Toroid Transformer Design

[Anais Wachowski]

Iwant to replace the transformer in LM5026EVM with a toroid core transformer. I rarely seen somebody using a toroid transformer for DC-DC converter. Even in TI Reference design or evaluation board none of the design has done with Toroid core transformer. Anybody can explain me why nobody preferring toroid core transformer?

It's almost impossible to put a gap in a toroidal transformer - I know that a gap is not necessary in a topology like the active clamp forward but sometimes a small gap is used to give the transformer some ability to carry a small DC bias.

Toroidal transformers don't use bobbins, this means that the windings come off a toroid as wires rather than being soldered neatly onto the pins of a bobbin. - they can of course be mounted on a carrier plate of some form. Having pins on the transformer makes testing easier and makes assembly into the finished product faster, and less error prone.

There may be some second order effects relating to EMI - it's difficult to shield nearby components from electrical fields coming off the windings of a toroidal transformer - although the stray magnetic field from a toroid should be a bit lower than that from a 'normal' transformer.

A smaller transformer can be used if the load is intermittent. Because the output power in this case significantly exceeds the nominal power, the secondary voltage drops below the voltages given. The voltage drop increases proportionately with the current being drawn.

The secondary voltages and currents are valid for normal output power. At partial load, the output voltage, as a function of transformer size, will be accordingly higher. The below figure shows the voltage increase for Talema standard toroidal transformers for partial loads.

As can be seen from the graphs below, Talema standard toroidal transformers are designed for a temperature rise of 60 C to 70 C at nominal load. When choosing a transformer size, the ambient temperature and heat sink coefficient of the mounting place must be taken into consideration. Figures show the typical temperature change which occurs as a function of output power or overload.

An autotransformer allows smaller dimensions and a more economical overall design in cases where galvanically separated windings are not required. The same transformation of voltage and current can be obtained with a single winding autotransformer as with a normal two winding transformer. There are two major differences:

Autotransformers have lower leakage reactance, lower losses, smaller excitation currents, and they can be smaller and less expensive than dual winding transformers when the voltage ratio is less than 2:1. And, of course, they provide no isolation.

The characteristics which give the toroidal transformer advantages also contribute to a disadvantage: high inrush current with initial application of power. Talema is successful at designing transformers with low inrush current.

where Vp-pk is the peak primary voltage, and Rp is the DC resistance of the primary winding, depending on the power capability of the transformer, and on how strongly the core was magnetized. This inrush current peak occurs for a short time during the first or second half period of the power sine wave.

The purpose of these devices is to cut off the transformer in the event of overheating. The one-shot fuse is used primarily for protection from internal transformer faults, tripping at a preset temperature. The auto-resettable thermal switch provides intermittent protection from internal transformer faults and external overloads. This device opens at a preset high temperature and closes at a preset lower temperature. These devices are mounted internally to the transformer and wired in series with the primary or secondary winding.

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Three coils, in magnetic series as you've drawn them, will not make a 3 phase transformer. There would be only one value for flux which would be common for all three coils, as each coil surrounds the entire core cross section.

In a three-phase transformer each primary and secondary pair are wound on the same "limb" or "branch". With the 120 phase difference on each branch the flux on one branch can always find a path on the other two so that there is always a flux circuit. For example, when the red phase (Fig. 1) is max upwards the yellow and blue will be 0.5 downwards.

You could build a three phase transformer out of torriods. However, you need unique magnetic flux in each and the only way you could do that is to stack three separate torriods on top, or beside each other. Basically you would have three single phase transformers in one box.

I am willing to bet that historically 3-phase transformers were indeed built as three separate transformers till someone figured out that, since the three phases are 120 degrees apart, the magnetic effects of the other two coils basically cancel out at the primary coil in question. By combining them on a single core you can significantly reduce the weight and cost of the entire transformer.

In general torroidal transformers are expensive. Not only is the core itself harder to produce, but the act of winding it requires either very expensive knitting machinery or manual winding. That is an order of magnitude more cost compared to simple machine wound bobbins installed on laminated cores.

However, power toroidal xformers are made by winding very thin metal almost foil made by quenching very quickly so it has incredibly high permeability (I remember when this was new - I'm really old). I think it was first called Metglass? So in equipment to be shipped, if you care about weight, you might use toroidals. I have seen industrial higher powered equipment with three separate toroids used as three phase step down. I don't think it scales up to the power levels of "pole pigs" for utility distribution, and would probably not be cost effective.

You could use the shape of a wheel with three spokes, one primary and secondary winding on each spoke for each phase and no windings on the torodial wheel. But this is the same topology as the conventional three phase transformer with the B-shaped core described in the answer given by Transistor.

Other answers already explained why a toroidal core is not suited for a compact three-phase-transformer. But even if that doesn't matter and you consider three single-phase-transformers, the toroidal core won't work in most applications involving three phases.

Three-Phase-Transformers are almost exclusively used for high-power applications, e.g. to connect generators and motors with the electrical grid and to transform voltages within the grid. In any case a high amount of energy is involved. To transport this energy you actually need leakage fluxes, which you (almost) won't have in case of a torodial core.

For an experiment I have wound a single toroidal core with a three phase winding and have produced a rotating filed within the core and all three phase currents were identical so I can say it can be done.

Now, for Laminated E cores transformers I have formulas for calculating the N turns needed for primary and secondary based on required input and output voltages... but for toroidal cores I didn't find any specific formula on Google and on my books...

There is no real difference between the formulas for designing a toroidal transformer vs. an E-I transformer. The core cross-sectional area is \$\pi r^2\$ rather than the L \$\times\$ W of the center post, but the exact shape of the core does not matter much.

Its very important to specify RF toroid like perhaps a center tapped balun for biasing a class AB amplifier, or a AC line power toroid like the xfrmr for my semi-audiophile old record player amp which fed a old fashioned linear regulator.

To a crude first approximation, you're going to be buying your core from somebody unless you're making your own, and that mfgr will have helpful data sheets and books, for free or for sale. Amidon and Palomar have excellent RF design books for their products. For another perspective MFJ's "Ferromagnetic Core Design & Application Handbook" will set you back about $20.

A few years ago I contracted with Microsoft as a test engineer. We bought a 1KVA toroidal transformer for mains isolation for some tests. But almost all of the time, when this transformer was connected to the mains, it would INSTANTLY (by human perception) trip the power service 15A circuit breaker. I fixed this by adding an inrush limiter. The problem is that the toroidal core has negligible magnetic gap and very low reluctance. The hysteresis loop is shaped such that the core can retain significant residual magnetic flux after the transformer is unplugged. If the phase of the AC mains is not quite right the next time power is applied, the magnetic core can saturate and (being a 1 KVA transformer) a huge current spike can result. By comparison, a transformer which is built with EI laminations tends to have more core gap and will have less residual flux so this problem may be less severe. Also a toroidal transformer will almost always be more expensive.

I have made quite a few transformers for switching power supplies but those operate all above 20 kHz and mostly above 50 KHz. At 12 KHz you could use ferrite core but there may be some type of iron lamination which would work. For winding a transformer by hand, I have found that the easiest thing is to use a round center leg core and bobbin such as a PQ core. Ferrite would work at 12 KHz although some other material might permit higher flux density and a smaller finished transformer. I think that Digikey sells Ferroxcube ferrite cores.

The equipment dates to the mid-'80s. Given the brand name on the outside (not naming it), this major OEM really should have known better! I think the small target audience and the low production requirement of this equipment played a part in the shortcuts taken.

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