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Audio modulated SSTC - Some design questions

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aonomus

Mon Oct 20 2008, 05:46PM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

Ok so for my next SSTC I'm planning on audio modulation, however I'm starting to run into a few roadblocks as I jump between SSTC and DRSSTC design documents/threads. Hopefully there aren't too many questions here, but the gist of this is that I'm trying to meld together some SSTC designs, while using a offboard modulator (that I can reuse for other projects).

1. PWM (TL494) or PLL (4046) - which one is better? I'm leaning towards using PWM with feedback, and an offboard external modulator.
2. Can I use a current feedback transformer on the secondary instead of an antenna for feedback?
3. Should I use primary current sense to prevent overloading my MOSFETs?
4. Should I build in a zero-current-switching circuit using a flipflop like out of the minibrute schematic so my interrupter does not cut off in the middle of a cycle?
5. While I'm planning on a Fres of 180khz and approximate power of 500W-1kW, should I bother to use fiber-optic input, or can I just get by using TTL over shielded coax with a BNC connector (shield grounded).

Edits to answer questions from IRC:
Target fullbridge voltage: 240V via a voltage doubler (with the possibility of a third stage if I'm insane).

Dalus

Mon Oct 20 2008, 05:50PM

Registered Member #639 Joined: Wed Apr 11 2007, 09:09PM
Location: The Netherlands, Herkenbosch
Posts: 512

1. PWM (TL494) or PLL (4046) - which one is better? I'm leaning towards using PWM with feedback, and an offboard external modulator.
At your Fres it doesn't matter much.
2. Can I use a current feedback transformer on the secondary instead of an antenna for feedback?
Yes but it will be unstable if you get groundstrikes, it will most likely blow your bridge when that happens.
3. Should I use primary current sense to prevent overloading my MOSFETs?
Always a good extra protection.
4. Should I build in a zero-current-switching circuit using a flipflop like out of the minibrute schematic so my interrupter does not cut off in the middle of a cycle?
Audiomod coils usually run in CW mode so you don't need to sync anything
5. While I'm planning on a Fres of 180khz and approximate power of 500W-1kW, should I bother to use fiber-optic input, or can I just get by using TTL over shielded coax with a BNC connector (shield grounded).
Shielded coax is better shielded then fiber optic transmitters and recievers.

uzzors2k

Mon Oct 20 2008, 06:47PM

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

As to question 2, it depends entirely on feedback scheme. You'll need to decide that first. With the PLL drivers using PC2 it works perfectly, with PLL using a XOR comparator you'll need to use an antenna for stable operation. Basically it's a matter of how phases are added up to get feedback.

HV Enthusiast

Mon Oct 20 2008, 11:20PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Go with the Class-E design that i think Reaching (i think that is who did it) did. He has the design documented on his website.

Very clever design, and a nice high-side modulated driver too.

aonomus

Tue Oct 21 2008, 02:30AM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

Hmm it would seem that his E-class SSTC page and the audio mod'd SSTC links don't work on his site...

Edit: Question about CT's - will any old torroid do, or must it be a high permeability core like those used for GDTs? At the rate winding torroids go I think I might end up just buying one for primary current protection...

uzzors2k

Tue Oct 21 2008, 12:33PM

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

I've used random cores before and they give enough signal to make things work, but a GDT toroid will definitely work.

aonomus

Tue Oct 21 2008, 03:31PM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

How well can the class E SSTC scale up? I've only seen small models yet I'm aiming for a line-powered 500W-1kW model....

At the same time I'm also aiming towards building a separate interrupter so I can use it to do PWM for a few other things. Should I go back towards fullbridge PWM drive, or is there some way a 4MHz class E can scale up....

Arcstarter

Tue Oct 21 2008, 04:33PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

aonomus wrote ...

How well can the class E SSTC scale up? I've only seen small models yet I'm aiming for a line-powered 500W-1kW model....

At the same time I'm also aiming towards building a separate interrupter so I can use it to do PWM for a few other things. Should I go back towards fullbridge PWM drive, or is there some way a 4MHz class E can scale up....

You could always just make a half or fullbridge.. It will no longer be class e though..

101111

Tue Oct 21 2008, 05:19PM

Registered Member #575 Joined: Sun Mar 11 2007, 04:00AM
Location: Norway
Posts: 263

I think Steve made a very powerful class E SSTC with a IGBT instead of a MOSFET not too long ago, there is a thread about it on the forum.

EDIT: I found the thread:

quite impressive I must say..

Arcstarter

Tue Oct 21 2008, 06:43PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

101111 wrote ...

I think Steve made a very powerful class E with IGBT not too long ago, there is a thread about it on the forum.

Yes, he did. He got some enormous sparks too, like a foot or somethin. I think all the changes he made to the class e he already had was use an igbt and higher voltage input.

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