Low power dc-dc converter transformer with multiple isolated secondaries

[Steve]

I'm designing an 8 channel, high performance thermocouple input module
for industrial applications. The specification requires full safety
isolation between the channels (250VAC). The problem is using a
separate dc-dc converter transformer for each channel takes too much
board space. Does anyone have any references or recommendations on the
design of a single transformer with 6-8 fully isolated secondaries? The
dc-dc input in 24VDC and the output requirement is +5V per channel.
Power rating is on the order of 2-3 watts.

 

[Klaus Kragelund]

I'm designing an 8 channel, high performance thermocouple input module
for industrial applications. The specification requires full safety
isolation between the channels (250VAC). The problem is using a
separate dc-dc converter transformer for each channel takes too much
board space. Does anyone have any references or recommendations on the
design of a single transformer with 6-8 fully isolated secondaries? The
dc-dc input in 24VDC and the output requirement is +5V per channel.
Power rating is on the order of 2-3 watts.

Since you are talking thermocouples you must also be talking low speed.
Perhaps use a capacitor coupled bridge to transfer power using a CMOS
push-pull stage (that is two Y2 caps to get safety isolation and a
bridge to rectify the pulses). Then communicate back to the primary
side using a clamp to short the bridge. Measure current consumption on
the primary side to get the info back. This is low speed, but it should
be do-able for your application

Costs = nothing, but R&D time is larger than a brute force transformer
solution

Regards

Klaus

 

[Joerg]

I'm designing an 8 channel, high performance thermocouple input module
for industrial applications. The specification requires full safety
isolation between the channels (250VAC). The problem is using a
separate dc-dc converter transformer for each channel takes too much
board space. Does anyone have any references or recommendations on the
design of a single transformer with 6-8 fully isolated secondaries? The
dc-dc input in 24VDC and the output requirement is +5V per channel.
Power rating is on the order of 2-3 watts.

Often I have to design such features into medical gear. There we need up
to 5KV of isolation but the method always boils down to this:

#77 or similar ferrite core. My core preference is the toroid but that
can get old if you need lots of windings. One primary winding that is
driven by an H-bridge plus some EMI muffling. This primary is made of
wire with a 100% certified insulation. The drive is somewhere between
40kHz and 200kHz, usually. For lower power apps I have used cheap audio
amps as a driver.

Then as many secondaries as desired. If they need high isolation against
each other you also need to use the good stuff for those windings. That
is rectified and regulated either by a linear circuit or another
switcher. Where every penny counts I often use two BAV99 in a bridge
configuration.

 

[John Larkin]

I'm designing an 8 channel, high performance thermocouple input module
for industrial applications. The specification requires full safety
isolation between the channels (250VAC). The problem is using a
separate dc-dc converter transformer for each channel takes too much
board space. Does anyone have any references or recommendations on the
design of a single transformer with 6-8 fully isolated secondaries? The
dc-dc input in 24VDC and the output requirement is +5V per channel.
Power rating is on the order of 2-3 watts.

http://www.highlandtechnology.com/DSS/V450DS.html

We just used a transformer per channel. Little bitty ones.

How big does your board have to be?

John

 

[[email protected]]

for industrial applications. The specification requires full safety

Often I have to design such features into medical gear. There we need up
to 5KV of isolation but the method always boils down to this:

#77 or similar ferrite core. My core preference is the toroid but that
can get old if you need lots of windings. One primary winding that is
driven by an H-bridge plus some EMI muffling. This primary is made of
wire with a 100% certified insulation. The drive is somewhere between
40kHz and 200kHz, usually. For lower power apps I have used cheap audio
amps as a driver.

Then as many secondaries as desired. If they need high isolation against
each other you also need to use the good stuff for those windings. That
is rectified and regulated either by a linear circuit or another
switcher. Where every penny counts I often use two BAV99 in a bridge
configuration.

What kind of certification does engineering medical equipment require..?
(and where to get 'life support' chips?)

 

[Joerg]

What kind of certification does engineering medical equipment require..?

Usually EN601, the whole enchilada. This also includes EMC, of course.
In my own designs I go a step further in that I insist that they be
defibrillator proof even if the regs don't require it.

(and where to get 'life support' chips?)

You almost have to make your own. Seriously, that's what one company I
know does. I am not aware of any large manufacturer that doesn't have a
life support exclusion. Of course, you can always negotiate but then
there needs to be a huge stash of money on the table or they won't even
talk to you about it.

 

[Joerg]

http://www.highlandtechnology.com/DSS/V450DS.html

We just used a transformer per channel. Little bitty ones.

Was that the telco transformer from the other thread? How do you manage
the 15KV ESD?

 

[John Larkin]

Was that the telco transformer from the other thread? How do you manage
the 15KV ESD?

Yup, the $1 ISDN things, with 2N7002 drivers. They're rated for the
1500 volt telecom transient. Some modest capacitors between the input
pins and ground (good for emi anyhow) divide down the human-body-model
zot to below 1500.

John

 

[Joerg]

Yup, the $1 ISDN things, with 2N7002 drivers. They're rated for the
1500 volt telecom transient. Some modest capacitors between the input
pins and ground (good for emi anyhow) divide down the human-body-model
zot to below 1500.

Aha. Caps. That's what I am usually not allowed to do on my medical
designs. They have to stomach the discharge of a 32uF cap at 5000V into
a body model via a huge inductor. When the agency folks want me to
demonstrate that their engineers move to the far corner of the room
before I push that big red button.

 

[Chris Jones]

I'm designing an 8 channel, high performance thermocouple input module
for industrial applications. The specification requires full safety
isolation between the channels (250VAC). The problem is using a
separate dc-dc converter transformer for each channel takes too much
board space. Does anyone have any references or recommendations on the
design of a single transformer with 6-8 fully isolated secondaries? The
dc-dc input in 24VDC and the output requirement is +5V per channel.
Power rating is on the order of 2-3 watts.

Is this any good to you? (Not 2-3 Watts, but maybe that isn't needed)

http://www.analog.com/en/prod/0,,760_1080_AD7401,00.html
http://www.analog.com/library/analogDialogue/archives/40-12/iso_power.html

Chris

 

[[email protected]]

What kind of certification does engineering medical equipment require..?

Usually EN601, the whole enchilada. This also includes EMC, of course.
In my own designs I go a step further in that I insist that they be
defibrillator proof even if the regs don't require it.

Is it a written exam or something else?

I guess 'defibrillator proof' means you can put the pads on your equipment and
it will still work.

You almost have to make your own. Seriously, that's what one company I
know does. I am not aware of any large manufacturer that doesn't have a
life support exclusion. Of course, you can always negotiate but then
there needs to be a huge stash of money on the table or they won't even
talk to you about it.

What about haveing an extra quality ensurance, no single point of failure plus
an insurance for any legal claims?

Makeing your own chips doesn't seem trivial considering todays complexity ..

 

[James T. White]

Was that the telco transformer from the other thread?

What other thread?

 

[Joerg]

Is it a written exam or something else?

Nope, its a set of standards by which we must live.

I guess 'defibrillator proof' means you can put the pads on your equipment and
it will still work.

There are several grades in terms of how the equipment is allowed react,
depending on its function during a procedure. All the way from "must not
electrically break down" to "must be fully operational again in x
seconds". I always make sure it's operational in under a second no
matter what. Never failed one test

What about haveing an extra quality ensurance, no single point of failure plus
an insurance for any legal claims?

That's the negotiating part. However, large manufacturers won't spend
that time with you unless there is a substantial business volume in the
cards for them.

Makeing your own chips doesn't seem trivial considering todays complexity ..

But it's done. Out of sheer necessity. The good thing is that there are
a lot more independent foundries than there were 20 years ago.

 

[Joerg]

What other thread?

Check John Larkin's June 17, 2005 posts in this thread:

http://groups.google.com/group/sci....n"+Joerg+"ISDN"&rnum=1&hl=en#507e2f6d9e0a5548

I believe there was another more recent disussion though.

 

[Steven Swift]

I'm designing an 8 channel, high performance thermocouple input module
for industrial applications. The specification requires full safety
isolation between the channels (250VAC). The problem is using a
separate dc-dc converter transformer for each channel takes too much
board space. Does anyone have any references or recommendations on the
design of a single transformer with 6-8 fully isolated secondaries? The
dc-dc input in 24VDC and the output requirement is +5V per channel.
Power rating is on the order of 2-3 watts.

Severall companies make multi-output transformers. This one has six windings:
http://www.cooperet.com/library/products/PM-4301 Versa-Pac.pdf

Maybe they have an 8-output one.

I often designed my own using multisection bobbins. One time I used a
toroid with wire loops from each section. That was a "driven" transformer
design. I used 1000V wire to get low capacitance and high breakdown. Drive
the primary push-pull from your 24v and all you'll need is a single turn
on the secondary.

Works.

Steve.