Daocheng Huang

A Universal Adaptive Driving Scheme for Synchronous Rectification in LLC Resonant Converters

In this paper, a universal adaptive driving scheme for synchronous rectification (SR) is proposed. The drain to source voltage of the synchronous rectifier is sensed so that the paralleled body diode conduction is detected. Using the proposed SR driving scheme, the SR turn-OFF time is tuned to eliminate the body diode conduction. The SR gate driving signal can be tuned within all operating frequency regions. Moreover, a simple digital implementation is introduced. Compared with analog ones, it enables more intelligent and precise SR control, improving converter efficiency. For rapid prototyping purposes, the digital SR tuning system is realized in Cyclone III field-programmable gate array (FPGA).

High switching frequency, high efficiency CLL resonant converter with synchronous rectifier

In this paper, a CLL resonant tank (CLL) is presented as an attractive option in the front-end applications as DC/DC converter. This topology can achieve zero-voltage switching (ZVS) from zero load to full load, low turn off loss, ZCS for output rectifier, and easy to realize secondary rectification. A novel design methodology of CLL resonant converter is proposed based on the trade-off among efficiency, hold up time and ZVS requirement. A transformer structure for better performance in high frequency application is proposed. And current type synchronous rectifier (SR) drive scheme is used in this topology. An 800kHz, 300W CLL resonant converter prototype is made to verify the proposed design method.

High-Frequency High-Efficiency

This paper proposes a CLL resonant dc-dc converter as an option for offline applications. This topology can achieve zero-voltage switching from zero load to a full load and zero-current switching for output rectifiers and makes the implementation of a secondary rectifier easy. This paper also presents a novel methodology for designing CLL resonant converters based on efficiency and holdup time requirements. An optimal transformer structure is proposed, which uses a current-type synchronous rectifier (SR) drive scheme. An 800-kHz 250-W CLL resonant converter prototype is built to verify the proposed circuit, design method, transformer structure, and SR drive scheme.

CLL

This paper proposes novel electromagnetic interference (EMI) suppression techniques for dc-dc converters. For low-voltage high-current applications, windings are paralleled and interleaved. Although low conduction loss and low leakage inductance can be achieved, the winding capacitances are considerably increased, thereby deteriorating the converters EMI and soft-switching performances. To solve these problems, a novel balanced choke concept is proposed, as well as other techniques. The advantages of the proposed concepts and strategies are verified and demonstrated on a 1-kW 1-MHz 400-V/12-V LLC resonant converter prototype. More than 52-dB noise attenuation and 75% equivalent winding capacitance reduction are achieved. Hence, EMI and soft-switching performances are significantly improved.

Resonant Converters With Synchronous Rectifiers

A digital implementation of synchronous rectification (SR) driving scheme for LLC resonant converter is proposed in this paper. Compared with analog ones, digital implementation enables more intelligent and precise SR control, improving converter efficiency. In the proposed SR driving scheme, the drain to source voltage of synchronous rectifier on the secondary side is sensed, so that the paralleled body diode conduction is detected. The SR turn-off time is digitally tuned based on the body diode conduction information. For rapid prototyping purposes, the digital tuning system is realized in Cyclone III FPGA.

Novel Techniques to Suppress the Common-Mode EMI Noise Caused by Transformer Parasitic Capacitances in DC–DC Converters

A systematic method is proposed to select desired multi-elements resonant topologies, like three-element, four-element or more elements resonant topologies. The intrinsic characteristics of multi-element resonant converters are utilized for this classification and selection method. Three-element resonant tanks are grouped. One group is chosen for illustration. The characteristics of topologies in this group are compared. This method is also used for four-element resonant converters to improve three-element resonant converters. A test board is built to verify proposed method.

Digital implementation of driving scheme for synchronous rectification in LLC resonant converter

In this paper, a novel burst mode control solution for LLC resonant converters is proposed for the highest efficiency and small output voltage ripple. The burst time is set to be constant, the off time is modulated by load conditions. During the burst time, three pulse switching pattern is implemented to keep output voltage low frequency ripple minimal. The switch first turned on is controlled to be last turned off to reduce the initial hard switching turn on loss when burst starts. The pulse width of first switch is optimized to settle LLC to the steady-state under the highest efficiency load condition in one pulse. Zero current detection (ZCD) of the resonant current is implemented to achieve first switch partial ZVS, further increasing light load efficiency.

Classification and selection methodology for multi-element resonant converters

In isolated converter, transformer is a main path of common mode current. Methods of how to reduce the noise through transformer have been widely studied. One effective technique is using shield between primary and secondary winding. In this paper, EMI noise transferring path and EMI model for typical isolated converters are analyzed. And the survey about different methods of shielding is discussed. Their pros and cons are analyzed. Then the balance concept is introduced and our proposed double shielding using balance concept for wire winding transformer is raised. It can control the parasitic capacitance accurately and is easy to manufacturing. Next, a newly proposed single layer shielding for PCB winding transformer is discussed. The experiment results are provided to verify the methods.

LLC resonant converter burst mode control with constant burst time and optimal switching pattern

In this paper, a high efficiency high power density matrix transformer structure for LLC resonant converters is proposed. Matrix transformer is help to reduce leakage inductance and the AC resistance of windings. Flux cancellation method is utilized to reduce core size and loss. Synchronous Rectifier (SR) devices and output capacitors are integrated into secondary windings to eliminate termination related winding losses, via loss and leakage inductance. An 1.6 MHz 390 /12 V 1kW LLC resonant converter prototype is built to verify the proposed structure.

Transformer shielding technique for common mode noise reduction in isolated converters

In this paper, the common mode (CM) EMI noise characteristic of three popular topologies of resonant converter (LLC, CLL and LCL) is analyzed. The comparison of their EMI performance is provided. A state-of-art LLC resonant converter with matrix transformer is used as an example to further illustrate the CM noise problem of resonant converters. The CM noise model of LLC resonant converter is provided. A novel method of shielding is provided for matrix transformer to reduce common mode noise. The CM noise of LLC converter has a significantly reduction with shielding. The loss of shielding is analyzed by finite element analysis (FEA) tool. Then the method to reduce the loss of shielding is discussed. There is very little efficiency sacrifice for LLC converter with shielding according to the experiment result.