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Department of Electrical EngineeringA resonant converter has been widely used in various industrial applications, since it has high power conversion efficiency. The increase of the power density is necessary to obtain high cost-effectiveness and design freedom on the electric products. A high switching frequency operation can be an effective method to obtain the high power density of power converters. In this dissertation, three topic will be discussed to obtain the high power conversion efficiency and the high power density for the resonant converter, as follows: First, the power stage and feedback loop are designed for the high switching frequency operation. The power stage is designed to obtain the high power conversion efficiency at the high switching frequency operation. In addition, the feedback loop is designed to guarantee the stability. Second, the control algorithm is proposed to obtain the tight output voltage regulation at the high switching frequency operation. The operational principle and design of control algorithm are analyzed to obtain the tight output voltage regulation. Third, the spread spectrum technique (SST) will be applied to the resonant converter to reduce the electromagnetic interference (EMI), which can improve the power density with small EMI filter size. In this research, the design constraint to implement the SST on the resonant converter is analyzed to obtain the dual functionality properly. In addition, the control algorithms are proposed to achieve tight output voltage regulation and EMI reduction, simultaneously. All the proposed design considerations and control algorithms are verified with the simulation and experimental results.clos

Design, Control, and Implementation of High Frequency LLC Resonant Converter

Department of Electrical EngineeringA high switching frequency operation has been introduced with much interest in research and industrial areas to improve the power density of power converters. However, its implementation is difficult for an elaborate switch mode power supply which has high efficiency and stable operation. In this paper, a power stage and a feedback controller design will be considered for proper operation, stability, and high power conversion efficiency of the high frequency LLC resonant converter. The power density can be improved by adopting high switching frequency which allows small sized passive components. At the high switching frequency, the size reduction of the passive components such as transformer, and output capacitor will be estimated to obtain the high power density design. In addition, the design method of the magnetizing inductance design method will be derived to achieve the zero voltage switching (ZVS) at the high switching frequency operation. In aspect of frequency domain, the smaller output capacitor which has small capacitance and low effective series resistance (ESR) changes the small-signal behavior of the converter???s power stage. It can make the converter unstable by increasing the crossover frequency in the loop gain of the small-signal model. The effect of the smaller output capacitance should be analyzed for stability analysis using a proper small-signal model of the LLC resonant converter. Therefore, the proper design methods of the feedback compensator are derived to obtain sufficient phase margin in the bode plot of the converter???s loop gain for its stable operation. The design considerations of the power stage and the feedback loop will be verified with the performance comparison of 100 kHz and 500 kHz switching frequency LLC resonant converters. Since the switching performance of state-of-art power switches has been improved, the power converter can operate over a 1 MHz switching frequency. In this paper, GaN E-HEMTs are used to achieve the high switching frequency operation due to its small channel resistance and small output capacitance. However, the GaN E-HEMTs also have different switching operation characteristics to other conventional silicon-based MOSFETs. Therefore, the high speed switching characteristics of the GaN E-HEMT should be analyzed to obtain proper operation for a half-bridge type LLC resonant converter using a boostrap gate drive circuit. Moreover, a soft start algorithm for the high switching frequency is analyzed to suppress inrush currents at the cold start operation of the converter. All the design considerations using the GaN E-HEMT are verified with a 240 W prototype LLC resonant converter operating at 1 MHz switching frequency.ope

PWM and PFM Hybrid Control Method for LLC Resonant Converters in High Switching Frequency Operation

High switching frequency is an effective method to improve power density for LLC resonant converters. However, conventional digital controllers, such as general-purpose digital signal processors and microprocessors, have limited frequency resolution, which induces high primary- and secondary-side current variation and leads to poor output voltage regulation. In this paper, a hybrid control method combining pulse frequency modulation and pulse width modulation is proposed to overcome the limited frequency resolution issue. The proposed hybrid control method focuses on steady state operation and its operating principles are introduced and analyzed. In addition, the proper magnetizing inductance and dead time duration are derived to ensure that the power MOSFETs achieve zero voltage switching with the proposed control method. The improved voltage regulation performance is compared with the conventional PFM control and verified through simulation and experimental results using a 240W prototype converter operating at 1 MHz switching frequency.clos

Design Methodology of Quasi-Resonant Flyback Converter with a Divided Resonant Capacitor

The leakage inductance in flyback converters can induce high-voltage spikes in power switches as well as power losses. Although conventional resistor-capacitor-diode (RCD) snubbers and active clamp circuits can be used to suppress these voltage spikes, the clamping operation of RCD snubbers increases power consumption, while the active clamps require additional power switches. To address these issues, a quasi-resonant flyback converter featuring a divided resonant capacitor is proposed in this article, in order to suppress voltage spikes in power switches and improve the power conversion efficiency. The operational principle of a divided resonant capacitor is analyzed to obtain the design methodology of the divided resonant capacitor. Furthermore, the validity of the proposed design method is verified via experiments, using 15-W prototype quasi-resonant flyback converter. The results indicate that the proposed design effectively reduces voltage spikes and improves power conversion efficiency

Improved control strategy of 1 MHz LLC converter for high frequency resolution

High switching frequency is one of the effective methods to improve power density for a LLC resonant converter. However, conventional controllers such as a digital signal processor (DSP) and an analog controller have a performance of the limited frequency resolution which introduces high primary and secondary side current variations and poor output voltage regulation at high switching frequency. In this paper, a hybrid control method combining the pulse frequency modulation (PFM) and the pulse width modulation (PWM) is proposed to overcome the limited frequency resolution performance. The proposed hybrid control method is analyzed with its control flow chart and operational principles. The improved voltage regulation performance is compared with the conventional PFM control and verified by simulation and experimental results using a 240 W prototype converter operating at 1 MHz switching frequency

Design Considerations of 1 MHz LLC Resonant Converter with GaN E-HEMT

LLC resonant converters with high switching frequency can show high power density by reducing the size of passive components, such as the output capacitor and transformer. However, it is difficult to operate the PWM generator and at a high switching frequency. Moreover, soft start operation requires much higher switching frequency than the nominal one. Therefore, this paper proposes a new soft start algorithm to suppress high inrush current with limited switching frequency. In addition, stable operation of the LLC converter at the high switching frequency is considered. GaN E-HEMTs are selected to achieve the high switching frequency operation due to its small drain-source resistance and small parasitic capacitance. However, GaN E-HEMTs also have different switching operation characteristics t. In this paper, the design and implementation of a 1 MHz LLC resonant converter are proposed to verify the improvement of power density reducing the passive component size. The soft start algorithm for high switching frequency is analyzed for small inrush currents at the cold start condition. Simulation and implementation are used to verify the validity of the soft start algorithm. The side effects of high switching frequency operation are analyzed to design the power components and PCB. The high speed switching characteristics of the GaN E-HEMT are also analyzed to obtain proper operation for a half-bridge type LLC resonant converter using a boostrap circuit. Simulation and experimental results are presented to show the validity of the proposed analysis and design methods with a 1 MHz prototype converter using GaN E-HEMTs

Power Stage and Feedback Loop Design for LLC Resonant Converter in High Switching Frequency Operation

As conveter switching frequencies are moving towards MHz frequencies for high power density, secondary leakage parasitics that were previously negligible have to be considered in mathematical modeling for LLC resonant converters. At high switching frequency operation, the power stage design must take secondary leakage inductance into account because it can affect the input-output voltage gain. In addition, the feedback loop design should consider the effect of the time delay caused by the performance limitation of a digital controller to improve the small signal model accuracy of the converter. Using the proposed power stage and feedback control loop design considerations, the LLC resonant converter can achieve high power conversion efficiency and stability enhancement at high switching frequencies. All the proposed methods are experimentally verified using a 240-W prototype LLC resonant converter operating at 1 MHz switching frequency.clos

Modeling and Feedback Control of LLC Resonant Converters at High Switching Frequency

The high-switching-frequency operation of power converters can achieve high power density through size reduction of passive components, such as capacitors, inductors, and transformers. However, a small-output capacitor that has small capacitance and low effective series resistance changes the small-signal model of the converter power stage. Such a capacitor can make the converter unstable by increasing the crossover frequency in the transfer function of the small-signal model. In this paper, the design and implementation of a high-frequency LLC resonant converter are presented to verify the power density enhancement achieved by decreasing the size of passive components. The effect of small output capacitance is analyzed for stability by using a proper small-signal model of the LLC resonant converter. Finally, proper design methods of a feedback compensator are proposed to obtain a sufficient phase margin in the Bode plot of the loop gain of the converter for stable operation at 500 kHz switching frequency. A theoretical approach using MATLAB, a simulation approach using PSIM, and experimental results are presented to show the validity of the proposed analysis and design methods with 100 and 500 kHz prototype convertersclos

Load-Adaptive Modulation of a Series-Resonant Inverter for All-Metal Induction Heating Applications

Conventional induction heating (IH) system have been developed to heat the pot of ferromagnetic materials, because the small resistance of non-ferromagnetic materials induces large resonant current to the power switch of a series resonant IH inverter. However, the heating capability for various materials is the most important to improve the functionality and the usability of IH products. In this paper, a load adaptive modulation (LAM) method is proposed to heat the pot made from non-ferro and ferromagnetic materials. The LAM method can change the magnitude of the input voltage of the IH coil and the operating frequency of the series resonant IH inverter according to the resistance of the pot. The operational principle and the design method are analyzed to implement the proposed LAM method and its power control. The validity of the LAM method is experimentally verified using a 2 kW prototype series resonant IH inverter

Bidirectional Current-Fed CLLC Resonant Converter Employing Asymmetric PWM

A power systems of wearable application requires high power conversion efficiency and bidirectional power flows to obtain the compatibility with various power sources and loads. The conventional flyback converter has a simple structure and bidirectional power conversion capability. However, it shows poor power conversion efficiency caused by hard-switching operation, high DC offset current on the transformer, snubber losses, and large peak current. In this paper, a current-fed CLLC resonant converter is proposed to obtain the high power conversion efficiency and the bidirectional power conversion capability. The soft-switching operation, zero DC-offset current, and small peak current can induce high power conversion efficiency. The asymmetric pulse width modulation (APWM) regulates the bidirectional power flows and the output voltage. The design methodology of the proposed converter is analyzed by introducing its operational principles. The performance of the proposed converter is experimentally verified with a 20 W prototype