Zhongming Ye

Circulating Current Minimization in High-Frequency AC Power Distribution Architecture With Multiple Inverter Modules Operated in Parallel

The control issue of multiple inverter modules operated in parallel is investigated for high-frequency alternative current (HFAC) power distribution architectures, where multiple high-frequency resonant inverters are connected in parallel to the high-frequency high-voltage low-current (HVLC) AC bus, to feed a number of point-of-use power supplies. The circulating current in the multiple inverter system is analyzed first. A novel control scheme is proposed based on the active and reactive current decomposition concept. In the proposed control, there are two loops: 1) the current sharing control loop and 2) the voltage feedback control loop. For the current sharing loop, the active current and reactive current are controlled separately. It is shown that the minimization of the circulating current can be achieved if both the active current and reactive current of the equivalent load are evenly distributed among activated inverter modules. This control method is superior to the scalar control where only the magnitudes of the currents are controlled. Performance is verified with both simulations and experiments on a prototype HFAC power distribution system where two two-stage resonant inverter modules of 500 kHz and 100 W are connected in parallel through small connection inductors to the 500-kHz 28-V rms HFAC bus.

A topology study of single-phase offline AC/DC converters for high brightness white LED lighting with power factor pre-regulation and brightness dimmable

Due to the recent advancement in the light emitting diode (LED) technology, high brightness white LED becomes feasible in residential, industry and commercial applications to replace the incandescent bulbs, halogen bulbs, and even compact fluorescent light bulbs. In these offline applications, high power factor, and low harmonics are of primary importance. Single stage power factor pre-regulation technology is preferred in these cost sensitive applications where power factor regulation is mandatory, while adding additional power factor correction controller will surely increase the cost. In this paper, we exam several single stage offline ac/dc power converters with power factor pre-regulation for LED lighting applications, including boost converter, SEPIC converter, flyback converter, and half-bridge converter. A boost converter is suitable for high efficiency, large LED string applications. A SEPIC converter has the best power factor correction performance, but the efficiency is not comparable with the other solutions. Flyback converter is the most commonly used topology for low power offline applications. For medium power applications, symmetrical and asymmetrical half bridge converters can be used with better power factor and high efficiency. The harmonics of the input line current is reduced and power factor is high if the current in the input inductor is discontinuous and the converter is driven with constant duty ratio. A current feedback loop can be used to control the LED brightness. The performance of these converters will be analyzed and compared in terms of power factor, efficiency, etc, based on the lab prototype boards.

A Full-Bridge Resonant Inverter With Modified Phase-Shift Modulation for High-Frequency AC Power Distribution Systems

The design of a resonant inverter for high-frequency ac (HFAC) power distribution systems is complicated by the following three factors: 1) A number of electronic loads located in different locations are connected to the resonant inverter, the impedance, and the power factor of the equivalent load of which varies over a wider range than a system with a certain load; 2) the resonant inverter is subject to an input-line voltage varying over a wide range; and 3) the characteristics of the resonant inverter depend on the load impedance. It is mandatory to operate the inverter with zero-voltage switching under various load conditions of different power factors and over wide input variations. It is further desirable that multiple resonant inverters can be paralleled with simple current-sharing control (CSC). A phase-shift- modulation (PSM)-controlled full-bridge series-parallel resonant inverter is proposed for the HFAC power distribution architectures. A new PSM method is proposed with which the phase angle of the inverter output voltage is independent of the modulation signal of the phase-shift modulator. Such a feature allows multiple resonant inverters to operate in parallel with a magnitude CSC. The resonant inverter is analyzed with a general nonresistive load model, and the design curves are developed. A prototype resonant inverter system is designed and implemented with an operation frequency of 1 MHz, a rated output power of 150 W, and a sinusoidal output voltage of 1-MHz 28-V rms. The proposed resonant inverter has the advantages of high efficiency over wide input/output line variations, high waveform quality of the output voltage, and phase-angle independence of the voltage-feedback/feed-forward control and CSC.

Design considerations of a high power factor SEPIC converter for high brightness white LED lighting applications

A current-shaper SEPIC converter for high brightness LEDs is presented. Due to the recent advancement in the light emitting diode (LED) technology, high brightness white LED becomes feasible in residential, industry and commercial applications to replace the incandescent bulbs, halogen bulbs, and even compact fluorescent light bulbs. In these offline applications, high power factor, and low harmonics are of primary importance. It is also important to step up/down voltage with simple single-stage conversion for low cost and high efficiency. A SEPIC converter is particularly suitable for non-isolated application since it is single-stage, and can step up/down, and of high power factor as it is run in discontinue conduction mode. In this paper, we proposed a high power factor SEPIC converter for the high brightness LED lighting applications with universal input voltages and dimmable LED light. The condition of discontinuous conduction mode is given. The harmonics of the input line current is reduced and power factor is high because the input inductor current follows the input voltage. A current feedback loop is proposed to control the LED brightness. The gate drive signal of the switch is generated by comparing the saw-tooth carrier signal of the PWM controller with the current feedback signal. This circuit has the advantages of one stage of power conversion, no need to sense the input voltage, simple feedback control, voltage step-up and down, high power factor and dimmable LED current. It is particular suitable for the offline LED applications. The critical design constraints and equations for both the power stage and control loop are highlighted and detailed. A practical evaluation board is developed to verify the proposed design.

Single-Stage Offline SEPIC Converter with Power Factor Correction to Drive High Brightness LEDs

An interleaved SEPIC converter with LED current dimmable and input power factor correction is proposed as a high performance driver for the high brightness white LEDs. The converter is controlled with voltage mode PWM and run in discontinuous conduction mode so that the inductor current follows the rectified input voltage. The critical design constraints and equations for both the power stage and control loop are highlighted and detailed. A practical evaluation board with 110V input and 60V, 700mA output is developed to verify the proposed design. The proposed converter can be used to drive a wide number of high brightness LEDs for industrial or commercial lighting applications.

A Robust One-Cycle Controlled Full-Bridge Series-Parallel Resonant Inverter for a High-Frequency AC (HFAC) Distribution System

Resonant inverters are connected to a high-frequency AC (HFAC) bus, where power is delivered to different locations for points-of-use power management. Such a power distribution system subjects to more perturbations and load uncertainties than inverters operating with single load. A novel voltage control method is proposed in this paper for a high-frequency full-bridge resonant inverter with series-parallel resonant tank. A modified one-cycle controlled phase-shift modulation is proposed to effectively compensate the input line variations. The uncertainty model of the high frequency resonant inverter is developed and analyzed with the resonant circuit component tolerance, input line and load variations taken into design considerations. The voltage feedback controller is designed based on the Hinfin robust control theory and is implemented with analog discrete devices. The proposed control scheme has the advantages of fast response for both input line and load perturbations. It also ensures a wide range of system stability and guarantees robustness of the power converter. Both simulations and experimental results are provided to verify with the theoretical analysis through an experimental prototype of a full-bridge resonant inverter with an output power of 150-W operating at 1 MHz and an output voltage of 28 V (rms).

Phasor-Domain Modeling of Resonant Inverters for High-Frequency AC Power Distribution Systems

The circuit modeling and analysis of resonant inverters is complex because the state variables such as inductor currents and capacitor voltages are AC dominant. The phasor dynamic modeling method maps the periodical time-varying state variables into stationary frame for each harmonic of interest. Correspondingly, the circuit is decomposed into two DC subcircuits, the state variables of which are the time-varying Fourier coefficients of the original AC variables. A small-signal model can be derived by applying small perturbation and linearization to the Fourier coefficients. A phasor-domain modeling method is used to investigate the resonant inverters in high-frequency AC power distribution systems. A resonant inverter system with five energy storage elements is modeled and simulated, and compared with switch simulation for both steady state and transients. The phasor model simulation matches the switch model simulation in both steady state and transients, but takes much less computing time. In addition, this model closely relates to the power converter topology in time domain, and therefore, keeps the physical meaning of the state variables. It can be used for high accuracy of modeling, simulation, and circuit analysis and control design. It can be extended to a higher order of resonant topologies including parasitic components. A high-frequency AC system with two pulse-phase-modulation-controlled resonant inverters is modeled and simulated, and the current distribution control is investigated with the phasor model. The model simulation is compared with switch-level simulation and experimental results.

A Two-Stage Resonant Inverter With Control of the Phase Angle and Magnitude of the Output Voltage

A high-efficiency two-stage resonant inverter with effective control of both the magnitude and phase angle of the output voltage was proposed in this paper for high-frequency ac (HFAC) power-distribution applications, where a number of resonant inverters need to be paralleled. In order to parallel multiple resonant inverters of the same operation frequency, each inverter module needs independent control of the phase angle and magnitude of the output voltage. It is also desirable that the output voltage has very low total harmonics distortion, as well as high efficiency over wide input and load ranges. The proposed resonant inverter consists of two stages. The first stage is a two-switch dc/dc converter with zero-voltage transition, and the second stage is a half-bridge resonant dc/ac inverter with fixed duty ratio. A series-parallel resonant tank is used to achieve high waveform quality of the output voltage. The magnitude of the output voltage is regulated through the duty-ratio control of the first stage with pulsewidth modulation. The phase angle of the output voltage is regulated through a pulse-phase-modulation control of the second stage. The proposed resonant inverter has the advantages of better waveform quality, wide range of input and load variations for soft-switching, and independent control of the phase angle and magnitude of the output voltage, making it an attractive candidate for applications where a number of resonant inverters need to be placed in parallel to the HFAC bus and a number of distributed loads are connected to the HFAC bus. The performance is verified with both simulation and experiments.

A Full Bridge Resonant Inverter with Modified Phase Shift Modulation

A full bridge resonant inverter is proposed as one of the potential building blocks for high frequency ac distributed power supply. Because the load impedance in ac distributed power system is unknown and the input line voltage has wide range of variation, voltage feedback and feed-forward control has to be applied to regulate the output voltage. However with the conventional phase shift modulation (PSM), the phase angle of the inverter output voltage shifts with the line and load perturbation. A modified symmetrical PSM is proposed where triangle waveform is used as carrier signal and the modulation signal is double edge modulated. With this method, the phase angle of the output voltage is independent of the line and load variation. The mechanism is described and analyzed. Performance during steady state and transient shows that the symmetrical PSM inverter is applicable to high frequency ac distributed power system

Multiple frequency modeling of high frequency resonant inverter system

Multiple-frequency modeling (MFM) offers a general small signal modeling methodology for resonant type converters. A DC/AC resonant inverter is decomposed into two pure DC sub-circuits for each harmonics, the excitations of which are orthogonal. However, such decomposition makes the definitions of active power P, reactive power Q, apparent power S and instantaneous power p difficult, while these concepts are important for resonant inverter system power flow analysis and controller design. We extend the MFM by giving general definitions of P, Q and S in terms of the state variables of the series of decomposed sub-circuits of MFM. We present a general circuit model for high frequency DC/AC resonant inverter system. With this model, power flow analysis can be conveniently performed. Performances are analyzed in detail for a high frequency inverter-ac bus system.