LLC Converter Hold-Up Operation Using Third Transistor
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Solution Overview
Problem
Conventional LLC series-resonant converters face challenges in extending hold-up time due to high conduction losses from small magnetizing inductance, and existing solutions either increase transformer size, add bulky components, or are not suitable for low-output voltage applications with synchronous rectifiers.
Innovation Solution
A half-bridge LLC converter design that uses a single additional switch (or switch and diode) to operate in PWM mode during hold-up, decoupling normal and hold-up operations, and maintaining output voltage with a resonant capacitor, suitable for low-voltage, high-current applications with synchronous rectifiers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If small magnetizing inductance is used to achieve high peak gain for wide input voltage range, then voltage gain is improved, but conduction losses on the primary side increase
Solution Approach 1:
The patent divides the operation into two distinct segments: normal operation mode and hold-up operation mode. During normal operation, the converter operates in frequency modulation mode with the additional switch off. During hold-up operation, the converter switches to PWM mode with the additional switch on, providing high voltage gain. This segmentation allows optimization for each mode separately, achieving wide input voltage range adaptability without excessive conduction losses.
Solution Approach 2:
The patent dynamically switches between two operating modes based on input voltage conditions. The additional switch is turned on during hold-up operation to provide high voltage gain, and turned off during normal operation to reduce conduction losses. This dynamic operation allows the converter to adapt to different input voltage ranges while minimizing energy losses in each operating condition.
2Adaptability or versatility
If additional secondary winding is added to increase secondary turns during hold-up operation, then voltage gain is improved, but transformer size and device complexity increase
Solution Approach 1:
The additional switch and existing transformer windings serve multiple functions. The additional switch can operate in PWM mode during hold-up operation to provide high voltage gain, or be turned off during normal operation. The existing transformer windings are used in both modes. This multi-functionality achieves high voltage gain during hold-up without requiring additional transformer windings, thereby avoiding increased transformer size and complexity.
Solution Approach 2:
The patent changes the operating parameters (switching mode and switch states) rather than changing the physical transformer structure. By switching the additional transistor on/off and changing the control mode (frequency modulation vs PWM), the converter achieves different voltage gains without modifying the transformer winding configuration. This parameter-based approach avoids the complexity of additional transformer windings.
3Loss of energy
If conventional LLC series-resonant converter topology is used to achieve high efficiency with ZVS, then efficiency is improved, but hold-up time capability deteriorates
Solution Approach 1:
The patent segments the operation into normal mode (using conventional LLC topology for high efficiency) and hold-up mode (using PWM mode with additional switch for high voltage gain). During normal operation, the converter operates with frequency modulation maintaining ZVS for high efficiency. During hold-up operation, the additional switch enables PWM mode providing the high voltage gain needed to extend hold-up time. This segmentation allows the converter to maintain high efficiency during normal operation while gaining hold-up time capability.
Solution Approach 2:
The additional switch acts as an intermediary element that enables the converter to operate in two different modes. During hold-up operation, this switch is turned on to provide the additional voltage gain needed to extend hold-up time. During normal operation, it is turned off to maintain the simple LLC topology for high efficiency. This intermediary switch allows the converter to achieve both high efficiency and extended hold-up time capability.
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
The solution achieves increased voltage gain with high efficiency and reduced complexity, avoiding the drawbacks of additional transformer windings and component bulkiness, while maintaining high power density and efficiency across a wide input voltage range.
Implementation Method 1
the stored energy capacitor Cbus provides power for the LLC series-resonant converter
Implementation Method 2
LLC series-resonant converters have been used because the primary switches can use zero-voltage switching (ZVS)
Implementation Method 3
a transformer including primary and secondary windings
Data Source
AI summary
A converter includes a half-bridge circuit including first and second transistors that are connected in series, the half-bridge circuit is connected in parallel with a voltage input and includes a node connected to both the first and second transistors; a resonant inductor connected to the half-bridge circuit and the primary winding of a transformer; a resonant capacitor connected to the half-bridge circuit and the primary winding; a third transistor with a first terminal connected to the half-bridge circuit and a second terminal directly connected to a first terminal of the resonant inductor; and a rectification stage that is connected to the secondary winding of the transformer and that includes first and second synchronous rectifiers. The rectification stage does not use discrete diodes to provide rectification, and during voltage boost operation, the third transistor is turned on and off to maintain an output voltage level.