LED Load Control with PWM–PFM Switching for Stable Dimming
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Solution Overview
Problem
LED light sources exhibit instability and undesirable characteristics when dimmed to low intensity levels or turned off, such as flickering, inconsistent brightness, or sudden intensity drops, due to limitations in existing dimming techniques like PWM and CCR.
Innovation Solution
A load control device with a power converter circuit, load regulation circuit, and control circuit that employs pulse width modulation and pulse frequency modulation to control LED intensity, switching between these methods based on intensity thresholds to maintain stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If pulse width modulation (PWM) or constant current reduction (CCR) techniques are used to dim LED light sources, then the LED intensity can be reduced, but the LED exhibits instability, flickering, and sudden intensity drops at low intensity levels
Solution Approach 1:
The patent applies dynamics by making the dimming control adaptive based on operating conditions. The system dynamically adjusts between CCR and PWM modes depending on the desired intensity level and operating conditions, rather than using a fixed dimming method. This dynamic adaptation resolves the contradiction by maintaining stability (reliability) across the full intensity range while still achieving dimming capability.
Solution Approach 2:
The patent changes the control parameter from a single fixed method to a dual-mode approach. By switching between constant current reduction (CCR) for higher intensity levels and pulse width modulation (PWM) for lower intensity levels, the system optimizes both stability and dimming performance. This parameter change resolves the contradiction by selecting the appropriate control mode based on the operating point.
2Illumination intensity
If existing dimming techniques are used to control LED intensity, then the LED can be dimmed, but inconsistent brightness and color across individual LEDs occur
Solution Approach 1:
The patent implements feedback mechanisms that monitor the actual performance of individual LEDs and adjust the dimming control accordingly. By measuring the brightness and color output of each LED and using this information to adjust the control signals, the system maintains consistent brightness and color across all LEDs even at low intensity levels, resolving the contradiction between dimming capability and brightness consistency.
3Device complexity
If PWM dimming is used with a fixed frequency, then the control is simple, but the LED becomes unstable at low intensity levels due to the minimum on-time requirement
Solution Approach 1:
The patent makes the PWM frequency dynamic rather than fixed. The system adjusts the PWM frequency based on the desired intensity level, using higher frequencies for low intensity dimming where minimum on-time constraints would otherwise cause instability. This dynamic frequency adjustment maintains both control simplicity and LED stability across the full dimming range.
Solution Approach 2:
The patent uses periodic PWM action with variable frequency to achieve stable low-intensity dimming. By adjusting the period (frequency) of the PWM signal dynamically, the system ensures that the on-time and off-time are appropriately balanced at each intensity level, preventing the instability that occurs with fixed-frequency PWM at low intensities.
Data Source
AI summary
A load control device for controlling the intensity of a lighting load, such as a light-emitting diode (LED) light source, may include a power converter circuit operable to receive a rectified AC voltage and to generate a DC bus voltage, a load regulation circuit operable to receive the bus voltage and to control the magnitude of a load current conducted through the lighting load, and a control circuit operatively coupled to the load regulation circuit for pulse width modulating or pulse frequency modulating the load current to control the intensity of the lighting load to a target intensity. The control circuit may control the intensity of the lighting load by pulse width modulating the load current when the target intensity is above a predetermined threshold and control the intensity of the lighting load by pulse frequency modulating the load current when the target intensity is below the predetermined threshold.