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

Powering terminal should prevent signal inflow from the communication lines. Powered terminal in most wire communication systems needs to separate ac signals and dc supply power in its input part connected to the communication lines. Our original communication system consists of low-power dc-dc converters. In our system, the direction of dc power is opposite in powering terminal at reception and powered terminal. Therefore, to use the same electronic choke for powering terminal and powered terminal, it is preferable that the electronic choke be bidirectional. So, we have proposed new electronic choke by the use of small inductors and an amplifier. From the viewpoint of power distribution, this system contains a parallel-connected dc power supply. In this paper, voltage-ripple attenuation characteristics at reception are presented on the dc power distribution system using buck-type converters and new electronic chokes. New electronic choke has an ability of good attenuation against signals. Even though switching frequencies are different, no beat is observed in the output dc voltage waveform of powered terminal. New electronic choke effectively removes circulation or interference by the ripple.

Ripple Reduction Characteristics in the Parallel-Connected DC Power Distribution System Constructed by New Terminals for Wire Communication

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

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. >

Improved variable capacitance device and its applications to resonant converters

A novel wire communication system which consists of low-power switched-mode DC-DC power converters is proposed in this paper. A converter acts as not only a power supply but also a transmitter. The main power unit in a conventional system can be removed and the system reliability is much increased, because converters construct a parallel-connected system with respect to DC power. Transmission of signals is performed by the use of output voltage ripple. At transmission, this ripple is increased intentionally and modulated. Both analog modulation and digital modulation are acceptable.

Wire communication system utilizing ripple-modulated DC-DC converter as signal transmitter and power supply

In our original wire communication system that consists of low-power dc-dc converters, a converter in the powering terminal operates as not only power supply but also transmitter. Signal is transmitted by the use of output-voltage ripple. At transmission of message, the ripple is intentionally increased and modulated by the signal. Steady state and transient characteristics on FM (frequency modulation) by the way of varying the switching frequency are investigated. FSK (frequency-shift keying) is presented as an example of digital FM transmission and reception. In addition, we introduce a new electronic choke used in each terminal.

https://ieeexplore.ieee.org/iel5/8870/28025/01252179.pdf

Digital-FM transmission and reception in a new wire-communication system that utilizes dc-dc converter as transmitter

In the authors' original wire communication system having low-power DC-DC converters, a converter in the powering terminal acts as a transmitter. A signal is transmitted by the use of the output-voltage ripple as carrier wave. At transmission of a message, the ripple is intentionally increased and modulated by the signal. Steady state and transient characteristics on the ripple are investigated in detail on the case that the capacitance of the output filter capacitor is varied. Experimental results on ASK (amplitude-shift keying) are presented as an example of digital modulation.

Akihiko Katsuki

Papers

Improved variable capacitance device and its applications to resonant converters

Wire communication system utilizing ripple-modulated DC-DC converter as signal transmitter and power supply

Ripple Reduction Characteristics in the Parallel-Connected DC Power Distribution System Constructed by New Terminals for Wire Communication

Digital-FM transmission and reception in a new wire-communication system that utilizes dc-dc converter as transmitter

Signal transmission characteristics of inductively loaded high-ripple DC-DC converter used in a new wire communication system