LED Driver Circuit Stabilizes Phase-Cut Dimmers
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Solution Overview
Problem
LED lamps connected to phase-cut power supplies, especially when multiple lamps are used together, experience instability and visible flicker due to 'early disconnect' issues with TRIAC dimmers, particularly leading-edge dimmers, which can lead to unacceptable light output and flicker.
Innovation Solution
An LED driver circuit that injects a current pulse between 200-700µs after a phase-cut operation to stabilize the dimmer, reducing oscillations and maintaining the TRIAC in a conductive state by altering the negative dI/dt, thereby preventing early disconnect and improving light output stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If LED lamps are connected to phase-cut power supplies with TRIAC dimmers, then energy consumption is reduced, but the TRIAC may disconnect early due to current dropping below holding current
Solution Approach 1:
The circuit proactively detects the phase-cut edge and pre-applies a hold current before the TRIAC current can drop below the holding current threshold. This preliminary action prevents early disconnect by ensuring the TRIAC remains in conduction state throughout the intended conduction period.
Solution Approach 2:
A controllable current source is introduced as an intermediary element between the phase-cut power supply and the LED lamp. This current source injects additional current during the conduction period to maintain the total current above the TRIAC holding current, thereby stabilizing TRIAC conduction without significantly increasing overall energy consumption.
2Productivity
If multiple LED lamps are connected to a single phase-cut power supply, then lighting coverage is improved, but visible flicker occurs due to oscillations
Solution Approach 1:
The circuit detects the phase-cut edge and uses this feedback information to control the timing and duration of the hold current injection. By synchronizing the hold current application with the phase-cut cycle, the circuit eliminates oscillations and prevents visible flicker while supporting multiple lamp connections.
Solution Approach 2:
The hold current is applied periodically in synchronization with the AC mains cycle and phase-cut timing. This periodic injection of current stabilizes the total load current throughout each half-cycle, preventing oscillations that would otherwise cause flicker when multiple lamps are connected.
3Reliability
If the conduction interval is extended to maintain TRIAC current, then early disconnect is prevented, but energy consumption increases
Solution Approach 1:
Instead of extending the entire conduction interval, the circuit applies a partial action by injecting hold current only during the critical period when the current would otherwise drop below the holding current threshold. This excessive current is applied selectively and temporarily, preventing early disconnect without significantly increasing overall energy consumption.
Data Source
Figure 1~2
Figure 3a~3b
Figure 3c~3d
AI summary
A circuit arrangement (1), a detection circuit (50, 70) and an LED driver circuit (100, 110) is disclosed for operating at least one lighting unit, such as an LED unit, with a phase-cut operating voltage. The circuit arrangement (1) comprising at least an input (6) for receiving a phase-cut operating voltage from said power supply and/or an output (7) for connection to said at least one lighting unit and a pulse injection circuit, configured to determine a phase-cut operation of said power supply and to draw a current pulse from said power supply within a delay time between 200 - 700mus after said phase-cut operation to provide a stable operation of said LED unit with the phase-cut power supply. The detection circuit (50,70) comprises at least an input (6) for receiving a phase-cut operating voltage from said power supply and a lamp compatibility detector (52, 72), configured to determine the presence of a parallel lamp (57), connected in parallel with the detection circuit (50, 70) to said phase-cut power supply during operation and to provide a compatibility signal to a LED driver circuit (100, 110) corresponding to the determination of said parallel lamp (57), so that said driver circuit (100, 110) is set between a normal operating mode and a compatibility mode in dependence of the presence of said parallel lamp (57).