Isolated power supply design

I think you should look in the DigiKey catalog where they have units available off the shelf with the safety/EMC certifications you need.

--
Namaste--

I expect the whole instrument will draw a maximum of about 30W. Most of the the circuits are on the same ground reference as the patient which is where the isolation requirement comes from. The patient site is only about disipating about 30mW. You can't put too much power here. It's bad form to burn an unconscious person. The rest of the power is going to displays and battery chargers and processors and a bunch of other bells and whistles.

If I don't put all the patient isolation into the mains power supply my architecture gets really complicated. I have been round and round on this with my quality assurance and regulatory guys and it really simplifies things if all the isolation is in one place. Assuming I can get it to work. If I can't, there are other solutions with multiple isolation boundries, but these get complicated and I would like to avoid that.

Regarding Pooh Bear's comments on triple insulated wire, that looks interesting. My concern is the capacitance between primary and secondary windings still needs to be low due to leakage currents. If I remember right, test is less than 10uA-ac-rms at 265V 50Hz, or about

125pF. But still, the wire looks like it is worth looking into for real.

Thanks,

Ethan

Ethan

Can you clarify something?

Do you need 30W at the patient applied part? Or does the entire instrument draw appx. 30W and you need less power at the patient site?

Design of a whole instrument for 10microamps is lots harder than design of a small sensor or other probe.

Regards PN2222A

"Been there. Done that. Got the 483."

Ok.

A 'universal input' supply such as you require means you need to use a switcher.

30W is definitely in the preferred range for a flyback type.

You can have any voltage you like. Even multiple supplies. If you're designing it yourself then simply choose the voltages you actually need.

There is *no* difference in this respect between a flyback transformer and any other transformer. Reinforced insulation can certainly be used. Increased leakage inductance isn't especially helpful though but there's a newish alternative....

If you're concerened about isolation ( sensible in your application ) then I suggest that you use triple insulated wire in the transformer ( in addition to any other methods that may be required for medical isolation requirements ).

Triple insulated wire is great but check the regulatory acceptance for this in your application.

Here's a link.

" Certified as Reinforced Insulation; the combination of any two layers can resist up to 3,000 ACV for one minute. "

Check out

and read all the app notes. You should be an expert by then !

You're likely to need to pay special attention to leakage currents I suspect in your case.

Graham

"Ethan" wrote

Burning the unconscious is exactly what some of my designs intend! But it is important to put the burn where the surgeon wants it.

Not that complex, really. Standard design for ECG amplifiers includes a transformer-isolated power supply at a few MW providing the power to the preamp, then some clever way of sending the signal back across the barrier. Or at least that was the way we did it a generation ago...

For 30mW, you don't really care about efficiency, just isolation (4KV standoff MINIMUM)and low,low leakage current. It's a handful of cheap components costing a few bucks.

Think about how much simpler any supply design is with 30 dB less power to handle.

I'm also thinking about the capacitance you get for free with a larger physical package 'inside the barrier'.

Regards PN2222A Tf (fall time) 60 nS See figure 2

Digi-Key: Elpac "MW" 24 W medical (12, 15, 18, or 24 V) $36/10, $27/100. If you need more power, the 40 W version is $42/10, $32/100. Mouser has some similar units at similar prices by Ault, Phihong, and Elpac. These are all external ones - there are internal ones as well.

Matt Roberds

I would rather not get into the architecture of my system in this thread. Partly because it would get complicated and hard to explain in a forum like this in a reasonable amount of time. But mostly because it is proprietary and my company is pretty sensitive about that kind of thing these days. Talking about it at all on this list is a little edgy.

So, for the sake of this discussion I would like to pretend it is a given that I need to design a universal input power supply with reinforced insulation. I know it is possible because as a couple people pointed out I can go to Digikey and buy one. And if they can do it, I should be able to do it too. Also my manufacturing quantities are pretty large so developing my own power supply is definitely worth while from a cost of goods vs NRE trade off point of view, and I am starting early enough that I have time to do this design without effecting time to market, etc.

The forward converter designs on the

app notes look pretty interesting. It is giving me ideas on how to take a fly back controller chip and put it into a design that is more like a resonant converter where leakage inductance is not much of an issue.

Does anybody know of app notes on resonant converter designs for universal input power supplies?

Thanks,

Ethan

My thoughts too. There are companies who specialise in medical spec switchmodes - look for US switchmode companies with Google. This removes a lot of work - do you really want to be a switchmode company ? Unless your product is very low priced and large volume. You will get to market a lot faster. Anyway, version 2 can have an in-house switchmode if you feel like it.

If you don't need medical grade insulation, the Asians can be cheap and good - I'll bet you could find an Asian 30W switchmode board with UL certification. Or cased, if you want.

Roger Lascelles

Hello Ethan,

Connecting system ground to the patient is usually not a good thing to do. Often it is prohibited. Considering that the system can be touched by personnel, just imagine what could happen if the patient went into a cardiac situation and would need to be defibrillated.

Patient isolation at the patient interface connection is usually not complicated, I do this all the time. What signals need to be transferred? Frequency range? Noise specs? If you are at liberty to tell, what's the system supposed to do?

What truly is complicated is procuring the parts to make the isolation barrier. Agencies usually want all the parts (such as wire insulation) certified as compliant by the manufacturer. I usually spec in wire where every single foot produced is HV tested. So, typically the time on the phone with the various vendors almost matches the design time. This strategy pays off when doing the agency tests. Labs such as UL often favor certificates and manufacturer documents over rationale. It's pretty similar to aeronautics.

If you go that route of isolating at the patient connection you also need to have a very meticulous assembly procedure and proper training of the folks doing it (must be documented). Plus Hipot and the usual works.

With a switcher that is going to be hard. As you mentioned you must also comply with EMC regs. Since you can't have much in terms of Y-caps it will be tough if using a switcher. With a linear supply you might get away with it, else it'll likely need a med grade iso transformer (no big deal). But again, doing a patient isolation solely at the power supply is something I have never done and would never do.

Regards, Joerg

Hello Ethan,

A resonant converter is the ticket if you absolutely have to have all the isolation there and must live with a somewhat leaky transformer. They are also usually lower in EMI. Best source as far as I can tell would be the old Unitrode app notes, now on the TI web server. They also have the chips.

Regards, Joerg

Hi Ethan,

You do not state the output power of the PSU, it does affect the architecture, i.e. Flyback, Foreward, Half Bridge, etc. You might also want to consider power factor correction too. A good book on PSU design which I have often refered to is by Abraham I. Pressman, ISBN

0-07-052236-7.

TI also have a list of their power supply (previously Unitrode) design course notes online, The latest course notes are listed as SEM1600 (and go back as far as SEM100). SEM1600 does discuss PSU safety and insulation. SEM600 discusses resonant converters. You can find this infomation on this url ...

Take care and good luck,

Mike Dawson.

You might consider buying one and reverse-engineering it to figure out what kind of topology and techniques they used.

what

Oooooooh, that's cheating...

...

I think you should find somebody who's already making medical equipment power supplies, and ask them for a quote.

Good Luck! Rich