Staggered PWM LED Driving for Constant Load Stability
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Solution Overview
Problem
Existing LED drive systems face challenges in maintaining a constant load when implementing local dimming, leading to fluctuations in current and voltage, which can limit brightness control and power efficiency in display devices.
Innovation Solution
The method involves generating PWM driving signals for LED strings with staggered timing, ensuring that no more than one string is driven simultaneously at a time, thereby maintaining a relatively constant load by arranging ON times serially within periodic switching cycles.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If independent control of each LED string is provided for local dimming, then brightness control range is improved, but load fluctuation increases
Solution Approach 1:
The patent implements periodic action by using PWM (pulse-width modulation) to control LED strings in a sequential manner. Each LED string is turned on and off in periodic cycles, with the duty cycle adjusted to achieve desired brightness levels. This periodic control allows independent brightness adjustment of each string while distributing the load over time, preventing simultaneous switching and reducing load fluctuations.
Solution Approach 2:
The patent applies preliminary action by pre-establishing a timing sequence that controls the switching of LED strings before actual operation begins. The controller is configured with a predetermined timing pattern that activates LED strings in a controlled sequence, ensuring that not all strings are switched on simultaneously. This preliminary timing arrangement prevents load spikes and maintains stable operation while enabling independent brightness control.
2Stability of the object's composition
If PWM currents are phase shifted to reduce load fluctuations, then load stability is improved, but control flexibility deteriorates
Solution Approach 1:
The patent implements dynamics by making the PWM duty cycle adjustable for each LED string independently. Rather than using fixed phase-shifted timing, the system dynamically adjusts the on-time proportion for each string based on brightness requirements. This dynamic control allows the system to maintain load stability through sequential switching while preserving full control flexibility to adjust individual string brightness levels as needed.
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 maximum load transients, allows for lower duty cycles, and enables wider brightness control ranges while maintaining a stable load on the drive circuit, improving display performance and power efficiency.
Implementation Method 1
a timing circuit 22 within LED drive circuit 10 operates the switching elements 20 in unison to pulse-width modulate (PWM) the currents conducted by the LED strings to which they are connected; the duty cycle of the PWM signals determines the brightness of the LEDs in the strings
Implementation Method 2
The DC voltage provided to the LED strings may be provided by, for example, a switching power converter (not shown)—most typically a boost-type power converter (referred to herein as a 'boost converter', which produces an output referred to herein as a 'boost voltage')
Data Source
AI summary
A multi-string LED driving method and system requires generating pulse-width-modulated (PWM'd) driving signals to respective LED strings to control their brightness levels, and staggering the timing of the driving signals such that the number of LED strings driven on simultaneously varies over time by no more than one LED string. The PWM'd driving signals are generated to, for example, achieve local dimming for a display device which employs a multi-string LED backlight system; the present method enables local dimming to be achieved while maintaining a relatively constant load on the drive circuit. The staggering of the timing of the PWM'd driving signals is preferably implemented by arranging the ON times of the driving signals such that they occur serially, such that the loading imposed by the LED strings is spread throughout each switching cycle.