Re: SSTC: Why it cant use a Bipolar Transistor?
J. Aaron Holmes, Sat Sept 08 2007, 03:50AM

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

Re: SSTC: Why it cant use a Bipolar Transistor?
Hazmatt_(The Underdog), Sat Sept 08 2007, 04:08AM

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.

Re: SSTC: Why it cant use a Bipolar Transistor?
Dr. Dark Current, Sat Sept 08 2007, 05:22AM

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

Re: SSTC: Why it cant use a Bipolar Transistor?
uzzors2k, Sat Sept 08 2007, 07:09AM

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.

Re: SSTC: Why it cant use a Bipolar Transistor?
Marko, Sat Sept 08 2007, 10:58AM

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

Re: SSTC: Why it cant use a Bipolar Transistor?
Steve Conner, Sat Sept 08 2007, 02:44PM

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.

Re: SSTC: Why it cant use a Bipolar Transistor?
MRacerxdl, Wed Sept 12 2007, 09:56PM

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

Re: SSTC: Why it cant use a Bipolar Transistor?
Marko, Wed Sept 12 2007, 10:20PM



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

Re: SSTC: Why it cant use a Bipolar Transistor?
MRacerxdl, Thu Sept 13 2007, 03:32PM

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)

Re: SSTC: Why it cant use a Bipolar Transistor?
Dr. Dark Current, Thu Sept 13 2007, 04:40PM

I suppose you have a transformer with ferrite core, that's not a Tesla coil. Tesla coil is an air cored resonant transformer.

Re: SSTC: Why it cant use a Bipolar Transistor?
MRacerxdl, Thu Sept 13 2007, 11:11PM

I didnt say tesla coil :p I know the difference. I only putted the Coil of a plasma globe, to test the mosfets hehe

I finishing my Tesla Coil now, I have some problems with GDT I think.

Re: SSTC: Why it cant use a Bipolar Transistor?
Marko, Sat Sept 22 2007, 09:37AM

Hi guys

I wondered about some things here, it's all about bridge design, CW SSTC with BJT's;

1. I can use a current transformer or feedback loops to sense the secondary current, but I fear it won't be reliable enough. Coupling would need to be precisely configured in order to acheive best of current-proportional drive. Am I right?

2. I wonder if there is anything that would prevent me from simply sensing the primary current, usng a single CT in 'true' proportional-current drive mode?

With enough coupling the primary current will track secondary current allowing oscillation, and transistors will always have assured class C operation with minimal losses.


3. If 2 fails for any reason I could use a current transformer with 'steer' windings just like hard switched designs in PC power supplies.

Then I would take feedback from secondary base and drive the PCD transformer via two diodes. (I don't know how much sense does that make).
That would basically be like replacing the TL494 with base-feedback current transformer and I don't see why wouldn't it work.

4. I believe bipolars can match mosfets in performance with proper driving tecchnique. With PCD, the charge actually starts draining off the base just as the current passes peak, and tracks it slowly to zero. This should further eliminate storage time and result in good efficiency.

I can also get some small, fast bjt's here for cheap, and might try something one day if I get enough xp.

Marko

Re: SSTC: Why it cant use a Bipolar Transistor?
Marko, Sun Oct 07 2007, 09:58PM

Hi guys

I just got around to try the proposed scheme 2.
Half bridge of but11A's driven directly by a Hfe proportional current transformer in series with primary - would that work?

Purely experimental, as I never tried something like that and want to get more into bjt's.