Multi-Transformer LLC Converter Flux Cancellation
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Solution Overview
Problem
Conventional LLC converters face inefficiencies due to high transformer core losses, synchronous rectifier losses, and non-zero-current switching issues, especially at frequencies below the resonant frequency, leading to increased volume and component losses.
Innovation Solution
The LLC converter design incorporates a resonant circuit, switching circuit, current-sharing circuit, and multiple transformers with series-connected primary windings and matching secondary windings, utilizing freewheeling diodes and a control circuit to achieve zero-current switching and magnetic-flux cancellation, reducing transformer size and losses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the transformer is designed to store significant energy in conventional LLC converters, then the transformer can operate at required power levels, but the transformer core size increases resulting in high conductive losses
Solution Approach 1:
The patent divides the single transformer into multiple transformers (e.g., two transformers with primary windings P1 and P2). Each transformer handles a portion of the total power, allowing for smaller individual core sizes that reduce conductive losses while maintaining the required total power transmission capability.
2Adaptability or versatility
If the LLC converter operates at frequencies below the resonant frequency, then the converter can adapt to varying load conditions, but non-zero-current switching occurs resulting in reduced efficiency
Solution Approach 1:
The patent introduces a current-sharing circuit with freewheeling diodes that prepares and maintains current paths in advance. This preliminary action ensures that when switching occurs below resonant frequency, the current can continue flowing through the freewheeling diodes, enabling zero-current switching and preventing the efficiency losses associated with non-ZCS operation.
3Speed
If the resonant half cycle is interrupted by the start of the other half of the switching cycle at frequencies above the resonant frequency, then the switching frequency can be increased, but power switch turn off losses and rectifier switching losses increase
Solution Approach 1:
The patent introduces current-sharing circuits with freewheeling diodes as intermediary elements that provide continuous current paths during switching transitions. These intermediaries allow the resonant half-cycle to be properly completed even at higher switching frequencies, enabling the power switches and rectifiers to turn off at zero current and eliminating the increased losses that would otherwise occur.
4Loss of energy
If multiple transformers with series-connected primary windings are used, then magnetic-flux cancellation reduces transformer size and core losses, but the device complexity increases
Solution Approach 1:
The patent combines multiple transformers with series-connected primary windings in a unified circuit configuration where the windings are connected in series and the secondary windings are connected in parallel. This merging approach enables magnetic-flux cancellation between the transformers, reducing core losses and transformer size, while the standardized connection method keeps the added complexity manageable.
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 design enhances efficiency by reducing magnetic core losses, MOSFET losses, and synchronous rectifier losses, allowing for smaller transformer volumes and improved performance even below the resonant frequency, with reduced conduction and core losses.
Implementation Method 1
LLC resonant converters are designed to operate with zero-voltage switching (ZVS) that requires a large magnetizing current
Implementation Method 2
a plurality of transformers, each including a plurality of primary and secondary windings
Implementation Method 3
Magnetic-flux cancellation can be used to reduce both core size and component losses. Magnetic-flux cancellation requires reducing high-magnetic core loss caused by high-magnetic flux excitation
Implementation Method 4
The primary windings of the plurality of transformers and the freewheeling diodes that connect the primary windings provide separate paths that allow the magnetizing-inductance-current energy stored in the transformers during a previous cycle to be transferred back to the DC input
Data Source
AI summary
An LLC converter includes a resonant circuit connected to a DC input voltage, a switching circuit connected to the DC input voltage, transformers each including primary windings and secondary windings, and synchronous rectifiers each connected to one secondary winding and to ground. The primary windings of the transformers include a first primary winding and a second primary winding. The first primary windings of the transformers are connected in series, and the second primary windings of each of the plurality of transformers are connected in series. The series-connected first primary windings and the series-connected second primary windings are directly connected in parallel with the resonant circuit. A first current from a first switch flows into the series-connected first primary windings, and a second current from a second switch flows into the series-connected second primary windings. Currents from each of the secondary windings are equal or substantially equal.