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I need to isolate my circuit from the power source of 5V dc. Since this is for medical use, and the device is powered by USB, the isolation needs to be at least 4kVrms, 5mm clearance, 8mm creapage distance. I searched for isolated DC-DC converters couldn't find any with those ratings and in small size. The remaining option I have, seems to be using 2 dc-dc converters in series. But that will be very inefficient and large in size. What could be the other ways to achieve this isolation?

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Current requirements? – Matt Young Aug 4 '14 at 12:56
@MattYoung The device requires very low current requirements ~ 10 mA. – zud Aug 4 '14 at 13:00

I don't agree that you need to be an expert in medical device design to accomplish this task, but you do need to do some research.

  1. Get a copy of IEC60601 and study it. There is a lot more to this type of project than just isolating the patient from the computer.
  2. You're going to need to treat your USB connection as if it were mains. Find a transformer that was designed to IEC60601, and matching driver. Design from there.
  3. Find a testing house that does IEC60601 compliance testing. They will test your device to the standards, and give you a 180+ page report.
  4. Go through the report, fix the problems, and test again.

Do not skimp on compliance testing. It will save your bacon.

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Hi, I have already gone through 60601. The values of 4kVrms, 8mm distance, I got from there and this is for working voltage of 250V (because of USB). I tried seaching for the suitable DC-DC converters in Farnell, Digikey but couldn't find a suitable one. Limited board space and the small current requirements are the main trouble. Otherwise I could use two converters to achieve the requirements. I am just looking for other alternatives now. One could be to use a transformer with a driver such as MAX845. But I have same problem finding medically approved small size transformer. – zud Aug 4 '14 at 13:56
Don't use the MAX845. It's getting long in the tooth and sometimes hard to source. Take a look at Wurth Elektronic transformers. – Matt Young Aug 4 '14 at 14:00
While in theory someone can learn all that is required to create a medical device starting from a basic level of electronics, this is really not a good idea. Knowing the regulations and compliance testing is only part of it. In the US, the paperwork that goes with filing a 510k is not trivial, and there are probably similar issues in other juridictions. You need to have someone on board that has been thru these processes before, else this will be a very expensive development effort with lots of delays. Don't skimp on expertise, it will save your bacon. – Olin Lathrop Aug 4 '14 at 14:19

No, Olin Lathrop is absolutely correct. I work for a FDA compliant medical device manufacturer, (also a pharmaceutical manufacturer). Designing your device is the least of your worries. Outside the scope of massproducing said device. If you still decide to build this "medical device" make sure you contact your business's insurance underwriter. if the failure of your device directly or indirectly affects any patient or provider, the legal actions sought against your company will never be worth what you got paid to make the device. And I agree, From an EE perspective, you either are trolling us.. or you are in over your head.

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This is absurd. You can't just design a medical device and have it go into production. It will never see a patient without first passing rigorous 3rd party testing and qualifications. He's asking how to start designing this. If he can't do it properly it will fail quality and never see use. That shouldn't mean we have to discourage him trying with snarky remarks. Medical device design starts somewhere. – ACD Aug 4 '14 at 19:04

If you have to ask this kind of basic question about a medical device here, the answer is: You don't. You get someone that knows what they are doing to do this. Then you can learn from what they do and maybe next time you're ready to do it yourself, but not this time.

This is not a do-it-yourself or learn-on-the-job project.

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The best way to avoid injury while skiing is not to ski... – ACD Aug 4 '14 at 15:23
-1 Share your knowledge and if necessary add dire warnings and disclaimers. – nalply Aug 4 '14 at 16:15
-1: Not an answer. – Armandas Aug 4 '14 at 20:33
@Armandas: Read carefully. Yes it is, just not one you and the OP want to hear. – Olin Lathrop Aug 4 '14 at 22:45

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