Felix A. Himmelstoss

Analysis of a Smith-predictor-based-control concept eliminating the right-half plane zero of continuous mode boost and buck-boost DC/DC converters

It is pointed out that the small-signal transfer function of a boost or a buck-boost converter shows a zero located in the right-half complex plane for continuous operation. This zero makes the stabilization of the system considerably difficult. The authors use a boost converter as an example to show that application of the principle of linear prediction (as originally proposed for the control of systems with dead times) yields the mirror image of the control circuit zero. This shift of the zero into the left-half s-plane is achieved with unchanged location of the poles. Thereby the small signal transfer function is split into two parts: a phase-minimum system and a nonphase-minimum system. The latter can be interpreted as a linear Pade approximation of a dead-time element. For the design of a robust control system it is necessary to investigate the shift of the poles of the closed system depending on the change of the operating point. How the predictor influences the small- and large-signal system disturbance responses for changes of the load and for input voltage changes is also analyzed.<<ETX>>

High Dynamic Class-D Power Amplifier

Class-D amplifiers have high efficiency and are used when high audio frequency power is necessary and also when low power consumption in battery powered equipment is required. A new kind of class-D amplifier is presented and analyzed. The ripple of the high power class-D amplifier stage is compensated by a small (low power) high dynamic amplifier, which can be realized either as an analog one or as a second power class-D converter. The main part of the power is handled by a half-bridge with relatively small switching frequency (compared to the output bandwidth) and therefore low switching losses. Furthermore, the compensation mechanism leads to no additional current or voltage stress in the main switching stage. In the presented example the switching frequency of the main power processor is selected at only twice the required output bandwidth of the amplifier. The resulting output ripple is compensated by using the transformer coupled supplement power processor. Only small filters are necessary. The concept is well suited for consumer as well as aerospace applications as due to the improved efficiency, and the battery lifetime can be increased without any quality reduction.

Low-loss converters with high step-up conversion ratio working at the border between continuous and discontinuous mode

The use of tapped inductors in the classical boost or buck-boost converter leads to a high step-up ratio thus avoiding the extreme duty cycles of the active switch and the high peak currents in the active and passive switches. Two converters are presented and analyzed. Due to the simultaneous regulation of the switching frequency and the duty cycle on one hand and the winding ratio of an autotransformer on the other, high conversion rates of 1:5 can easily be realized. The dimensioning of the converters, their dynamical behavior, and the component stresses are analyzed.

Analysis of a boost converter with tapped inductor and reduced voltage stress across the buffer capacitor

A step-up converter with tapped inductor is analyzed. Compared to the classical boost converter, the structure has been slightly modified which offers the advantage of reduced voltage stress across the buffer capacitor and due to a transformer the voltage transformation rate is changed. The classical and most common converter uses a simple inductor, leading to the disadvantage of extreme duty ratios when high voltage transfer ratios are needed. To meet such requirements, a tapped inductor (autotransformer) can be used instead of the ordinary inductor thus avoiding excessive duty ratios. After basic analyses in the continuous inductor current mode, important data for the dimensioning of the components, like the voltage and the current stress, and the equations for the component values are given. Moreover, a state space model and linearized transfer functions for the control of the converter are derived. When transformed into a bidirectional converter, it can be used for coupling two voltage links and as a two quadrant chopper for DC motors. To verify the proper function of this modified boost topology, a small test converter has been designed. Measurement results and a more precise model are given in the appendix.

Modelling and simulation of a synchronous machine

A computer-aided simulation of a synchronous machine is described with respect to applications for power electronics. The model is written in the modeling language MAST and will be used for analysing transients. Starting from data sheet values the process of generation of the model parameters is described. A two axes rotor fixed description of the machine is used to get the mechanical behavior. Finally the three phase currents are calculated out of the two axis model by the inverse Park transformation. Modelling results are shown.

Analysis of a quality class-D amplifier

The amplifier consists of a half-bridge MOSFET device with appropriate driver, an output filter and a modulator. The load is connected between the center of the half-bridge and the center of the symmetrical supply. The dimensioning of the output filter and the modulator is especially treated. The frequency, step and burst response for an example are shown.

Analysis and comparison of half-bridge bidirectional DC-DC converters

Ten half-bridge (seven fourth order and three second order as reference) PWM DC/DC converters are treated and a survey of important data (maximum voltage and current ratings for the elements, RMS-values for the semiconductor devices and a rough approximation of the losses) of the circuits are given. Furthermore, realistic converter models with simple device-models based on duty ratio averaging are established. Finally, dimensioning equations for the inductors and the capacitors are given. The results make it possible to estimate the applicability of the given converter structures and offer sufficient material for the calculation and analysis of these circuits.<<ETX>>

Analysis of a new high-efficiency DC-to-AC inverter

A high-efficiency DC-to-AC inverter for stand-alone and direct AC mains connection is described. This structure uses only off-line switches to generate the AC voltage. Due to safety reasons, the input voltage (e.g., the voltage of solar cells or batteries) is limited. Therefore, high currents on the primary side are necessary to obtain the required output power. The overall efficiency of the inverter depends to a high degree on the number of switching elements in the current path. The structure described here has only one active switch in the main path. This can be obtained by applying a diode with controlled turn-off capability (like a thyristor or other derivatives). High duty cycles (d near one) of the main switch at high currents lead to low duty cycles (1-d near zero) of the switchable diode. For that reason, a series connection of two active switches in the main path has to be avoided. The new inverter leads to improved efficiency and is therefore well suited for solar and renewable energy as well as for aerospace applications. The improvement of the efficiency is due to the new topology and not due to overdimensioning of the semiconductors. Hence, an additional potential of efficiency improvement is still available.

State space control for a step-up converter

Based on the equivalent circuit of the boost converter the state space representation of the converter is derived and the transfer function is determined. Then the dependency of the system pole and zero locations on the duty ratio and on the load is investigated. Based on this system analysis a state space controller is determined for the rated system operating point using he pole placement approach. Two ways of controller design are shown. The advantage of the state space approach in general, especially as given here, is that the system behavior can be chosen to a large extent by selection of the pole positions.<<ETX>>

Flux measurements and modeling of the magnetic hysteresis for a zero-voltage switching dc-to-dc converter

A half-bridge bidirectional step-up dc/dc converter with zero voltage switching is introduced. The losses in the semiconductors are reduced in this concept but the higher current ripple leads to increased losses in the MnZn ferrite inductor. The resulting unsymmetric hysteresis including minor loops due to switching oscillations is predicted using reversible and irreversible energy contributions.