switching transformer

[MacIntoshCZ]

The tittle speaks for itself.
I tried to wind the pulse transformer at 50khz with only one wire to test the skin effect. It must be said that the diameter of the wire was 0.5 mm. So size wasn't that critical. The primary was wound with one wire, the secondary with two. The target power should be 200W. Primary voltage approx. 300V, 39 primary turns, 16 secondary. At full power, all components were beautifully cold, I already thought I had won, but then I noticed that the transformer was 70 degrees. What is the safe temperature of pulse transformers at full load? 70 degrees seems to me enough due to the isolation between the primary / secondary. I did the insulation with electrical tape, so I'm bad at the temperature.

 

[MacIntoshCZ]

Core details
air gap 0,2mm

https://semic.cz/!old/files/pdf_www/CF297.pdf

https://semic.cz/!old/files/pdf_www/Lj_297ETD4917_CF.pdf

BTW i used this topology

source:

https://www.wcmagnetics.com/wp-content/uploads/2015/02/appnote11.28.10.pdf

 

[MacIntoshCZ]

Skin effect calc

Round Wire ac Resistance Calculator

Losses in primary lower than aprox 0,13W.
Socondary is two wires + half lenght so 0,13/2/2 and current is aprox (200W/120V) 1,7A -> 0,1W

 

[MacIntoshCZ]

Is it core itself?

 

[MacIntoshCZ]

It should be around 200mT i calculated it to 250mT around 50khz with 50% duty cycle for 390V. And i use 300V so it should be lower

 

[ronsimpson]

It is important to know if the heat is from the core or the wire. (should be 50/50 more or less)
Where are you measuring the temperature? The outside is cooler than the inside.
Some times I measure the primary resistance cold then hot to get a idea if primary temperature. (resistance change with temp)
Seems a little hot. Might be 100C down inside.
When I got above 50khz I moved to Litz wire.

 

[MacIntoshCZ]

I measured it on secodary winding. Core outside was cold. Maybe Middle column ? It is probably wire.

 

[MacIntoshCZ]

Voltage on secondary is right. But some resonance show up

 

[MacIntoshCZ]

I would like to wind a current transformer so that I can measure the current on the secondary. I have a toroidal core. If I have one primary turn and 33 turns on the secondary, at 1A the current on the secondary will be 33 times smaller. If I choose 3.3 ohms as the sensing resistor on secondary and multiply the measured voltage by ten I get one volt, right?

 

[rjenkinsgb]

That sounds about right.
Or you could double or triple the turns to keep the ratio but possibly with better accuracy, if working with low currents.

For low frequencies a normal E-I type laminated iron core will also work fine. We use a custom made current transformer in one of our products that has a three turn primary on a small conventional style core & bobbin. That's for up to 40A primary current.

 

[MacIntoshCZ]

I think i am to lazy to wind 100 turn in toroidal core. I have big core so long wire will be needed. But with higher turns losses are lower... Or i can cut core in half and than wind it. That will be much easier...
thanks

 

[rjenkinsgb]

If it's a big enough core, wind the wire on a small spool first and pass that through the core.

Or make a shuttle-style tool, a bit of flat plastic or wood that will go through the core, notched at each end and wind the wire endways around that to get it through the toroid?

 

[MacIntoshCZ]

Measured on secondary
aprox Imax = 12A
Duty cycle 17% 50Khz
Ontime = 0.17/50000 = 3,4e-6
area of pulse = ontime*imax/2 = 2,04e-5
Irms = area of pulse * 50khz = 1,02A
Am i right?

Irms (with cursors) = 1,14A
I used 30 turns and 30 Ohms

BTW does rigol has any math function for own equations?
Like integrals, derivative and so on (except integrating traces...)

 

[MacIntoshCZ]

Also how much amps per 1mm2? I used 6A per 1mm2.
1mm2/6A -> 0,167 mm2 per 1A
I used 0,5mm wire -> 3,14*0,25*0,25= 0,196 mm2
(1A/167mm2)*196mm2 = 1,174A max
I used two wires on secondary -> 2,34A max
Iprimary = Imag +Iseconday/(transformer ratio)

 

[MacIntoshCZ]

Do you think heat is on secondary winding?
I know of one weak point. The cores are temporarily fastened with a cable tie. They are not tight enough, however, the gap cannot be seen ...

 

[MacIntoshCZ]

Checker Once again And heat comes from top.
Worth mention that i have unconnected auxilary winding. Its same as primary - 39 turns.

 

[MacIntoshCZ]

I made another Transformer with same turns primary secondary but i increased frequency to 75khz becouse core Is two times smaller.
This one got even hotter.

 

[MacIntoshCZ]

I missed this from post #5: I though its in watts....
200mT is equal to 300kw/m3
Effective volume of first transformer is 0.000 024 m3
0.000 024 * 300kW -> 7,2W
So should i increase air gap?

 

[MacIntoshCZ]

I tried Transformer on lower voltage becouse core losses Are proportional to voltage And frequency. On 75v AC main 75khz 100w way Transformer Also very hot.
I have no idea if Its duel core or winding. But core was Warmer

 

[MacIntoshCZ]

Current on primary side ,second transformer.
Some resonance when transformer should reset.
5A*3,8us/2*75khz = 0,71A