First SSTC design - Need some critique / Tesla Coils / Forums

Forums

4hv.org :: Forums :: Tesla Coils

« Previous topic | Next topic »

First SSTC design - Need some critique

first previous 2 3 4 5 next last

Move Thread

LAN_403

Dr. Dark Current

Fri Jun 27 2014, 01:56PM

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

What does the cap on the output of the 74HC14 do? o_0

P.S. If you use the coil as an DRSSTC with no loop delay compensation (as your current circuit) I highly suggest you to use IGBTs with ultrafast antiparallel diodes rather than MOSFETs. This is because the switching transitions happen after current zero crossings, which forces recovery of the free-wheeling diodes. The body diodes of MOSFETs are relatively much too slow for this.
On the other hand, using it as a single-resonant SSTC, FETs are a better choice, because the magnetizing inductance of the primary moves the switching transitions before current zero crossings (usually).

DerStrom8

Fri Jun 27 2014, 02:49PM

Registered Member #3704 Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92

Dr. Dark Current wrote ...

What does the cap on the output of the 74HC14 do? o_0

P.S. If you use the coil as an DRSSTC with no loop delay compensation (as your current circuit) I highly suggest you to use IGBTs with ultrafast antiparallel diodes rather than MOSFETs. This is because the switching transitions happen after current zero crossings, which forces recovery of the free-wheeling diodes. The body diodes of MOSFETs are relatively much too slow for this.
On the other hand, using it as a single-resonant SSTC, FETs are a better choice, because the magnetizing inductance of the primary moves the switching transitions before current zero crossings (usually).

The 100pF cap filters out the extremely high frequencies that I don't really want going into the 27425. It's not critical, but one of those "nice to have" things.

You actually bring up a point that I have been considering. To this point I have been designing this coil around MOSFETs because it's what I have in my parts bin. I realize they are slower and probably not the best for high-frequency circuits, so if I do turn this into a DRSSTC I will swap them out with IGBTs. For the time being, though, I think I'll just stick with the MOSFETs and get a working TC. I can then make modifications from there.

Oh, and they are not shown in the schematic, but yes--each of the chips (the UCC27425 and the 7414) have decoupling caps. I simply opted not to show the power pins on the schematic, so I did not show the caps.

Thanks,
Matt

Dr. Dark Current

Fri Jun 27 2014, 05:18PM

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

MOSFETs are faster devices than IGBTs. It's just their body diode which is slow. However the FETs can not take such high pulsed currents, so they are better for coils with lower peak powers and higher duty cycles.

I don't see why you do want to slow down the transitions into the gate drivers. Having them as fast as possible is not a bad thing, often it is required. The cap will just increase your loop delay and present unnecessary load onto the inverter gate's output.

DerStrom8

Fri Jun 27 2014, 08:26PM

Registered Member #3704 Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92

Dr. Dark Current wrote ...

MOSFETs are faster devices than IGBTs. It's just their body diode which is slow. However the FETs can not take such high pulsed currents, so they are better for coils with lower peak powers and higher duty cycles.

I don't see why you do want to slow down the transitions into the gate drivers. Having them as fast as possible is not a bad thing, often it is required. The cap will just increase your loop delay and present unnecessary load onto the inverter gate's output.

The 100pF cap is supposed to filter out the MHz harmonics, but can be omitted if necessary. I actually borrowed it from loneocean's SSTC2 design.

I have always found IGBTs to be more expensive than MOSFETs, but I came across something once that said it was the other way around? The MOSFETs I have now are junk-bin ones and are really only going to be used for testing. They're IRF350s in TO-3 packages. The datasheets say they are rated for 400V @14A, though the on resistance is quite high--0.3 ohms--and I'm not certain about the switching frequency. The datasheet says it has a 35nS turn-on time and a 150nS turn-off time, so if I did my math right it should be able to switch fast enough, though I'm not 100% sure on this. Eventually I do hope to swap all of them out with proper IGBTs.

Hi again, I've been thinking--

Shouldn't the feedback match the frequency of the secondary? If so, how would primary feedback work in a regular SSTC if it is not actually in tune with the secondary? I guess this is a little confusing to me.

On another note, do you have recommendations for winding the GDT? I'm thinking of making two transformers with paralleled primaries, and two secondaries on each core. The other option would be to wind all of the coils on one core, but I think that would require a larger toroid, which would be more expensive. I'm just wondering how other people have wound their GDTs.

Thanks for all the help guys, things are really coming together now.

Regards,
Matt

Dr. Dark Current

Fri Jun 27 2014, 10:52PM

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

Primary feedback will not work in an SSTC, I thought this has been already said a bit.

-> If the circuit is to be used as an DRSSTC, a primary current feedback is a much better option than secondary base current feedback.

loneoceans

Fri Jun 27 2014, 10:57PM

Registered Member #4098 Joined: Fri Sept 16 2011, 09:26PM
Location:
Posts: 236

DerStrom8 wrote ...

Hi again, I've been thinking--

Shouldn't the feedback match the frequency of the secondary? If so, how would primary feedback work in a regular SSTC if it is not actually in tune with the secondary? I guess this is a little confusing to me.

On another note, do you have recommendations for winding the GDT? I'm thinking of making two transformers with paralleled primaries, and two secondaries on each core. The other option would be to wind all of the coils on one core, but I think that would require a larger toroid, which would be more expensive. I'm just wondering how other people have wound their GDTs.

Thanks for all the help guys, things are really coming together now.

Regards,
Matt

I think there a little bit of confusion in the thread since you mentioned you wanted to build a DRSSTC at the beginning. I'd recommend going with an SSTC first since it's just easier to get running. For DRSSTCs, primary feedback is usually used, and secondary for normal SSTCs (as you mentioned, no resonance going on in the primary for a normal SSTC, so you cannot use pri feedback). In fact, antenna works pretty well too in my experience. I never encountered any problems with or without the 100pF cap so no need to put it in. You don't need a bit GDT for your full bridge (and they're not expensive either) - just a few dollars online for a 1" core which will be enough for you if you use thinner wire.

If I had a small core, I'd wind it with 6 thin wires twisted together. 4 of them go to the transistors, and two of them will be in parallel and wired to the driver. Just make sure your phasing it correct! For sure you could wind two cores - it'll be exactly as described on my page, with the primaries in parallel. Or you could just go for a half bridge first which will save you half of the parts! Again most of the components are really not expensive - suitable IGBTs or MOSFETs all run for around $3 or less on digikey which would work great!

There are several reasons why people like IGBTs for DRSSTC use because of the lower losses (proportional to I in IGBTs but squared for mosfets), higher current handling capability and usually nice fast integrated body diodes. As Dr. Spark said, they're also usually slower, but some of the nice new ones on the market today are approaching the speeds of older Mosfets! I've pushed TO220 igbts to 800kHz in DRSSTC operation and they seem to be fine. :o

DerStrom8

Fri Jun 27 2014, 11:16PM

Registered Member #3704 Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92

Dr. Dark Current wrote ...

Primary feedback will not work in an SSTC, I thought this has been already said a bit.

-> If the circuit is to be used as an DRSSTC, a primary current feedback is a much better option than secondary base current feedback.

loneoceans wrote ...

I think there a little bit of confusion in the thread since you mentioned you wanted to build a DRSSTC at the beginning. I'd recommend going with an SSTC first since it's just easier to get running. For DRSSTCs, primary feedback is usually used, and secondary for normal SSTCs (as you mentioned, no resonance going on in the primary for a normal SSTC, so you cannot use pri feedback). In fact, antenna works pretty well too in my experience. I never encountered any problems with or without the 100pF cap so no need to put it in. You don't need a bit GDT for your full bridge (and they're not expensive either) - just a few dollars online for a 1" core which will be enough for you if you use thinner wire.

If I had a small core, I'd wind it with 6 thin wires twisted together. 4 of them go to the transistors, and two of them will be in parallel and wired to the driver. Just make sure your phasing it correct! For sure you could wind two cores - it'll be exactly as described on my page, with the primaries in parallel. Or you could just go for a half bridge first which will save you half of the parts! Again most of the components are really not expensive - suitable IGBTs or MOSFETs all run for around $3 or less on digikey which would work great!

There are several reasons why people like IGBTs for DRSSTC use because of the lower losses (proportional to I in IGBTs but squared for mosfets), higher current handling capability and usually nice fast integrated body diodes. As Dr. Spark said, they're also usually slower, but some of the nice new ones on the market today are approaching the speeds of older Mosfets! I've pushed TO220 igbts to 800kHz in DRSSTC operation and they seem to be fine. :o

I had a feeling that was the cause of the confusion. I should have specified that I am starting with an ordinary SSTC and eventually will probably modify it later on to be a DRSSTC. I apologize for misleading you. Since this is the case, I will be using the secondary feedback (which makes much more sense to me!

) that I originally planned on. I will probably change it when I convert it into a DRSSTC.

I found examples of people using Ethernet wires for the GDT, which I think I may do--they are cheap and easy to source, and there are 8 wires in each cable already. I will probably do the same for the secondary feedback winding.

Again, this is all great info, so thanks very much!
Cheers,
Matt

I understand how the losses tend to be more significant in MOSFETs. Some day (hopefully soon) I will look into getting some IGBTs, but again, I already have some FETs that will probably work, so I may as well just start with them

GrantX

Sat Jun 28 2014, 06:16AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

http://thedatastream.4hv.org/gdt_index.htm This site has been invaluable in helping me understand proper GDT design. It covers the important theory elements, as well as core selection and winding techniques. Very nice to keep in your bookmarks.

EDIT: Sorry, I didn't see that your questions regarding feedback had already been answered.

You're definitely getting there.

Primary feedback: DRSSTC - both the primary and secondary are resonant tank circuits, ideally both tuned to almost the same frequency. In a DRSSTC you need to see how far the primary current is ringing up; since the inverter is switching at the primary circuit's resonant frequency the current gets bigger each half cycle (the simplified version as I currently understand it, I'm still learning as well).

Secondary base feedback: standard SSTC - only the secondary is being driven at it's resonant frequency, thus the primary current stays pretty much steady. Less turns on the primary means less impedance and thus more input current. Again, simplified version.

Designing a single-resonant SSTC seems very similar to other "normal" switch mode power supplies.

Steve Conner

Sat Jun 28 2014, 12:25PM

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

If you want to use that 100pF capacitor to filter out high frequencies, it should probably be on the input of the logic gate, not the output.

DerStrom8

Sat Jun 28 2014, 12:49PM

Registered Member #3704 Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92

GrantX wrote ...

I'll definitely take a look at the link, thank you very much!

Yes, I understand why you'd want to use a feedback transformer on the primary in a DRSSTC and why you can't on a regular SSTC. For a little while I was thinking I had mentioned that I wanted to start with a regular SSTC (which apparently I hadn't yet) and someone recommended a primary feedback transformer, and I couldn't understand how that would work. I see that it was a lack of communication on my part though, so things seem to fit my understanding and assumptions better now.

It's funny you should mention SMPSs--Every time I look at a SSTC schematic I think of a SMPS, and vice versa. They are very similar in a lot of ways. Just another idea of Tesla's that's leaked into modern technology! He really was the inventor of modern electronics

Steve Conner wrote ...

If you want to use that 100pF capacitor to filter out high frequencies, it should probably be on the input of the logic gate, not the output.

This has been noted, thank you!

I just need to get a few more parts and I should be able to start prototyping and testing before doing the full build.

Hi all, just thought I'd post some of my calculations here to make sure I'm doing things right.

Years ago I built a spark gap Tesla coil and I hope to re-use the secondary from it. The coil is about 19 inches tall wound around 4" ID PVC, giving the coil a diameter of about 4.5". Calculating the inductance using the formula,

L = (r^2 * t^2)/(9*r+10*l)

where L is the inductance, r is the radius, t is the # of turns, and l is the length, I get an inductance of 34.54mH.

For the topload I plan on starting with some aluminum dryer duct (also the same topload I used on my SGTC) with has a large diameter of 12.5" and a small diameter/height of 4 inches. This gives me a calculated capacitance of around 13.9pF.

Then, using the formula

f = 1/(2*pi*(L*C)^(1/2))

where f is the frequency, L is the inductance, and C is the capacitance, I get a resonant frequency of around 230KHz. I think this will probably be reasonable. How do you guys feel about these figures?

Cheers,
Matt

first previous 2 3 4 5 next last

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob