Publications

This paper describes a high-rel transformer design and the design iterations required to meet the severe environments of military grade transformers. It describes a method of reducing the mechanical stress caused when a ferrite pot core is encapsulated in a rigid epoxy. Stresses are due to differences in coefficient of thermal expansion between the two materials. The mechanical design optimization of a small flyback transformer designed to charge an energy storage capacitor up to 6 kV from a low voltage source is described. The basic design uses a 2616 manganese zinc ferrite pot core. The goal was to eliminate the core cracking problem. The purpose for writing and presenting this paper is to document a proven process that evolved out of necessity, and to present to the industry a method which reduces stresses on the core, eliminates cracking of the core and provides the insulation necessary for small high voltage transformers.

In the discharge of a capacitor the current was measured with a current viewing transformer (CVT). In addition to measuring the current flowing through the CVT primary, a 51 MHz noise signal was added to the primary current. When the CVT was covered with a gold shield, the noise was eliminated. Analysis of the measured results indicate that the gold layer reflected the electromagnetic that was generated by current flowing in the primary and that the capacitance between the shield and the CVT secondary had no measurable effect on the CVT output.

Sandia National Laboratories has been encapsulating magnetic components for over 40 years. The reliability of magnetic component assemblies that must withstand a variety of environments and then function correctly is dependent on the use of appropriate encapsulating formulations. Specially developed formulations are critical and enable us to provide high reliability magnetic components. This paper discuss epoxy, urethane, and silicone formulations for several of our magnetic components.

Many Sandia components for military applications are designed for a 20-year life. In order to determine if magnetic components meet that requirement, the parts are subjected to selected destructive tests. This paper reviews the re-design of a power transformer and the tests required to prove-in the re-design. The re-design included replacing the Epon 828/Mica/methylenedianiline (curing agent Z) epoxy encapsulant with a recent Sandia National Laboratory (SNL) developed epoxy encapsulant. The new encapsulant reduces the Environmental Safety and Health (ES and H) hazards. Life testing of this re-designed transformer generated failures; an open secondary winding. An experimental program to determine the cause of the broken wires and an improved design to eliminate the problem was executed. This design weakness was corrected by reverting to the hazardous epoxy system.

This paper describes the use of Formula 456, an aliphatic amine cured epoxy for impregnating coils and high voltage transformers. Sandia has evaluated a number of MDA-free epoxy encapsulants which relied on either anhydride or other aromatic amine curing agents. The use of aliphatic amine curing agents was more recently evaluated and has resulted in the definition of Formula 456 resin. Methylene dianiline (MDA) has been used for more than 20 years as the curing agent for various epoxy formulations throughout the Department of Energy and much of industry. Sandia National Laboratories began the process of replacing MDA with other formulations because of regulations imposed by OSHA on the use of MDA. OSHA has regulated MDA because it is a suspect carcinogen. Typically the elimination of OSHA-regulated materials provides a rare opportunity to qualify new formulations in a range of demanding applications. It was important to take full advantage of that opportunity, although the associated materials qualification effort was costly. Small high voltage transformers are one of those demanding applications. The successful implementation of the new formulation for high reliability transformers will be described. The test results that demonstrate the parts are qualified for use in DOE weapon systems will be presented.

This document, after giving a summary of Sandia`s mission, describes technology transfer efforts and accomplishments at Sandia. This includes information on their user facilities: the combustion research facility, the national solar thermal test facility, and the electronics quality and reliability center.

This paper describes the use of {open_quotes}Formula 456{close_quotes} an aliphatic amine cured epoxy for impregnating coils. Methylene dianiline (MDA) has been used for more than 20 years as the curing agent for various epoxy formulations throughout the Department of Energy. Sandia National Laboratories began the process of replacing MDA with other formulations because of regulations imposed by OSHA on the use of MDA.

This paper describes a method of reducing the mechanical stress caused when a ferrite pot core is encapsulated in a rigid epoxy. the stresses are due to the differences of coefficient of thermal expansion between the two materials. A stress relief medium, phenolic micro-balloon-filled, syntactic polysulfide, is molded into the shape of the pot core. The molded polysulfide is bonded to the core prior to encapsulation. The new package design has made a significant difference in the ability to survive temperature cycles.