Martin F. Schlecht

Issues related to 1-10-MHz transformer design

Issues related to the design of high-frequency transformers are discussed. An analysis of skin and proximity effects in the conductors and measurements of permeability and hysteresis loss in the magnetic material are combined in a computer-assisted study of the relationships between size, efficiency, and frequency. Experimental data are presented for a prototype transformer. >

Design considerations for micromachined electric actuators

Abstract This paper provides a perspective on the design and fabrication of surface-micromachined actuators. An analysis of electromagnetic to mechanical energy conversion indicates that electric drive is preferable to magnetic drive for these microactuators. Planar rotary and linear microactuators can be fabricated by selectively etching multi-layer thin-film sandwiches, with the potential for gaps on the order 1 μm and lateral dimensions on the order of 300 μm. Prototype designs are presented for rotary variable-capacitance and induction micromotors. Rotor speeds of 2.4 × 10 5 rad s −1 (2.3 × 10 6 rpm) and accelerations of 2.9 × 10 9 rad s −2 appear feasible for both. In conclusion, some of the research problems in developing a microactuator technology are identified, including warpage of microstructural films due to residual stress, friction between micromachined surfaces and electric breakdown in small gaps.

A study of three microfabricated variable-capacitance motors

Abstract This paper discusses the design, microfabrication, operating principles and experimental testing of three types of rotary variable-capacitance micromotors. The advantages and disadvantages of these motors are discussed. The three motor types are top-drive, side-drive and harmonic side-drive. In this work, the micromotors are surface micromachined using heavily-phosphorus-doped polysilicon for the structural material, deposited oxide for the sacrificial layers and LPCVD nitride for electrical isolation. Frictional forces associated with electric pull-down forces on the rotor are dominant in the side-drive and harmonic side-drive motors fabricated and tested to date. Air drive and electric excitation have been used in studying these effects. Side-drive micromotors have been successfully operated by a three-phase electrical signal with the rotors air-levitated. With air levitation, successful operation is achieved at bipolar excitations greater than 80 V across 4 μm air-gap motors having eight rotor and twelve stator poles, with only half of the stator poles excited. Motor operation is sustained indefinitely.

Active Power Factor Correction for Switching Power Supplies

The harmonic-free utility/dc interface provides to the computer industry a means to extract more power from the wall outlet than the normal rectifier, with its heavily distorted input current, will allow. Due in part to the very high switch stresses in this interface, however, the cost appears to be too high to justify its use at this time. An alternate approach is proposed that focuses on power factor correction rather than harmonic reduction. Compared to the harmonic-free interface, the switch stress of this new approach is approximately a factor of two smaller.

A high-frequency, low volume, point-of-load power supply for distributed power systems

A resonant version of the DC-DC forward converter is presented in which both the primary-side switch and the secondary-side diode are either on or off at the same time. Such a topology is capable of operation in the 10-MHz range, and can be used for very-low volume point-of-load conversion in distributed power systems. This type of converter takes advantage of a very low transformer leakage inductance to achieve zero-voltage switching of all its power semiconductor devices. Its resonant ring is also independent of load current. A 50-W prototype operating at 3.6 MHz is presented along with a discussion of the changes necessary to achieve 10 MHz. >

Design of active ripple filters for power circuits operating in the 1-10 MHz range

A fundamental analysis of active ripple filters for powder circuits operating in 1-10 MHz range is presented. The improved understanding derived from this analysis allows filter circuits with gains five to ten times higher than previously reported to be built. Besides providing further reduction in the size of 1 MHz ripple filters, these higher gains make the active filters valuable for power circuit switching near 10 MHz. >

Comparison of the square-wave and quasi-resonant topologies

The waveforms of a square-wave and a quasi-resonant dc-dc converter are examined in detail so that a comparison between the switching and conduction losses for each topology can be made. Using data from commercially available semiconductor devices, conservative estimates are then given for the switching frequency at which the resonant approach becomes advantageous. The effect of an isolation transformer on this comparison is also addressed.

Active Filters for 1-MHz Power Circuits with Strct Input/Output Ripple Requirements

As the switching frequency of a dc-converter is raised from 100 kHz to 1 MHz, one would expect the energy storage elements of the circuit to be correspondingly reduced in size. If the circuit must meet input and output ripple requirements of the nature of MIL-STD-461B CEO3, however, the advantage of a higher switching frequency is significantly offset by the higher filter attenuation levels required. The use of active filters, in conjunction with minimal passive filters, to achieve the required ripple levels with a much smaller overall volume is discussed. Working active filters with gains in excess of 100 at 1 MHz are also presented.

A line interfaced inverter with active control of the output current waveform

A line interfaced inverter for photovoltaic systems is presented. This inverter employs a high frequency dc-chopper for shaping the output current, providing unity power factor operation and minimizing the size of the output filter. The isolation of the solar array from the utility is accomplished with a high frequency transformer, eliminating the need for large, 60 Hz magnetics. The dynamics of the power circuit are analyzed to determine proper compensation of the output current control loop. This analysis shows that a minor loop is required to stabilize the power circuit and that system response varies during the 60 Hz cycle. A Parity Simulation of the power circuit/control system is performed to verify this analysis. The results of this simulation are presented and compared to waveforms from a test inverter. The special considerations required for properly loading a photovoltaic array with a line interfaced inverter are discussed.

Issues related to 1–10 MHz transformer design

Issues related to the design of high frequency transformers are discussed. An analysis of skin and proximity effects in the conductors and measurements of permeability and hysteresis loss in the magnetic material are combined in a computer assisted study of the relationships between size, efficiency, and frequency. Experimental data is presented for a prototype transformer.