ferrite rings

When you hear “ferrite rings,” the first thing that comes to mind is some kind of magic donuts that are “put on wires.” and everything immediately becomes better. In fact, if you dig, it turns out that most even engineers do not always understand what type of ferrite is needed for what task. Personally, about ten years ago I also thought that the main thing was the internal diameter for the wire, and the rest was a secondary matter. Until I came across a project where noise filtering in a switching power supply came down to the wrong choice of core material. That's when I really started to figure it out.

Materials: not all ferrites are created equal

Look, the most common myth is that a ferrite ring is just a piece of magnetic material. In fact, the difference between, say, brandsferrite ringsfrom Mn-Zn and Ni-Zn - it’s like between night and day photography. The first are for low and medium frequencies, up to a couple of megahertz, the second are for high-frequency applications, where minimal eddy current losses are important. I remember how we tried to use rings from old Soviet equipment in a new circuit with PWM at 500 kHz - the result was disastrous, the heating was significant, and the efficiency dropped. I had to order specialized ones, fortunately, now it’s easier.

And here it is important not just to choose a brand, but to understand temperature stability. For some inexpensive ferrites, when heated to 70-80 degrees, the magnetic permeability begins to “float,” and with it the winding inductance. In one of the projects for the mining industry, where equipment operates in unheated rooms with a large temperature difference, this nuance became critical. I had to recalculate and reorder the batch, which resulted in delays and extra expenses.

By the way, about supplies. When you are looking for qualityferrite ringsfor critical applications, you often come up against the issue of origin and quality control. Not all manufacturers, especially in the CIS market, can ensure the stability of parameters from batch to batch. Here, for example, you can pay attention to the products suppliedLONGI Corporation. They, as I heard from colleagues, work with trusted factories and do the rejecting themselves, which is a must have for industrial use. Their websiteljmagnet.ruIt’s worth keeping in your bookmarks if you need not just “donuts”, but guaranteed components.

Geometry: why size and shape are not just about space on the board

Inner diameter, outer diameter, height - everything seems simple. You take a reference book, look at AL (inductance per turn) and wind it. But in reality there are nuances. For example, for power chokes in converters, it is often not so much AL that is important, but the ability to accommodate large cross-section wires and ensure efficient heat dissipation. Tolstoyferrite ringwith a large internal diameter may have excellent magnetic properties, but if it cannot be effectively mounted on a heatsink or chassis, all effectiveness will be lost due to overheating.

One of our failures was connected precisely with this. We made a prototype of a current stabilizer for a welding inverter, calculated everything, and selected rings based on the maximum permissible induction. We assembled it, launched it - during tests under load, after 20 minutes the ferrite became so hot that the solders on the terminals floated. It turned out that we did not take into account the shape of the magnetic field during one-sided winding and local overheating in “hot” ones. core points. I had to change the design, switch to an assembly of two rings and uniform distribution of the winding.

Another point is mechanical processing. Sometimes, to fine-tune the inductance or reduce the gap (which is critical for some types of filters), the rings are ground. Do it handicraft, “on your knees?” - a sure path to microcracks and subsequent destruction of the core under vibration. It is better to immediately look for ready-made solutions or suppliers who provide professional processing services. On the same siteljmagnet.ruatLONGI Corporation, judging by the description of their capacities, they must cope with such tasks - they have a production area of 140 thousand square meters, and more than 60% of their staff have higher education, which indirectly speaks about the possibilities of technological support.

Application in “heavy” conditions: not only electronics

Often the conversation about ferrite rings ends up focusing on printed circuit boards. But there is a whole layer of applications in power engineering, and especially in mining technology, where the requirements are much more stringent. Vibration, dust (sometimes conductive or magnetic!), changes in humidity and temperature. Here it is no longer enough to simply solder the component - you need special impregnation of the windings, sealing of the entire assembly, and sometimes shielding of the ferrite itself from external fields.

We once developed a current sensor based onferrite ringsfor excavator engine monitoring system. It would seem like a classic scheme. But the constant vibration from operation led to the fact that after a month of operation, the varnish on the winding cracked, an interturn short circuit began, and the sensor failed. The solution was found in cooperation with manufacturers who specialize in industrial equipment. What was needed was a winding wire material with increased abrasion resistance and vibration-resistant impregnation. This is the case when the component must be part of an engineering system, and not just a radio component.

It is precisely for such tasks that a company likeLONGI Corporation. If you look at their description, they have been developing and producing mining equipment since 1993, producing up to 4,000 units per year. This is a scale that requires a deep understanding of the harsh operating conditions. I think if then, with that sensor, we had immediately turned to specialists who understood how their ferrite assemblies would behave not on a laboratory bench, but in a rattling workshop or in an open pit, many problems could have been avoided.

Measurements and control: what practice involves

Theory is one thing, but when you start measuring the real parameters of the wound ring, another reality opens up. The simplest example is that same inductance. You measure it on a bridge at 1 kHz and you get one value. You raise the frequency to 100 kHz - it’s already different, because parasitic winding capacitances and losses in the ferrite itself turn on. For EMC filters, this is a key point, because they operate precisely at high frequencies, where they “live?” interference.

We had a curious case with noise suppression via a 220V network for a frequency converter. We installed a filter on rings calculated using data from the datasheet. When checked with a combined oscilloscope-spectrum analyzer, it turned out that in the region of 3-5 MHz the filter not only does not attenuate, but, on the contrary, resonates and enhances the interference! This is all due to the fact that the resonant frequency of the core itself in this winding configuration turned out to be precisely in this range. I had to experimentally select the number of turns and the winding method (in bulk, probably sectioning) in order to move this point.

Hence the conclusion: without a normal measurement base and understanding of how parametersferrite ringsbehave in a real circuit, and not in the ideal conditions of the manufacturer, there is nothing to do. Especially when it comes to mass production. We need selective control of batches. And here again it is important who you work with. If a supplier such asLONGI, itself a major equipment manufacturer, it is logical to assume that they have established incoming control of components, including ferrites. Their experience of a team of 1,200 people and the emphasis on professional education of employees indirectly indicates this - defects in their products are unprofitable for them.

Instead of a conclusion: ferrite as part of the ecosystem

So what is all this for?Ferrite rings- this is not a universal spare part that can be pulled out of one device and stuck into another. This is a calculated component whose properties are highly dependent on everything that surrounds it: from the frequency and shape of the current in the winding to the ambient temperature and mechanical loads. The most valuable experience is not successful projects, but precisely those where something went wrong and we had to figure out the reasons.

Now, when choosing a ferrite core for a new development, I no longer look only at price and dimensions. I look at the manufacturer, the availability of complete and detailed datasheets with graphs of losses depending on frequency and temperature, and the reputation of the supplier. And if the project is industrial, with the prospect of a series, then I immediately think about those who work “in the field?” — with mining equipment, machine tools, powerful drives. Because their understanding of reliability is applied, hard-earned.

This may be why companies with a history such asLONGI Corporation, and remain on the market. They have gone from an idea on paper to hardware that has been working in difficult conditions for years. Their website -ljmagnet.ruis not just a catalog, but a reflection of this experience. And when you see that they have been in operation since 1993, producing 4,000 units of equipment per year, you understand that their specialists have probably encountered the same problems of ferrite overheating, vibration and interference as we do. And, most likely, they found their own working solutions. And this is worth a lot in our work.