Michael Jaritz

Analytical model for the thermal resistance of windings consisting of solid or litz wire

In this paper a method to calculate the thermal resistance of windings is presented and validated by measurements. Two formulas have been developed for widely used wire types in power electronics being round and litz wire. With the presented approach it is possible to describe the thermal resistance of arbitrary wire arrangements. This analytical approach can either be used in fast forward designs of magnetic devices, respectively can also be integrated in automatized optimization procedures to improve the designs of magnetic components. The calculated results are showing good coincidence with the measured values.

Optimal Design of a Modular Series Parallel Resonant Converter for a Solid State 2.88 MW/115-kV Long Pulse Modulator

Modern accelerator driven experiments like linear colliders or spallation sources are supplied by RF amplifiers using klystrons. The cathode voltage for these klystrons can be generated by long pulse modulators generating highly accurate voltage pulses in the length of milliseconds. Conventional designs using pulse transformers become huge for long pulses. The series-parallel resonant converter (SPRC) topology avoids this drawback as the transformer is operating at high frequencies. This paper presents a comprehensive design approach for SPRC modules, which is based on an optimization procedure containing an electrical model of the resonant circuit and a thermal model of the semiconductors. In addition, an insulation and a leakage design procedure and a thermal model of the transformer are also presented. Finally, this procedure provides the optimal design parameters of the resonant circuit elements. With these parameters, a global optimizer chooses the optimum amount of modules connected in series and/or in parallel to fulfill the given restrictions. The efficiency of a basic SPRC-module is 94.7% with a pulsed power density of 4.63 kW/l.

Control of a modular series parallel resonant converter system for a solid state 2.88MW/115-kV long pulse modulator

In this paper, a control strategy of a modular serial parallel resonant converter (SPRC) modulator system is presented and verified by simulations. The system is based on two SPRC modules forming an input series output parallel (ISOP) stack. To obtain the given output voltage of 115kV, 8 of these ISOP stacks are connected in parallel at the input and in series at the output, forming an input parallel output series (IPOS) system. For designing the control of this system, a large signal model is derived and the influence of component tolerances is investigated. The performance of the controller is shown with simulation results. The simulation model of a single module is validated by measurement results.

Optimal design of a modular 11kW serial parallel resonant converter for a solid state 115-kV long pulse modulator

Modern accelerator driven experiments like linear colliders or spallation sources are supplied by RF amplifiers using klystrons. The cathode voltage for these klystrons can be generated by long pulse modulators generating highly accurate voltage pulses in the length of milliseconds. Common designs like Bouncer Modulator topologies using pulse transformers become huge for long pulses. The series parallel resonant converter (SPRC) is a modular topology which avoids this drawback as the transformer is operated at high frequencies. In the considered application, the required nominal pulse voltage amplitude is 115kV with a pulse power of 2.88MW and a pulse length of 2.8ms. The pulse to pulse reproducibility of 0.02% and a voltage ripple at top of less than 0.05% are highly demanding. Additionally, the energy delivered to the load in case of an arcing klystron should not exceed 10J and the time of the converter should exceed 109 pulses. In order to meet these highly demanding specifications, the modulator is based on interleaved SPRC modules. Because of the high number of degrees of freedom as geometric parameters of the transformer, number of parallel semiconductors, design of the resonant tank the optimization procedure presented in [1] has been developed for designing the modulator. A SPRC module contains a full bridge connected to a series parallel circuit followed by a transformer a rectifier and a filter capacitor. In this paper, an optimal design of a single module according to the design considerations in [1] which are based on an electrical model of the inverter, a magnetic, a thermal and an isolation model of the transformer is presented and the design trade-offs/alternatives are discussed in detail. For validation of the models and the optimization procedure, a prototype of a single module has been built and is tested under full load conditions. There the focus is on evaluating the thermal behaviour of the transformer and the isolation of the transformer, which is especially crucial for a series connection of the modules. In order to meet the highly demanding requirements on the ripple and the reproducibility, a comprehensive small signal model based on [2] and control strategy for optimal interleaving of the series connected modules is presented in the paper. This enables to minimize the output voltage ripple and reducing the filtering effort at the same time.

Design procedure of a 14.4 kV, 100 kHz transformer with a high isolation voltage (115 kV)

In this paper, the design procedure of a 14.4 kV output voltage, 100 kHz transformer with an isolation voltage of 115 kV using Litz wire is presented. All design models, including a generalized magnetic model for the leakage and the loss calculations as well as an electrical model for the parasitic capacitance estimation for the transformer are derived and proven by measurements. For designing the insulation, a comprehensive design method based on an analytical maximum electrical field evaluation and an electrical field conform design is used. The resulting design is verified by long and short term partial discharge measurements on a prototype transformer.

Experimental Validation of a Series Parallel Resonant Converter Model for a Solid State 115-kV Long Pulse Modulator

Medium and high beta cavities used in linear colliders or spallation sources are supplied by klystrons or inductive output tubes amplifiers. The cathode voltage for these amplifiers can be generated by long-pulse modulators generating highly accurate high voltage pulses in the length of milliseconds. With existing modulator topologies, all the demanding requirements like fast pulse rise time, high accuracy, and low voltage ripple hardly can be satisfied at the same time. Common designs like bouncer modulator topologies using pulse transformers become huge for long pulses. The series-parallel resonant converter (SPRC) avoids this drawback as the transformer is operated at high frequencies. In this paper, the comprehensive multidomain model of an SPRC including an electrical model of the inverter, a magnetic model, and an isolation design procedure of the transformer is verified with a prototype of a single module operated under full-load conditions. In addition, a comparison between the predicted parasitics like leakage inductance and stray capacitance of the transformer and the measured values are given. An evaluation of the isolation of the transformer, which is especially crucial for a series connection of the modules, is also performed. In addition, different possibilities to realize the desired series inductance are discussed.

Design and Optimization Procedure for High-Voltage Pulse Power Transformers

A design and optimization procedure for high-voltage pulse transformers is presented. The procedure is enhanced by the integration of core loss measurements under pulsed excitation, by electrical peak field calculations and by checking of the isolation distances, which are compared to scaled high-voltage breakdown data. The procedure is applied with specifications of the compact linear collider, and the sensitivity of system parameters such as the number of primary turns, core material, transformer oil, and high-voltage cable length is investigated by comparing their Pareto fronts. With amorphous core material, the highest efficiency is achieved. Replacing mineral oil by natural ester results in an efficiency reduction of 0.5%, and an increase in HV cable length from 1.5 to 5 m reduces the efficiency by 0.6%. Finally, the optimized transformers pulse shape is investigated over the entire load range.

Isolation design of a 14.4kV, 100kHz transformer with a high isolation voltage (115kV)

In this paper, the isolation design procedure of a 14.4kV output voltage, 100kHz transformer with an isolation voltage of 115kV using Litz wire is presented. For designing the isolation, a comprehensive design method based on an analytical maximum electrical field evaluation and an electrical field conform design is used. The resulting design is verified by long and short term partial discharge measurements on a prototype transformer.