synchronized system.
Solution Approach 2:
The patent changes the parameter of clock signal phase relationship from independent/asynchronous to synchronized/phase-shifted. By adjusting the phase parameters of the clock signals fed to each PWM controller, the system achieves both independent control and brightness stability. The phase shift parameter is specifically set to 360°/n where n is the number of light source strings, resolving the contradiction through parameter optimization.
2Illumination intensity
If large capacitors are used to smooth ripple in the power supply, then brightness fluctuations are reduced, but device complexity and cost increase
Solution Approach 1:
The patent substitutes the mechanical/electrical approach of using large capacitors to smooth ripple with a control-based approach using synchronized PWM switching. Instead of relying on passive capacitor filtering, the system uses active phase-shifted clock signals to coordinate switching operations, thereby reducing ripple without requiring large capacitors. This resolves the contradiction by replacing component-based solutions with control-based solutions.
Solution Approach 2:
The patent converts the harmful effect of PWM switching ripple into a beneficial outcome by using the ripple characteristics to inform the phase shift timing. The synchronized switching with 360°/n phase shifts is designed to exploit the ripple patterns, converting what would normally be a problem into an advantage for reducing overall brightness fluctuations while minimizing the need for large capacitors.
3Illumination intensity
If phase-shifted clock signals are used to synchronize PWM controllers, then brightness fluctuations are eliminated, but additional control circuitry is required
Solution Approach 1:
The patent implements a universal clock signal generation circuit that serves multiple functions: it generates the base clock signal, creates the phase-shifted variants for different PWM controllers, and coordinates the switching of all light source strings. By making this single circuit multi-functional, the system achieves brightness stability without proportionally increasing device complexity. The universal circuit handles what would otherwise require multiple separate control circuits.
Data Source
AI summary
A circuit arrangement may include an input for coupling to a supply voltage; n semiconductor light source units comprising a driver device; wherein the units are coupled in parallel, wherein each driver device comprises a PWM controller. A respective controller is designed to provide a PWM signal at a respectively predefinable frequency to a control electrode of a respective converter switch. The arrangement includes current measuring devices which are designed for measuring the current through a respective string having at least one semiconductor light source; and a control device having control outputs, wherein each controller has a clock input, wherein a respective control output is coupled to a respective clock input of the controllers, wherein the control device is designed to provide clock signals at its control outputs, said clock signals being phase-shifted by 360°/n with respect to one another.