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Old 7th August 2005, 04:41 PM   #11
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Quote:
Originally posted by mzzj
Forgot to mention that toroids are usually? only class-I approved so you need grounding of primary side also.
Edit: and with little extra money Class-II
I already had a look at the assortment from Buerklin,
they offer potted, magnetic shielded and class II suitable torroids
for around EUR 100,- for 250VA or 300VA.
But this does not look for me like a bargain and also their primary
is a single winding, suitable for 240V only....



Quote:
Originally posted by Lars Clausen
EVA: I think you should be very careful to encourage people to ignore safety regulations. These voltages can be dangerous, so i think Chokoholic's respect for EN60022 (safety for IT equipment) is very well considered.

Chocoholic: Have you thought about a resonance mode push pull converter? IME they are suitable for high power jobs. But forget the 12 V input to the same converter, you will have too much copper on your main transformer. Make 2 separate converters and join them on the secondary side.
Is there a reason why you consider EN 60022? During my search I came to the conclusion that EN60065 would be the perfect match, its title is definitely adressing to Audio-, Video- and similar electronic gear.
Don't worry, not even EVA could convince me to ignore the safety standards. She's got the typical southern relaxed position in this regard, and please note: She also implements 5mm creepage her designs.
I am more the accurate German. I don't mind to work with non isolated kVs. But after closing the casing, it must be baby save.

..."...forget the 12V input..."
Uhps, you calculated that I would get current peaks of 300A on my
primary right?
I agree, that 500W PFC-flyback from 120V....274V/AC to 12V/DC is
is crazy. And honestly speaking in the moment this fact would be the only reason for me to try.
With respect to size, weight etc., my clear brain would indicate a
50Hz transformer as the heavy Electrolytic caps for 12V are necessary anyway....

Resonance push pull converter:
Honestly speaking: No, I did not consider this up to now.
I considered a traditional PFC-boost converter followed by an
halfbridge which is switching to an transformer with reasonable leakage inductance to allow regulation of the +/-50V output.
But it's an two stage design. Furtheron it does not use the existing 2F energy storage, so I would need about 200F/500V at the output of the boost converter......
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Old 7th August 2005, 05:56 PM   #12
mzzj is offline mzzj  Finland
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Quote:
Originally posted by Eva
Oh, it's always very healthy to have some safety regulations, but something is going terribly wrong when they are routinely ignored for certain classic applications and by big companies without nobody complaining. I have to go now but I will put some examples later.

Hmm, I Am intrested to hear about these, at least here at Finland local authorities are withdraving hundreds of products from markets every year nowadays(mostly chinese CE-****). Quality of electrical products has dropped dramatically in here after harmonised european safety regulations, maybe thanks to southern europeans
With Chinese pieces of **** it is enough that some underage slaveworker is sticking CE-stickers to their products and nobody is checking it for real. Then in here TUKES is doing some supprise test for products in market and quite often ordering importer to withdraw their s*it.

Ever seen cheap 6-20 euro multimeter like this?
http://www.tukes.fi/kuvat/myyntikiel...40210-04_y.jpg

Have only 3.5mm creepage distances on PCB, reguired is 20mm for 1000V classII meters. Will explode before reaching 1kV
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Old 7th August 2005, 06:47 PM   #13
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Quote:
Originally posted by Eva

Anyway, I think that Chocoholic got the point.
I think I got it. "No need to be cleaner than the pope".

But safety ... I am not afraid about my own safety, because
I know what I am handling with.
But friends? Children? Or if I decide to sell the stuff in some years?


mzzj:
Don't you know the Chinese interpretation of CE?
"China Export."
Or about copyright?
"Copy's right."
...somehow I like these guys *lol*
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Old 7th August 2005, 11:42 PM   #14
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Chocoholic,

I don't know but quite sure it's the same in Germany as in some other Northern european countries, you are NOT allowed to sell or even give away or let it be operated by others a design you have made in private that is not approved with the electrical safety regulations if it's going to be connected to the mains, so you don't have to worry about your friends etc.

Flyback for 500W, you will have huge currents through the secondary diode, maybe upto 200 A peak... the problem with Flyback and high power is switchng losses.

BTW I was a bit puzzled LC mentioned EN 60022(eg. CISPR 22) when talking about electrical safety, its an EMC standard dealing with radiated and conducted emissions.

I think Choco got it right, heres a note of the exact versions to be used:
The last date for evaluation and production to EN60065:1998 is 1st March 2007, when it too will be superseded by EN60065:2002 (which will apply until further notice.





In general:

BTW, talking about creepage and clearance distances, when designing transformers for SMPS one must leave a distance towards the sidewalls of the coilformer, which means one have to put yhe winding layers without support from surrounding walls.
This have made me wondering for many years how we can trust such winding will stay in place.

Distance are needed because it prevent dielectric breakdown between electrodes caused by the ionization of air. The dielectric breakdown level is further influenced by relative humidity, temperature, and degree of pollution in the environment, so one have to also take into account the pollution degree.

Well, but theres a solution, one can always pour the transformer in a plastic pot which will give much smaller creepage and clearance distances, and there wont be any problem with the pollution degree class as in such case its the lowest.

However at least 3 layers between primary and secondary with a good tape and preferably a suitable glue/silicone between primary and secondary at the peripheral "winding package" in the coilformer(not inbetween the winding layers) will do a lot for DIY:ers.

Michael
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Michael 29th January 2010
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Old 8th August 2005, 10:06 AM   #15
Kenshin is offline Kenshin  China
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Default Re: Torroid vs. SMPS

How did cellphone chargers pass safty standards? The whole transformer in them have a size only less than 20mm. Also...laptop adaptors are small.

Maybe use some special insulation wire / material?

Quote:
Originally posted by ChocoHolic
-The safety standard EN 60065 (Table 11) would require enormous creepage distances for reinforced isolation. Safety class II is a must for me as I do not want to have a mandatory earth connection.
In case of a flyback I would end up in a minimum creepage of
12.6mm and in case of PFC-Boost + Halfbridge around 10mm.
...values which make a suitful small sized transformer difficult...
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Old 8th August 2005, 10:28 AM   #16
mzzj is offline mzzj  Finland
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Default Re: Re: Torroid vs. SMPS

Quote:
Originally posted by Kenshin
How did cellphone chargers pass safty standards? The whole transformer in them have a size only less than 20mm. Also...laptop adaptors are small.

Maybe use some special insulation wire / material?

Vacuum potting helps with creepage distances as you dont need to care about creepage in trafo after it is vacuum impegrnated in epoxy
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Old 8th August 2005, 04:41 PM   #17
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Yes, but it is sometimes hard to pass the certification with vaccuum potted designs.
Typically the certification institute cuts the transformer into thin slices and if they find any bubble, you can go home and try another improvement to manufacture it really perfect.... ...experienced this in my old company and our vaccum potting was definitely not a poor potting !!!

Aside potting there are special wires and special bobbins.
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Old 8th August 2005, 05:18 PM   #18
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Quote:
Originally posted by Ultima Thule
Chocoholic,

I don't know but quite sure it's the same in Germany as in some other Northern european countries, you are NOT allowed to sell or even give away or let it be operated by others a design you have made in private that is not approved with the electrical safety regulations if it's going to be connected to the mains, so you don't have to worry about your friends etc.

I am not a lawyer, but I thought you are allowed to give away or sell. But you will be responsible with your life for anything that might happen. If you ignore safety standards and something happens, then you are in trouble.


Quote:
Originally posted by Ultima Thule


In general:

BTW, talking about creepage and clearance distances, when designing transformers for SMPS one must leave a distance towards the sidewalls of the coilformer, which means one have to put yhe winding layers without support from surrounding walls.
This have made me wondering for many years how we can trust such winding will stay in place.

Michael
The unused volume is often filled with 'margin tape'.
It is simply wound at the side wall, or can substitute the side wall if fixed properly. The isolation between the layers must extend throughout this margin tape (See my pics above).

Instead of margin tape there a special creepage spacers, which can be clipped on the bobbin.

If you are able to wind without margin tape or spacers, you ensure
that the windings will stay in place by a simple impregnation with varnish. Varnish also helps to reduce noise and improves humidity properties.
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Old 8th August 2005, 06:14 PM   #19
Eva is offline Eva  Spain
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These are some evidences showing that safety standards are usually ignored in the equipment we routinely use in real life.

Most AT and ATX PC power supplies doesn't include any creepages in their transformers, altough their labels are loaded of approval symbols:

This one has CE marking...
Click the image to open in full size.

But no creepages at all...
Click the image to open in full size.
Click the image to open in full size.


This one has a bunch of seals...
Click the image to open in full size.

But no creepage at all...
Click the image to open in full size.


This other one had also a lot of seals...
Click the image to open in full size.

But it not only lacked creepage but its common-mode EMI filter was also missing! Note the jupered common-mode choke and absent capacitors:
Click the image to open in full size.

mzzj you should check the PSU of your computer(s), you may have been using a unit without creepages for years without even knowing it, like most of us.


But, no doubt, where creepage is always mising is in 50Hz E-I mains transformers. I have still never seen a 50Hz transformer with creepage in my whole life, and these are the ones used in most commercial audio equipment and in most DIY projects.

This one is from my preamplifier, I bought it brand new in an electronics store some time ago:
Click the image to open in full size.

No creepage at all...
Click the image to open in full size.


This other one is an isolation transformer from a 12V 50W halogen lamp:
Click the image to open in full size.
Click the image to open in full size.


And concerning these toroid transformers that everybody uses for audio assuming they are absolutely safe...

These two pictures are from a 1000VA unit that suffered from an intermittent short between adjacent turns in one of the secondary windings. I stripped the secondary windings to further investigate the effects of overheating on insulation.

Note how the plastic tape (not mylar, maybe PVC?) has melt badly leaving primary-side and secondary-side magnet wires in direct contact far before the wires had any chance to overheat (temperature probably never exceeded 90C):
Click the image to open in full size.

Also note how the insulating tape (and magnet wire enamel) is always badly stretched on the corners of 50Hz toroids, leaving a very thin insulation layer and increasing leakage dramatically:
Click the image to open in full size.


Why are we not dead? We are still alive because creepage requirements are outrageous. Magnet wire enamel has usually over 3KV dielectric strenght (more like 5 or 7KV) and mylar tape also whitstands more than 3KV per layer. Furthermore, in order to start an arc between two needle shaped electrodes placed 5mm apart, more than 5KV are required, and this is the worst case, since 50KV or so would be required in order to start an arc between insulated round electrodes placed 5mm apart (like magnet wires).

This means that even a transformer without any creepage will pass flawlessly all classic '3,750V for 1 minute' insulation tests, it may even pass a 10KV or even 20KV test depending on construction.

This also means that any multimeter with 3.5mm creepage between +V and -V PCB tracks is probably going to reliably measure 2KV or even 4KV without PCB arcing, altough resistors will suffer breakdown with such high voltages. So in case any government decides to whitdraw them from the market, it would be actually for any reason other than objective reliability, probably economics and company fights.

(check this out as a refference http://www.kronjaeger.com/hv/hv/msr/spk/ )


Chocoholic:

Now try to figure out how much voltage is required to get an arc between two insulated magnet wires placed 10mm apart

And even though, I'm sticking to 5mm creepage, and this already makes my transformers far safer than most ones everybody is using for audio. Then again, does tube gear dealing with 480V use 10mm creepage? No, it doesn't, and their chasis isn't even earthed!

Anyway, as Ultima Thule mentioned, even if you build equipment following all safety regulations, if anybody (other than you) uses that equipemt, it will be a violation of law and you may be lawsuited and jailed. It won't matter if your equipment is objectively safe or not, what matters is that you have to pay dozens of thousands of euros to certification agencies in order for your equipment to be lawful. Nowadays safety is a question of money, not creepages, you must buy it.



kenshin:

I've just opened the off-line battery charger from my old Nokia 3210 phone. It's a self-oscillating flyback. It uses a special magnet wire with a very thick insulation for the secondary winding and standard magnet wire for the rest. Creepage is 5mm (total) and it's certified as a double insulated product. It doesn't have a primary-to-secondary common-mode shunting capacitor.



Safety regulations should be dictated by objective physics, not by economics, politicians and convenience. That's why production and design is progressively shifting towards China: Safety regulations in USA and Europe are dictated and strongly biased by big companies and by the own safety certification companies (these ones that charge you 30.000 euro for each approval), with the only purpose of preventing small companies from growing in order to avoid competence.

On the other hand, Chinese companies are free to grow in their country, until they can afford approval costs in order to enter our market. Ironically, their electronics market is free while ours is under a strong dictatorship.
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Old 8th August 2005, 06:47 PM   #20
Eva is offline Eva  Spain
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kenshin:

Sorry, creepage in that Nokia charger is actually 4mm, I've taken some pictures:

Note the thick secondary wire...
Click the image to open in full size.

Also note that the secondary is shifted 4mm from the primary connections and a small flux band is used to reduce the stray field created. Standard wire is used in the primary side.
Click the image to open in full size.

Overall PCB view showing a 4mm spark-gap.
Click the image to open in full size.
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