Self-Oscillating LLC Converter for Power Factor Correction
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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 dealing with wide input voltage ranges and varying load conditions, which complicates feedback control and leads to issues like false triggering and asymmetrical output currents.
Innovation Solution
The implementation of a self-oscillating LLC converter circuit with a control circuit that uses electrical feedback parameters to generate gate drive signals for the high side and low side switches, employing outer and inner control loops to set threshold levels and compare feedback parameters, allowing for threshold-based control without the need for balancing controllers and enabling efficient power factor correction and stable operation across a wide range of conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional resonant LLC converters use feedback control with threshold settings, then power factor correction can be achieved, but the system becomes sensitive to threshold variations and experiences false triggering
Solution Approach 1:
The converter uses self-oscillating operation where the resonant tank circuit automatically generates oscillations without external threshold control. The system serves itself by using the natural resonance characteristics to determine switching timing, eliminating the need for manual threshold settings and reducing sensitivity to threshold variations.
Solution Approach 2:
The patent implements feedback control by detecting the voltage across the resonant capacitor and using it to control the switching timing. The feedback mechanism naturally adapts to load and voltage variations, providing stable power factor correction without requiring fixed threshold values. The feedback loop automatically adjusts the switching instant based on the actual resonant tank state.
2Adaptability or versatility
If the converter operates with fixed switching frequency, then control is simplified, but the system cannot adapt to wide input voltage ranges and varying load conditions
Solution Approach 1:
The converter operates with dynamic switching frequency that automatically adjusts according to the input voltage and load conditions. The resonant tank circuit's natural oscillation frequency changes with operating conditions, and the control system follows these changes, enabling adaptation across wide voltage ranges without requiring complex frequency modulation circuits.
Solution Approach 2:
The system changes the switching frequency parameter dynamically based on operating conditions. By allowing the switching frequency to vary with input voltage and load, the converter maintains optimal resonant operation across different conditions. This parameter change is achieved through feedback control that adjusts the switching instant based on the resonant capacitor voltage.
3Reliability
If the converter uses self-oscillating operation with feedback control, then false triggering is avoided and symmetrical duty cycle is maintained, but the control circuit complexity increases
Solution Approach 1:
The resonant capacitor voltage serves as an intermediary signal that mediates between the power stage and the control logic. By using this intermediate voltage to trigger the switching transitions, the system achieves stable operation without direct threshold comparisons. The intermediary voltage naturally reflects the resonant tank state and provides reliable triggering information.
Solution Approach 2:
Instead of using traditional threshold-based control where a reference voltage triggers switching, the patent inverts the approach by using the resonant capacitor voltage itself to control the switching. This inversion eliminates the need for threshold settings and balancing controllers, as the system uses the natural oscillation waveform to determine switching instants.
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 allows for efficient power factor correction and stable operation by reducing the complexity of threshold control, avoiding false triggering, and maintaining a symmetrical duty cycle, thereby improving the converter's ability to handle varying input voltages and loads while maintaining high power factor and low harmonic distortion.
Implementation Method 1
converters which comprise an LLC resonant circuit having two inductances and one capacitance
Implementation Method 2
a transformer having a magnetizing inductance which also acts as one of the inductances of a series LLC resonant circuit
Data Source
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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.