Peak Primary current in small DRSSTC

[Jesperb123]

Hi!
I'm planning on building a small DRSSTC, but my progress has slowed to a halt. I've tried my best to follow Mads Barnkob's guide on kaizerpowerelectronics.dk to determine some of the parameters:

Secondary design:
75mm diameter secondary
375mm tall
1700 turns (0.2mm cablediameter)

Topload
shape: toroid
Rminor = 3.75cm
Rmajor = 18.75cm

Primary
shape:flat spiral coil
4 turns
86mm inner diameter
10mm inbetween turns
6mm copperpipe with 1mm thickness
tapped at around 3 turns if 0.45uF capacitance is used (to match fres of secondary LC-circuit)


This should give a resonantfrequency of 202.5kHz and a reactance of 50k at resonantfrequency. I've read that 0.15uF to 0.45uF capacitance is suitable for a coil  of this size (http://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/mmc-tank-capacitor/). Here's my problem: I need to know how large the current (peak and RMS) in the primary LC-circuit will be to determine which capacitors to use, but I have no idea how to come up with these values. Since i'm on a relatively small budget I figured that the transistors i use will put an upper limit on this value. Some people seem to go about it this way: select a peak current and then design a MMC (and buy transistors) that can handle this current. Since i've got no prior experience with DRSSTCs I don't know what kind of currents I can expect to run in a system like this, hence why i'm stuck.

 If anyone could point me in the right direction I would be very grateful.
//Jesper

[AstRii]

That depends on many factors.. tuning, ratio of L:C (number of turns), resistances of your connections etc..
You can also control your peak primary current with some maximum ontime, or a pulse-skip OCD driver.

You are only using 4 primary turns, this will result in pretty high primary current, for a coil like this i'd approximate you'll have 300A on the primary after about 40us.

Usually any pulse rated capacitors like WIMA or TESLA capacitors will do just fine for coil this size. You should be more concerned about what voltage rating the capacitors should have.
You can calculate that easily; Xc = 1/(2*pi*f*C) , Vpk = Xc * Ipk

Also make sure to look at the AC voltage rating of the capacitors, not DC

If you haven't made your primary too permanent, i would recommend you to use at least 7 turns, to increase inductance so you can decrease capacitance of your MMC. This will result in smaller peak currents, larger ontimes and it's more budget-friendly.

[Jesperb123]

If you haven't made your primary too permanent, i would recommend you to use at least 7 turns, to increase inductance so you can decrease capacitance of your MMC. This will result in smaller peak currents, larger ontimes and it's more budget-friendly.

Hi!
Thanks alot for the reply 
Since i'm only in the planning stage I can still change alot about the project. The suggested change seems like a good idea. If i use 0.15uF instead of 0.45uF i'll need about 4.12uH from the primary, which according to this calculator (https://www.circuits.dk/flat-spiral-coil-inductor/)  (with the same pipediameter and spacing) I would need at most 5 turns. Is there some other reason for why would I would need more turns? or should I choose a smaller capacitance which increases the turns required?

You can calculate that easily; Xc = 1/(2*pi*f*C) , Vpk = Xc * Ipk

This was why I was struggling. I if I don't know Ipk I can't choose the right voltage rating on the caps for example. Is there some calculation one can do to find the peak current or is it impossible? (sparkloading?). Is 300A a good Ipk number to design MMC and transistors around?

[AstRii]

With higher amount of turns, the current rises slower and usually settles as lower values, which enables higher ontimes and that usually results in bigger sparks with lower primary current.
5 turns is still not that many, i would recommend at least 6.5 to 8 turns.

300A is i would say pretty okay assumption, you can always assume a bit more just to be safe

Don't forget to always include some headroom, since your primary can sometimes go a bit higher even with OCD (eg. during ground strike)

PS: I'm assuming you use 230VAC as your input. With 120VAC, you will have lower primary currents, but higher ontime.
 

[Mads Barnkob]

Hi Jesper and welcome to HVF!

My small DRSSTC3 ( http://kaizerpowerelectronics.dk/tesla-coils/kaizer-drsstc-iii/ ) peaks at about 300'ish Ampere primary peak current. So for your coil that is about 1/3 larger, I would go with 400A for the OCD settings and design it for 500A to have some 20% headroom.

More or less turns with the resulting low or high capacitance of the MMC all comes down to the preference of low or high impedance Telsa coil systems. While the 50k impedance is a good land mark, it is not a goal to strive for! I do stress in the guides that experimentation is encouraged Most important for you right now seems to be the budget, so choose from this:
Low impedance = high cost
- Higher peak currents, overall construction needs to minimize stray inductance
- Bigger and more expensive MMC
- Bigger and more expensive IGBTs
+ Lower switching losses

High impedance = low cost.
+ Lower peak currents, overall construction can slack more
+ Smaller and cheaper MMC
+ Smaller and maybe cheaper IGBTs
- Higher switching losses

Did you play around with the MMC calculator? http://kaizerpowerelectronics.dk/calculators/mmc-calculator/

I also got for helical and spiral coils http://kaizerpowerelectronics.dk/calculators/helical-coil-calculator/ and http://kaizerpowerelectronics.dk/calculators/spiral-coil-calculator/

Looking forward to see your coil being built and make sparks

[Jesperb123]

Hi Mads!
Thanks for the warm welcome and your guides, I would not have started this project if it wasn't for them

Maybe i'm dull but the coil you linked is way bigger than mine. Did you make a smaller version of it?

My small DRSSTC3 ( http://kaizerpowerelectronics.dk/tesla-coils/kaizer-drsstc-iii/ ) peaks at about 300'ish Ampere primary peak current. So for your coil that is about 1/3 larger, I would go with 400A for the OCD settings and design it for 500A to have some 20% headroom.

Then i'll design the bridge and MMC with this in mind. Yesterday I read the threads in the "DRRSTC information and other valuable thread archive", and it seems like one either has to use proper IGBT bricks or multiple TO-247 IGBT's to build the bridge. I'll have to do some more research before I can come to a conclusion, but I've read that IGBTs aren't really suited for high switching speeds, and since my resonant frequency is quite high i'm wondering if I might have to use mosfets instead?

High impedance = low cost.
+ Lower peak currents, overall construction can slack more
+ Smaller and cheaper MMC
+ Smaller and maybe cheaper IGBTs
- Higher switching losses

I'll keep this in mind.

Did you play around with the MMC calculator? http://kaizerpowerelectronics.dk/calculators/mmc-calculator/

I've just started taking a look at it. I'm sure it will be very useful when designing the MMC.

[Mads Barnkob]

Hi Mads!
Thanks for the warm welcome and your guides, I would not have started this project if it wasn't for them

Maybe i'm dull but the coil you linked is way bigger than mine. Did you make a smaller version of it?

My small DRSSTC3 ( http://kaizerpowerelectronics.dk/tesla-coils/kaizer-drsstc-iii/ ) peaks at about 300'ish Ampere primary peak current. So for your coil that is about 1/3 larger, I would go with 400A for the OCD settings and design it for 500A to have some 20% headroom.

That should have been My small DRSSTC2 ( http://kaizerpowerelectronics.dk/tesla-coils/kaizer-drsstc-ii/ )

I am glad that you find help in my guides, please do ask all the questions you have or suggestions on how to make the articles better to read for people with less knowledge about the subjects.

[Hydron]

200kHz is OK for some of the faster 600/650V TO-247 parts, though you'll need to parallel up a couple for current handling, which is has been done successfully a few times with forced current sharing. (Has anyone got experience running TO-247 parts in parallel at 300-500A without forced current sharing?)

[davekni]

Hydron,

Yes, my DRSSTC uses 40 TO247 IGBTs, 10 in parallel for each switch, with no forced current sharing.  I do feed bus power to one end of the array and make output (to MMC) connection at the other end.  That way voltage drop down the array on VBus is somewhat compensated for by voltage drop down the H-Bridge output planes.  So far I'm running 2600A OCD and have hit that plenty of times.  Project post is:
https://highvoltageforum.net/index.php?topic=798.msg5285#msg5285

In my case, bricks would have made more sense, running around 80kHz.  At the time I had not found this forum or other information indicating that bricks could run well at 80kHz due to ZCS operation.

[Jesperb123]

Just a quick question

How come we use the Vdc rating when selecting capacitors for the MMC? I think I'll use the 942C20P15K-F (2 parallel strings of 2 caps in series) and the datasheet states that they have a Vdc of 2kV while Vac is only 600V.

Edit: After calculating the RMS current (ontime = 200uS, BPS = 200, Ipk =500A) I get 50A, which the previous (2 parallel strings of 2 caps in series) MMC wouldn't be able to handle. I would need 4 strings of 2 capacitors in series to get the RMS-current rating up, but then I'd have too much capacitance (0.3uF instead of 0.15uF) which would mean I would need a fewer turns on my primary. I'd like to be able to run the teslacoil long enough to play music on it eventually, so I believe I have to take the RMS current rating seriously?

[Hydron]

We use Vdc for calculating as you can normally get away with pushing it a bit (at least for low duty cycles). Or you can go for something that's actually specified for low duty cycle over-voltage, such as the parts I'm using in my QCW:
https://www.tdk-electronics.tdk.com/inf/20/20/db/fc_2009/MKP_B32671L_672L.pdf (these probably don't come in large enough values for your use unfortunately)

That said, regarding DC voltage rating I haven't run the numbers on your coil but I am a little surprised that you think you can get away with only two of those caps in series. Maybe worth looking at a 3S * 3P config to give the same 0.15uF but at higher voltage and current. Also consider using cheaper caps and more of them - the CD parts you're looking at are overkill for a DRSSTC (their popularity is due to their success in spark-gap coils, though if you have them already there's no reason not to use them!).

[Jesperb123]

Hi hydron!
Ok then I understand why 2 in series might not be enough. I calculated the voltage across the MMC with:

Vpeak = Zc * Ipeak and since Zc is almost equal to abs(Xc) if the esr is low I get:
Vpeak = abs(Xc)* Ipeak

plugging in the numbers abs(Xc) = 5.25 ohms and Ipeak = 500A I get
Vpeak ≈ 2620V.

Using the Vdc rating for the caps mentioned earlier gave me 2*2000V = 4kV. But I guess what you mentioned about Vdc not being equal to Vac shows why two in series might not be enough.

That said, regarding DC voltage rating I haven't run the numbers on your coil but I am a little surprised that you think you can get away with only two of those caps in series. Maybe worth looking at a 3S * 3P config to give the same 0.15uF but at higher voltage and current. Also consider using cheaper caps and more of them - the CD parts you're looking at are overkill for a DRSSTC (their popularity is due to their success in spark-gap coils, though if you have them already there's no reason not to use them!).

Then I'll try to find some other capacitors that might work. Thanks alot for your help.

[Hydron]

OK so it looks like you have done the sums (unlike me!), I wasn't really thinking about the fact that Xc would be fairly small at 200kHz. 2 are likely be just fine in the end with ~2600Vpk - it just seemed low at first glance.

I think you might get away with it in terms of RMS current too with 2 caps in parallel, more likely with 3 - I get more like 35A per string (23A per string with 3) when I do the sums (I think you used peak as the RMS current, so there is a factor of root-2 off in there), and I assume you're running at less than 70C ambient which will also give some more margin over the datasheet number. Unless it's for a museum display or something you can push things a bit - we're not going for years of continuous operation! I've never used these particular caps though, so some advice from those who have wouldn't go astray (has anyone cooked them with RMS current?)

Finally not saying you shouldn't use the 942C20P15K-F caps, just that they might not be the best value if you're trying to make a coil on a tight budget. Especially if you only need 4 or 9 to meet your requirements and time is valuable then just go for it - save the penny pinching for when making a massive MMC.

[Jesperb123]

So I've been doing some searching around and found these for 4,12 euro:
Wima FKP4U031507H00KYSD

They have:

• high dv/dt = 9000V/us
• very low dissapation factor (<6*10^-4)
• 2kVdc
• Negative temperature coefficient for capacitance ==> Xc becomes larger for increasing temperature, better currentsharing?

So I think I should be able to use them in a 2S*2P configuration.