Some questions about DRSSTC's, and guidance to build one.

[mrsebe]

I have a couple questions since I am looking to make a drsstc. I have been building and tinkering with electronics for several years, and have built a small sstc and sgtc before, so I have a basic understanding. Where should I start with building a drsstc? They seem very complicated, and I can’t find any guides online that show a step by step process.

A couple questions off the top of my head that I couldn’t easily find on the internet:

1. Do all drsstc’s use a dedicated driver board, such as Steve wards universal driver? Could I build my own, or should I buy a driver board?

2. Most of the drsstc’s I’ve seen online have ~2000 turns on the secondary, as opposed to the ~1000 that sgtc’s have. Why is this?

3. Should I build a half bridge or full bridge? I am planning on using to-247 igbts, as the big “brick” ones seem a little too expensive for my first drsstc. What are the best IGBT's (to-247) to use? I'm looking to make one similar to this one: https://www.loneoceans.com/labs/drsstc1/

4. Since the primary and secondary tank circuits have to be in tune, what is the circuit designed around? Put another way, do you create a secondary coil first, then design the rest of the primary tank circuit based on the resonance of the secondary? How do you decide the number of turns for the secondary, and is there a height/width ratio that gives the best performance?

5. Does the driver switch the IGBT's at the resonate frequency? In some videos I see that the driver board has an antenna or some sort of feedback device, is this to determine the resonant frequency? Is lower resonant frequency better for drsstc's?

6. This question is pointed towards sgtc's: I'm having trouble finding an nst that does not have secondary ground fault protection. Does anyone have an nst that they'd be willing to sell, or where to source one?

[AstRii]

Hello!

To answer your questions:

1.)
Very few DRSSTCs use their very own driver, and if so, it's usually based on Steve Ward's driver anyway. I think the best thing to do is to buy a plain driver board and solder your components by your own. This way you get some sense of how it works by looking at the traces and ICs on the board and you do not just blindly use an existing driver.

2.)
I don't really think that's the case. I would say that most DRSSTCs operate in the range of 1000 - 1500 wingings. It's always a compromise... More windings = higher windings ratio which leads to higher voltages. But lower amount of turns usually coresponds with a thickness of the wire. The thicker ther wire, the lower its resistance and thereore higher Q factor which again leads to higher voltages...

3.)
If this is your first DRSSTC i would probably go for half bridge as this way the IGBTs will operate at lower voltages which means less likely to blow up.

4.)
You usually design your secondary first. Don't go too crazy with height/width ratio. I would say that 3:1 - 5:1 is the optimal ratio. Then you usually make your topload minor diamater to match the diameter of the secodary and topload major diameter to match the height of the secondary.

5.)
DRSSTC drivers always have a feedback system, usually a feedback transformer sensing the current on the primary coil. Lower resonant frequency is usually better as you can push your IGBTs further, but the frequency depends on the size of your coil, a small table top DRSSTC will have a frequency in the range of few 100kHz while a large IGBT brick coil will be in the range of 30-70kHz.

Good luck with your DRSSTC and dont forget to share your results with us!:)

[MRMILSTAR]

Even though JAVATC was made for spark gap coils you can still use much of it for other types of Tesla coils such as transistor coils and vacuum tube coils. Just ignore the spark gap output portions.

[davekni]

If this is your first DRSSTC i would probably go for half bridge as this way the IGBTs will operate at lower voltages which means less likely to blow up.

AstRii,  I agree with all your suggestions, excepting this one.  A half-bridge generates half the drive voltage to the primary tank circuit, but each IGBT still sees full bus voltage (plus switching spikes).  For a given bus voltage, IGBT voltage is the same between half and full bridges.  Full bridges are a bit easier on bus bypass capacitors, as all ripple current is at 2x operating frequency (and higher harmonics).  Also, a full bridge can output the same power using half the IGBT current, since output voltage is double.  Or, it can generate twice the power for a given IGBT current rating.

Of course, if something goes wrong, a half-bridge has half as many IGBTs to fry

Whether making a half or full bridge, I suggest taking a look at my low-inductance bridge design guide:
https://highvoltageforum.net/index.php?topic=1324.msg9795#msg9795
A good low-parasitic-inductance bridge layout will save a lot of headaches later with switching spikes and fried IGBTs.

[mrsebe]

Hello!

To answer your questions:

1.)
Very few DRSSTCs use their very own driver, and if so, it's usually based on Steve Ward's driver anyway. I think the best thing to do is to buy a plain driver board and solder your components by your own. This way you get some sense of how it works by looking at the traces and ICs on the board and you do not just blindly use an existing driver.

2.)
I don't really think that's the case. I would say that most DRSSTCs operate in the range of 1000 - 1500 wingings. It's always a compromise... More windings = higher windings ratio which leads to higher voltages. But lower amount of turns usually coresponds with a thickness of the wire. The thicker ther wire, the lower its resistance and thereore higher Q factor which again leads to higher voltages...

3.)
If this is your first DRSSTC i would probably go for half bridge as this way the IGBTs will operate at lower voltages which means less likely to blow up.

4.)
You usually design your secondary first. Don't go too crazy with height/width ratio. I would say that 3:1 - 5:1 is the optimal ratio. Then you usually make your topload minor diamater to match the diameter of the secodary and topload major diameter to match the height of the secondary.

5.)
DRSSTC drivers always have a feedback system, usually a feedback transformer sensing the current on the primary coil. Lower resonant frequency is usually better as you can push your IGBTs further, but the frequency depends on the size of your coil, a small table top DRSSTC will have a frequency in the range of few 100kHz while a large IGBT brick coil will be in the range of 30-70kHz.

Good luck with your DRSSTC and dont forget to share your results with us!:)

Thanks for all of this information, I don't think I could ask these questions anywhere else on the internet haha.

[Mads Barnkob]

I have a couple questions since I am looking to make a drsstc. I have been building and tinkering with electronics for several years, and have built a small sstc and sgtc before, so I have a basic understanding. Where should I start with building a drsstc? They seem very complicated, and I can’t find any guides online that show a step by step process.

You should start by reading my DRSSTC design guide, start with the FAQ https://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/

A couple questions off the top of my head that I couldn’t easily find on the internet:

1. Do all drsstc’s use a dedicated driver board, such as Steve wards universal driver? Could I build my own, or should I buy a driver board?

A DRSSTC needs a dedicated driver to ensure zero current switching, else exploding IGBTs.

2. Most of the drsstc’s I’ve seen online have ~2000 turns on the secondary, as opposed to the ~1000 that sgtc’s have. Why is this?

From a old theory on "more turns is more sparks", while it holds somewhat true, driving waveform envelope shaping has proven more efficient. Else look at the secondary design guide link below.

3. Should I build a half bridge or full bridge? I am planning on using to-247 igbts, as the big “brick” ones seem a little too expensive for my first drsstc. What are the best IGBT's (to-247) to use? I'm looking to make one similar to this one: https://www.loneoceans.com/labs/drsstc1/

It is easier to get away with mistakes on a 200A IGBT brick than a TO-247 package, they will explode at minimal overloads (2-3 times rating for short times, a brick will most likely endure that) Check out the list at bottom here: http://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/igbts/

4. Since the primary and secondary tank circuits have to be in tune, what is the circuit designed around? Put another way, do you create a secondary coil first, then design the rest of the primary tank circuit based on the resonance of the secondary? How do you decide the number of turns for the secondary, and is there a height/width ratio that gives the best performance?

I wrote a long guide on secondary design and has some good tables to make choices from, all these designs are viable and will work fine: http://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/secondary-coil/

5. Does the driver switch the IGBT's at the resonate frequency? In some videos I see that the driver board has an antenna or some sort of feedback device, is this to determine the resonant frequency? Is lower resonant frequency better for drsstc's?

Antennas as only used in SSTC that has no resonant primary circuit to get a feedback from, from a CT. Lower resonant frequency = lower switching losses in IGBTs. The bigger the brick, the slower it is, the lower frequency is needed to not kill it.

6. This question is pointed towards sgtc's: I'm having trouble finding an nst that does not have secondary ground fault protection. Does anyone have an nst that they'd be willing to sell, or where to source one?

NSTs are often too heavy to ship around the world, its best to find a local neon shop and just buy one, even if it seems expensive, its properly cheaper without shipping