In this article, based on the LLC resonant converter fundamental harmonic analysis model, the modulation strategies for LLC resonant converter can be categorized into the following three groups: 1) input voltage fundamental harmonic modulation; 2) resonant tank elements modulation; and 3) secondary equivalent impedance modulation. Operational principles and control diagrams for different modulation strategies are given. Comprehensive comparisons between these modulation strategies in terms of topology complexity, control complexity, and voltage gain range are presented with respect to the same system specifications. The advantages and disadvantages for each modulation strategy are summarized to provide guidance for engineers when analyzing and designing an LLC resonant converter. The hybrid modulation strategies are categorized into different groups based on the specific applications. Brief introductions of these hybrid modulation strategies are presented. Future trends regarding the modulation strategies of LLC resonant converters are presented.

Overview of Modulation Strategies for LLC Resonant Converter

The capacitance of voltage-dependent capacitors can be defined in two ways: as “total capacitance” Ct ( v ) = Q / v and “local capacitance” Cd ( v ) = dQ / dv . The former is applicable to cases when the capacitance is measured by a charge injection or when the total capacitance is derived from the properties of the dielectric material. The “local capacitance” is applicable in cases when the capacitor is measured by a small test signal for various bias voltages. Based on the capacitance definitions, SPICE compatible models are implemented either by applying local and integral operators or by a nonlinear reflection of a linear capacitor. These are demonstrated by PSPICE behavioral-dependent sources. When properly emulated, the Ct and Cd models are applicable for simulating both small and large signals across the nonlinear capacitor. It is further brought up that both models suffer from convergence problems that can be partially alleviated by slowly increasing the level of the excitation signal and by reducing the maximum step time.

On SPICE Simulation of Voltage-Dependent Capacitors

The possibility of applying standard commercial ceramic capacitors for controlling a resonant LLC converter was studied analytically and experimentally. Using a two serially connected ferroelectric capacitors assembly, experiments were run on a mockup of an LLC converter that was switched at 90kHz to a power level of 5W. The voltage dependent capacitors had a range of 95nF-320nF for a bias span of 0–100V. A DC bias driver with a high output impedance that is suitable for controlling the voltage dependent capacitors is proposed and discussed. The theoretical considerations and experimental results of this study suggest that ferroelectric ceramic capacitors could be a viable control element of resonant LLC converters running at a fixed switching frequency.

Ceramic capacitor controlled resonant LLC converters

An entirely new type of AC voltage regulator is proposed by the use of our original variable capacitance device. This circuit theoretically makes no distortion of output voltage waveform, because only linear reactance devices are utilized in the power stage. Here, the variable capacitance device was employed as a linear capacitor to control output voltage. A boost-type DC-to-DC converter was used for a high-speed driver of this device.

Distortion-free regulated AC power supply using variable capacitance device

An entirely new hype of AC voltage/current regulator is proposed by the use of our original variable capacitance devices. This circuit theoretically makes no distortion of output waveform, because only linear reactance devices are utilized in the power stage. For output voltage/current control and input power-factor correction, two variable capacitance devices were employed as a linear capacitor. Two small boost-type DC-to-DC converters were used as a high-speed driver for these devices. These converters deliver no power in steady state. Input current of the proposed power supply becomes almost zero with the load shorted.

Power-factor-corrected distortion-free constant-voltage constant-current AC power supply using variable capacitance devices

A new variable capacitance device for use in power electronics is proposed. The capacitance of this device can be varied by DC control voltage. In the new device, constructed by ceramic capacitors, four well-balanced capacitors are bridge-connected. Therefore, the AC leakage current which flows from the control voltage terminals becomes extremely small. The series resistance for suppressing the leakage current can be removed unlike the former three-terminal device. This brings a marked decrease in the time constant as seen from the control voltage terminals. The transient response times are investigated in the case when this device is used as the resonant capacitor of resonant power converters. >

M. Fujiwara

Papers

Improved variable capacitance device and its applications to resonant converters