RF Suppression for your Switch Mode Power Supply
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- čas přidán 24. 11. 2022
- In this video I discuss how to suppress RF interference from a switch mode supply. I also discuss how a common mode choke operates and the difference between a linear and switch mode supply.
Toroid Jaycar part number LO1238
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Comments: Good presentation Sir.
However The RFI noise generated when investigated with 250MHz CRO & 10:1 probe sniffing. Switching spikes on circuit ripple.
Used a keyhole camera snake tip & dentist mirror to get the transistor type numbers. Looked up at the manufacturers recommended application bare bones which appears to have been followed but without what was mentioned about drive & by pass suggestions.
Closer inspection revealed also a switching ringing on the spike base.
Driver cct to drive xfmr driving two mosfets in push pull.
The switch ringing is commonly associated with over driven push pull output stage.
In my case I cut the respective tracks leading to each final's gate and placed a smd 20 ohm resistor in drive supply line, which only reduces the total drive slightly. Also placed across the push pull final mosfets', Gate & Source tracks a 39nF 100v capacitor.
CRO Result showed a much cleaner output, dramatically knocked the over drive ringing cause.
Dramatic reduction in cross over spikes such reduced to general probe noise pick up floor level.
There was no noticeable difference in the finals operating dissipating temp.
On another branded chinese manufactured switch mode dc supply was of similar design. There a 25 ohm resistor was placed in the Gate drive supply track to each of the push pull mosfets and 47nF caps used across each Gate Source. Problem eradicated at the cause.
The dc output choke, 220uF + 0.001uF 100v green cap really makes a reduction difference on HF, but those spikes & hash appear in the noise floor beyond GHz.
Like lots of shacks running HF VHF , UHF & SHF. The problem causes needed addressing.
VK2BPP
Wow…. Thanks for this very detailed account of your investigations. I have to admit my understanding of SMPSU is limited to the basic theory of operation. Nipping the problem at the bud is a good way to go if you are knowledgeable. Being as I’m not that clever I rewound a couple of microwave xformers and built a simple 20 amp linear PSU 😝Linear 20A 13.8v DC HAM SHACK PSU - Made with rewound microwave transformers@TheArtofEngineering
czcams.com/video/uV5gD_t0V6w/video.html
nice demo, i bought my rtl-sdr v3 on a wim, just to visually see the band conditions ect, after a few days it was my best £20 investment, not only did it detect mains born rfi, i was experimenting with any rf noise and was suprised with the results, 12v smart car chargers were jamming all of hf/vhf, solar garden lights, led bulbs, ... but remember the rtl-sdr is connected to mains earth through the usb gnd,
after building a dedicated sdr scope using a tablet, i managed to reduce my noise floor to almost silent, eliminating mains power altogether, 3 identical 100amh lead batteries in parallel, 12v dc equipment, and an old solid state 6amp car battery charger, before i went live the charger was off, if anything is mains driven, you are connected via earth/ground sharing all devices local,
after all that, i then used sdr to compare antenna's, rst, noise floor and perfomance, ...... sdr rx'r is the most important tool i have in the shack,
Yes I have thought about going all battery. Looking for noise is something I need to do! Thanks for the informative comment….envious of your noise free situation 73 de VK2AOE.
@@TheArtofEngineering no worries thanks m1dls
This is just the solution I've been looking for. I bought one of those cheap chinese voltage boosters to give me B+ on my valve radio but the RF noise is atrocious. I'll pop in to jaycar and get a toroid ring. Thanks for the video
Glad it was helpful 73 de VK2AOE
Thanks
73 you are most welcome!
Nice George. They should have put better filtering in it, in the first place. You should be able to place your filter board within the case I imagine.😊
I would suggest paralleling a .001 cap with the electrolytic cap.
Yes.....part 2/2 has the Drew diamond design which adds more caps (as you suggest) and also filters the mains too! TU fer the comment 73. link to vid here - czcams.com/video/oJaPQWJDdp4/video.html
Thanks VK2AOE
You are most welcome.
I have an astron that just started giving me trouble like this. I would get chopping between 7.1ish to 7.22mhz when the supply was on and powering something. I had my g106 on the switched and my 991a on the linear and the noise on the 991 was horrible. So I wrapped a few turns of the power lead in a mix43 for the g106 and the 991 noise dropped right to normal. I'll have to di into this some more. I wonder what happened to my Astron SS-30m though.
Can you able to help me I am running a HAM radio on a power supply from Jaycar ic725 when I go down to a.m. annoyed gets very bad when I go to 27 mz the noise is ok can I add the use it for transmitting what the noise come over it
The noise might be local generated rather than from the supply? When you say “go down to AM” do you mean, listening on broadcast AM, or using AM mode? This filter only gets rid of RF generated by the switch mode. If your IC725 is noisy on transmit it could be supply related….but could also be something else? 73 de VK2AOE
Is your power supply under any serious load when you test the choke mod ?
No…. But when it is loaded …. I would be transmitting….. so noise on receive is of little consequence. It would be interesting to see if it effects noise though….. presently I now only use it on test bench as I’m using another Drew Diamond VK3XU design, the Miser DC 20 A linear 13.8V PSU (I’m using 2 rewound microwave xformers in it) Linear 20A 13.8v DC HAM SHACK PSU - Made with rewound microwave transformers 73 de VK2AOE @TheArtofEngineering
czcams.com/video/uV5gD_t0V6w/video.html
I'm glad you got this to work. However, I disagree with our explanation of how the common choke works, and your claim that magnetic flux has reactance. The workings of the common choke is such that common noise in the two ports create induced EMF that opposes current of the noise. Effectively, the induced EMF cancels the noise due to the polarity of the induced EMF due to the other coil.
Yes you are correct! I do discuss opposing flux for differential current cancelling and addition of flux for common mode current …. The inductive reactance “excursion” is most likely not a good way of explaining it. But if it has nothing to do with it … then what “chokes” the common mode noise? The common mode noise “sees” a big magnetic field produced by a big coil …. The noise is RF …. Varying ….. so is there inductive reactance?
@@TheArtofEngineering If we examine how EMF is induced in one coil due to current in the other coil, we can determine that common noise is cancelled by opposing EMF induced form one coil to the other. It's best to think of the common mode choke as a transformer (because that's what it is) If you draw a schematic while noting the "dots" are on the same side, then you can easily see that transient current in one coil cancels common transient current in the other coil. And that's the "magic" of the common mode choke. While analyzing the operation, remember that flux due to induction is the opposite of the flux due to current in the coil, as required by the law that says you can't get more energy than you put into a device. Consequently, a common mode choke doesn't work that same way for differential current. Now, having said all of that, I'm not an expert on magnetism. I watch these videos to cement my understanding of the subject, but often find myself disagreeing.
@@dannylong6966 I think your explanation sounds more correct than mine! Especially the “thinking of it as a transformer” …. Thanks for taking the time to explain this! The Drew Diamond design which also filters the mains AC works even better …. Video will be posted shortly. As for not being an expert on magnetism ….. you seem to have a good grasp of the concepts. Are you studying engineering? If not you should be ! 😀Also are you a ham? If not ….. join the cult of ham. 73 de VK2AOE
@@TheArtofEngineering Hi again. Thanks for the stimulating conversation. It's not often I get to have a discussion like this. No, not a ham, but my father loved radios. I've been an electrical engineer for almost 30 years, but I don't often have any work that involves chokes. A few years ago, I was trying to find a better way to connect analog and digital grounds on a signal board, and was looking into using a common-mode choke to do this. But I never finished that project. I will one day, if I can find the time. I'll look for your videos in the future. It was hard to find my way back to this one.