Boost Converter Current Feedback for LED String Stability
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Solution Overview
Problem
Conventional power supplies for series-connected LEDs, such as buck and boost converters, face challenges in maintaining stable current and power delivery, especially at low input voltages, leading to power losses and complex circuitry requirements in boost converters due to the need for current measurement and amplification.
Innovation Solution
A method and converter that form a robust current feedback signal by combining measurements of the main switch current and LED string current as weighted averages, eliminating the need for operational amplification and allowing lower resistance measurements, thus reducing power losses and simplifying the circuitry.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a measurement resistor is used to directly measure current through LEDs in a boost converter, then current measurement accuracy is improved, but power losses in the resistor increase significantly
Solution Approach 1:
The patent combines the current measurement function with the existing main switch current measurement. Instead of using a separate measurement resistor for LED current, the system measures the main switch current and uses it as a proxy for LED current, since they are equal in the boost converter topology. This eliminates the need for an additional measurement resistor and its associated power losses.
Solution Approach 2:
The existing main switch current measurement infrastructure serves dual purposes: it measures both the main switch current and indirectly the LED current. The system uses the measurement already taken for switch current control to also determine LED current, eliminating the need for separate measurement components.
2Loss of energy
If an operational amplifier is used to amplify the measurement voltage, then power losses in the measurement resistor are reduced, but circuit complexity increases
Solution Approach 1:
The patent extracts the amplification function entirely from the measurement path. Instead of using an operational amplifier to amplify small voltages from a low-resistance measurement resistor, the system directly uses the voltage from the main switch current measurement, which is already at a sufficient level for control purposes.
Solution Approach 2:
The measurement system is segmented into two independent measurements: main switch current measurement and LED string current measurement. Each measurement uses its own resistor optimized for its specific function, avoiding the need for amplification and complex signal conditioning.
3Device complexity
If a buck converter is used to supply current to series-connected LEDs, then current control is simplified, but the number of LEDs that can be connected in series is limited by input voltage
Solution Approach 1:
The patent uses a boost converter with dynamic duty cycle control to adapt the output voltage to match the requirements of varying numbers of series-connected LEDs. The controller dynamically adjusts the switching duty cycle to maintain stable current regardless of the LED string configuration or input voltage variations.
Solution Approach 2:
The system changes the operating parameters of the boost converter (switching frequency, duty cycle, inductance value) to optimize performance for different LED configurations. By adjusting these parameters, the converter can drive different numbers of LEDs in series while maintaining stable current control.
4Measurement precision
If separate current feedback signals from LED string and main switch paths are used, then measurement accuracy is improved, but control complexity increases
Solution Approach 1:
The patent merges the control of the main switch into a single integrated control loop that uses the main switch current measurement. The controller simultaneously performs duty cycle control for voltage regulation and current limiting functions using this single measurement, eliminating the need for separate feedback paths and reducing control complexity.
Data Source
Figure 1~2
AI summary
The present disclosure describes a boost converter for supplying current to a series connection of light-emitting diodes (LEDs). A duty ratio of the main switch of the converter is configured to be controlled on the basis of a current feedback signal. The current feedback signal is formed as a weighted average of at least two signals including a first signal representing the current through the series connection of LEDs and a second signal representing a current through the main switch.