LED Drive Circuit Voltage-Compensated Current Limiting
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Solution Overview
Problem
LED drive circuits that use full-wave rectified waveforms experience brightness fluctuations due to voltage amplitude changes, leading to inconsistent LED illumination.
Innovation Solution
An LED drive circuit with a bridge rectifier circuit, an LED string, and a current limiting circuit that adjusts the upper limit current based on voltage fluctuations, using components like transistors, operational amplifiers, and zener diodes to manage current flow through the LED string, thereby stabilizing brightness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a full-wave rectified waveform is applied to the LED string, then the LED can be driven by commercial alternating-current power source, but brightness fluctuates when voltage amplitude changes
Solution Approach 1:
The patent employs a feedback mechanism where the operational amplifier continuously monitors the voltage across the LED string and adjusts the current limit accordingly. When the input voltage increases, the feedback loop detects this change and reduces the current limit to maintain constant LED brightness, and vice versa. This closed-loop control system resolves the contradiction by automatically compensating for voltage fluctuations.
Solution Approach 2:
The patent dynamically changes the current limit parameter based on the input voltage level. By making the current limit a variable parameter rather than a fixed value, the system adapts to different voltage conditions. The operational amplifier adjusts the current limit parameter in real-time according to the detected voltage, ensuring stable brightness while maintaining compatibility with AC power sources.
2Reliability
If a constant current diode is used to limit current, then the current during LED on-period is constant, but brightness changes due to fluctuating on-period
Solution Approach 1:
The patent transitions from a static current limit to a dynamic current limit that changes with input voltage conditions. The operational amplifier creates a dynamic current limit that is proportional to the input voltage, ensuring that the LED current remains stable relative to the voltage changes. This dynamic adjustment compensates for the fluctuating on-period, maintaining consistent brightness while preserving the current stability during on-periods.
Solution Approach 2:
The patent applies preliminary anti-action by preemptively adjusting the current limit in response to voltage changes before they can cause brightness fluctuations. The operational amplifier detects voltage variations and modifies the current limit in advance to counteract their effect on LED brightness, preventing rather than correcting brightness instability.
3Device complexity
If the current limit is fixed, then the circuit is simple, but brightness fluctuates with voltage changes
Solution Approach 1:
The operational amplifier serves multiple functions within the circuit: it acts as a voltage detector, a current regulator, and a feedback controller simultaneously. By making this single component perform multiple roles, the patent achieves voltage-compensated current limiting without adding excessive circuit complexity. The operational amplifier's multi-functionality resolves the contradiction by providing stable brightness control while maintaining relatively simple circuit architecture.
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 solution effectively suppresses brightness changes caused by voltage fluctuations in the full-wave rectified waveform, maintaining consistent LED illumination by dynamically adjusting the current limit in response to voltage changes.
Implementation Method 1
a bridge rectifier circuit (17), an LED string (20) in which a plurality of LEDs (20a) is connected in series, and a current limiting circuit (29)
Implementation Method 2
The zener diode 115 is used as a voltage drop element
Implementation Method 3
the current limiting circuit decreases an upper limit current value in accordance with an amount of rise in the voltage when the voltage at the current input terminal viewed from the current output terminal rises
Data Source
AI summary
An LED drive circuit in which a bright change of an LED is suppressed even if the voltage of a full-wave rectified waveform fluctuates is provided. The LED drive circuit includes a bridge rectifier circuit, an LED string made up by a plurality of LEDs being connected in series and supplied with a current from the bridge rectifier circuit, and a current limiting circuit that has a current input terminal and a current output terminal connected onto a path that starts from the bridge rectifier circuit and returns to the bridge rectifier circuit via the LED string, which limits a current that flows through the LED string to an upper limit current value or smaller, and further, which decreases the upper limit current value in accordance with the amount of rise in the voltage when the voltage at the current input terminal viewed from the current output terminal rises.