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Need Low EMI / RFI Emitting Step-up Power Supply.

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LAN_403

Patrick

Thu Mar 06 2014, 12:18AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Im needing a PS for a special purpose.

I need a step-up PS from 5v input to 12ish V out, at 70mA. But it must have ultra low EMI and RFI emission, so im thinking this rules out typical small transformers and inductive type PS.

So, im wondering if a capacitive multiplier is a solution? is my reasoning sound, or should i use a conventional PS with super-dooper shielding. I'd rather not have radiation emited in the first place though.

It must be very near (inside my radio transmitter) so hence the need for not sabotaging my 2.4GhZ TX/RX signals...

Advise / opinions ....

Sulaiman

Thu Mar 06 2014, 06:21AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

12v x 70mA = 840 mW ... just nice for a dc/dc converter in a can.

Patrick

Thu Mar 06 2014, 01:36PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

So should I use a transformer?

GeordieBoy

Thu Mar 06 2014, 02:43PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Buy a ready made DC-DC converter from a reputable manufacturer like Traco, Aztec, etc. This will come in a metal can so you can mount it over a PCB with ground plane underneath to complete the shielding. Then add any additional common-mode chokes to input and output as necessary to further reduce EMI.

I wouldn't contemplate designing your own SMPSU unless you already have a good appreciation on the power electronics and EMI challenges ahead. It's simply not worth the time when you can buy something ready made.

-Richie,

Dr. Slack

Thu Mar 06 2014, 08:21PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

+1 for buying a packaged solution, boring as that sounds. They're cheap as chinese chips on fleaBay. If when you test one you find that the EMI is too large, then adding filtering and putting it ina can are probably going to be easier than designing one from scratch, then adding filtering and putting it in a can.

The key to low noise filtering is to realise that getting the right value filter components, by a filter design program or SPICE simulation is only a necessary part of the battle, it's not usually sufficient. The other thing you have to do is to mount the components so that their parasitics, and wiring parasitics, don't undo all the good work that the components are doing. This usually means analysing the paths that will be taken by the GROUND currents, and keeping those short and not coupling with anything else. Most people aren't used to thinking like that, and unfortunately that is what can still bite you, even after you think you have the right values soldered in. If you run into this sort of trouble, post photographs, as it's very difficult to give general guidance beyond the above

2Spoons

Thu Mar 06 2014, 09:33PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

What exactly do you mean by "low EMI"?
I'm a proponent of designing your own supply - that way you can choose your switching frequency such that the harmonics will miss your 2.4GHz, but far more importantly miss the IF frequencies used in your radios. You have no control over this buying readymade. And as for buying cheap and applying filter bandaids .. that's a tough road to follow, and probably a false economy. You be better to buy a milspec supply - emi regs for military gear are extremely tough.
Low noise topologies: I'd be looking at either resonant or cuk. At that power level a cuk converter would work well.

Patrick

Sat Mar 08 2014, 05:47AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

ive decided after considering your valued input, to go with adding a second battery pack, its simpler and cheaper, and guarantees no EMI or RFI.

Thank you all.

Dr. Slack

Sat Mar 08 2014, 08:02AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

The BBC (British Broadcasting Company) certainly used to run all their studio sound equipment from banks of lead-acid batteries to avoid power supply noise, and head off all sorts of ground loop problems. I don't know whether they still do, probably not.

Sulaiman

Sat Mar 08 2014, 10:17AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I am (very slowly) making a hf (usb only) transceiver for 14 to 50 MHz
(possibly to 70 or 145 MHz if I'm lucky)
with one IF at 1.4 MHz (found some lovely crystal filters on eBay)
where smpsu noise is very likely to be a problem.

I have put all the 'noisy' stuff (2x vfo, 2x xto, battery management etc.)
in one area and hope to screen that area from the 'sensitive' areas.

Even with battery power to eliminate switching noise,
there are plenty of other potential noise sources to consider
so I think that a commercial (ready made) dc/dc converter
if given some thought, would be ok.

So far my biggest problem has been power line decoupling
it's not easy at 50 MHz, it must be even worse at GHz !

Other than where commercial pressures exist
the most common approach to microwave construction is to
put each module or sub-module in it's own screened enclosure
since even a short conductor can be a fairly efficient antenna
for transmitting or receiving interference.
Of the commercial stuff I have examined
(transceivers, cell-phones, cellular radio mast equipment etc.)
great attention has been given to screening sections from each other.
The general principle being to assume that there is all kinds of external interference
and shield each module / sub-section.
External smpsu noise will be negligible compared to other problems I expect.

Having said all that, I intended to operate on one 12 Vdc battery
but for higher power I am using 2x 12 Vdc battery for when I want
higher power. (my license allows for up to 50W transmit)
and one battery for 'QRP'.
The inverter that I built for 12 V to 24 V dc WAS noisy,
but it was over 100x the power you are considering.

In my work I (try to) repair industrial electronics
and even very sensitive analogue sections
are powered by dc/dc converters nearby on the same pcb
and I normally see negligible noise problems.

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