Cheol-O Yeon

High-Efficiency LLC Resonant Converter With High Voltage Gain Using an Auxiliary LC Resonant Circuit

To design an LLC resonant converter optimally in the wide input voltage range, the LLC resonant converter with high efficiency and high voltage gain using an auxiliary LC resonant circuit is proposed. In this paper, the auxiliary LC resonant circuit operates as a variable inductor according to the change of the switching frequency, and it is presented as an effective magnetizing inductance. In the nominal state, since the effective magnetizing inductance increases, the primary circulating current is decreased. Thus, the turn-off switching loss of the primary switches and the primary conduction loss are minimized. During the hold-up time, the effective magnetizing inductance decreases so that the proposed converter has a high voltage gain. As a result, an optimal design of the LLC resonant converter over the wide input voltage range is possible. The proposed converter is verified by experimental results with a 330-390 V input and 350 W(56 V/6.25 A) output prototype.

Improving the Light-Load Regulation Capability of LLC Series Resonant Converter Using Impedance Analysis

Generally, an LLC series resonant converter (LLC SRC) is an attractive topology for applications, which require wide input variation and high conversion efficiency because of its wide gain capability and soft-switching capability. However, there is a regulation problem in which the output voltage increases as the load current decreases. In this paper, Bode plot and impedance asymptote analysis were conducted to obtain an intuitive sense of the regulation characteristic of LLC SRC under the light-load condition. Moreover, to improve the regulation capability, a new resonant tank with an additional capacitor is proposed. Its design guidelines were determined by Bode plot and impedance asymptote analysis. Therefore, the proposed LLC SRC achieves very light load regulation, while it maintains the advantages of typical LLC SRCs.

LLC resonant converter with high voltage gain using auxiliary LC resonant circuit

To design a LLC resonant converter optimally in the wide input voltage, the LLC resonant converter with a high voltage gain using an auxiliary LC resonant circuit is proposed. The auxiliary LC resonant circuit operates as a variable inductor, and it is represented as an effective magnetizing inductance combined with the magnetizing inductance. In the nominal state, the auxiliary LC circuit provides a high effective magnetizing inductance to minimize the primary circulating current for a high efficiency. During the hold-up time, the auxiliary LC circuit leads the effective magnetizing inductance to be reduced to obtain the high voltage gain. Therefore, an optimal design of the LLC resonant converter, which reduces the primary conduction loss and switching loss, is possible. The proposed converter is verified by experimental results with a 330-390V input 56V/350W output prototype.

A high efficiency half-bridge LLC converter with simple hold-up compensation circuit

The half-bridge (HB) LLC converter has been widely used in medium power applications because of low component count, no transformer dc-offset current, and wide zero-voltage-switching (ZVS) range. However, the HB LLC converter should be designed with small transformer magnetizing inductance to obtain high voltage gain in wide-input-voltage applications with the hold-up time conditions. It causes the HB LLC converter to have large conduction and switch turn-off losses in the primary side at nominal input where a high efficiency is required. To overcome these problems, a simple control method of the HB LLC converter with one additional switch and capacitor in the primary side is proposed in this letter. At nominal input, since the proposed method enables the HB LLC converter to operate with large transformer magnetizing inductance, it can reduce the conduction and switch turn-off losses in the primary side, which makes a high efficiency. On the other hand, during the hold-up time, since the proposed method increases the resonant capacitance by turning on one additional switch, the HB LLC converter can obtain high voltage gain. To confirm the validity of the HB LLC converter with the proposed method, a 325-385V input and 56V/350W output laboratory prototype is built and tested.

A new LLC resonant converter with resonant frequency change for high conversion efficiency and high power density

A LLC resonant converter is a converter, which achieves Buck or Boost gain with Pulse Frequency Modulation, and it is a suitable topology for high power density. This LLC resonant converter has many advantages, such as wide soft-switching range, high switching frequency capability, and low voltage stress of active devices, however, it also has large RMS current and circulating current. From these results, the usage of LLC resonant converter had been limited in low voltage and high current output applications. In this paper, a new LLC resonant converter, which can achieve high conversion efficiency while maintaining high power density, is proposed. Since the proposed converter can be reduced the conduction loss and turn-off loss by large magnetizing inductance, it can achieve higher conversion efficiency than the conventional converter. In addition, in order to regulate the output voltage, the proposed converter changes the resonant frequency using the small size of auxiliary circuit under hold-up time condition. Therefore, the proposed converter is a topology that can satisfy both high conversion efficiency and high power density.

Bode plot and impedance asymptotes for light-load regulation of LLC series resonant converter

In general, the LLC series resonant converter (LLC SRC) is an attractive topology for the applications which require the wide input variation and high efficiency especially at light load condition. This is because of its wide gain capability with pulse-frequency modulation and zero-voltage-switching over the entire load condition. However, there is a regulation problem that the output voltage increases as the output current decreases. In this paper, Bode plot and impedance asymptotes analyses have been conducted to obtain an intuitive sense for the regulation characteristic of LLC SRC under light-load condition. Moreover, in order to improve the regulation characteristic, a new resonant tank with an additional capacitor is proposed and its design guideline is also provided by Bode plot and impedance asymptotes analyses. Therefore, the proposed LLC SRC achieves the very light-load regulation while it can maintain the advantages of typical LLC SRC.

Asymmetric half-bridge resonant converter having a reduced conduction loss for DC/DC power systems with a low input voltage

A new asymmetric half-bridge (HB) resonant converter for DC/DC power system with a low and wide input voltage is proposed in this paper. The proposed converter is based on the switch integration technique, merging the Active-Clamp Forward (ACF) circuit and the HB LLC resonant converter. By adopting the ACF circuit in front of the HB LLC resonant converter, higher input voltage of LLC resonant converter stage can be achieved. As a result, the primary conduction loss can be significantly reduced. In addition, an asymmetric pulse width modulation (APWM) control is applied to cover wide input voltage range and to mitigate the design limitation for a high efficiency and. Consequently, the proposed converter can achieve not only the small conduction loss and the optimal design for high efficiency, but also high power density and low cost due to the switch integration technique. The validity of the proposed converter is confirmed by the experimental results of a prototype converter with 36-72VDC input and 300W (12V/25A) output.

Wide ZVS Range Asymmetric Half-Bridge Converter With Clamp Switch and Diode for High Conversion Efficiency

A conventional asymmetrical half-bridge (AHB) converter is widely used for dc-dc stage in low-to-medium power system. However, since the asymmetric operation of AHB converter causes the low efficiency over entire load condition, the conventional AHB converter is not usually considered for the candidate of server power system. In order to overcome the problems of the conventional AHB converter, a wide zero-voltage-switching (ZVS) range AHB converter with a clamp switch and a clamp diode is proposed in this paper. The proposed AHB converter (PAHBC) replaces a low-side clamp diode with a MOSFET switch and adds an auxiliary winding, which changes the transformer turns-ratio. From these modifications and the pulsewidth-modulation (PWM) control of clamp switch, the PAHBC can be designed optimally at nominal input condition and obtains additional dc gain at hold-up time condition. These advantages achieve the high conversion efficiency over the entire load condition. The operational principle and analysis of the PAHBC are presented in this paper and verified by a 340-400 V input and 50 V/500 W output laboratory prototype.

Three-switch LLC resonant converter for high efficiency adapter with universal input voltage

In this paper, an LLC resonant converter using three metal-oxide-silicon field-effect transistor (MOSFET) switches is proposed for high efficiency with universal input voltage. At high line input, the proposed converter operates same as the conventional half-bridge (HB) LLC converter. At low line input, by changing the operation of three switches, the resonant tank of the proposed converter is divided into two resonant tanks and turns-ratio of the proposed converter is changed to half. As a result, the DC voltage gain is doubled for the low line input, which enables the proposed converter to be designed with large magnetizing inductance (Lm). Therefore, the proposed converter has low conduction loss and low turn-off switching loss over the entire input voltage range. The proposed converter is verified by experimental results with 90Vac∼264Vac input and 45W/19V output prototype.

An improved current compensation method for high PF and low THD in digital boost power factor corrector

Continuous conduction mode (CCM) boost power factor corrector (PFC) is widely used for high power applications such as server power supply. PFC performs AC/DC conversion with nearly unity power factor (PF) and low current harmonic distortion. However, the electromagnetic interference filter capacitors in front of the bridge diode and input filter capacitor behind the bridge diode incur phase leading of input current. This phenomenon reduces PF. In this paper, an improved digital compensation method for phase leading of input current in CCM boost PFC is proposed. By using this method, the power system can achieve high PF and low total harmonic distortion. Moreover, it is very simple to implement and has short computation time in digital controller. The principle and analysis of the proposed method are presented. The performance and feasibility are verified by experimental results from the high line, i.e. 230vrms input, 750W/400V output prototype.