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.

A conventional induction heating (IH) system has been developed to heat the pot of ferromagnetic materials because the small resistance of nonferromagnetic 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 nonferromagnetic 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.

Arc fault detection is important technology to guarantee the safety of power systems and is therefore essential for producing practical power systems for real-world applications. However, fuses and arc fault detection devices (AFDD) struggle to detect series arc faults in DC systems, because the series arc fault induces small current variation between the normal and abnormal conditions. In addition, switching noise from the grid-connected inverter makes detecting arc fault conditions even more difficult. This paper proposes arc fault detection algorithm based on the relative comparison of current variability in terms of frequency spectrum and time series. The operational principle of the proposed algorithm is analyzed to detect the arc fault condition. In addition, the investigation of arc fault impedance using the small-signal modeling can obtain the resonant frequency of arc fault condition at low frequency range. From the impedance model, the frequency analysis range can be designed to avoid the switching noise of inverter. The performance of proposed arc fault detection algorithm is verified with a 3.8 kW grid-connected PV system and arc fault generator.

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.

A spread spectrum technique has been introduced to mitigate electromagnetic interference (EMI) in power converters. However, this technique is difficult to apply to resonant converters that use a pulse frequency modulation (PFM), because the spread spectrum can induce large output voltage fluctuations by PFM's switching frequency variations. In this letter, a hybrid control method using the PFM and phase shift modulation is proposed to obtain tight output voltage regulation under the spread spectrum operation of an LLC resonant converter. The performance of the proposed hybrid control method is experimentally verified using a 500 W prototype LLC resonant converter.

Arc fault detection is an important process for ensuring the safety of PV and grid-connected inverters and is essential for producing PV systems in real applications. However, it is difficult to detect series dc arc faults using conventional fuses because these faults produce only small current variations. In addition, the arc fault detection devices have limited performance, because they focused on arc fault detection between PV arrays and inverter. Moreover, they require the additional current and voltage sensors to implement the arc fault detection algorithm. This article proposes an arc fault detection algorithm employing differential power processing (DPP) structure. The proposed algorithm only uses intrinsic voltage sensors of DPP and inverter, which can improve the cost-effectiveness of PV systems. In addition, the proposed algorithm can integrate the functionality of maximum power processing for each PV panel and arc fault detection. The voltage relationship between DPP and inverter is analyzed to detect the arc fault condition according to the DPP voltage sensing type. The performance of DPP units and its voltage relationship are verified with the control hardware-in-the-loop system. The arc fault detection performance is verified with the prototype PV system.