LED Driver Feedback Sampling for Stable Low-Duty PWM Current
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Solution Overview
Problem
Existing semiconductor light-emitting device driving apparatuses face instability in driving current when the duty cycle of PWM modulation is small, leading to inaccurate feedback and unstable current supply due to variations in temperature and device characteristics.
Innovation Solution
A semiconductor light-emitting device driving apparatus comprising a switching power supply, a switching element, a PWM modulator, and a feedback circuit that includes a comparator, capacitor, sampling switch, detection resistor, and sampling signal generator, which generates a sampling signal to average the detection voltage only during current flow, allowing accurate feedback and stable current supply even at low duty cycles.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If PWM modulation with small duty cycle is used to control luminance, then power consumption is reduced, but driving current becomes unstable due to inaccurate feedback
Solution Approach 1:
The patent introduces a sampling switch as an intermediary component that selectively samples the detection voltage only during the current flow period. This sampling mechanism acts as a mediator between the PWM modulation signal and the feedback control system, extracting meaningful current information even when the duty cycle is very small, thereby maintaining feedback accuracy and driving current stability while enabling low power consumption operation
Solution Approach 2:
The patent implements a feedback control mechanism where the sampling switch samples the detection voltage during current flow, the capacitor integrates these sampled values to generate an average voltage, and this average voltage is fed back to control the switching power supply. This closed-loop feedback system ensures that even at small duty cycles, the driving current remains stable by continuously adjusting based on actual current measurements
2Reliability
If average current value is used for feedback control, then power supply voltage variations are suppressed, but feedback accuracy deteriorates at small duty cycles
Solution Approach 1:
The patent employs periodic sampling action where the sampling switch periodically samples the detection voltage during each current flow period. This periodic sampling, synchronized with the PWM modulation cycle, ensures that feedback information is continuously updated with high precision even at small duty cycles, while the capacitor integrates these periodic samples to provide stable feedback control against power supply voltage variations
Solution Approach 2:
The patent performs preliminary sampling of the detection voltage during the current flow period before the current actually flows in subsequent cycles. By sampling and integrating the voltage in advance during each PWM on-period, the system prepares accurate feedback information that compensates for upcoming current variations, maintaining both feedback accuracy and power supply stability
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
The apparatus ensures stable and accurate driving current supply to semiconductor light-emitting devices, even at duty cycles less than 5%, by effectively averaging the detection voltage and controlling the switching power supply to maintain a target current value, thereby suppressing the influence of device characteristic variations.
Implementation Method 1
the capacitor is charged with a difference voltage between a voltage of the detection resistor when the switching signal is turned on and a reference voltage
Implementation Method 2
a detection resistor connected in series... By controlling the FET to be turned on and off in accordance with a switching signal from outside, a current flowing in the semiconductor light-emitting device is subjected to PWM modulation
Data Source
AI summary
A semiconductor light-emitting device driving apparatus is disclosed with a semiconductor light-emitting device. In the semiconductor light-emitting device driving apparatus, a switching power supply outputs a driving current for driving the semiconductor light-emitting device, and a switching element controls the driving current to be turned on and off. A PWM modulator generates a PWM modulation signal for controlling the switching element to be turned on and off based on a PWM setting value inputted from outside, and a feedback circuit drives and controls the switching power supply based on the driving current and a target current value that is externally inputted. The feedback circuit drives and controls the switching power supply such that an average value of the driving current for an interval when the driving current flows is equal to the target current value.