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Current per Unit of Area in a Transformer or inductor turn.

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Patrick

Sun Nov 22 2015, 07:46PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

So ive been told and read that some makers of transformers use the 1.25 to 0.75 ma per circular mill area for their calculations. I have no idea if that's bogus or not. It seems like voltage drop per length should be a concern too.

Id like to be of the cooler side of things, but can anyone offer other opinions or measures?

Sulaiman

Sun Nov 22 2015, 08:52PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

it depends upon purpose and cooling,
medical use transformers, industrial control power transformers etc. are conservatively rated
most consumer ratings are for non-long-life products.
Large transformers are difficult to cool the interior windings .....
I use 2.5 A/mm2 for small transformers but I know that is conservative, usually.

Dr. Slack

Sun Nov 22 2015, 09:43PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

The typical Amps/mm2 varies with the size of the transformer, as it's cooling dependent. Fortunately, it only varies fairly slowly, so hobby size transformers, say 50VA to 500VA, all tend to be in the sort of 3A/mm2 ballpark.

The main thing to notice is it's not the same current density that you would use for supply cable, which is 10A/mm2, due to the rather better cooling.

Other makers of transformers, MOTs for instance, use higher than 3A/mm2, due to the fan cooling, intermittent use, disregeard for efficiency, and maxed-out cheapskating of the design.

Patrick

Sun Nov 22 2015, 10:36PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Ok this helps. I'm thinking planar ferrites may help at least a bit over EE cores. The ferrite hot spot might be more favorable.

DerAlbi

Sun Nov 22 2015, 11:48PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Can anyone tell me what the positve effect of a cool transformer is?
Afaik its dependend on the insulation... usual boundaries are 130Â°C, 180Â°C and 220+Â°C. As long as you are within the specification of the insulation.......

Is the breakdown voltage lower?
Does the ferrite saturation current drop at high temperatures?

I do know what one feels more comfortable with low temperatures but issnt that actually a waste of potential energy density?
The worst thing that happens is actually the increase of ESR which only contributes to more heat, but given that heat if one is still within reasonable limits...

And sorry to pirate the thread but: does anyone know where i get premagnetized cores? I would like to have a ferrite with 0.4T saturation...
so going from -0.2T to +0.2T seems much more attractive in a flyback since it doubles the current and quadrples the energy density. The thing is: how to get the -0.2T at zero current?

Patrick

Mon Nov 23 2015, 12:49AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

though I ask for reasons of avoiding hot spots, I am really interested in high efficiency which means its the design has failed even if the transformer continues to work. (if its overheating represents the percent of loss I don't want?

DerAlbi

Mon Nov 23 2015, 01:05AM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

A hot component does not mean bad efficiency. A SOT23-Transistor is hot even with small loss while a TO220-Package is still cold having the same power dissipation.
Having a big transformer does only increase its cooling area, so the heat disipates easier. There is no difference concerning the losses if you measure it in Joules or Watts. A smaller transformer with the same losses might get hotter... so what

Btw: i found in german wikipedia the information that ferrite has its lowest core loss around 100Â°C.

Specially if you choose a bigger core due to the heat seems contraproductive in this point of view: a smaller core might have had shorter wire due to less circumfence which can enable the use of thinner wire.
At the end: it becomes hotter, but it might weigh half. So why not. As long as you stay within specification...
but ok. it stresses the surrounding components... i get that.

PS: In my experience efficiency is inhertent to a concept, not a single component btw. of course everything must fit together.. but.. dont hang up on a transformer if the switching losses are dominant.

hen918

Mon Nov 23 2015, 01:38PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

DerAlbi: At higher temperatures most things, including copper, have a higher resistance. This is why efficiency is usually inversely exponential. Keeping things cool usually results in keeping things efficient.

DerAlbi

Mon Nov 23 2015, 03:15PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

And core losses are measured in Watts/cm^3 - Power/Volume. Keeping things small usually results in keeping things efficient

Specially when core loss seems to have a negative temp.coeff.
Just couple all power diodes of the design to the core too, and they have also less los with increasing temperature as long they are not schottky.
Increasing the temp of copper by 130K results only in 1.5 times the resistance. As long as the room-temp. resistance is ok, the higher resistance will be fine too. Its not that we talk about orders of magnitude here..
I am sorry but i really have no topology in mind where the resistive inductor losses are of dominant nature if built correctly..
There are allways worse switching losses (hard turn off in flyback / step-up) or bad coupling factor (flyback).
Other topologies use cores with higher inductance per turn (AL-Value) and have intrinsic high quality factor inductances - so switching or rectifying will be the dominant loss again.
Thats basically why you see heat sinks on semiconductors but no heatsinks on transformers.

Wolfram

Mon Nov 23 2015, 03:37PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

hen918 wrote ...

DerAlbi: At higher temperatures most things, including copper, have a higher resistance. This is why efficiency is usually inversely exponential. Keeping things cool usually results in keeping things efficient.

Efficiency is rarely inversely exponential with temperature, but often lifetime is, especially for semiconductors and electrolytic capacitors. Ferrite cores are often designed for maximum efficiency around 100 degrees centigrade, so it makes sense to run them around this temperature.

N87 (which is a common power ferrite) has about a 1/3 of the losses at 100 degrees centigrade compared to room temperature, at 100 kHz, while copper has only 50 % higher resistivity. If the AC resistance is dominated by skin effect losses (Rac >> Rdc) then the additional copper loss will only be 22 % higher compared to room temperature (skin depth increases with resistivity). Ferrite losses will be 1/3 compared to room temperature, so unless copper losses are much larger than ferrite losses (which often indicates a bad design), efficiency will increase with temperature. At higher frequencies, the losses don't decrease so drastically with temperature, but the lower mean-length-per-turn can still mean lower copper losses.

[Source: N87 datasheet

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