Self-Oscillating LLC Converter Control for Power Factor Correction
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional resonant LLC converters face challenges in achieving efficient power factor correction and stable operation due to high gain ratio requirements and sensitivity to threshold variations, especially when used as AC/DC converters, which complicates feedback control and leads to issues like false triggering and asymmetrical output currents.
Innovation Solution
The implementation of a self-oscillating LLC circuit with a control circuit that uses electrical feedback parameters to generate gate drive signals, setting thresholds based on converter output voltage or current and rectified input voltage and current, and employing inner and outer control loops to derive gate drive signals, allowing for threshold-based control without the need for balancing controllers and enabling accurate duty cycle control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If conventional resonant LLC converters are used for AC/DC conversion with power factor correction, then the converter can perform both functions, but the high gain ratio requirements and sensitivity to threshold variations complicate feedback control and cause false triggering
Solution Approach 1:
The feedback control is segmented into two independent control loops: an outer control loop that sets the threshold level based on output voltage/current and input voltage/current, and an inner control loop that compares the electrical feedback parameter with the threshold to generate gate drive signals. This segmentation isolates the threshold-setting function from the switching control function, reducing the complexity of feedback control while maintaining dual function capability.
Solution Approach 2:
An electrical feedback parameter (such as voltage across a capacitor in the LLC circuit) is introduced as an intermediary to represent the input current. This intermediary allows the control system to indirectly control the input current waveform to achieve power factor correction without directly measuring or controlling the input current, simplifying the feedback control structure.
2Ease of operation
If threshold-based control is used in resonant LLC converters, then the control is simplified, but the system becomes sensitive to threshold variations causing false triggering and asymmetrical output currents
Solution Approach 1:
The threshold level in the outer control loop is dynamically adjusted based on feedback from the converter output voltage or current and the rectified input voltage and current. This feedback mechanism compensates for threshold variations and ensures accurate duty cycle control, preventing false triggering and asymmetrical output currents while maintaining simple threshold-based control operation.
Solution Approach 2:
The control system transitions from static threshold control to dynamic threshold control where the threshold level adapts to changing operating conditions through the outer control loop. This dynamic adjustment maintains control simplicity while ensuring reliable operation across varying load and input voltage conditions.
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 enables efficient power factor correction and stable operation by reducing complexity and sensitivity to threshold variations, avoiding false triggers, and ensuring symmetric operation, thus improving the reliability and efficiency of resonant LLC converters in AC/DC conversion.
Implementation Method 1
Resonant converters which comprise an LLC resonant circuit having two inductances and one capacitance are well-known. Such converters have the advantage that energy-efficient operation with relatively low switching losses is possible.
Implementation Method 2
The implementation of a self-oscillating LLC circuit with a control circuit that uses electrical feedback parameters to generate gate drive signals
Implementation Method 3
The converters can be configured or operated as a constant current source or a constant voltage source. A constant current source can be used to drive an LED arrangement directly, thus enabling a single stage driver.
Data Source
AI summary
Various improvements are provided to resonant DC/DC and AC/DC converter circuit. The improvements are of particular interest for LLC circuits. Some examples relate to self-oscillating circuit and others relate to converter circuits with frequency control, for example for power factor correction, driven by an oscillator.