What is the best way to learn how to design and make SMPS transformers?

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Clem

Thanks Clem, very useful links. Peter

I don't know if there is some material on the web (I expect that there is) but I once attended a very good TI seminar about power supplies with a very good lecture on transformer/inductor design.

MK

Hi Michael, it is always interesting to get advice from someone who has some good experience, which is why I asked the question. I can find lots of info about transformers, and TI website is full of reference designs.

I am thinking about buying an evaluation kit such as this:

http://uk.farnell.com/power-integrations/rdk-242/top266vg-psu-30w-eval-board/dp/2311547

I could make my own transformer and learn from a few mistakes perhaps.

Peter

Peter,

Unless you need that evaluation kit for the electronic converter, I'd say away from it as It looks like it has a ready-made transformer in it. If you want to design and build your own transformer, that's not too hard. I'd Google transformer design. I've seen several DIY articles on transformer design. I design and manufacture transformers and DC-DC and DC-AC converters/inverters and I can help you a little here. Do look at the TI/Unitrode seminars in pdf form on the TI website. Look for article on magnetics or transformer design by LLoyd DIxon. These are very good, but are at a moderately high level engineering wise. If you want to get a kit, try the CoilCraft C-356 planar transformer design kit. They even replace the materials in the kit as you use them if you ask. It's about $ 120 for 1 kit. The power level is up to 120 watts per transformer. Go to the Coilcraft website and search for the C356 kit if you're interested. This is a planar kit, so the windings are circuit boards and copper stampings and it has only one size core. You can design your own transformer from scratch by using a manufacturer's core datasheets. I have no ides what power level you want and what frequency you're thinking of, or else I could help you with an initial core selection. Once you have that, it's easy to calculate whether or not the wire will fit or not. If not, something has to give. Before you can compute the "build" as we call it, which is percent build or percent of the available window area, you must compute the number of primary turns required and this depends on a lot of factors, but here is a general equation:

Np= Ein*10^8/(4*Fo*Ac*Bmax) where Np = primary turns, Ein = primary voltage, Fo = frequency, Ac=Core cross-sectional area in cm^2, and Bmax= maximum applied flux density in Gauss, which is your choice to keep the core from saturating, based on the manufacturer's core material datasheets. The factor of 4 is only for bipolar square wave form applied and it would be 4.44 for a sine wave. For a unipolar pulse, the equation would change to:

Np=(E*10^8)*Ts/(Fo*Ac*Bmax) where Ts is the on time per pulse for a unipolar pulse. This assumes that the off-time and/or circuit for transformer core reset is such that the core will be reset under all conditions. For if it is not, it will saturate, often blowing up your power converter.

Hi Kamran, thanks for the comprehensive answer. The attraction of a kit is that it is a complete circuit and will work, which is not a bad starting point as it can be quite confusing getting a new technology up and running. I would attempt to wind my own transformer and use with the board.

Buying a transformer kit is a good idea. The CoilCraft kit sounds okay but a planar transformer is less easy to use as a first lesson, I suspect winding wire is more flexible place to start, and more representative of transformers you find in SMPS. I found this from a search Design Kit Demoboard for Flyback Transformers it is a wurth kit that uses Linear Tech controllers. Again that may come with ready made transformers but it would be a basis to make my own and try them out.

I'll take a look for the Lloyd Dixon application notes. I remember them from years ago, they were classics in their time. Unitrode was a great company and it was a sad day when TI took them over.

Peter

This is a rather difficult entry point for a beginner.  There is no such thing as a 'flyback transformer.'  They are coupled inductors called 'transductors.'  Transformers' output is nearly exactly in phase with their input.  Transductors are out-of-phase.  This means that the magnetic storage in the transductor core must be much greater than in the transformer.  Transductors are prone to saturate catastrophically.  They use materials that soft-saturate like MPP, rather than ferrite, often.

thanks for that advice/caution...Peter

SMPS is fascinating and at the cutting edge of linear design.  If one wants to get into it I recommend designing small supplies, in the 1 - 10 Watt regime and keeping below, say, 24V at first, and not leaving one's circuits energized and unattended at first.  There are different rules for insulation when transformers are used in higher-power regimes, and regulations to follow.  If one takes on the challenges of more sophisticated topologies, higher operating frequencies, and higher-power regimes stepwise he will benefit.  My opinion is that this isn't somewhere where one should start at the top.  It is its own discipline.

Peter, you're welcome.

Well, there's some mis information here. There certainly are flyback transformers and while there may be a few people calling them transductors (I've never heard the term before in my 35 years of power conversion design), the engineers in the industry call them flyback transformers. One could say that every transformer is a coupled inductor; it's not really instructive. A flyback transformer is different than most transformers in that a lot of energy is stored in the core while the main (usually) MOSFET switch is on and no power is being transferred to the secondary. When the switch is opened, the energy stored in the core is then transferred to the secondary as the polarity of the secondary voltage changes nearly instantaneously when the switch gets opened. In most flyback transformers, energy is stored in air gaps since a ferrite or other non-gapped core material cannot store much energy. SOme cores have what's called a distributed (air) gap and are made from a type of powder that has a filler between magnetic particles to provide the gap. Other's use a standard ferrite core, but with a fixed gap between where the core halves come together. This can be made by milling the center core (preferred method) or by inserting a suitable material, like Nomex between the core halves.