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ShawnLG

Tue Apr 03 2007, 05:34PM

Registered Member #286 Joined: Mon Mar 06 2006, 04:52AM
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Posts: 399

Since the Micro RGTC was a sucess, why not build a solid state version as suggested in the other thead. The only problem making a SS coil this small is it's high operating frequency. The secondary has a low inductance and the topload has a miniscule capacitance because of their size. To overcome this, I would need to wind a new seconday with many more turns with finer wire than the micro RGTC had. Below is a schematic I drew up. I need someone to look it over before I start etching boards. This circuit would need to operate in the 1 to 2 mhz range. I may not have any GDT cores that operate at that frequency.

[Edit: Fixed image size

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thedatastream

Tue Apr 03 2007, 10:37PM

Registered Member #505 Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329

That's an interesting approach to driving a GDT, removes the need for UCC type driver chips.

Winding the GDT for reliable operation at that frequency and in that configuration would not be particularly easy. I would wind 4 turns in parallel, have one for each output and centre-tap the remaining two.

I would also have the GDT primary driven by the outputs of the TL494 in an open collector configuration as these would be better suited for driving FET gates at high frequencies than 2N3904s

I would be very interested to see what results you get with this configuration

James

p.s. There should be no need for D1 and D2 in series with the FET drains
p.p.s Also, a small gate resistor (2 ohm) should be used just to take the edge of the ringing you will get unless the gate drive layout is nice low inductance

...

Wed Apr 04 2007, 12:35AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

hmm, I didn't actually think you would go for it

In any case, for some decent ideas, check the worlds smallest thread, although there is a lot of noise in that thread

Also, I might suggest going for a simpler approach. A tl494, gdt, etc is going to be sorta big for the size of the coil... Your best bet might even be driving the primary directly off a ucc37321/ucc37322 pair, and heck you could probably even get away with just an antenna connected across the inputs of the ucc's and run self resonant

Have fun!

ShawnLG

Wed Apr 04 2007, 06:29AM

Registered Member #286 Joined: Mon Mar 06 2006, 04:52AM
Location:
Posts: 399

"I would also have the GDT primary driven by the outputs of the TL494 in an open collector configuration as these would be better suited for driving FET gates at high frequencies than 2N3904s"

A TL494 IC would be too big unless they make them in a SMT package. I thought about using a 7406 to directly drive the GDT but it's only rated 48 ma of sink current. I could always replace the 2N3906s with the 2N7000 series but I only have one of them in an smt package. I will need to keep looking.

"Your best bet might even be driving the primary directly off a ucc37321/ucc37322 pair, and heck you could probably even get away with just an antenna connected across the inputs of the ucc's and run self resonant "

I do not have any UCCs.

"In any case, for some decent ideas, check the worlds smallest thread,"

Wow, I didn't know there was a heated competition on mini SSTC.

Ken M.

Wed Apr 04 2007, 02:06PM

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

Technicaly the UCC chips differ from 442X chips just by the enable correct? If thats teh ONLY difference then why not just use a high frequency and gate or just make 1 from schottky diodes, or any diode that can switch relatively fast!

...

Wed Apr 04 2007, 02:24PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Many companies make gate drivers, of the top of my head (in order of increasing power available) microchip, farchild, TI, ixys... There are many 'standard' gate drivers that you can get from any of them, and then there are some monsters made by ixys that can do amazing things. Like swing a 4nf gate in 3ns. We have sorta established that the ucc chips are capable of driving just about anything we will encounter, and they are reasonably priced (you can even sample), the enable is handy for drsstc's, etc.

BTW, put in a side by side test, a tc4422 will run hotter than a ucc37322, but you can get the 4422s in to-220 package which can make up for the difference. But the gate waveforms always look a little nicer with the 327322'a.

As to your original question, any ferrite core will take 1mhz no problem. Once you get >5mhz you really really have to watch your layout and things, but any old core (that isn't powdered iron) will take the frequency.

thedatastream

Wed Apr 04 2007, 08:48PM

Registered Member #505 Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329

Didn't realise that size was a consideration
SMT gate drive ICs are available for reduced size
2N2222 would be a good transistor to use, they are quite quick IIRC
Keep your GDT core size small to reduce the stray inductance
Running at 1-2MHz you can probably use about 250uH GDT inductance without too much problem
I recommend (naturally)

;)

ShawnLG

Thu Apr 05 2007, 06:38PM

Registered Member #286 Joined: Mon Mar 06 2006, 04:52AM
Location:
Posts: 399

I used datastream's saturation calculation to find a good number of turns for my GDT. I built a prototype using breadboard size components. Below you can see a pic of the setup containing a 1Mhz clock. The IC is a six schmitt trigger inverting hex gates. This give me an inverting and noninverting output for the 2N7000 sitting next to it. I choosed the schmitt trigger because I can easley build an oscillator with it also. Right now the GDT is not working properly. The 250pf capacitor on the GDT output, it is not charging. There is only a small ringing in it.

By the way, the 2N7000 themselfs are working properly. I even tested them on 14.3181Mhz.

...

Thu Apr 05 2007, 06:47PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Could you post a pic of the scope across the GDT primary?

Also, I don't see a dc blocking cap in series with your gdt, that shouldn't cause your problems but it is a good idea to have it regardles.

ShawnLG

Fri Apr 06 2007, 06:01AM

Registered Member #286 Joined: Mon Mar 06 2006, 04:52AM
Location:
Posts: 399

I have found the problem. I had miss crossed one of the primery wires during GDT assembly. Now it works find.

"Also, I don't see a dc blocking cap in series with your gdt, that shouldn't cause your problems but it is a good idea to have it regardles." I don't understand. The primery uses alternating DC pulses.

Below is a picture of the two GDT outputs with their 270pf capacitive load on them.

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