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Thomas W

Fri Sept 21 2012, 01:42PM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
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Posts: 1276

Hello,
could someone please help me with the maths, in desigining a ferrite transformer,
im using 2 rather large U cores, specifications known:

were being used at about 50Khz,
each of the U cores has this size:

100mm wide
57mm long
25mm deep
the space between internal and external dimensions is 25mm all the way around,
the internal dimensions are:
50mm wide
32mm long
25mm deep

could i have help working out the core saturation, best turns ratio and anything else i should know,
im planning on creating it to give out

15Kv - 0 - 15Kv
(230v 25A Mains input)

Thankyou,
Thomas Williamson

Daedronus

Fri Sept 21 2012, 02:34PM

Registered Member #2329 Joined: Tue Sept 01 2009, 08:25AM
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Posts: 370

There is a flux calculator around the middle of the page...

Thomas W

Fri Sept 21 2012, 05:17PM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

Thanks Daedronus,
i put the numbers in, so if if i did it correctly, this is it:
choosen 300v since its 230v smoothed, as for the flux density, i made a half-educated guess x3

Ash Small

Fri Sept 21 2012, 06:12PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

24 turn primary sounds in the right ballpark, my thread on this subject is here:

You may find it of some use.

Thomas W

Fri Sept 21 2012, 06:38PM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

Cool, had a little read through, im guessing, that since i dont have much information o nthe core itself other then whats listed above, i can never totally mathmaticly calculate the whole deal for the best results, oh well ^^

thanks,
Thomas Williamson

Ash Small

Fri Sept 21 2012, 08:19PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

The gist of it seems to be 'wind 25-30 turns on the primary, and see how hot it gets', but you'll need your H-bridge and mains rectifier first. Apparently you don't need a secondary to try this.

Daedronus

Fri Sept 21 2012, 08:38PM

Registered Member #2329 Joined: Tue Sept 01 2009, 08:25AM
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Posts: 370

core saturation does not mean the core get's hot, instead the primary will look like a dead short to the switching device, and you will get a huge current spike.

If you measure with a scope the primary current and you slowly lower the frequency at some point you will see the current suddenly raping upward, quite nice to see this real time.

klugesmith

Fri Sept 21 2012, 08:53PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Ash Small wrote ...
... Apparently you don't need a secondary to try this.

Yup, core loss and saturation effects do not depend on the presence of a secondary coil, much less on the secondary circuit current. By testing the primary circuit without a secondary, you can avoid the nuisance and risk of high voltage.

There are some real numbers and charts in the following thread, written about a steel core rather than a ferrite one, so the saturation flux density is higher by a factor of perhaps 5. (?)

Bottom line: test your intended primary winding with your HF inverter at intended frequency. Crank up the input voltage with a variac. If you reach full voltage without excessive current, monitor the temperature for up to 15 minutes.

If the core gets too hot, you should increase the primary turns count.

If it gets only warm (not hot at all) then you are underloading the core. That's generally bad, because it forces you to wind a needlessly large number of secondary turns and suffer excessive I2R loss in the secondary winding. In a commercial product it would mean your transformer is too big, heavy, and expensive.

Thomas W

Fri Sept 21 2012, 09:26PM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

Daedronus wrote ...

core saturation does not mean the core get's hot, instead the primary will look like a dead short to the switching device, and you will get a huge current spike.

If you measure with a scope the primary current and you slowly lower the frequency at some point you will see the current suddenly raping upward, quite nice to see this real time.

I have never scoped a current, infact, i havent really touched my scope to start with, just had nothing to use it with, my whole project life is in abit of a mess at the moment,
i have like 50 projects unfinished / abandoned and little money to compleate any of them, i want to get this one finished. As ash mentioned, im gonna need to make a driver
and rectifier first, is there any reccomendations for the acctual logic, i havent got a probelm with the actual H-bridge and GDT, but the TC494 ect, hows the best way to deal with that,
should i add things like over current protection to the entire system from the start, or at a later date?

thanks,
Thomas Williamson, the eternal noob

Patrick

Sun Sept 23 2012, 06:29AM

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

Ash small, JPsmith123 and I have all made efforts in this area, id look over every thread weve post onb this matter and simulate many possibilities, before you start buying materiels...

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