Welcome to part two of our five-part blog series on EMC basics!
In the previous post, we provided an introduction to EMC.
In this post, we will discuss the details of chip bead ferrites and how to pick them out according to the specifications from a datasheet. We'll talk about what they are, how they work, how to pick out the right part, and how to use them.
A chip bead ferrite is nothing more than an inductor. It is used to remove unwanted noise at high frequencies on supply voltage lines, ground planes, and data signals.
Remember the law of conservation of energy, which states that energy can be neither created nor destroyed — it can only be converted. That’s exactly what a chip bead ferrite is doing in this application. It is transforming unwanted signal energy into thermal energy. In other words, it's converting the unwanted signal into heat.
In order to maximize the impedance of an unwanted signal, you need to choose the appropriate material with the best characteristics to absorb the specific frequency you’re trying to attenuate. This material must also be non-conductive to ensure it does not short the cord. All of Wurth Electronics Midcom's chip bead ferrites use nickel zinc because of its non-conductive material.
What you end up with at the end of the day is a frequency-dependent resistor. If you take a look at this image, you can see the parasitics of a chip bead ferrite. We need to always be cautious of these because a chip bead ferrite has inductance, capacitance, and resistance. We'll get into this further as we move along.
This image gives you an X-ray view of a chip bead ferrite. Keep in mind that the windings are not insulated and the core is non-conductive.
As you can see, the winding of the high-current chip bead ferrite needs to be thicker. That's because it has to handle higher current; therefore, you have the ability to wind it horizontally.
The high-frequency chip bead ferrite needs to maximize the number of windings, so it utilizes the space by being wound vertically.
Picture these lying on their sides, and you can see that it’s best to use the amount of space you have for the application that you're trying to use: either high current or high frequency.
Inductive reactance is the result of a primary current fighting with the induced current in a helix-type of winding. This is based off of Lenz's law, which states that the induced current must flow in the opposite direction of the primary current.
As you can see in this image, the inductive reactance (or XL) is a function of frequency and inductance.
In any inductance, first you have a primary current going through the wire of the inductor. This creates a magnetic field coupled with the winding, which then creates an induced current flowing in the opposite direction of the primary current. This ultimately causes the opposing current, which we call inductive
So a chip bead ferrite has an inductance, a resistance, and a capacitance. You can see these characteristics shown in blue, green, and yellow on the graph here. What you're seeing is a typical datasheet of a chip bead ferrite. Three curves are shown: Z is the impedance, XL is the inductive reactance, and the R is the resistivity of the part.
The crossover frequency is where the inductive reactance and the resistivity meet, shown here as the red and blue dotted lines. Anything below the crossover frequency, and the inductive reactance is higher than the resistivity; thus, the part is better used as an inductor.
Anything above the crossover frequency, and you see more resistivity; thus, the part is better used as a filter at those frequencies — right up to the self-resonant frequency, or the SRF. SRF is the highest point where there is no inductive reactance, and your impedance and resistivity are at their highest points.
The main purpose of the chip bead ferrite is to block unwanted AC noise frequency. Anything over the SRF would become capacitive, which passes AC and blocks DC. So if you need a filter, you don't want to be in the capacitive region of the chip bead ferrite. Said differently, you could use a chip bead ferrite as a capacitor, but it's not the intended end use.
As with other magnetic technology, when you’re choosing a chip bead ferrite, it’s important to play close attention to heat.
Remember, you're converting an electrical energy to a thermal energy and dissipating it as heat. That means there will be a temperature rise of the part, and you must also be aware of the ambient temperature in which you're operating.
Sometimes it will be necessary to derate the current because the core is very sensitive to temperature. For commercial-grade products, the core material we use has a max operating temperature of 125℃. That’s because above 125℃, we come close to the Curie temperature, and the permeability drops down to one. As a reminder, the Curie temperature is where the material starts to lose its magnetic property.
Here, we’ll look at a specific example of a product that has an ambient temperature above 85℃. We specify on our datasheets as 40° temperature rise above ambience when used at five amps.
Remember that since the part has an 85° ambient temperature, with a 40° temperature rise, that would result in that max operating temperature of 125℃. So you could not go any higher, or you would be exceeding the recommended max operating temperature.
In this case, we have an ambient temperature of 105℃. When you add in the 40° temperature rise, now we're exceeding that 125° max operating temp, which means we have to derate the part. If you follow the graph, you can see that at 125℃ we must rate the current by 50%. Follow that line to 105℃, and if you go over to the Y-axis, you can see that leads you to 50%.
So at 2.5 amps, you get a 20° temperature rise, according to our datasheet. The results of the ambient temperature of 105℃ and a 20° temperature rise puts you safely at 125℃ max operating temp.
We used some very simple numbers here to provide an easy example. Your numbers may be more complex. Still, this is the type of process you have to go through to make sure your part will be under the max operating temperature and functioning correctly.
We specify our chip bead ferrites with zero bias. That means we only show you the impedance you'll get with no current flowing through the part.
For example, this image shows 330 ohms of impedance at 0 amps. If you add the current flowing through the part, you would be saturating the core.
Once you add the current, the impedance drops, and the frequency shifts. We do have LTspice models for these situations, but only for our high-current chip bead ferrites.
So, the big question: how do you choose the proper chip bead ferrite for you?
The answer is that it depends, like many other situations you run into. Based on your application, you need to take into consideration the frequency range of the noise, the source of the noise, the amount of attenuation needed, the environment, the electrical parameters of the circuit, and — as with everything else that you encounter — space and cost.
So it becomes a trade-off among all these considerations, based on the priorities of your situation and the design you're doing.
Find more articles about the EMC basics in the following blogs:
