Switching power supply in high magnetic field, is an air coil OK? - Page 1
Forum section: electronics / projects

author: cdwijs
author_rank: Regular Contributor
timestamp: June 09, 2015, 11:33:09 am
content: Hi All,I have to create some electronics that works while a high magnetic field (MRI) is present. I wonder if it's possible to use a switchmode powersupply. I think I can't use inductors with a ferrite core, as the magnetic field will saturate the core.What happens when I use an air coil? Is this unaffected by the magnetic field? Will the switching power supply actually work correctly under those conditions?Kind regards,Cedric

author: nctnico
author_rank: Super Contributor
timestamp: June 09, 2015, 12:10:26 pm
content: How much power? You could consider switched capacitor voltage conversion.

author: TimFox
author_rank: Super Contributor
timestamp: June 09, 2015, 12:49:30 pm
content: How will you deal with RF interference to the sensitive receive electronics in the MRI system?

author: coppice
author_rank: Super Contributor
timestamp: June 09, 2015, 12:55:36 pm
content: There are a number of applications which need a switch mode supply that is completely immune to powerful static magnetic fields, which would saturate even a well screened ferrite cored transformer. So, quite a few people do what you are trying to do. Your additional complexity might be the RF field. You will need to look into how well that might need to be screened from transformer.A switch mode supply with an air cored transformer might only give 20% to 30% efficiency, but it gets the job done..

author: cdwijs
author_rank: Regular Contributor
timestamp: June 09, 2015, 01:34:19 pm
content: Quote from: TimFox on June 09, 2015, 12:49:30 pmHi TimFox,My electronics will be in a 3mm die-cast aluminum housing. I don't think the MRI will see me :-)Kind regards,Cedric

author: cdwijs
author_rank: Regular Contributor
timestamp: June 09, 2015, 01:40:10 pm
content: Quote from: coppice on June 09, 2015, 12:55:36 pmHi Coppice,How is the efficiency of an air cored inductor? I use it in a buck converter, non-isolated. Is this the same as an ferrite cored inductor?Where can I buy a 10uH air cored inductor? The biggest inductor I see at farnell is only 0.5uH. Must I make my own?http://nl.farnell.com/webapp/wcs/stores/servlet/Search?catalogId=15001&langId=31&storeId=10168&categoryId=700000005469&pageSize=25&showResults=true&pf=111468899Kind regards,Cedric

author: amyk
author_rank: Super Contributor
timestamp: June 09, 2015, 01:46:01 pm
content: Could you keep the SMPS out of the MRI and run some wires to the device?

author: cdwijs
author_rank: Regular Contributor
timestamp: June 09, 2015, 02:05:53 pm
content: Quote from: amyk on June 09, 2015, 01:46:01 pmNope, wires are visible as disturbances on the MRI image, therefore I power the thing from Li-ion cells.Kind regards,Cedric

author: langwadt
author_rank: Super Contributor
timestamp: June 09, 2015, 02:19:15 pm
content: Quote from: cdwijs on June 09, 2015, 02:05:53 pmand an aluminium box or even worse magnetic won't?

author: coppice
author_rank: Super Contributor
timestamp: June 09, 2015, 03:45:18 pm
content: Quote from: amyk on June 09, 2015, 01:46:01 pmThis is the strategy used for the MRI itself. For example, the motors inside the machine are ultrasonic motors, made from alloys that have no magnetic issues. The high voltage drives for the ultrasonic transducers are kept outside the strong field.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 09, 2015, 09:55:29 pm
content: Another issue is having magnetic-field-tolerant switching and control devices.Silicon is sensitive to magnetic fields, due to the Hall effect. While most devices are designed to be insensitive to this (with magnetic sensors being the obvious opposite case!), strong enough fields will eventually cause problems of a much more complicated and subtle nature: for example, the transistor structures inside a device might even be designed to cancel out the Hall drift effect through balanced design; but in a strong enough field, the drift is so extreme that the sheer conductivity of the device becomes impaired, and balanced compensation fails.I would guess bipolar devices are less sensitive than MOS, but really, you'll want to ask the manufacturer if their device is qualified for the field intensities you'll be seeing. You'll probably want to talk to them anyway regarding medical approvals.

author: BennVenn
author_rank: Regular Contributor
timestamp: June 09, 2015, 10:27:19 pm
content: As for you Air cored inductor, take a look at Low/Band/High pass filters for sub/mids/tweeters. I've seen some massive inductors not sure of the actual value though...

author: moffy
author_rank: Super Contributor
timestamp: June 09, 2015, 11:54:15 pm
content: You can purchase piezo electric transformers that will isolate and provide your voltage e.g.http://www.unictron.com/wordpress/piezo-elements/transformers-2/

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: June 10, 2015, 12:03:31 pm
content: lots of these calculators onlinehttp://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/indtor.htmluse 1 for air core relative permeabilityair core solenoid/cylinder probably leak mag field too far to be used in a compact and conductive enclosure

author: TimFox
author_rank: Super Contributor
timestamp: June 10, 2015, 12:46:38 pm
content: Quote from: cdwijs on June 09, 2015, 01:34:19 pmWhen I worked with MRI back in the 1980s, I found that commercially-available batteries always had ferromagnetic materials in the housings (regardless of internal chemistry). I haven't tried newer Li-based batteries. We used to keep an old speaker magnet at the workbench to check all components before installing them in preamp boxes, etc.You may also need to add finger-stock or other RF gasketing to your aluminum box to keep the switching garbage inside.

author: zombiess
author_rank: Newbie
timestamp: June 13, 2015, 08:04:30 am
content: Wouldn't an aluminum box get really hot in the moving magnetic field due to generating an eddy current? would it not also generate its own magnetic field opposing that which it is in? Diamagnetism.

author: dom0
author_rank: Super Contributor
timestamp: June 13, 2015, 12:22:19 pm
content: The strong field in a MRI is constant and not changing. Only the smaller fields are changing afaik.