LLCC-SOR Power Converter Light Load Efficiency
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Solution Overview
Problem
LLC resonant power converters experience low efficiency at low power levels due to large resonating currents, which degrade their light load efficiency.
Innovation Solution
The LLCC Secondary Overtone Resonant (LLCC-SOR) power converter tunes the transformer secondary to an odd-order overtone of the upper primary switching frequency and employs secondary duty cycle control when the upper frequency limit is reached, allowing the transformer circuit to resonate in a different mode, maintaining zero-voltage switching (ZVS) and reducing power draw.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If LLC resonant power converter operates at light loads, then high load efficiency and ZVS are maintained, but light load efficiency deteriorates due to large resonating current
Solution Approach 1:
The patent introduces a dynamic switching mechanism that transitions between two resonant modes based on load conditions. A secondary resonant circuit with adjustable capacitance is coupled to the primary resonant circuit, allowing the system to dynamically switch between standard LLC mode (for heavy loads) and secondary overtone resonant mode (for light loads). This dynamic reconfiguration reduces resonating current at light loads while maintaining ZVS operation.
Solution Approach 2:
The patent changes the resonant parameters of the circuit by introducing a secondary resonant circuit with adjustable capacitance. By varying the capacitance value in the secondary resonant circuit, the system can tune the resonant frequency and impedance characteristics to match different load conditions. At light loads, the secondary resonant circuit is activated with specific capacitance values that reduce the resonating current while maintaining the required output voltage.
2Loss of energy
If primary switching frequency is increased to improve light load efficiency, then conversion efficiency improves, but electromagnetic interference and switching losses increase
Solution Approach 1:
The patent utilizes resonant vibration principles by introducing a secondary resonant circuit that operates at an odd-order overtone frequency (3rd, 5th, 7th, or 9th harmonic) of the primary switching frequency. This secondary resonance creates a vibrational mode that efficiently transfers power at light loads without requiring excessive increase in the primary switching frequency, thereby reducing EMI and switching losses while maintaining high conversion efficiency.
3Loss of energy
If standard LLC mode is used, then simple circuit structure is maintained, but light load efficiency is poor due to constant resonating current
Solution Approach 1:
The patent implements a nested resonant circuit structure where the secondary resonant circuit is coupled to the primary resonant circuit. The secondary circuit contains an inductor and capacitor that form a resonant tank, which is nested within the overall LLC converter architecture. This nested structure allows the system to maintain the simple standard LLC configuration for heavy loads while adding the capability for secondary overtone resonance at light loads, achieving improved light load efficiency with minimal increase in overall circuit complexity.
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 significantly enhances light load efficiency by increasing the resonant circuit gain, reducing primary current, and maintaining high conversion efficiency even at the lightest loads, as the primary current returns to near zero after each switching transition.
Implementation Method 1
a parallel LC resonant tank circuit on the transformer secondary is approximately tuned to an odd order overtone (harmonic) of the primary switching frequency
Implementation Method 2
The zero-voltage switching (ZVS) condition is maintained on the primary switch
Data Source
AI summary
An LLCC Secondary Overtone Resonant (LLCC-SOR) power converter obtains dramatically higher efficiency with light loads by providing a resonance in the transformer secondary that is approximately tuned to an odd order overtone of the upper primary switching frequency, an upper frequency limit of the primary switching frequency, and a secondary duty cycle control that engages once the upper primary switching frequency limit is reached. The transformer circuit resonates in an LLCC-SOR mode that regulates the output voltage when the maximum frequency limit is reached. In operation, the gain of the resonant circuit is raised above its regulation point under light loads, forcing the controller into duty cycle mode. The secondary current completes an odd number of oscillations per single oscillation of the primary current, and the primary current returns to near zero after each switching transition. Also, a zero-voltage switching condition is maintained on the primary switch.