Patent Application
20050134399
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The present invention relates generally to the microwave ferrite devices and more particularly to an improvement of heat transmission from a surface mount termination that is the main heat source in isolators, to the mounting base, which, in operation, is installed on a heat sink. It specifically relates to isolators having a resistive termination to shunt the reflected energy to the ground in combination with a ferromagnetic housing for closing a magnetic loop in magnetic chamber.
There are contradictory requirements to the isolator housing in conducting the magnetic flux, and, at the same time, increasing the heat transmission. For a magnetic flux the housing made of mild steel is needed—the material having high magnetic susceptibility (μ>1000) but low coefficient of heat transmission (0.00062 BTU per second). For a good heat transmission the materials like copper or aluminum are needed (0.00404 and 0.00203 BTU per second, respectively) which do not have any magnetic susceptibility (they are diamagnetic). Therefore, many attempts have been done in the past to improve isolators' performance by combining these two materials in the most effective way. For example, Naohiko Kanbayashi teaches (U.S. Pat. No. 3,621,476) a nonreciprocal device in which some portions of a heat dissipating plate or heat sink are introduced into a magnetic chamber through apertures thereof and are made in close contact with the chamber (which houses microwave ferrite elements and a center conductor). This structure, however, is pretty complex and does not cover the isolators wherein the resistive element, a termination—the most substantial source of the heat, is situated outside of the magnetic chamber.
Also known is a prior art where one portion of the housing having a steel magnetic chamber with ferrite elements and a center conductor, and the other portion made of copper or aluminum where the termination is located. This prior art is shown in
Prior art with one-portion housing isolator is also known (see
Thus, what is needed is an isolator that can provide both good magnetic susceptibility in the magnetic chamber and high coefficient of the heat transmission in the heat path from the termination to the mounting base of a device. This isolator should be of simple structure, easy to assemble and reliable in operation.
In accordance with the present invention an isolator's housing, made of a material having good magnetic susceptibility, for example mild steel, includes a top surface, a mounting base, and also a magnetic chamber. A surface-mount resistive termination to be grounded in operation is situated on the top surface. The mounting base to be contacted with a heat sink in operation, has a through hole right under the termination. This hole is completed with a plug made of a material having high coefficient of heat transmission, for example copper/aluminum. The plug connects the termination, both mechanically and electrically, with the mounting base of the housing. The magnetic chamber contains a central stuck incorporating the magnets, ferrites and the center conductor.
The plug is shaped as a cylinder and can be pressed into the hole in the housing from the mounting base side. Pressing with a flat plate during the assembling (common practice) allows positioning the plug exactly flush with the mounting base surface. Thus, required coplanarity to the mounting base with a tight tolerance on flatness can be easily obtained. At the same time, the press fit helps avoiding the voids in the area of dissimilar metals contact, and, by this, excludes forming a detrimental galvanic couple.
The termination is secured to the plug on the top surface of the housing, for example by soldering. For the best results, the plug should be flush with or slightly above the top surface of the housing. In this case, the solder fills the entire area under the termination and creates a reliable electrical and mechanical contact with the plug.
Thus, a simple and inexpensive isolator structure of single-portion ferro-magnetic housing in combination with material capable of effectively transmitting the heat from termination to a mounting base is in accordance with the present invention.
It is an object of the present invention to have a structure of isolator wherein good magnetic susceptibility and high ability to transmit a heat are effectively combined.
It is a further object of the present invention to have a structure of an isolator wherein the termination body would be connected to the mounting base of the housing both mechanically (for heat transmission) and electrically (for grounding).
It is a further object of the present invention to have a structure of heat transmission path wherein the presence of two dissimilar metals would not create a detrimental galvanic couple leading to the corrosion.
It is a further object of the present invention to have the isolator with tight tolerance to the flatness of its mounting base where different parts are exposed, which could be achieved in a simple and inexpensive way Oust by pressing and without any secondary machining).
It is an advantage of the present invention that the pressed-on plug actually excludes any voids in the area of bimetallic contact, because for pressing fit the high quality surfaces and tight tolerances are intrinsically needed. That is easily achieving in the present invention by pressing a cylindrical plug and a round hole.
It is another advantage of the present invention that the flatness of the mounting base with tight tolerance requirement, or coplanarity, is easily achieved by pressing the plug into the housing using just the flat press plates. The usage of the flat plates in the pressing practice is very common and does not invoke any additional expenses.
Referring to
There is a through hole in the housing 1 completed with the plug 2 made of material having high coefficient of heat transmission, for example copper/aluminum. The plug 2 has a cylindrical shape with both ends perpendicular to its longitudinal axis. On top end (as shown) of the plug 2 the termination 3 is secured, for example, by solder. Bottom end of the plug 2 is coplanar with the bottom surface (as shown) of the housing 1 which is a mounting base of the isolator. In operation, the isolator is mounted to a heat sink (not shown) which makes contact with a mounting base and, accordingly, with the lower end of the plug 2.
The length of the plug 2 is equal to or a slightly higher (by applying appropriate tolerances in designing) than the length of the hole in the housing 1. With coplanarity of the lower end to the mounting base, it assumes that the upper end of the plug 2 will be either flush to or slightly: above of the upper surface of the housing 1 in the area of location of the plug 2. This provides the optimal conditions for soldering.
The plug 2 is inserted into the hole with press fit. In order to provide a coplanarity of the lower end of the plug 2 with the mounting base of the housing 1, at the assembling the plug 2 is rammed into the housing 1 by flat pressing plate. The size across the pressing plate shall be substantially larger than that of the plug 2. If that is the case (commonly it is, unless it is deliberately changed) the pressing process ends when the pressing plate stops when it meets the mounting base of the housing 1 and, accordingly, the low end of the plug 2 is flush with the base. This is common practice in pressing process and is described here only to illustrate how easily the coplanarity can be achieved in the structure according to the present invention.
In operation, the isolator is installed on a heat sink providing a contact with the housing's 1 installation base. The plug 2 transmits the heat from the termination 3 to the heat sink in the most efficient way because of the high coefficient of heat transmission in copper/aluminum material.
One of the possible embodiments of the structure in accordance with present invention has shown in
Thus, a simple and inexpensive structure to reconcile contradictory requirements to isolators having a housing with a good magnetic susceptibility and, at the same time, a high coefficient of heat transmission is proposed. While the invention having been described in detail, it is clear that there are variations and modifications to this disclosure here and above which will be readily apparent to one of ordinary skill in the art. For example, one of the obvious variations is having the entire housing made of copper/aluminum with pressed-on magnetic chamber made of mild carbon steel, as described above as an another embodiment. To the extent that such variations and modifications provide an adequate path from heat source to heat sink in one-portion housing isolators and, at the same time, have good magnetic susceptibility in the magnetic chamber, which result in better performance and cost-labor savings, such are deemed within the scope of present invention.
