LED Converter Primary Side Control for Load Detection
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Solution Overview
Problem
LLC resonant converters used in LED lighting systems typically operate as constant voltage converters, requiring a secondary side current source and control loop, which increases costs and complexity, and fail to meet safety requirements when no load is connected, as voltages rise excessively.
Innovation Solution
Implementing a primary side control circuit that senses electric parameters to control the switching frequency of a half-bridge resonant converter, allowing operation in burst mode to detect and prevent overvoltage when no LED is connected, and using a feedback path from the secondary side to the primary side to monitor output voltage and activate voltage protection, eliminating the need for secondary side components and ensuring safe operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an LLC converter is operated as a constant voltage converter with a secondary side current source, then the converter provides good load rejection ratio and soft switching capability, but the device complexity and cost increase due to required secondary side control circuit
Solution Approach 1:
The patent extracts the control circuit from the secondary side and relocates it to the primary side. The secondary side control circuit is completely removed, and only a simple voltage detection circuit remains on the secondary side to detect overvoltage conditions. The main control functionality is moved to the primary side control circuit, which monitors the auxiliary winding voltage and adjusts the switching frequency accordingly. This extraction eliminates the complex secondary side control while maintaining the constant current conversion capability.
Solution Approach 2:
The patent merges the control functionality into a single primary side control circuit that performs multiple functions: frequency adjustment based on auxiliary winding voltage detection, LED current regulation, and overvoltage protection. The voltage detection circuit on the secondary side is merged with the primary side control through the transformer coupling, creating a unified control system that simplifies the overall device architecture while maintaining reliable constant current operation.
2Device complexity
If an LLC converter is operated without a secondary side control circuit, then the cost and complexity are reduced, but the converter cannot maintain constant current output and requires additional control mechanisms
Solution Approach 1:
The patent inverts the conventional control architecture by placing the intelligent control circuit on the primary side instead of the secondary side. The primary side control circuit monitors the voltage on the auxiliary winding, which is magnetically coupled to the primary winding, and uses this information to adjust the switching frequency. This inversion allows the system to maintain constant current output capability while eliminating the need for a complex secondary side control circuit, as the control decisions are made on the primary side based on reflected voltage signals.
3Loss of energy
If the switching frequency is reduced to transfer more power when no LED is connected, then the power transfer efficiency improves, but the output voltage rises excessively creating safety hazards
Solution Approach 1:
The patent implements a feedback mechanism where the voltage on the auxiliary winding is continuously monitored and fed back to the primary side control circuit. When no LED is connected, the feedback signal indicates high voltage conditions, and the control circuit responds by adjusting the switching frequency to prevent excessive voltage rise. This feedback loop enables the system to optimize power transfer efficiency while maintaining safe operating voltages, as the control circuit dynamically adjusts frequency based on the actual load conditions reflected through the auxiliary winding voltage.
Solution Approach 2:
The patent employs preliminary action by detecting voltage conditions on the auxiliary winding before excessive overvoltage occurs. The primary side control circuit monitors the auxiliary winding voltage and proactively adjusts the switching frequency to prevent dangerous voltage levels from developing. This preliminary detection and correction mechanism allows the system to maintain efficient power transfer while preventing safety hazards before they occur, rather than reacting after overvoltage conditions have already arisen.
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 solution allows for cost-effective, safe operation of LLC converters as constant current sources without secondary side control circuits, enabling reliable power supply and detection of LED connections while preventing excessive voltages, thus meeting safety standards.
Implementation Method 1
A transformer connected to or being part of the resonant converter then transfers power over a galvanic barrier, e.g. a SELV barrier (separated or safety extra low voltage barrier), from a primary side of the galvanic barrier to a secondary side of the galvanic barrier
Implementation Method 2
LLC converters serve as a constant voltage converter, which supply a secondary side DC bus. There, a current source, typically a buck converter, generates the constant current that is required to drive high power LEDs. As known to the skilled persons, a LLC converter is a resonant half-bridge converter that uses two inductors (LL) and a capacitor (C)
Data Source
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AI summary
In a first aspect, the invention provides an LED converter, comprising: a resonant converter with a switching regulator, preferably a clocked half -bride converter, a galvanic barrier, the primary side of which being supplied by the switching regulator and the secondary side of which being arranged for providing directly or indirectly power to terminals for connecting one or more LEDs, a control circuit on the primary side of the galvanic barrier of the resonant converter, a sensing means on the secondary side of the galvanic barrier sensing a secondary side electric parameter indicative of an LED load, when connected to s,aid terminals, a feedback path from the sensing means to the control circuit adapted to feedback the secondary side electric parameter from the secondary side of the galvanic barrier to the primary side of the galvanic barrier, wherein in case no LED string is connected to the LED converter, the control unit is adapted to cyclically control a switching frequency of switching regulator switches, in particular switches of the clocked half-bridge, from a high frequency to a lower frequency, and to monitor whether the secondary side electric parameter reaches a predetermined threshold value, and to, in case the predetermined threshold value is reached, deactivate the switching regulator switches and to activate a voltage protection switch to pull the secondary side electric parameter to ground.