| Magnetics: what to do about |
Probably the least understood technology learned by engineers because of limited instruction in engineering schools, magnetics design is a special field where a few engineers spend every day designing and studying these devices.
Magnetic devices are an integral part of power electronics due to their energy storage capabilities, and the fact that you can "roll your own," unlike capacitors that you must purchase.
You can purchase transformers and inductors from several manufacturers as long as you can find what you want. Unless the shop will make something special to your requirements, you are stuck with catalog parts showing few specs but not enough to fully evaluate. For example: what core does the device use? Without this info, efficiency cannot be calculated. And there is much more. One can build a breadboard with the purchased part, amd empirically determine this, but calculations are better. If you rationalize the use of a purchased part, it would be merely necessary to mount it on a p.c. board with the associated circuitry and test it out.
On the other hand, if you really want to control what goes on with your design, you should learn the fundamentals of magnetics and design your own. After you are happy with it, your assembly plant winds the parts, or you can send it out as a spec to a magnetics shop to fabricate.
To learn the fundamentals, take a course or buy a book on the subject. Power Electronic Systems (PES) teaches a short magnetics course for engineers. One core vendor, Magnetics, Inc. http://www.mag-inc.com has exceptional practical design literature available in their catalog, plus separate papers on the subject.
This paper won't go into an actual design, but will give the reader an appreciation of some of the design steps required:
1. Pick a core type (ferrite, silicon steel laminations).
2. Select a core size by power handling capability.
3. Pick a part with mechanical and electrical details, most of which you will use.
4. Calculate turns.
5. Assign wire type.
6. Check wire build to make sure all the wire will fit on the core or bobbin.
7. Calculate DCR, inductance, efficiency with known parameters from the circuit this device goes into.
8. Get a core sample from vendor.
9. Wind and test in circuit.
10. Make alterations as required to device, whether re-doing the calculations or changing windings.
If all this seems futile, it is best to leave the design to the experts, such as a consultant or a company like PES, who can tailor the magnetic device to the circuit requirements. This would be the fastest way if the product schedule does not allow a learning period. It is difficult because so many cores are available with so many options to select from.
Or you could buy a standard part and hope it works, or get a magnetics vendor to design it for you, and you can pay for the R and D costs.
Book reference:
Transformer and Inductor Design Handbook
- Col. William T. McLyman,
1978 or later ed. (Marcel Dekker Inc., New York)