Analysis of the ferroresonant transformer with a rectified outputin the low-line-voltage minimum-line-frequency full-load condition

Most previous analyses of the ferroresonant transformer address the unrectified-output AC-load case, and do not include some or all of the important loss mechanisms. The analysis reported in this paper addresses the rectified-output DC-load case and includes the stray losses. The low-line-voltage minimum-line-frequency full-load condition is the focus of this analysis because it is the worst-case condition for supplying power to the load. Assumptions about the operation of the transformer in this worst-case condition lead to a simplified equivalent circuit. Given equivalent-circuit element values, a special-purpose time-domain computer simulation analyzes transformer performance in the periodic steady-state condition. This analysis technique is used to find equivalent-circuit element values for which the transformer winding currents are a minimum and the input power factor and conversion efficiency are at a maximum     

Analysis of magnetic field and circulating current for HTS transformer windings

In a high-temperature superconducting (HTS) transformer, the leakage magnetic field decreases the critical current and increases the ac loss in the tapes. Moreover, because of nearly zero resistance of HTS tapes, a slight unbalance of the branch inductances of the windings might result in heavy circulating current. So, the numerical analysis of the leakage magnetic field and circulating current is especially necessary for an HTS transformer design. In this paper, the influence of the winding configurations on the stray field and circulating current is studied. That is, the magnetic field distribution is analyzed by finite-element method and then, based on the inductance matrix obtained after a magnetic field analysis, the circulating current is calculated by circuit analysis. Some measures for improving the leakage field and circulating current distribution are also proposed to make HTS transformers more efficient.     

A Computer Model of Magnetic Saturation and Hysteresis for Use on SPICE2

Magnetic saturation and hysteresis are simulated on SPICE2 using a model containing two nonlinear controlled sources. The model is applied to inductors and transformers made from a variety of core materials, and procedures are given for determining the model parameters directly from magnetic core catalog data. Four examples of circuit applications are given: a saturating instrumentation transformer, a saturating snubber inductor, a ferroresonant regulator transformer, and a Royer saturation oscillator.     

Sliding Transformer With Long Magnetic Circuit for Contactless Electrical Energy Delivery to Mobile Receivers

A new type of sliding transformer with long magnetic core and movable secondary winding is presented. The basic transformer configurations and wiring are described. The magnetic flux distribution along the magnetic circuit and its influence on transformer parameters are discussed and examined. Also, the leakage inductance of the transformer primary winding, as the main cause of voltage drop and energy losses, has been discussed. The main experimental results of transformer model investigation are presented in this paper     

A novel method for the synthesis of magnetically coupled networks considering the frequency-dependence of the impedances

A new methodology for the representation of a set of magnetically coupled coils by means of an equivalent circuit is presented in this paper. The proposed method makes use of an equivalent circuit made up of a primary group of RL branches, which are the input terminals-pairs of the model, and a group of RL loops that are magnetically coupled with the primary branches, constituting a multiport model of a system of coupled coils that allows representing its frequency-behavior very accurately. This can be seen as a generalization of the classic methodology for the synthesis of RL driving-point impedances (developed by Foster and Cauer) considering the more general case of having multiple input terminals-pairs. This paper contributes an appropriate tool for the accurate modeling of transformer windings, a historically very difficult problem due to the frequency-dependence of the inductances and losses. The new method is first applied to the simple case of a single coil and was used afterward to the representation of a coil system, proving in both cases the mathematical validity of the method. The usefulness of the method is shown through its application to the modeling of a transformer winding.     

一种低漏感平面变压器

变压器的漏感是电磁干扰的主要来源之一,这是因为开关管在高速关断时,在变压器的漏感上产生感应电动势,叠加在变压器绕组的关断电压上,形成关断电压尖峰,这些电压尖峰不但造成电磁干扰,还会使开关管的电压应力增大,重者可能击穿开关管,并增大开关损耗,降低开关电源的效率。本文提出一种分布磁路结构的低漏感平面变压器,其原边绕组的匝数降低为一匝,副边绕组的等效匝数降低为小于一匝,因而漏感显著减小,这种分布磁路结构可以用于低压大电流电源的变压器,其有效性通过Maxwell 3D得到验证。     

A single-ended SMR converter topology with optimized switchingcharacteristics

The analysis and design of an improved single-ended switched-mode rectifier (SMR) converter topology is presented. The novel feature of this topology is a nondissipative LC-type subcircuit which provides transformer flux balancing and improves converter efficiency. This is achieved by minimizing switching losses and by returning the energy stored in the transformer leakage and circuit stray inductances to the source and the load. Theoretical predictions and SMR converter design procedures are verified experimentally on a 1 kW 20 kHz prototype circuit     

高重复频率纳秒级脉冲发生器研究

为实现高重复频率纳秒级脉冲输出,提出了采用射频功率MOSFET,基于感应叠加拓扑的脉冲发生器。脉冲发生器采用15个模块化组件,每个组件输出670 V/50 A 脉冲,每个组件的输出脉冲在感应变压器次级串联叠加,得到10 kV/50 A 高压脉冲。为实现脉冲前沿小于5 ns,必须尽量降低脉冲变压器漏感以及组件和系统的回路电感。感应脉冲变压器采用圆柱形同轴结构,初次级均为单匝,并且和脉冲发生器单元一体化设计,以减小漏感以及组件分布电感。采用大功率驱动电路和同步触发器,实现MOSFET 开关的快速导通和关断,以及触发的一致性。仿真结果显示设计能够满足指标要求。     

高频反激变压器绕组模型优化设计

反激变换器中高频变压器是核心部件,其效率直接关系到变换器的效率,因此优化设计高频变压器就成为提高效率的关键。通过对反激变压器绕组采用不同结构时所带来不同的涡流损耗和漏感进行分析,得到本文所设计绕组结构二维模型。利用有限元分析软件进行数值仿真,获得的数据证明此模型是可行的。制作出实验样机对其进行实验比较,验证了所设计的高频变压器绕组结构合理,漏感小,效率高,输出的电压的谐波含量低。     

Elimination of Leakage Effects Related to the Use of Windings with Fractions of Turns

Obtaining exact output voltages from transformers is a difficult task due to discrete turn ratios. This becomes even more difficult in modern switching power supplies operating above 100 kHz because the number of turns per winding is reduced. For multichannel applications different methods of postregulation have been employed, resulting in increased power dissipation and power supply cost. A new method of dealing with this problem has been developed, offering a higher degree of flexibility. This technique allows the use of fractions of turns, while maintaining low leakage inductances. Theoretical analysis of leakage effects related to fractional turn windings has been derived and tests on an experimental transformer have been performed.     

HF Characterization and Nonlinear Modeling of a Gapped Toroidal Magnetic Structure

The frequency dependent characteristics of a gapped toroidal structure are extracted empirically over a bandwidth that exceeds 30MHz. The analysis is complicated due to nonlinear flux distributions, magnetic properties of the core material, leakage inductance, stray capacitances, and eddy currents in the windings. A permeance model of the core is implemented to model the magnetic circuit. The model includes a linear lumped element equivalent circuit to approximate the nonlinear complex permeability of the core, which was measured empirically. Stray capacitance and inductance of the winding are also modeled. A gyrator is used to couple the electric and magnetic models for circuit simulation. The measured and simulated results of open-circuit impedance from the secondary winding and the transimpedance gain (V/A) of the current sensor are compared and discussed.     

Coaxially wound transformers for high-power high-frequencyapplications

Design considerations for transformers utilized in high-power high-frequency DC/DC converters are addressed. Major areas of concern are core-material selection, minimization of copper losses due to skin and proximity effects, and the realization of controlled leakage inductances. Coreless characteristics for various high-frequency materials are presented, and the influence of various conventional winding arrangements on the copper losses and leakage field is also demonstrated. Coaxial winding techniques (used commonly in high-frequency transformers) are investigated next as a feasible solution for containing the leakage flux within the interwinding space, thus preventing it from permeating the core and resulting in lower core losses and the avoidance of localized heating. Added benefits of this technique are reduced forces within the transformer, lower copper losses, and robust construction. The performances of two experimental single-phase 50 kW, 50 kHz units are reported. A three-phase version of coaxially wound transformers is also presented     

A high-frequency two-switch forward converter with optimizedperformance

The analysis and design of a high-frequency two-switch forward converter topology with transformer flux balancing and extended duty cycle capability are presented. To improve converter performance, an auxiliary circuit connected in parallel with each power switch is proposed. This auxiliary circuit uses a low-power switch or a nonlinear resistor connected in series with a capacitor. As a result, the DC component of the magnetizing current is minimized, and the converter provides the means of recovering the energy associated with the parasitic inductances of the circuit components. Thus, higher than usual efficiency and higher operating frequencies are obtained. Experimental results are presented for a 4 kW, 40 kHz prototype unit     

一种新颖有源箝位ZVS正激变换器的研究

介绍了一种中心抽头全波整流有源箝位ZVS正激变换器的工作原理及主要参数计算。有源箝位电路由一个箝位开关管和箝位电容组成。变压器磁芯实现无损复位,励磁能量和漏感能量全部传递到负栽.磁芯利用率高,功率开关管承受电压应力降低。通过变压器漏感与开关管输出电容的谐振,主开关管与箝位开关管都可以实现ZVS开通,提高了变换器工作效率。文章首先分析了变换器工作原理,然后给出了主要参数的计算方法,最后通过样机(48V输入5V/20A输出)实验验证了该拓扑的高效性能。     

A New Model of Synchronous Machine Internal Faults Based on Winding Distribution

A synchronous machine internal faults model based on the actual winding arrangement is described in this paper. Based on the winding function approach, the machine inductances are calculated directly from the machine winding distribution, thereby the space harmonics produced by the machine windings are readily taken into account. Moreover, the calculation of the machine inductances is made easier by the use of the machine electrical parameters instead of the geometrical ones. Simulation results for internal faults on a laboratory generator are compared with experimental results to verify the accuracy of the proposed model     

A forward converter topology employing a resonant auxiliary circuit to achieve soft switching and power transformer resetting

This paper presents a forward converter topology that employs a small resonant auxiliary circuit. The advantages of the proposed topology include soft switching in both the main and auxiliary switches, recovery of the leakage inductance energy, simplified power transformer achieving self-reset without using the conventional reset winding, simple gate drive and control circuit, etc. Steady-state analysis is performed herein, and a design procedure is presented for general applications. A 35-75-Vdc to 5 Vdc 100-W prototype converter switched at a frequency of 200 kHz is built to verify the design, and 90% overall efficiency has been obtained experimentally at full load.     

Essential-coupling-path models for non-contact EMI in switching power converters using lumped circuit elements

This paper proposes a simple lumped circuit modeling approach for describing noncontact EMI coupling mechanisms in switching power converters. The resulting model assumes a minimum number of noise sources and contains essential coupling paths that allow easy physical interpretations. Essentially, all capacitive couplings are represented by an equivalent noise voltage source and six coupling impedances, whereas all inductive couplings are represented by an equivalent noise current source and three coupling impedances. The resulting coupled noise appears as currents flowing into the terminals of the line-impedance-stabilization-network (LISN). The equivalent voltage source can be conveniently approximated as the switching-node-to-zero voltage, which is typically a rectangular pulse of a few hundred volts. The equivalent current source can be modeled as the current flowing around a loop containing the equivalent voltage source and parasitics such as winding capacitance of the power transformer, the snubber capacitance and connection inductances. Also, the coupling impedances can be estimated by making simplifying assumptions about the geometry of the components and tracks, or by direct measurements. Simulations and experiments verify how inductive and capacitive couplings through each path may produce substantial EMI measured by the LISN. Being based on a lumped circuit approach, the proposed model is easy to apply in practice for understanding, diagnosing and approximating EMI behaviors.     

Voltage equaliser for Li–Fe battery

In this article, a voltage equaliser is proposed for a battery string with four Li–Fe batteries. The proposed voltage equaliser is developed from a flyback converter, which comprises a transformer, a power electronic switch and a resonant clamped circuit. The transformer contains a primary winding and four secondary windings with the same number of turns connected to each battery. The resonant clamped circuit is for recycling the energy of leakage inductance of the transformer and for performing zero-voltage switching (ZVS) of the power electronic switch. When the power electronic switch is switched on, the energy is stored in the transformer; and when the power electronic switch is switched off, the energy stored in the transformer will automatically charge the battery whose voltage is the lowest. In this way, the voltage of individual batteries in the battery string is balanced. The salient features of the proposed voltage equaliser are that only one switch is used, the energy stored in the leakage inductance of the transformer can be recycled and ZVS is obtained. A prototype is developed and tested to verify the performance of the proposed voltage equaliser. The experimental results show that the proposed voltage equaliser achieves the expected performance.     

Experimental determination of stray capacitances in high frequency transformers

This paper presents practical techniques for determining stray capacitances in a two-winding high frequency transformer for circuit simulation and computer-aided design purposes. These techniques fall into two categories: The two-port network approach; and the step-response approach. The first approach can be employed for high frequency transformer circuit models with the effect of stray capacitances modeled as a /spl pi/-shape network of three lumped stray capacitances. The second approach is useful for the transformer circuit model with the overall effects of stray capacitances modeled as lumped stray-capacitance connected cross the primary side. These techniques have been verified in the modeling and numerical simulation of a 500 W 25 kHz two winding E-core transformer. The merits and limitations of these techniques are also discussed.     

高频变压器铜损的三维模拟仿真与分析

对高频变压器建立三维电磁分析模型,采用有限元法对变压器铜损进行数值求解.模拟分析次级绕组段间距、初级绕组以及磁芯漏磁对次级绕组损耗的影响,并引入了矩形导线和圆导线之间的等效因子.分析表明,在考虑次级绕组段间距、初级绕组以及磁芯漏磁后,高频变压器次级铜损为246.086W,大于Dowell模型的理论计算值107.500W,这为变压器的优化设计提供了参考.