J.B. Klaassens

Resonant Contactless Energy Transfer With Improved Efficiency

This paper describes the theoretical and experimental results achieved in optimizing the application of the series loaded series resonant converter for contactless energy transfer. The main goal of this work is to define the power stage operation mode that guarantees the highest possible efficiency. The results suggest a method to select the physical parameters (operation frequency, characteristic impedance, transformer ratio, etc.) to achieve that efficiency improvement. The research clarifies also the effects of the physical separation between both halves of the ferromagnetic core on the characteristics of the transformer. It is shown that for practical values of the separation distance, the leakage inductance, being part of the resonant inductor, remains almost unchanged. Nevertheless, the current distribution between the primary and the secondary windings changes significantly due to the large variation of the magnetizing inductance. An approximation in the circuit analysis permits to obtain more rapidly the changing values of the converter parameters. The analysis results in a set of equations which solutions are presented graphically. The graphics show a shift of the best efficiency operation zone, compared to the converter with an ideally coupled transformer. Experimental results are presented confirming that expected tendency.

Phase-staggering control of a series-resonant DC-DC converter with paralleled power modules

A method of decreasing the ripple on the output voltage of high-power AC-DC or DC-DC series-resonant converters without increasing the internal converter frequency or the capacity of the energy storage elements is discussed. This improvement is accomplished by subdividing the converter into two or more series-resonant power modules operated with a constant relative phase shift (phase-staggering control). The method of eliminating the harmonic components in the input and output currents of the conversion system, without increasing the internal pulse frequency, is justified by Fourier analysis of the current waveforms. The frequency spectra of the source and output waveforms for the continuous and discontinuous resonant current mode are shifted to higher frequency ranges, as computations show for both one single module and multiple paralleled modules. Inadequacies in the phase-staggering control method applied to series-resonant converters are indicated in relation to the dominant harmonic component, in particular for two modules and supported by experimentally acquired waveforms. High-frequency current components to the source and to the load are reduced. Resulting in smaller input and output filters. This improves the resolution of the control of the flow of energy from the source to the load, resulting in a faster system response. >

Efficient resonant power conversion

The DC analysis of a series-resonant converter operating above resonant frequency is presented. The results are used to analyze the current form factor and its effect on the efficiency. The selection of the switching frequency to maximize the efficiency is considered. The derived expressions are generalized and can be applied to calculations in any of the switching modes for a series-resonant circuit. For switching frequencies higher than the resonant frequency, an area of more efficient operation is indicated which will aid in the design of this class of converters and power supplies. It is pointed out that (especially for power MOSFETs where ohmic losses dominate) it is more attractive to select switching frequencies that are higher than the resonant frequency because of the possibility of nondissipative snubbers. Slowing down the rise of the gate voltage and, hence, the slow decrease of ON resistance during turn-on is also not a drawback to high-frequency switching. Because of this safer operation, the standard intrinsic diode of the power MOSFET could be used at high frequencies instead of the more expensive FREDFET. >

Series resonant AC-power interface with an optimal power factor and enhanced conversion ratio

A new method of power pulse modulation with internal frequencies of tens of kHz which is also suitable for multikilowatt power levels is applied to a series-resonant converter system for generating synthesized multiphase bipolar waveforms with reversible power flow. The power circuitry of the converter is provided with one single high-frequency resonant link in the direct energy path. Natural current commutation of the thyristors is obtained by the use of a series-resonant circuit for power transfer and control.

Side effects in low-speed AC drives

Contemporary converter-fed AC machines are being increasingly applied in low-speed industrial applications. This is a demanding task for AC drives with respect to the power topology and the applied control. The side effects inherent in all categories of AC drives have a significant influence on low-speed performance. This paper discusses some of these effects such as the influence of the voltage drop over semiconductor switches, the DC offset in the system and the system asymmetries. Their common feature is that at near zero rotor speeds, assuming the closed-loop control of the machine, they might cause instabilities in the system thus worsening its dynamical behavior. Suitable examples are presented and analyzed to give the proper proportion of the problem. Also, troubleshooting hints and suggestions helpful to its solution are given.<<ETX>>

Three-phase AC-to-AC series-resonant power converter with a reduced number of thyristors

Power pulse modulation with internal frequencies of tens of kHz and suited for multikilowatt power levels is applied to a series-resonant converter (SRC) system for generating synthesized multiphase bipolar waveforms with reversible power flow and low distortion. The high pulse frequency allows the application of the principle of modulation and demodulation for fast system response. The use of an SRC for power transfer and control obtains natural current commutation of the thyristors and the prevention of excessive stresses on components. Switches with bidirectional current conduction and voltage blocking ability are required. The conventional series-resonant AC-AC converter applies a total for 24 antiparallel thyristors. An alternative circuit configuration for the series-resonant AC-AC converter with only 12 thyristors is presented. Use of the converter results in a higher efficiency and lower costs. >

Transformer-induced low-frequency oscillations in the series-resonant converter

The article demonstrates the existence of a number of modes of transformer-induced low-frequency oscillations (TLOs) which can be observed in the series-resonant power converter with a transformer in the resonant circuit, operating under conditions of cyclic stability. The TLO phenomena are mathematically analyzed and the conditions of existence are determined. Experimental observations confirm the outcomes of the mathematical analyses. The critical aspects of the TLO phenomena with respect to the converter performance are explored.

Steady-state analysis of a series-resonant DC-DC converter with a bipolar power flow

A time-domain analysis for the steady-state model of a series-resonant power interface for both step-up and step-down modes is presented. The exchange of electrical energy between a source and the resonant circuit in order to stabilize the stored electrical energy is defined. The characteristics of a series-resonant converter with bilateral power flow are presented in normalized form, described by the output characteristics. The results obtained in a four-quadrant motor drive illustrate the characteristics of a high-frequency power interface. >

A multiple switch high-voltage DC-DC converter

A high-voltage DC-DC power converter with multiple series-connected devices is proposed. Series connection of power devices has evolved into a mature technique and is widely applied in HVDC systems. Static and dynamic voltage balance is ensured by shunting individual devices with (dissipative) snubbers. The snubber losses become pronounced for increased operating frequencies and adversely affect power density. Capacitive snubbers do not exhibit these disadvantages but they prerequisite zero-voltage switching mode. Super-resonant power converters facilitate the principle of zero-voltage switching. Furthermore they allow the application of nondissipating snubbers to assist the voltage sharing between the multiple series-connected devices and to lower turn-off losses. Simulation results obtained with a circuit simulator are validated in an experimental converter. The behaviour of the series connection will be examined for MOSFETs and IGBTs by both experimental work with a 1 kW prototype and computer simulation.

Novel three-phase AC-AC soft-switching series-resonant converter topology with six switches

The authors present a novel converter topology with only six switches. The switch current is a half period of the sine wave as generated by the resonant network. The delta i/ delta t is limited during the switching interval. The switching loss equals the product of switch voltage and switch current and is consequently low. The principle of soft switching can reduce the total power losses by one-third to one-half compared to previous series resonant converters without soft switching. The converter controls the output voltages as well as the input current. The harmonic distortion of both waveforms is low. The power factor of the input currents is controlled. The results of simulation confirmed the characteristics of this class of series resonant AC-AC power converters.<<ETX>>