Rf Toroidal Transformer Design

[Yamila Comejo]

Nowfor 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.

I want 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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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.

The formulae given in these standards are not available for free but you may opt for some useful calculators available online. You can directly use them for creepage distance calculations depending on the voltage of your design.

You need to check isolation for the components coming in direct contact with the power supply (in your case, pin 1 of the relay K1 and pin 4 of K1). It will appear like pin 4 is open but it will be connected to the main supply when the relay is not connected to pin 3. At the input of the transistors Q5 and Q4, there is a connection coming to pin 1 from the relay (12V). So, you need to check the isolation for this line too.

That changes a lot. My home project of the last several years is a potentially dangerous product which (as it is) would likely be cause for a serious lawsuit if it were to go into production. If you are comfortable with the risks of what you are doing then go for it!

Reduce the cost of your equipment by using toroidal transformer as they are lighter and smaller in size. A small transformer is less expensive to manufacture and reduces the your overall system cost by allowing chassis design. The transformer core, designed from a single strip of steel, will save cost by providing you the option of infinite configuration. The efficiency or losses at each point in the system determines the overall system operating cost. Minimizing these losses lowers the cost of the product and provides long term cost savings to your customer.

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Quite a bit of this article results from recollections of my early foray into designing and making my own transformers for guitar and bass amps (we're talking 50 years ago at the time of writing). I quickly discovered that I couldn't buy off-the-shelf transformers that would provide the voltages I needed or handle the current. One attempt at getting a custom transformer made was both a success and a disaster - it worked, but cost way too much, and was enormous (and very heavy). At that point, I ordered laminations and the best winding wire available (designed for high temperature operation) and proceeded to teach myself transformer design.

Not one of my transformers ever failed, even though it wasn't uncommon for bass players (in particular) to decide to load the amp with far too many speakers. One had 4 8Ω quad boxes in parallel (2Ω), on a 200W amp designed for 4Ω. Both the amp and transformer survived this ordeal, but he was told it was a no-no once I found out.

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