[BillKay]

in reply to: All Things Fair-Rite #25825

MOSFET gate ringing can come from several sources but the only way to calm the stray inductance of the gate circuit, beyond just resistance, is with a ferrite. Fair-Rite 43 material a good choice for high frequency issues. A hole diameter of 1mm is the start – B dimension. Length is second – C dimension. This brings us to P/N 2643000501. If you find you need a little more impedance try our 2673000501 – it shifts the peak impedance to a lower frequency and supplies more impedance between 1 and 10MHz than the 43 material. Curves are available on the website for both.
Good luck!
Bill

[BillKay]

in reply to: Audio & Home Entertainment #25786

A gentleman asked my advice on how to reduce noise spikes he say on his +/- analog voltage rails. The spike were occurring at 120kHz and causing beat frequency problems.

Hello,
At 120kHz there are two possible ways to deal with the problem. The first is a classic filter where an inductor blocks the unwanted noise signal. We know that these pulses contain a lot of harmonics so the inductor would need to act as an inductor beyond a couple megahertz. Our 43 material provides good permeability up to this band and then becomes lossy – which is a good thing. We make a lot of different product with that material for you to choose from.

Alternately you can use a suppression material to ‘consume’ the energy in the spike. Our 73 material is designed to become lossy at a low frequency. At the 120kHz is will add a good amount of resistive impedance out to about 100mHz. Our product offering has lots of smaller bead products made from this material. So the way this works is that the material acts like a frequency dependent resistor that has resistive losses and insertion impedance at these frequencies. Exactly how many ohms you need is a function of amplitude and source and load impedance. EMI gets attenuated and the loss ends up as tiny amounts of heat dissipated in the ferrite material.

If this is common mode noise, and it probably is because those are linear voltage regulators, then the positive and return conductors can go through the ferrite and you don’t have to worry about saturation.

Unfortunately most EMI components are great at helping with 10mHz and higher so 0805 SMD components are not going to help you much. You are going to need a ferrite bead. I’d start with the second option.

[BillKay]

in reply to: Equivalent inductance of a ferrite core #25756

*** Corrections: the part you mentioned becomes resistive above about 25MHz, not 100mHz. Equiv solid core is 2646626402 ***
Sorry for the mistake and any confusion it may cause.

The inductance value is highly frequency dependent, at the frequencies of concern. We can, however calculate the inductance (at a frequency) from the impedance curve. For example, XL at 20mHz is about 65 ohms. Using Z=2PI*F*L we get 517nH. Note at that same point the resistive losses are also about 65 ohms. The resulting vector impedance is 90 ohms. Above 200mHz the inductance drops to 0 quickly and the component is purely resistive.
Impedance curve: https://www.fair-rite.com/product/round-cable-snap-its-446167281/

Feel free to give me a call if I can be of help trying to get the smallest solution.

[BillKay]

in reply to: Equivalent inductance of a ferrite core #25754

Ferrite cores designed for EMI suppression show inductive characteristics up to some frequency but then turn resistive. This is ‘complex permeability’. Devices made from these materials are not considered inductors but rather suppressors and are not rated in henries but in impedance (at frequency). The resistance, as the name implies, has little phase lag so power is absorbed and dissipated as small amounts of heat. With your part this mode begins above 100MHz. If you look at the Fair-Rite web site for that part an impedance curve is shown. So you really can’t replace it with an inductor, per se.

Fair-Rite makes solid core equivalent parts to the snap on version that you have. The equivalent would be 2661626402.The solid core equivalents tend to be noticeably smaller, for the same impedance, because they do not have to compensate for the losses generated by splitting the core in half. In this case, over 40% smaller.

The frequencies that the part is suppressing is not identified nor is how much suppression is required so if the solid core part still isn’t small enough then some experimenting will be required to get another solution. Smaller beads around only the offending wires might be considered.

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