Franz C. Zach

Influence of the modulation method on the conduction and switching losses of a PWM converter system

The dependency of the conduction losses of a bridge leg of a PWM power converter system with a high pulse rate on the shape of the phase modulation functions is explored. This is done for modulation methods which are optimized with respect to minimum harmonic current RMS values. The results are compared to the results gained for simple sinusoidal modulation. Besides the conduction losses, the switching losses of the electric valves are calculated. Deviations from the classical sinusoidal modulation are only obtained for modulation methods for which the output voltage is formed by a cyclic change via only two active and a third nonswitching bridge leg. As the calculations show, these modulation methods allow a significant increase of the effective switching frequency. This effect is dependent on the phase angle between the fundamental of the converter output phase voltage and the converter output phase current; for this comparison, equal switching losses, as for the simple sinusoidal modulation, are assumed.<<ETX>>

A novel three-phase utility interface minimizing line current harmonics of high-power telecommunications rectifier modules

Based on the combination of a three-phase diode bridge and a DC/DC boost converter, a new three-phase three-switch three-level pulsewidth modulated (PWM) rectifier system is developed. It can be characterized by sinusoidal mains current consumption, controlled output voltage, and low-blocking voltage stress on the power transistors. The application could be, e.g., for feeding the DC link of a telecommunications power supply module. The stationary operational behavior, the control of the mains currents, and the control of the output voltage are analyzed. Finally, the stresses on the system components are determined by digital simulation and compared to the stresses in a conventional six-switch two-level PWM rectifier system.

A novel multicell DC-AC converter for applications in renewable energy systems

This paper presents a novel DC-AC converter for applications in the area of distributed energy generation systems, e.g., solar power systems, fuel-cell power systems in combination with supercapacitor or battery energy storage. The proposed converter is realized using an isolated multicell topology where the total AC output of the system is formed by series connection of several full-bridge converter stages. The DC links of the full bridges are supplied by individual DC-DC isolation stages which are arranged in parallel concerning the dc input of the. total system. Therefore, all switching cells of the proposed converter can be equipped with modern low-voltage high-current power MOSFETs, which results in an improved efficiency as compared to conventional isolated DC-AC converters. Furthermore, the cells are operated in an interleaved pulsewidth-modulation mode which, in connection with the low voltage level of each cell, significantly reduces the filtering effort on the AC output of the overall system. The paper describes the operating principle, analyzes the fundamental relationships which are relevant for component selection, and presents a specific circuit design. Finally, measurements taken from a 2-kW laboratory model are presented.

Design and experimental investigation of a three-phase high power density high efficiency unity power factor PWM (VIENNA) rectifier employing a novel integrated power semiconductor module

The development of guidelines for the practical application of a new power module (IXYS VUM25-E) realizing a bridge leg of a three-phase/switch/level PWM (VIENNA) rectifier system with low effects on the mains is discussed. The inner circuit structure of the power module is formed by a bidirectional bipolar switch and of two free-wheeling diodes. In a first step the switching losses of the power MOSFET and of the free-wheeling diodes are determined by measurement in dependency on the switched current for characteristic values of the junction temperature. The isolated driving stage of the MOSFET is designed for minimum switching losses considering the occurring switching overvoltages and the ringing between the parasitic circuit elements. The conduction losses of the semiconductor elements are calculated directly via simple analytical approximations of the mean and rms values of the device currents. Based on the knowledge of the dependency of the main loss contributions of the semiconductors of the power module on the operating parameters (mains voltage, output voltage, heat sink temperature and switching frequency) the thermally maximum allowable mains current amplitude is calculated. Furthermore, for different switching frequencies an overview over the power loss contributions of the semiconductor elements is given. Also, the reduction of the efficiency caused by the total semiconductor losses is determined. Finally, the overall efficiency of a PWM (VIENNA) rectifier system realised by using the IXYS VUM25-E module is estimated and further possible developments of this module are discussed.

Space vector-based analytical analysis of the input current distortion of a three-phase discontinuous-mode boost rectifier system

In this paper the low frequency harmonic distortion of the mains current of a three-phase single-switch discontinuous-mode boost-rectifier is calculated. The system analysis is based on application of space vector calculus and on substitution of discontinuous time shapes within a pulse period by quasicontinuous time shapes. The quasicontinuous time shapes are defined by averaging over the pulse period. The dependency of the shape of the input currents on the voltage transformation ratio is given for various control methods in analytical form. The results of the theoretical analysis are verified by digital simulation and by measurements on a laboratory model. A good consistency of the results has been found. >

Novel aspects of an application of 'zero'-ripple techniques to basic converter topologies

Based on a short summary of the theoretical basics of the zero-ripple current phenomenon a special zero input current ripple boost converter topology as presented in the literature is investigated. It is shown that the complete suppression of the input current ripple of the system is only given in a theoretical extreme case. For a practical realization only such a ripple reduction of the input current of the basic converter structure is obtained as corresponds to a simple low-pass input filter. This is proven by a detailed analysis also for a zero-ripple Cuk and for a zero-ripple SEPIC converter structure. Furthermore, it is shown that the realization of a zero-ripple Cuk or zero-ripple SEPIC converter is not linked to a magnetic coupling of the input and output inductors, but can also be achieved by a simple rearrangement of the elements of the basic converter structures. There one can see that the operating behavior of a zero input current ripple SEPIC converter is equivalent to the operating behavior of a buck-boost converter stage with LC input filter. Finally, the advantages and disadvantages of the different realization approaches of zero-ripple topologies are compared. Also, an outlook towards the planned further treatment of the topic is given.

A comprehensive design approach for a three-phase high-frequency single-switch discontinuous-mode boost power factor corrector based on analytically derived normalized converter component ratings

In this paper the peak, mean, and RMS values of the component currents of a three-phase single-switch discontinuous inductor current mode boost rectifier are calculated analytically. The values are given in rated form in dependency on the output power and on the ratio of output voltage to the amplitude of the mains voltage. Furthermore, the influence of the voltage transfer ratio on the shape of the mains currents and on the power factor of the system is analyzed. The theoretical analysis is verified by digital simulation and a good consistency is achieved. Finally, the approach of the converter dimensioning based on the graphical representation of the calculation results is described and illustrated using a specific example. The correctness of the dimensioning is verified by measurements on a laboratory model. >

Self-adjusting input current ripple cancellation of coupled parallel connected hysteresis-controlled boost power factor correctors

As this paper shows, the magnetic coupling of the inductances of two hysteresis-controlled single-phase AC-to-DC boost power converters connected in parallel leads in connection with a delayed switching of the power transistors to a self-adjusting staggered switching of the partial systems. Thereby, a significant reduction of the ripple of the resulting mains current as compared to the uncoupled case is obtained. Based on the analytical description of the current shapes within a pulse period the dependency on the circuit parameters of the occurring phase displacement of the switching functions of the partial systems is calculated. Furthermore, the effect of an unsymmetry of the power converters on the system behavior is analyzed and the automatic adjustment of the phase shift within a mains period is investigated by digital simulation. The theoretical considerations are verified by measurements on a laboratory model. Finally, the possibility of a substitution of the magnetic coupling of the input inductances of the partial systems by a coupling of the current controls of the power converters is discussed.<<ETX>>

A novel three-phase single-switch discontinuous-mode AC-DC buck-boost converter with high-quality input current waveforms and isolated output

In this paper, a new three-phase single-switch AC-DC flyback converter system is presented. The system operates in the discontinuous mode. The simple structure of its power and control circuit, low mains current distortion, and resistive fundamental behavior, as well as the high-frequency isolation of the controlled output voltage, have to be pointed out. Besides the analysis of the stationary operating behavior, the dependencies of the peak values, average values, and rms values of the device currents, and of the maximum blocking voltages across the power electronic devices on the circuit parameters, are given as analytic approximations. The theoretical analysis is verified by digital simulation. >

Current handling capability of the neutral point of a three-phase/switch/level boost-type PWM (VIENNA) rectifier

In this paper the stationary operational behavior of a three-phase/switch/level PWM rectifier is analyzed for asymmetrical loading of the output partial voltages. For the considerations a sinusoidal mains current shape, resistive behavior of the mains current fundamental and constant pulse frequency are assumed. Based on analytical calculations it is shown that the average value of the neutral point current (which has to be formed for asymmetrical load) has an approximately linear dependency on the distribution of the overall relative on-time of the switching states of the system being redundant concerning the voltage formation between begin and end of a pulse half interval. The maximum admissible load of the neutral point (capacitive output voltage center point) is calculated. This load is given as a function of the amplitude of the mains current and the voltage transfer ratio of the system. The calculations are checked by results of a digital simulation of the system behavior. Furthermore, the increase of the rms value of the ripple of the mains current of the rectifier is analyzed as it results in forming of an average value of the neutral point current. As a basis for comparison, the ripple rms value is used which results in harmonic-(sub)optimal control and negligible average value of the neutral point current. Finally, the current stress on the power semiconductors and on the output capacitors of the system are compiled in the form of diagrams which can be applied directly for dimensioning the system.