LED Driver Inductor Pre-Charging for High PWM Dimming
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing PWM dimming systems for LEDs face limitations in achieving small dimming duty cycles due to initial current ramp-up delays in switching regulators, leading to non-linear brightness control and potential LED failure at low duty cycles, with prior art solutions complicating pre-charge calculations and synchronizing regulator operations.
Innovation Solution
The technique involves pre-charging the inductor of a switching regulator to reach the target current level before the PWM ON-time, isolating the output capacitor during pre-charge, and using a sample and hold circuit to maintain the inductor current state, allowing for a minimal ON-time of one switching cycle or less, ensuring constant current supply and expanded dimming ratios.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the regulator starts up from zero current at the onset of PWM ON-time, then the inductor current ramps up to target current, but this causes initial current ramp-up delay and requires many switching cycles (greater than 20) to reach steady state
Solution Approach 1:
The inductor is pre-charged to a predetermined current level before the PWM ON-time begins. This preliminary action stores energy in the inductor magnetic field in advance, so when the PWM signal transitions to ON-state, the regulator immediately has current available to supply the LED load, eliminating the ramp-up delay and reducing the number of switching cycles needed to reach steady state.
2Reliability
If the PWM minimum ON-time is limited to ensure sufficient regulator switching cycles for steady state current, then the regulator achieves target direct current, but this limits the minimum duty cycle to 10% or more, preventing very dim light levels
Solution Approach 1:
By pre-charging the inductor before each PWM ON-time, the system eliminates the need for multiple switching cycles to build up current. This allows the PWM ON-time to be extended down to a single switching cycle or less, enabling duty cycles below 10% and even achieving dimming ratios up to 20,000:1, thus greatly expanding the adaptability for very dim light levels.
3Use of energy by moving object
If the output capacitor supplies current to the LED load during inductor pre-charge, then the capacitor voltage changes during pre-charge time, but this complicates pre-charge calculations and synchronizing regulator operations
Solution Approach 1:
The output capacitor is electrically isolated from the inductor during the pre-charge period by opening a switch between them. This extraction of the capacitor from the pre-charge circuit prevents it from discharging into the inductor, maintaining a fixed voltage across the inductor terminals. This simplifies pre-charge calculations and synchronizing regulator operations, as the pre-charge current can be precisely controlled without worrying about capacitor voltage changes.
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 enables precise control of LED brightness down to very low levels with a dimming ratio of up to 20,000:1, maintaining constant current throughout the ON-time and avoiding color shifts, while simplifying synchronization with other circuitry.
Implementation Method 1
pre-charging the inductor of a switching regulator to reach the target current level before the PWM ON-time
Implementation Method 2
Such high frequency pulses are smoothed by an output capacitor so the current through the LEDs is essentially a direct current
Data Source
Figure 1
Figure 2
Figure 3
AI summary
In a method for controlling a current regulator for dimming an LED load, a dimming signal has a duty cycle that controls the LED ON-time and LED OFF time at a fixed frequency. The dimming signal controls a switch in series with the LED load. Prior to the LED ON-time, the regulator is controlled to pre-charge the inductor so that the inductor current at the beginning of the ON-time substantially matches a stored value of the inductor current measured at the end of the previous ON-time. The regulator's feedback loop is frozen during the OFF-time to not change its feedback control signal. Upon the next ON-time, the regulator begins supplying current to the LED load with the pre-charged inductor current, so there is no initial decrease in the delivered LED current. Therefore, the current pulse magnitudes are constant even with very low duty cycles.