First time DRSSTC Build

[Saattvik24]

I am attempting to build a DRSSTC for the first time, having built a small half bridge SSTC before. After having gone through many resources and forums and using JavaTC and other calculators, I have chosen the following parts and specifications for the DRSSTC, I wanted to know if I have chosen them correctly and just needed some feedback before ordering the parts. Thank you.

Bridge - SKM100GB12T4 in Full Bridge
Bridge Supply - 0-260VAC, 35A Bridge Rectifier, 4700uF 400V EPCOS Electrolytic Capacitor (ESR - 28mohm, Ripple Current - 12.6A)
Primary Coil - Spiral Primary - 6 turns of 1/4 inch OD copper tube with 15mm spacing and an Inner diameter of 150mm
Secondary Coil - 110mm diameter, 513mm long, 1600 turns, 28AWG enameled copper wire
MMC - 2 strings in parallel of 2 in series of Cornell Dubilier 942C20P15K-F for 0.15uF at 2000VDC
Snubber - 2uF 1kVDC 827V/us dv/dt (KEMET C4BSNBX4200ZBLJ)
Top Load - Toroid -  Minor Diameter-127mm   Major Diameter-530mm   Capacitance-22.951pF
Driver - UD2.7C Thru Hole
Resonant Frequency - Around 130kHz

[AstRii]

MMC - 2 strings in parallel of 2 in series of Cornell Dubilier 942C20P15K-F for 0.15uF at 2000VDC

Is that 2000VDC a typo? I found those caps to be individually rated for 2kV, giving 4kV for 2 in series. Anyway 4kVDC still seems quite low. I assume those bricks could be easily pushed to 500A-700A OCD, which would result to 4080V - 5700V AC on your capacitors at 130kHz. I would put at least 4 in series if your build needs to be reliable.

[Saattvik24]

Yes sorry that was a typo. oneKone who built a skm100 coil which is in the list of drsstc builds here, he used a 165nF 6kv kemet snubber capacitor mmc, so should I go the same route as him or stay with the cornell dubilier capacitors? The kemet snubbers will be much cheaper, so I would prefer that.

[AstRii]

MMCs are in general one of the most expensive component of a DRSSTC. I would probably not try to save money on it. However, it's up to you how reliable you want your setup to be.
If you want just 30seconds of fun, heck even 2kV MMC might be enough.
If you want something cool to show your friends and family on special occasions, 4kV would probably do.
If you want something to show often on school events for example, I would go with 6kV.
If you need the coil to work 24/7 reliable in a museum for example, I'd go for 8kV MMC.

It also depends on how high you want your OCD setpoint. I usually stick to datasheet pulse rating values, which means quite low (in your case that would be 300A). And in return run longer on-times with pulse-skipping drivers.
This saves you some money on your MMC.

[flyingperson23]

I second the pulse current rating and long on times. If you haven't made your driver yet, I highly suggest making the UD2.9 as its basically just the 2.7 with pulse skipping. If you're looking for cheap MMC, try aliexpress dawncaps. As far as I can tell they're mostly made for Chinese induction heaters and seem a bit sketchy, but they work great for drsstcs. My skm150 coils (and franzolis alphaDR 2/3) use a single 100nf 80A 3000vac dawncap and it's worked perfectly so far.

edit: this is the capacitor: https://www.aliexpress.us/item/2255799830104555.html?gatewayAdapt=glo2usa4itemAdapt
 I have one skm150 full bridge coil that runs 300us 450A and one SKM150 half bridge coil that runs 1-2ms 300A, both with UD2.9, both use 1 of those for the mmc and i haven't had any problems with them

[Manz]

Hi
I also made a UD2.9 skm100 coil and my mmc is 97.5nF 8kv.
I usually run it at 300A 100-300us in MIDI mode

I used this capacitor type in a 4S 10P configuration without any problem, maybe this can help you
https://www.digikey.it/it/products/detail/kemet/R75UN239050H3J/12144310

[Saattvik24]

Thank you for the help. I also wanted to know if I need to use TVS Diodes as I already have snubber capacitors. Don't they serve the same purpose? And if I do then will 440V TVS diodes be enough? KaizerPowerElectronics used the same in his DRSSTC. Thank you.

[AstRii]

I also wanted to know if I need to use TVS Diodes as I already have snubber capacitors.

In general I think TVS diodes are not a good idea. I've personally had an experience where a whole bridge blew because of them. The problem is that the the bridge itself is very low impedance, all the traces are thick and there is nothing that limits the current in case a TVS diode catches a voltage spike. Chances are the current is going to be very high, combine that with the huge forward voltage drop of the TVS diode, and you got yourself enormous power which the diode is not able to withstand not even for a brief duration of the spike. The diode will not survive and go short circuit, shorting the bridge and destroying everything.

I think the only part of a DRSSTC where TVS diodes can be used, is between the Gate and Collector of IGBTs. Fairly low currents of GDT drive combined with low voltage, makes it very unlikely that a short voltage spike destroys the diode.

I also think that if your bridge shows problematic voltage transients, a voltage clamping is not a solution. It's probably caused by some non ideal switching. Snubber capacitors or better design of the circuit is needed.

[Saattvik24]

Also I wanted to know which is a good and simple MIDI Interrupter to build? The OneTesla interrupter? Thank you