Jae-Hyun Kim

Analysis on Load-Adaptive Phase-Shift Control for High Efficiency Full-Bridge LLC Resonant Converter Under Light-Load Conditions

Recently, the full-bridge (FB) LLC resonant converter is getting more attention due to its zero-voltage switching of the primary switches, zero-current switching of rectifier diodes, and high power capability. However, the conversion efficiency is degraded under light-load conditions due to the core loss in magnetic components and the switching loss in semiconductor devices. In this paper, a new control method is proposed for the FB LLC resonant converter. In the proposed method, the frequency control is basically used to regulate the output voltage over entire load conditions. Moreover, the proposed method utilizes phase-shifted gate signals between switch legs, based on predetermined optimal duty ratio under light-load conditions. At this point, the optimal duty ratio is determined to get minimum power loss under each light-load condition through the loss analysis of all components. Therefore, the proposed method makes high efficiency under light-load conditions. To confirm the validity of this paper, the prototype of the network power supply with 320-385 V dc input and 48 V/720 W dc output is tested.

A High-Efficiency PFM Half-Bridge Converter Utilizing a Half-Bridge LLC Converter Under Light Load Conditions

Recently, the various types of the half-bridge (HB) converters with the output inductor have been developed, and they exhibit a good performance in medium power applications such as the server power supplies and personal computer power supplies requiring high output current. However, they have common problems such as the primary and secondary switch turn-off losses and snubber loss in the secondary side caused by the output inductor, which degrades light load efficiency. To relieve these limitations of the conventional HB converters, a new HB converter, which employs one additional switch and capacitor in the secondary side, is proposed for a high efficiency under light load conditions in this paper. Since the proposed converter operates like the HB LLC converter with below operation by turning on additional switch under light load conditions, the switch turn-off losses and snubber loss can be minimized, and the zero-voltage switching (ZVS) capability can be improved. Consequently, the proposed converter can achieve a high efficiency under light load conditions. To confirm the operation, features, and validity of the proposed converter, a 330-400 V input and 12 V/300 W output laboratory prototype is built and tested.

A Novel Accurate Primary-Side Control (PSC) Method for Half-Bridge (HB) LLC Converter

Until now, several researches have been progressed on the primary side control (PSC) methods which decrease the size and cost of the overall system. However, all of them have been applied to the flyback converter, and it is difficult to apply them to the half-bridge (HB) LLC converter due to the large voltage across the secondary leakage inductor of the transformer. In this letter, a new PSC method for the HB LLC converter is proposed to obtain accurate output voltage. In the proposed method, the output voltage is regulated by obtaining the voltage across the primary side of the transformer when the external resonant inductor voltage becomes 0V. At this time, since the voltage across the transformer secondary leakage inductor is small, the proposed method can accurately regulate the output voltage. A 400V input and 20V/85W output laboratory prototype is built and tested to verify the effectiveness of the proposed PSC method.

Analysis for LLC resonant converter considering parasitic components at very light load condition

The LLC resonant converter has a critical problem that the output voltage cannot be regulated at very light load condition, although switching frequency is widely changed. This letter proposes time domain analysis considering parasitic components at very light load condition. Based on the proposed analysis, the essential reason of the regulation problem of the LLC resonant converter at very light load condition is described and the clear solution is suggested. To confirm the validity of the proposed analysis, the experimental results with 200W prototype is presented.

Load Adaptive Gate Driving Method for High Efficiency Under Light-Load Conditions

In this paper, a new gate driving method to improve light-load efficiency is proposed for the power conversion system with a standby converter. The proposed method reduces the gate driving voltage of power MOSFETs and the operating voltage of controller ICs under light-load conditions by adjusting the auxiliary output voltage of the standby converter. Therefore, the gate driving loss on MOSFETs and the loss on controller ICs are significantly reduced without the degradation of performance in the main power stage. To confirm the validity of this paper, the dc/dc server power supply with an input of 48 V, an output of 12 V/60 A for the dc/dc stage, and an output of 5 V/3 A for the standby stage is experimented.

A simple control scheme for improving light-load efficiency in a full-bridge LLC resonant converter

In this paper, a load adaptive phase-shift control is proposed for the full-bridge LLC resonant converter to improve light-load efficiency. The proposed method reduces the effective duty ratio and the magnetizing current. Therefore, the core loss on transformer and the turn-off switching loss on switches are significantly reduced. To confirm the validity of this study, the prototype with 320~400V DC input, 12V/40A DC output is experimented.

A high efficiency critical mode boost PFC using a variable inductor

In low-to-mid power applications, a critical conduction mode (CRM) boost power factor corrector (PFC) is widely used due to its simple control and a reduced switching loss. One of the important characteristics of CRM boost PFC is that the switching frequency is not constant and the range of variation is wide. Due to this variability of the switching frequency, there is a design guideline, which must be satisfied. That is, the minimum switching frequency must be higher than the audible frequency. In that reason, a small boost inductance is required to increase the switching frequency. However, this increases the switching loss resulting in a reduced efficiency. In this paper, a new method of improving the efficiency using a variable inductor is proposed. A variable inductor can be implemented by a simple auxiliary circuit. At nominal, where a high efficiency is required, the proposed method operates with a large inductance, which results in the improved efficiency with a decreased switching frequency. On the other hand, when the switching frequency decreases to the audible frequency, the inductance is changed to a smaller value, keeping the frequency higher than the audible frequency over all the range of input line voltage.

Resonant Capacitor O n/ O ff Control of Half-Bridge LLC Converter for High-Efficiency Server Power Supply

In this paper, a simple control method of the half-bridge (HB) LLC converter with one additional switch and capacitor in the primary side is proposed for wide-input-voltage applications with the hold-up time conditions. 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 it highly efficient. 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 325-385 V input and 56 V/350 W output laboratory prototype is built and tested.

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.

Zero-voltage-switching totem-pole bridgeless boost rectifier with reduced reverse-recovery problem for power factor correction

A novel totem-pole bridgeless boost rectifier with ZVS operation is proposed for power factor correction. In the proposed rectifier, conduction losses are lowered by eliminating full-bridge diode rectifier. Moreover, the proposed rectifier can achieve zero-current-switching (ZCS) turn-off of diodes, which eliminates the reverse-recovery problems of the body diode. In addition, since MOSFETs can achieve zero-voltage- switching (ZVS) turn-on, the proposed rectifier has low switching loss. In this paper, a theoretical analysis and design considerations are presented in detail. Finally, the simulation results are discussed to verify validity of the proposed rectifier.