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View Full Version : How do medical power supplies achieve such low leakage current??



Jon Gerow
06-30-2018, 06:07 PM
Honestly, I've never had a chance to tear one apart, but in your typical desktop PSU, I've not seen lower than 3.5mV. Medical PSUs can be as low as .2mV.

To reduce EMI, we tend to put a Y capacitor to ground, and that's where you get most of your leakage. But it's not like medical PSUs don't use a Y cap and say "EMI be damned!"

What's the secret sauce?

ashiekh
07-01-2018, 10:03 AM
https://378m1o49xyfcl4may47q66uc-wpengine.netdna-ssl.com/App-slpower/images/whitepapers/An113%20Leakage%20Current.pdf

" ...important to substantially reduce the capacitances that cause leakage currents. This poses a special design problem for the power supply and especially EMI filters."

Jon Gerow
07-01-2018, 12:17 PM
That document does not answer my questions at all.

I know what leakage current is and how we reduce it today. :D

I want to know how they reduce it as much as they do in medical PSUs and still pass EMI. :D

ashiekh
07-01-2018, 04:56 PM
Same document

"Condor's medical designs and patented EMI filtering techniques have overcome these problems."

Jon Gerow
07-01-2018, 06:25 PM
Same document

"Condor's medical designs and patented EMI filtering techniques have overcome these problems."

No offense, but I don't know what your deal is. LOL!

Let's forget this document and move on because clearly it's not going to answer my question.

I mean, if they had a diagram and component list of what "Condor's medical designs and patented EMI filtering techniques" comprise of... Otherwise, you're answering my questions with ambiguous statements that merely taunt my original question.

Seriously. It's like if I asked how the Prius regenerative braking system works and you replied with "when you slow down the car, it generates electricity."

It's not what I'm asking for.

ashiekh
07-01-2018, 06:33 PM
I just don't have the time to track down their patent but thought you might if you knew of its existence, given your question. It didn't give you the answer but rather a clue as to where to find it; thought that might be of interest.

Now do you know what my deal is?

Stefan Payne
07-01-2018, 07:35 PM
Buy a Medical PSU, tear it down and post pics of it here?

Maybe they don't use Y Caps.
Maybe they do use Y-Caps but "special ones"...

Jon Gerow
07-01-2018, 09:42 PM
Buy a Medical PSU, tear it down and post pics of it here?

Maybe they don't use Y Caps.
Maybe they do use Y-Caps but "special ones"...

I plan to eventually, but they're so expensive.

ashiekh
07-01-2018, 11:11 PM
An isolation transformer is one solution, although probably not the one being used.

Jon Gerow
07-02-2018, 12:30 AM
An isolation transformer is one solution, although probably not the one being used.

Yeah. That's a solution, but you wouldn't have 80 PLUS Titanium medical PSUs with an isolation transformer on the primary side. :D

ashiekh
07-03-2018, 12:08 AM
Of course, one wonders what would happen if one just didn't ground the Y caps

Jon Gerow
07-03-2018, 12:12 AM
Of course, one wonders what would happen if one just didn't ground the Y caps; some common mode noise would get out?

Do I have a clue what I'm talking about... nope.

Yeah. Eliminate the Y cap and all EMI approval goes out the window.

ashiekh
07-03-2018, 12:20 AM
"No offense, but I don't know what your deal is."

Did I say eliminate the Y caps? you are putting words in my mouth;
keep in mind that the neutral is tied to the ground.

Microflop
07-03-2018, 04:44 PM
The only other way except filtering common mode currents is reducing the capacitance to earth of all nodes in SMPS that have jumping potentials.

You can also achieve a lot by using an optimized winding scheme of the main transformer, so the critical winding capacitances will be lowered but other capacitances might rise that don't matter as much.

Stefan Payne
07-03-2018, 07:02 PM
I plan to eventually, but they're so expensive.

Used, phased out/used ones from eBay??

Stefan Payne
07-03-2018, 07:04 PM
"No offense, but I don't know what your deal is."

Did I say eliminate the Y caps? you are putting words in my mouth;
keep in mind that the neutral is tied to the ground.
You can't do that where SCHUKO connectors are used because they are not polarized connectors like your US or the UK Connectors.

So its very likely that you switch Neutral and Live around for varous reasons.


I don't understand why they dont use the french plug when polarisation is important...

ashiekh
07-04-2018, 10:18 AM
You can't do that where SCHUKO connectors are used because they are not polarized connectors like your US or the UK Connectors.

So its very likely that you switch Neutral and Live around for varous reasons.


I don't understand why they dont use the french plug when polarisation is important...

The input filter circuit is symmetric between live and neutral