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Homemade high-freq HV transformers

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Marko

Mon Apr 09 2007, 10:04AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Update: scratch this before, here's what i'm with now:

I changed the primary to 5+5 and tightened the airgap, now having a frequency of about 20kHz unloaded and 70kHz shorted secondary. 20kHz is really low!

Still, even when running from 14V battery charger (43V, ~8V/turn) the core is gets hot as quickly as it did before, up to seome 50degrees C and stays there.

Airgap should go back in...

Anyway I doubt this particular transformer will ever reach the voltage I wanted it to with such kind of winding technique. Core small as this simply *must* use magnet wire and dinky insulation, just in orderto fit more turns in.

Seeing rickr having sucess with magnet wire, I may just try it.

I'l be able to use only 2 groves, but still actually more turns than before... if I come to use this transformer.
Or maybe I could switch to flyback converter in the end..? (if I ever find a use for this transformer)..

uzzors2k

Mon Apr 09 2007, 07:12PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Beautiful flyback Firkragg!

I made a poorly insulated flyback transformer for my table top SGTC. Since the spark gap is set for 1kV it has survived. I used about 150 turns and got 10kV driven in flyback mode with 12V supply voltage. I used a standard E core from an ATX, and the 3kV rated tape used to insulate the windings.

After playing with the Mazzilli driver again I feel the need for a monster flyback, so I'll try making one this week. How about parralleling two cores for twice the cross sectional area? That would half the primary turns needed and double the output voltage. Are there any problems associated with that?

Sulaiman

Mon Apr 09 2007, 07:30PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Firkragg, most power ferrites have their lowest losses somewhere between 50 and 90 Celcius,
so running the core at c50 Celcius is GOOD.

Marko

Sun Apr 15 2007, 09:30PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi

I played some more with these things over a weekend. I lashed up a halfbridge in order to operate from mains and stop bothering about power supply.

The stacked transformer was a dissapointment; i bit and went to rewind it with 0,3mm magnet wire (only 2 of 4 grooves as with rickr's design) only to find out that it arced over and welded somewhere, practically no output and one winding getting hot.. I've had enough with it for the day.

To ease frustration, I sawn off a part of a small damaged SSTC secondary and put it onto core. It had only about 350 turns and I couldn't get more than 2-3kV from it at most.
If I used thinner wire (0,05mm) I could maybe get 6kV or so, but not more.
Using a wider secondary form and primary under it may also be an improvement.

Still, it's very robust and actually only way I would recommend magnet wire to be used in HV transformers.

preliminary halfbridge test...

This is how it looks like.. it's SG3525, with simple scr-based overcurrent protection.

arc from singlelayer transformer.. it is ballasted to stop the overcurrent protection from tripping with heavy arcs... Core gets warm but stays at about 50degrees. I'm only running about 7V/turn here. Note the burning insulation on magnet wire used as target.. arc melts it in seconds!

Dr. Shark

Mon Apr 23 2007, 12:45PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Very nice work Marco, I never got that long arcs from similar self-wound transformers, so I guess your driver is a lot better. Is it a double-sided board with the switches on the bottom? What kind of drive are you using there anyway, are you doing proper PWM or is the chip just used as an oscillator? Is it a (half) bridge or a single switch?

Ken M.

Mon Apr 23 2007, 08:17PM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

It looks like it might be a ZVS driver.

Marko

Mon Apr 23 2007, 08:34PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I'm using two IRFP450's in halfbridge wired to a SG3525.
Small current transformer triggers an SCR wich shuts down the controller and light a led in case of overcurrent of about 6..7amps.
I wasn't botheret to draw a schematic for it

Plugging into mains and deliberately shorting the output couldn't blow it up.

When I get caps for a proper MMC I may even wind a better transformer and try to power a SGTC with it...

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