Surface mount power supplies for standard assembly process

Details Surface mount power supplies for standard assembly process Surface mount power supplies for standard assembly process

2001-09-25

2004-06-08

Cuneo, Kamand (Department: 2827)

Electricity: electrical systems and devices

Housing or mounting assemblies with diverse electrical...

For electronic systems and devices

C361S758000, C361S770000, C361S803000

Reexamination Certificate

active

06747875

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates generally to a surface mountable power supply and specifically to an interconnect and method of interconnection that provides a robust physical and electrical connection between the power supply module and an end user circuit card.
2. Prior Art to the Invention
A distributed power architecture employs many small board mounted power supply modules in place of a few larger and more centralized power supply modules. A power supply module is mounted directly to the end user's circuit card that requires the power. A need exists for a method of placing the power supply module on the end user circuit card in an economical fashion. In other words, the method should minimize the number of manufacturing steps required for the placement. However, the method must also provide a robust physical connection to the end user circuit card, so that subsequent handling does not dislodge it. The connections between the power module and the end user's circuit board must also be robust enough to handle high current loads.
Conventional power supply modules may be constructed as a unitary, encapsulated package, having one or more rows of leads, with the power supply module enclosed in a metal case. The leads allow the module to be coupled to a circuit card while the metal case contains attachment mounts for an external heat sink. The power supply module often includes one or more power devices (e.g., transistors or diodes) in thermal communication with the metal case, one or more magnetic devices (e.g., transformers or inductors) providing electrical isolation and energy storage and one or more circuit boards containing passive electronic devices to provide, among other things, control and monitoring functions.
Power devices and magnetic devices that require thermal management due to their high power dissipation may be mounted on a metal circuit board employing insulated metal substrate technology, for example, a THERMAL CLAD substrate manufactured by the Bergquist Corporation of Minneapolis, Minn. Electronic devices such as passive devices that do not require thermal management may be mounted on either the metal circuit board or on a conventional FR4 circuit board. The FR4 circuit board may then be mechanically and electrically coupled to the metal circuit board to facilitate electrical communication and power flow between the various parts of the power supply module.
The leads of the power supply module are mechanically and electrically coupled to either the FR4 circuit board or the metal circuit board. The power supply module is typically encapsulated in a plastic or metal case that is filled with an encapsulant to protect the internal components of the power supply module from contaminants and perhaps to improve heat flow between the internal components and the case.
The aforementioned encapsulated package design, however, suffers from a number of deficiencies. At least two circuit boards, the metal circuit board and the FR4 circuit board, are required to accommodate the constituent components of the power supply module. Employing multiple circuit boards increases both complexity and cost of the power supply module. Further, the encapsulation can place additional stress on the component solder joints, leading to premature joint failure.
Conventional encapsulated and open frame power supply modules are often mounted to the end user's circuit card via through-hole pins. For example,
FIGS. 1 and 2
illustrates a through-hole pin connection. An end user's circuit card typically contains a large number of surface mount components. In fact, the power supply module is often the only through-hole mounted component on the circuit card. A separate or additional manufacturing step is thus required to mount the power supply module to the circuit card, thereby increasing the complexity and overall cost of the electronic assembly incorporating the circuit card.
FIG. 1
illustrates an exploded isometric view of prior art power supply
100
. The power supply
100
advantageously employs planar magnetic devices (one of which is designated
130
) to decrease the size of the power supply
100
. The power supply
100
includes a conventional FR4 circuit board
110
containing conductive traces for interconnecting electronic devices mounted thereto. The planar magnetic device
130
has windings
120
formed from a portion of the conductive traces. A core of the planar magnetic device
130
(including first and second core halves
133
,
136
) is disposed through apertures of the circuit board
110
and proximate the windings
120
.
The power supply
100
further includes a metal case
140
having a base and four side walls, thus forming a five-sided reservoir to receive the circuit board
110
therein. Alternatively, the metal case
140
may be formed without one or more of the side walls to allow air flow through the power supply
100
. The air flow may advantageously assist in dissipating heat from the constituent components of the power supply
100
. In the illustrated embodiment, the metal case
140
is an insulated metal substrate such as a THERMAL CLAD substrate, having an electrically insulating layer
145
and an electrically conductive layer
150
, in addition to the metal layer. Portions of the electrically conductive layer
150
may be removed, allowing the remaining portions to form conductive traces for interconnecting the electronic devices. The metal case
140
further has attachment points for coupling to an external heat sink. In the illustrated embodiment, the attachment points are internally threaded posts (one of which is designated
185
) protruding from the metal case
140
.
The power supply
100
further includes a plurality of semiconductor power devices (e.g., a switching device, one of which is designated
160
) having a body coupled in thermal communication with the metal case
140
and terminals (one of which is designated
165
) couplable to the conductive traces of the circuit board
110
. The power device
160
may be soldered to the metal case
140
via a conductive pad
155
, with the solder joint providing thermal coupling between the power device
160
and the metal case
140
. Soldering the power device
160
to the metal case
140
enhances an automatic assembly (e.g., for repeatability purposes) of the power supply
100
.
The power supply
100
further includes an encapsulant
170
, located within the reservoir, that provides a thermally conductive path to direct heat generated by the electrical components of the power supply
100
towards the metal case
140
. The encapsulant
170
may be located in only a portion of the reservoir or may substantially fill the reservoir.
FIG. 2
illustrates a side cut away view of the power supply
100
of
FIG. 1
mounted on an end user's circuit card
10
. The power supply
100
is designed to be through-hole mounted on the end user's circuit card
10
using conventional soldering techniques. Since the end user may subject the circuit card
10
to the reflow soldering process after the power supply
100
has been mounted thereon, it would be advantageous for the power supply
100
to be able to survive the reflow soldering process. The power supply
100
, therefore, employs high temperature solder. The end user's reflow temperature profile may then be set to a temperature sufficient to melt the solder between the power supply
100
and the circuit card
10
, but not sufficient to melt the high temperature solder joints within the power supply
100
. Further, the encapsulant
170
within the power supply
100
can cause solder displacement during solder reflow processes, resulting in component displacement.
While the use of high temperature solder will allow the power supply
100
to survive the reflow soldering process, the constituent components of the power supply
100
are necessarily subject to high temperatures during the assembly of the power supply
100
as the components are soldered to the circuit board
1

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