Gregory Ivensky

Generic operational characteristics of piezoelectric transformers

The universal attributes of piezoelectric transformers (PT) were derived by an approximate analysis that yielded closed form equations relating the normalized load resistance to the voltage gain, output power per unit and efficiency. Based on the results of the study, a calculation procedure is developed for specifying a PT for any given application and is demonstrated by considering the design of a fluorescent lamp driver. It is suggested that the closed form formulae, developed in this study, could be invaluable when studying, specifying and designing practical PTs applications.

Approximate Analysis of Resonant LLC DC-DC Converter

This paper presents an approximate analysis of LLC resonant converter with capacitive filter operating above and below resonance. An equivalent ac resistance model of the rectifier valid for discontinuous as well as continuous conduction modes is proposed. The dc voltage conversion ratio is then obtained using the fundamental harmonic approximation analysis method. Based on the analysis, LLC converter design plots and guidelines are suggested. The theory is verified by simulation and experiment.

A comparison of piezoelectric transformer AC/DC converters with current doubler and voltage doubler rectifiers

The objective of this study was to develop recommendations for an optimal design of piezoelectric transformer (PT) AC/DC converters. The paper presents a comprehensive comparison of the two commonly used rectifier topologies in a PT based power converters: current doubler and voltage doubler rectifiers. The advantages and disadvantages of the two rectifiers were investigated and the range of their applications with respect to output current, voltage, power capability, load resistance etc.-was delineated. Generic parameters are proposed and used to derive normalized and closed form equations that can help choosing a PT for a given set of requirements. Simulation and experimental results were found to be in a good agreement with the derivation of the theoretical analysis.

An RC load model of parallel and series-parallel resonant DC-DC converters with capacitive output filter

A novel analytical methodology is proposed and applied to investigate the steady-state processes in voltage-fed parallel and series-parallel resonant DC-DC power converters with a capacitive output filter. In this methodology, the rectifier, output capacitor and load are replaced by an equivalent circuit which includes a capacitor and resistor connected in parallel. Excellent agreement was obtained when comparing numerical values calculated by the proposed model to cycle-by-cycle SPICE simulation and to the numerical results of earlier studies.

An isolated DC-DC converter using two zero current switched IGBTs in a symmetrical topology

A symmetrical, current-fed DC-DC boost converter was investigated theoretically and experimentally on a 300 W prototype in the frequency range: 40 to 110 kHz. The converter is built around an L-type half-bridge IGBT inverter. Zero-current-switching is achieved by a series resonant LC-network placed between the collectors of the IGBTs.<<ETX>>

A novel RC model of capacitive-loaded parallel and series-parallel resonant DC-DC converters

A novel analytical methodology is proposed and applied to investigate the steady-state processes in voltage-fed parallel and series-parallel resonant DC-DC power converters with a capacitive output filter. In this methodology, the rectifier, output capacitor and load are replaced by an equivalent circuit which includes a capacitor and resistor connected in parallel. Excellent agreement was obtained when comparing the numerical values calculated by the proposed model to cycle-by-cycle SPICE simulation and to the numerical results of earlier studies.

Application of integrated magnetics in resonant converters

The physical size of inductors in resonant converters was examined in terms of inductor to transformer size ratio. Following the general discussion of the issue, the inductor size in a series resonant DC-DC converter is examined in detail. It is shown that the inductor to transformer size ratio is a function of the peak voltage of the inductor to the RMS voltage of the transformer and is independent of the inductance. The physical size of the inductor could be much larger than the transformer when the converter operates near resonance. Away from resonance, the inductor size becomes close to, and even smaller, than the transformer. Leakage inductance of the transformer can be used as the resonant inductance but if the leakage is associated with the secondary windings of the transformer, it will increase the peak reverse voltage of the rectifier diodes when a center-tapped rectifier topology is used. To solve this problem, an integrated transformer-inductor element with high primary leakage inductance and low secondary leakage is proposed and tested. The experimental unit was built for a power level of 1 kVA and switching frequency range 30-60 kHz.

Analysis and modeling of a voltage doubler rectifier fed by a piezoelectric transformer

Piezoelectric transformers (PZT) can be used advantageously in high output voltage dc-dc converters. In such applications the output section includes a voltage doubling rectification scheme to help increase the output voltage. This topology was modeled and analyzed by considering the expected voltage and current waveforms under the first harmonics approximation. The results were then used to build a linear ac equivalent circuit that emulates the ac-dc stage. The proposed model was verified against simulation and experimental results.

A resonant DC-DC transformer

The characteristics of a push-pull parallel resonant converter (PPRC) when operated as a DC-DC transformer were investigated theoretically and experimentally. In the DC-DC transformer region, the voltage transfer ratio of the PPRC was found to be practically constant and independent of the input voltage and load. In this mode, all the switching elements operate in the zero voltage switching (ZVS) condition. Another important feature of the proposed DC-DC transformer is the ability to drive it by an arbitrary switching frequency, provided that the latter is lower than the self-oscillating frequency. This permits the synchronization of the converter to a master clock. The analytical expressions for voltage and current stresses, as well as the other key parameters derived, are applied to develop design guidelines for the DC-DC transformer. The proposed topology was tested experimentally on a 100-W unit which was run in the 200-kHz frequency region. >

Analysis and evaluation of a lossless turn-on snubber

A lossless turn-on snubber was studied analytically, by simulation and experimentally. Limits were found for the proper operation of the snubber and are expressed as permissible duty cycle as a function of input current. It was found that a modified version of the snubber, which includes a tapped main inductor, improves the performance by allowing a wider operational range. The experimental results confirm the theoretical analysis. A reduction of power losses of about 19 W was observed when the snubber was implemented in a 1 kW boost power converter.