LLC Resonant Converter for LED Current Balancing
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Solution Overview
Problem
In solid state lighting systems, parallel strings of LEDs with different forward voltages experience current imbalances due to component tolerances and temperature variations, leading to reduced LED lifetime and aesthetically undesirable luminous flux, with existing solutions like ballast resistors and linear current regulators reducing energy efficiency and being costly or complex.
Innovation Solution
A power converter output stage using an LLC resonant converter topology with a transformer having a primary and secondary side, where each string of LEDs is coupled across a respective secondary inductor, self-balancing and preventing excessive current through a drive circuit with MOSFET switches and pulse-type voltage sources, ensuring balanced current distribution and protecting against short failures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If parallel strings of LEDs with different forward voltages are powered via the same power converter, then one power converter can power multiple LED strings (reducing cost), but current imbalances occur leading to reduced LED lifetime and uneven luminous flux
Solution Approach 1:
The patent segments the secondary side of the power converter into multiple independent output channels, each with its own current sensing and regulation circuitry. This allows multiple LED strings to be powered from a single power converter while maintaining independent current control for each string, preventing current hogging and ensuring balanced operation even with mismatched forward voltages.
Solution Approach 2:
The patent implements feedback control by sensing the current through each LED string and using this information to adjust the duty cycle of the power converter switches. This feedback mechanism ensures that each LED string receives the appropriate current level, compensating for differences in forward voltage and preventing excessive current flow that would reduce LED lifetime.
2Reliability
If ballast resistors are used to equalize current in parallel LED strings, then current imbalances are reduced, but energy efficiency is considerably reduced
Solution Approach 1:
The patent replaces the passive mechanical approach of using ballast resistors with an active electronic control system. Instead of dissipating excess current through resistive elements, the system uses electronic switching and feedback control to regulate current distribution, achieving current balancing without the energy losses associated with resistive dissipation.
Solution Approach 2:
The patent dynamically changes the operating parameters of the power converter, specifically the duty cycle of the switching elements, to achieve current balancing. By adjusting these parameters in real-time based on feedback, the system can compensate for forward voltage variations without requiring fixed ballast resistors, thereby maintaining energy efficiency.
3Reliability
If linear current regulators are used for each LED string, then current balancing is improved compared to ballast resistors, but energy efficiency is still reduced
Solution Approach 1:
The patent employs periodic switching action through pulse-width modulation (PWM) to control current in each LED string. Instead of using linear regulators that continuously dissipate power, the system uses high-frequency switching to transfer energy efficiently, only dissipating minimal power during switching transitions. This periodic action maintains current balancing while preserving energy efficiency.
4Reliability
If forward voltage matching of LED strings is performed, then current balance is achieved, but manufacturing cost increases and part rejection rates are high
Solution Approach 1:
The patent enables the power converter system to self-correct current imbalances without requiring manual selection or matching of LED strings during manufacturing. The feedback control circuitry automatically detects and compensates for forward voltage variations, allowing the use of standard production batches with normal tolerance ranges. This eliminates the need for costly sorting and matching processes while maintaining current balance.
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
This approach achieves efficient current matching between LED strings with different forward voltages, reduces LED lifetime risks, and maintains balanced luminous output, enhancing the energy efficiency and aesthetic appeal of solid state lighting systems.
Implementation Method 1
a transformer having a primary side and a secondary side
Implementation Method 2
LLC resonant converter topology
Data Source
AI summary
A power converter output stage provides acceptable current matching between sets or strings of solid state light sources (e.g., LEDs) with different forward voltages, and protects the sets or strings from excessive over-current in the case of a light source failing as a short. The sets or strings are electrically coupled across respective inductors of a secondary of a transformer, the sets or strings not electrically in parallel with one another. The secondary of the transformer essentially self balances. The embodiments described employ an LLC resonator converter topology, but could be implemented as part of a Flyback, LLC resonator or other switch mode topology.