LED Driver Resonant Frequency Control for Low Voltage Transfer
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Solution Overview
Problem
The half-bridge resonant topology for driving LEDs faces challenges in energy transfer when the input voltage is lower than the LED threshold voltage multiplied by the transformer turn ratio, leading to long zero current periods and poor power factor, which cannot be effectively addressed by reducing the transformer turn ratio without causing undesirable current increases and mismatch with the LED.
Innovation Solution
A circuit arrangement that includes a parallel resonant converter with capacitors and inductors forming a resonant circuit with the transformer's magnetizing and leakage inductance, boosting the voltage across the LED during zero-crossing instants by varying the switching frequency of the controller to match the resonance frequency, ensuring energy transfer even at low input voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by stationary object
If the transformer turn ratio is decreased to enable energy transfer at lower input voltages, then the power factor is improved, but the primary side current increases strongly and LED adaptation mismatch occurs
Solution Approach 1:
The patent applies dynamics by making the switching frequency variable rather than fixed. The controller dynamically adjusts the switching frequency of the half-bridge arrangement to track the resonant frequency of the resonant circuit, which changes with operating conditions. This dynamic frequency adjustment enables the system to maintain resonant operation across different input voltages, allowing energy transfer at low voltages without requiring a fixed low turn ratio that would cause excessive current.
Solution Approach 2:
The patent changes the operating parameter of switching frequency to match the resonant frequency of the circuit. By adjusting the switching frequency to coincide with the natural resonant frequency determined by the inductance and capacitance values, the system achieves resonant operation that enables voltage boosting and energy transfer at low input voltages without the need to change the transformer turn ratio, thus avoiding excessive primary current.
2Use of energy by stationary object
If the transformer turn ratio is decreased to enable energy transfer at lower input voltages, then the power factor is improved, but LED adaptation mismatch occurs
Solution Approach 1:
The patent uses dynamic frequency adjustment to adapt to different operating conditions. By varying the switching frequency to track the resonant frequency, the system can maintain proper voltage transformation ratio for the LED regardless of input voltage variations, eliminating the need for a fixed low turn ratio that would cause LED adaptation mismatch.
3Device complexity
If a fixed turn ratio transformer is used, then the circuit is simple, but energy transfer cannot occur when input voltage is lower than LED threshold voltage multiplied by turn ratio
Solution Approach 1:
The patent introduces dynamic frequency control to enable the fixed turn ratio transformer to operate effectively across a wide input voltage range. The controller dynamically adjusts the switching frequency to maintain resonant operation, which allows the transformer to transfer energy even when input voltage is below the LED threshold voltage multiplied by the turn ratio, without changing the transformer itself.
Solution Approach 2:
The patent exploits resonant oscillation in the resonant circuit formed by the transformer's magnetizing and leakage inductance and the capacitance. By driving the circuit at its resonant frequency, voltage amplification occurs across the LED, enabling energy transfer at low input voltages while maintaining a simple fixed turn ratio transformer design.
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 enables efficient energy transfer to the LEDs at low input voltages, improving the power factor and reducing electromagnetic interference, while maintaining compatibility with safety and EMI regulations, and allowing for compact, cost-effective LED drivers.
Implementation Method 1
A circuit arrangement is provided including a parallel resonant converter with capacitors and inductors forming a resonant circuit with the magnetizing and leakage inductance of a transformer. The resonant circuit boosts the voltage across the LED during zero-crossing instants by varying the switching frequency of the controller to match the resonance frequency.
Data Source
AI summary
A circuit for driving LEDs includes: —a transformer with a secondary winding for driving the LEDs and a primary winding, —a half-bridge arrangement fed with an input voltage and coupled to the transformer, —a resonant circuit between the half-bridge arrangement and the primary winding of the transformer, the resonant circuit having a given resonance frequency, and—a controller configured for switching the half-bridge arrangement with a switching frequency variable between first and second values. The second value is closer than the first value to the resonance frequency of the resonant circuit created between the half-bridge arrangement and the primary winding of the transformer. This boosts the voltage fed towards the LEDs via the transformer. The LEDs are preferably included in the form of series of LEDs connected to the secondary winding of the transformer via a rectifier placed outside the housing of the power-supply.