The present invention relates to a surface mountable, toroid magnetic device (e.g. a transformer or inductor) which can have improved thermal performance.
Toroid transformers and inductors are often used in switch mode power supplies due to their good magnetic performance. Such toroid magnetic devices are normally mounted using one of the following approaches:
Regardless of the mounting approach, the device temperature rise can be taken to be a function of surface area, one standard equation being:
Temperature Rise (° C.)=[Power Dissipation (mW)/Surface Area (cm2)]0.833
With the increased desire for automated assembly, surface mount applications are becoming more common. However, in harsh environment applications with high vibration requirements (such as aerospace engines) the mass of surface mount components should be minimised, which is in direct contradiction to the need to increase the size of the component for improved thermal performance.
There is therefore a need for improved thermal management of surface mount toroid magnetic devices to enable use in safety critical, high temperature, harsh environment applications.
Accordingly, in a first aspect, the present invention provides a surface mountable, toroid magnetic device having a potting filling the central hole of the toroid, the potting extending axially beyond the base of the toroid to form a contact surface which, in use, contacts a mounting body for the device, whereby heat generated by the device flows by conduction through walls of the toroid defining the central hole into the potting and thence through the contact surface into the mounting body.
Advantageously, the potting provides a path for conductive heat flow which can improve thermal performance of the device, allowing the use of smaller devices with higher power densities. Further the device can be compatible with automated surface mount manufacture.
In a second aspect, the present invention provides an electrical circuit including a mounting body to which is mounted the device according to the first aspect.
In a third aspect, the present invention provides a switch mode power supply including the electrical circuit of the second aspect.
In a fourth aspect, the present invention provides an aerospace engine electronic controller having the switch mode power supply of the third aspect.
Optional features of the invention will now be set out. These are applicable singly or in any combination with any aspect of the invention.
The mounting body can be a PCB (printed circuit board).
The potting may also extend radially over the base of the toroid. In this way the contact area with the mounting body can be increased, improving heat flow into the body.
The contact surface may be substantially planar, e.g. for compatibility with a corresponding planar surface of the mounting body. The contact surface may be substantially perpendicular to the axis of the toroid.
The potting may fill the central hole of the toroid to at least ¾ of the depth of the hole. In this way, more heat can be drawn out of the device by the potting.
A top portion of the central hole may be filled with an elastomer, for example silicone. This can facilitate manipulation of the device using standard automated pick and place technology, e.g. used for building PCBs, as the top surface can be made compliant and even for compatibility with machine vacuum nozzles. The upper surface of the elastomer can be parallel with the contact surface to further facilitate the automated pick and place task. The upper surface of the elastomer may be level with the upper surface of the toroid, or indeed may extend radially over the upper surface of the toroid.
The potting can, conveniently, be formed of a resin loaded with thermally conductive particles, such as aluminium or graphite. The resin may be epoxy resin or another curable resin material.
The thermal conductivity of the potting may be at least 1 W/mK.
The coefficient of thermal expansion of the potting can be substantially the same as that of the magnetic core of the toroid. This can help to reduce mechanical stresses during thermal cycling.
The device may further have a carrier which supports the toroid, the carrier including fixing members (e.g. pins) for fixing (e.g. soldering) the device to the mounting body, wherein the coefficient of thermal expansion of the carrier is substantially the same as the coefficient of thermal expansion of the mounting body. The fixing members can be contacts for electrically connecting the device to electrical conductors of the mounting body.
Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:
Conventionally, heat loss from a toroid magnetic device is via convection and radiation. The present invention provides an additional thermal conduction path which can be compatible with automated surface mount manufacture.
The potting 1 extends axially beyond the base of the toroid 4, and preferably also extends radially over the base of the toroid, to form a contact surface 6 which, in use, is held in close contact with the PCB. Typically the contact surface is planar and normal to the axis of the toroid. In this way, a thermal conduction path is created between the windings of the toroid 4 and the PCB that the transformer is mounted to, with the PCB acting as a heat-sink. This can remove a need for a separate additional heat sink for the transformer.
The carrier can have fixing members 5, e.g. in the form of pins, for soldering the transformer to the PCB. Indeed, the fixing members may be contacts for electrically connecting the transformer to electrical conductors of the PCB. Further fixing members may be added to the carrier for additional mechanical support with no electrical connection. To ensure that the contact surface 6 makes close contact with the PCB, the contact surface 6 can be co-planar with the undersides of the fixing members.
The potting 1 preferably fills the central hole of the toroid to at least ¾ of the depth of the hole. In this way, the potting can help to draw more of the heat generated by the toroid windings and core losses out of the device. However, the top portion of the hole may be filled with an elastomer 2, such as high temperature silicone, which can stick to the uneven outside surface of the wound toroid to become an integral part of the transformer. The upper surface of elastomer can be planar and parallel with the contact surface 6. The elastomer can also extend radially over the upper surface of the toroid. The elastomer facilitates manipulation of the transformer using standard automated pick and place technology for building PCBs though providing an even compliant surface compatible with vacuum nozzle pick and place machines.
Alternatively, as a second stage in the fabrication process, the elastomer can simply be poured into the central hole and allowed to self-level under gravity.
The toroid transformer described above, with its improved thermal management, is particularly applicable for use in the high-value aerospace electronics (e.g. PCBs for the engine electronic controller). However, it can also have other harsh environment applications, such as in oil and gas industry downhole applications.
While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. For example, the device described above is a toroid transformer, but the principals defined extend to other toroid magnetic devices such as inductors. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.
Number | Date | Country | Kind |
---|---|---|---|
1419162.1 | Oct 2014 | GB | national |
Number | Name | Date | Kind |
---|---|---|---|
3246272 | Wiley | Apr 1966 | A |
7821374 | Harrison et al. | Oct 2010 | B2 |
7847662 | Saito | Dec 2010 | B2 |
8624702 | MacLennan et al. | Jan 2014 | B2 |
20080018196 | Enomoto | Jan 2008 | A1 |
20090146769 | Feng et al. | Jun 2009 | A1 |
20110025441 | Tien et al. | Feb 2011 | A1 |
Number | Date | Country |
---|---|---|
3321721 | Dec 1984 | DE |
3522740 | Oct 1986 | DE |
198 14 897 | Oct 1999 | DE |
298 23 886 | Mar 2000 | DE |
19854642 | Jun 2000 | DE |
2 128 815 | May 1984 | GB |
2154068 | Aug 1985 | GB |
H08250343 | Sep 1996 | JP |
H11144977 | May 1999 | JP |
2002280234 | Sep 2002 | JP |
2005198147 | Jul 2005 | JP |
2005052964 | Jun 2005 | WO |
Number | Date | Country | |
---|---|---|---|
20160118176 A1 | Apr 2016 | US |