Hwa-Pyeong Park

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 (PFM) 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 240 W prototype converter operating at a switching frequency of 1 MHz.

Design and implementation of high switching frequency LLC resonant converter for high power density

To improve the power density of switch-mode power supplies, high switching frequency operation is an effective method to make the size of passive components small. The new design methodology of magnetizing inductance for zero voltage switching condition, and the size reduction of passive components should be proposed for 500 kHz high switching frequency operation. To verify the proposed methodology, the simulation and experimental results which include power conversion efficiency, and temperature of each passive and active component will be presented. Using those results, dominant power losses will be investigated according to the comparison of 100 kHz and 500 kHz operations. In addition, the small-sized output capacitor which has small output capacitance, and small effective series resistance induces the unstable operation problem. To analyze these control issue, the frequency response of small signal will be discussed to design the optimal feedback loop compensator for high switching frequency LLC resonant converter.

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 converter switching frequencies are moving toward megahertz 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.

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 converters.

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 and Implementation of 500 kHz High Frequency LLC Resonant Converter for High Power Density

In order to decrease the size of a switch mode power supply, high switching frequency can be an efficient way to reduce the size of passive components in the converter. In this paper, a 500-kHz high-frequency LLC resonant converter is proposed with an accurate design method of magnetizing inductance, as well as the relationship between the switching frequency and the size of the passive components. Simulation and experimental results are presented to verify the proposed methods and equations, including the temperature data of each passive and active device of the converter. Using those results, dominant power losses in the prototype converter under 500-kHz high-frequency operation are investigated, compared with the results from a 100-kHz converter. In addition, operating waveforms and power conversion efficiency will be shown to obtain design considerations for the high switching frequency LLC resonant converter.

Tightly regulated dual-output half-bridge converter using PFM-APWM hybrid control method

The conventional dual-output converter requires secondary side post regulator (SSPR) and/or additional circuitries to tightly regulate each output voltage. However, those dual-output methods can induce high cost and low power density. In this paper, a hybrid control method combining a pulse frequency modulation (PFM) and an asymmetrical pulse width modulation (APWM) is proposed to obtain tightly regulated output voltages using only output voltage sensors. The operational principles of the proposed hybrid control algorithm will be introduced and analyzed to derive its design methodology. The improved voltage regulation performance will be verified using simulation and experimental results with a 190 W prototype converter.

Design Considerations of Resonant Network and Transformer Magnetics for High Frequency LLC Resonant Converter

This paper proposes the design considerations of resonant network and transformer magnetics for 500 kHz high switching frequency LLC resonant converter. The high power density can be effectively achieved by adopting high switching frequency which allows small size passive components in the converter. The design methodology of magnetizing inductance is derived for zero voltage switching (ZVS) condition, and the design methodology of the transformer and output capacitance is derived to achieve high power density at high operating frequency. Moreover, the structure of transformer is analyzed to obtain the proper inductance value for high switching operation. To verify the proposed design methodology, simulation and experimental results will be presented including temperature of passive and active components, and power conversion efficiency to evaluate dominant power loss. In addition, the validity of magnetics design will be evaluated with operating waveforms of the prototype converter.

Design methodology of dual active bridge converter for solid state transformer application in smart grid

A design methodology of a dual active bridge (DAB) converter for solid state transformer applications is proposed using an elaborate mathematical model of the converter. The DAB converter is popular in bi-directional power conversion applications because of soft switching capability and seamless control in bi-directional power flows. However, several design considerations should be considered to overcome its demerits such as high RMS current at heavy load condition and limited soft switching capability at light load condition. Design methodology of the power stage will be discussed to minimize the conduction loss and to enlarge the soft switching region. In addition, the algorithm for soft start will be discussed to reduce overcurrent stress of the power switches during a start-up sequence. Experimental results of a 3.3 kW prototype DAB converter demonstrate the validity and effectiveness of the proposed methods.