LED Current Source PWM Control With Synchronized ADC Sampling
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Solution Overview
Problem
Existing ballast devices for LED lighting modules face challenges in complexity and dynamic characteristics of the feedback control loop, particularly due to the need for low pass filters which increase costs and affect performance.
Innovation Solution
A ballast device with a self-resonance power electronic circuit operating at high switching frequency, using pulse width modulation with a frequency smaller than the switching frequency, and synchronizing feedback signal sampling with the PWM period to eliminate or reduce the need for low pass filters, thereby improving dynamic characteristics and reducing costs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a low pass filter with low cutoff frequency is arranged in the feedback loop to avoid aliasing errors, then aliasing errors are eliminated, but device complexity and cost increase due to additional components
Solution Approach 1:
The patent removes the low pass filter from the feedback loop by extracting the aliasing problem solution from the traditional filtering approach. Instead of filtering the signal, the system uses synchronous sampling at the PWM frequency to inherently avoid aliasing errors, thereby eliminating the need for additional filter components and reducing device complexity.
Solution Approach 2:
The patent replaces the mechanical/electrical low pass filter with a digital signal processing approach using synchronous sampling. The ADC samples the feedback signal at the PWM frequency, and digital processing handles the signal conditioning, substituting analog filtering components with digital methods that reduce hardware complexity.
2Reliability
If a low pass filter with low cutoff frequency is arranged in the feedback loop to generate DC signal, then aliasing errors are avoided, but dynamic characteristics of the control loop deteriorate
Solution Approach 1:
The patent extracts the aliasing protection function from the low pass filter and implements it through synchronous sampling at the PWM frequency. This removes the need for the filter's low cutoff frequency characteristic, allowing the control loop to respond dynamically without the bandwidth limitations imposed by traditional filtering.
Solution Approach 2:
The patent changes the sampling frequency parameter to match the PWM frequency, creating a synchronous sampling system. This parameter change allows the system to maintain reliability by avoiding aliasing while simultaneously improving dynamic characteristics, as the sampling occurs at the optimal frequency point without the need for low pass filtering.
3Ease of operation
If self-resonance high frequency operation is switched on and off to set LED current, then current control is achieved, but significant ripple in output LED current occurs
Solution Approach 1:
The patent employs periodic PWM switching at a frequency lower than the self-resonance frequency to control the LED current. By using periodic on-off action with duty cycle modulation, the system achieves smooth average current control while the high-frequency self-resonance circuit naturally filters out ripples, providing both ease of control and current stability.
Solution Approach 2:
The patent introduces an output capacitor as an intermediary element between the self-resonance circuit and the LED load. This capacitor acts as a filter that smooths the high-frequency ripple current while allowing the PWM control to effectively regulate the average LED current, thus resolving the contradiction between control capability and current stability.
Data Source
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AI summary
A LED luminary includes a ballast device for providing a load current to a lighting module, in particular an LED module. The ballast device comprises a current source circuit, for example a switched converter or self-resonant current source, which generates a current, a pulse width modulation circuit providing a pulse width modulated load current generated based on the current via an output capacitor to the lighting module, and a control circuit for controlling the pulse width modulation circuit based on a feedback signal representing the load current. The ballast device has a specific control loop structure with a feedback loop circuit, which samples a measurement signal in synchronization with a pulse width modulation period in order to generate the feedback signal.