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Forum Dealing with Unexpected Emissions: The Fellowship of the Ferrite Ring
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Forum Thread Details
  • Replies 21 replies
  • Subscribers 106 subscribers
  • Views 4352 views
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  • ferrite ring
  • MXO4
  • emc
  • MXO 4
  • spectrum analyzer
  • toroid
  • choke
  • oscillator
  • ferrite core
  • Fair-Rite
  • ferrite
  • inductor
Related

Dealing with Unexpected Emissions: The Fellowship of the Ferrite Ring

shabaz
shabaz over 2 years ago


I was experimenting with bits of circuit, hopefully, to become a power supply eventually. It is a work in progress.

At this stage, I’m using bits of scrap copper-clad board, each with a sub-module. It’s too early for a PCB.

image

However, during the prototyping, I noticed (amongst other issues that I need to solve!) a particular artifact on the ‘scope trace when looking at the output.

This situation ended up being a good example of where it pays to zoom out and look at a lot more of the spectrum view than expected!

Admittedly this was a lot easier to identify using the MXO 4 ‘scope, but it would be possible to spot with other test equipment, such as a spectrum analyzer and a near-field probe. I just used the normal 'scope probe, I didn't do anything special.

With the MXO 4, the anomaly was zoomed in, and then using a logarithmic-frequency spectrum view, it was possible to see the extent of the problem, with the left side of the spectrum view showing the wanted oscillation (fundamental frequency) and its harmonics, and the right side showing the spurious output.

image

Now I could try to do something about it!

I used tiny ferrite rings (legend has it they were forged by Fair-Rite, part code 2643001501 ) on the base input of the transistors. Essentially, they act as a bit of additional resistance for the higher frequencies. The rings cost $0.02 currently (although more expensive in the UK).

image


Back to the ‘scope:

image

Much better!

As mentioned, I still have other problems to deal with, but I thought it was a nice example of why it makes sense to do testing to frequencies far higher than expected, just in case there are spurious emissions, and why it can be a good idea to possess the rings because one never knows when they are needed.

Thanks for reading!

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Top Replies

  • Jan Cumps
    Jan Cumps over 2 years ago in reply to anniel747 +3
    or nail polish. Also good to fix broken-off enamel.
  • robogary
    robogary over 2 years ago +2
    nice. I wouldnt have looked that closely to notice the ring.
  • anniel747
    anniel747 over 2 years ago in reply to shabaz +2
    Or conventional varnish dip.
  • wolfgangfriedrich
    wolfgangfriedrich over 2 years ago

    I am sure the company is called Fairy-Rite, because their parts do some real magic.

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  • shabaz
    shabaz over 2 years ago in reply to wolfgangfriedrich

    Yes! : ) They should write a book, "Fairy-Rite Tales that Came True", about how circuit problems were solved.

    Sold with a bunch of ferrites on golden thread : )

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  • Jan Cumps
    Jan Cumps over 2 years ago

    2 NPNs. No push-pull. Half-bridge? 


    If yes, how do you deal with the shoot-through? Is the oscillation artifact the resonance frequency of silicon getting cooked by shortcut current?

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  • anniel747
    anniel747 over 2 years ago

    Schematic? Frequency response of the toroid used?

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  • shabaz
    shabaz over 2 years ago in reply to Jan Cumps

    Thankfully it's not half-bridge, that indeed would be a difficult problem to solve with the few components. The circuit is pretty much this (I'm still experimenting so I've not narrowed down to any particular values yet), where each transistor is driving each end of the winding to ground, centre-tapped.  The artifact is due to so many windings and stray capacitances (some varying), causing resonances that are getting amplified. I think I can reduce it further, since I randomly used the only small ferrites I had at hand. The improvement is significant, but I hope to try to improve that further.

    image

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  • shabaz
    shabaz over 2 years ago in reply to anniel747

    I've tried several toroids so far (I didn't take photos or screenshots of them all, so the first PCB photo shows a different toroid to the last PCB photo! : ) because I wanted to use a commonly-available one so the circuit can be easily replicated. This was hard though, because in toroid form most ferrites are for chokes. As a result I'm seeing efficiencies barely approaching 50% at best so far, and quite an ugly waveform under load.

    I have ordered some hopefully more suitable ferrites to see if that can be improved, although I don't really mind if it is 50% efficient (I'm aiming for cheapness and low noise). 

    II only started on this yesterday, and the board is scratched-out copper-clad) but have already constructed perhaps 5 or 6 transformers, pulling off the windings and reattempting each time. I barely have any power toroids to experiment with here, although I have a fair amount of cores for chokes.

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  • anniel747
    anniel747 over 2 years ago in reply to shabaz

    Any specs on the chokes used? Those are pulls if understand correctly?

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  • shabaz
    shabaz over 2 years ago in reply to anniel747

    Do you mean component pulls? No, I'm trying to use easily-available parts, to make it repeatable. It's just unfortunate that most of the toroids I have (all with a part code so they can be purchased) are lossy, and therefore more suitable for chokes.

    I didn't record all the results so far since most were pretty bad. Here is an example which I did happen to record. It approached 50% efficiency at some settings, but I'm not happy with it, so this core is ruled out. This was with a type T38 core (B64290L0044X038)

    image

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  • anniel747
    anniel747 over 2 years ago in reply to shabaz

    You chose one labeled EM and you need one labeled Resonance (the ones that end by X001). 

    product.tdk.com/.../list

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  • robogary
    robogary over 2 years ago

    nice. I wouldnt have looked that closely to notice the ring. 

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