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SSTC: Why it cant use a Bipolar Transistor?

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MRacerxdl

Sat Sept 08 2007, 02:47AM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

I am new on the forum, I found the site on google, and i resolved to get here, since I have build so many Tesla Coils (Spark Gap ones).

I need a little bittle help, Why I need to use mosfets on the SSTC's? Why I can't use normal transistor (Bipolar)?

I have maded some tests here, and I didnt get any sparks on the secondary, And I was thinking, what is the really diference from MOSFETS and BiPolar Transistors??

PS: I Maded the circuit of tesla coil based on 4hv wiki page, if someone needs the circuit that I have tested, its here:

J. Aaron Holmes

Sat Sept 08 2007, 03:50AM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

I'm sure I'm the wrong person to answer, being a "spark gapper", but AFAIK many SSTCs *do* use bipolar transistors because that's what IGBT's are (IGBT = Insulated Gate Bipolar Transistor). Now, as to why "regular" bipolar transistors aren't used, I believe it's because IGBTs are more easily found with attractive voltage and pulse current ratings. I'm sure you "could" make a SSTC based on a 2N2222, but it probably wouldn't perform very well

Cheers,
Aaron, N7OE

Hazmatt_(The Underdog)

Sat Sept 08 2007, 04:08AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

2N2222 I don't think so, but my monster SDQ300AA100's yes.

I think the reason is that once the IGBT was introduced, there was no benefit to use a BJT even if it were a Darlington configuration, so industry moved on.

Dr. Dark Current

Sat Sept 08 2007, 05:22AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Most bipolar transistors are extremely slow (several uS fall time) for a SSTC.

uzzors2k

Sat Sept 08 2007, 07:09AM

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

Mosfets are much easier to drive directly form a GDT. A regular bipolar transistor needs extra components, while a gate (mosfet or IGBT) can be driven directly and easily.

btw, your circuit won't work. The high side transistor is floating, so it needs special drive circuitry.

Marko

Sat Sept 08 2007, 10:58AM

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

From all my theoretical knowledge, bipolars definitely *can* be used to drive SSTC's.

They need special drive scheme called proportional current drive. If circuit is done properly I think it might be made to run with really minimal number of components, with feedback directly taken from secondary base.

Since they are current driven devices the current transformer alone is perfect for a base driver.
Current transformer must be designed such way that it keeps ratio of collector and base current *just under* trasnistor's expected minimal gain. Then transistor will acheive fastest transition times and efficient class C operation. (It doesn't matter that base current is actually sine).

Marko

Steve Conner

Sat Sept 08 2007, 02:44PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I think the original SSTC in the 1970s used horizontal output transistors from a TV, which are of course bipolars. +1 on all Marko's comments.

MRacerxdl

Wed Sept 12 2007, 09:56PM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

Ah ok, I understand now :)

I have found some Mosfets here (K2049) 50A 80V Continuous and 200A peak, i think there is good for SSTC's :D

Marko

Wed Sept 12 2007, 10:20PM

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

Steve Conner wrote ...

I think the original SSTC in the 1970s used horizontal output transistors from a TV, which are of course bipolars. +1 on all Marko's comments.

Hey, thanks Steve. I don't know how would these ideas work, but someone could test it out BTW. I actually think that BJT's may in some cases be even more efficient than mosfet's in a series resonant circuit.

Lucas: Running things from mains can also be desirable for higher powers if you don't have a good low voltage supply. From other side it can be practical to run small coils from batteries in some cases.

Apart from that, mosfets are fine... Look around the archives and ask for anything else.

Marko

MRacerxdl

Thu Sept 13 2007, 03:32PM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

Thanks for the help :), in all, i have tested the mosfets here, all is working, i have 12 here :P

And the performance is incredible better, i have tested one with a 100kHz Oscilator and a Coil from a Plasma Globe (the smaller than Flyback ones), and I get incredible 4cm Sparks with only 24V. (Versus 2cm sparks with a MJE13007 Bipolar)

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