Inverter question

[RoamingD]

Hello, I am starting to work on my first Drsstc
I plan to use an half bridge inverter with a single BSM150GB60DLC IGBT module so I started looking at different schematics and I have a question about capacitors C4 and C5/
Are they really needed? Because in full bridge inverter schematics that I saw, similar capacitors were not found.
Also, I plan to run the OCD set at about 300-350A, I dont know if this information helps, but I want to give as many details as I can.
Thank you for your time, dear reader 

[davekni]

Are they really needed?

Good circuit analysis.  You are correct that in an ideal world C4 and C5 could be left out.  (Presumes value of remaining C7 cap is reduced to achieve correct primary resonant frequency.)

In the real world, additional caps have value.  Electrolytic capacitors C1 and C2 are likely not rated to handle DRSSTC primary resonant current (high frequency and high current).  Also, wire inductance from IGBTs to C1 and C2 causes voltage spikes at the IGBTs, which can be enough to damage parts.  Smaller film capacitors are easier to connect directly adjacent IGBTs to minimize parasitic inductance.  If IGBT rated max voltage is far above bus voltage, spikes are less likely to damage IGBTs, but still increase switching energy loss.

Even with film capacitors, excess lead length can lead to issues.  If you want to be a perfectionist about this, here's my tutorial thread on low-inductance construction:
https://highvoltageforum.net/index.php?topic=1324.msg9795#msg9795

There are other topologies for adding film capacitors.  You could start with your version leaving off C4 and C5, then add film capacitors in parallel with C1 and C2 directly at IGBT terminals.  If you go that route, a further option is to split the resonant capacitor (C7 in your version, often called MMC) in half, with one half to each side of Vbus supply.  That reduces peak current through caps across Vbus (C1, C2, and mostly added film caps in parallel with these).  Since MMC is usually made from an array of smaller caps, splitting it in half is often free and easy.

Have fun with your build!

[RoamingD]

Thank you so much for the help, sadly I can't implement the last idea of a split mmc as I already have a big resonance capacitor (I took inspiration from Franzoli Electronics last small DRSSTC)
I think i will buy a few WIMA film capacitors to use them as snubbers to suppress the spikes

[MRMILSTAR]

Does anyone know detailed specs of these Dawncaps? They look like ceramic doorknob capacitors. Are they? Those ridges on the side make me wonder. If so, are they Class 1 or Class 2?

[RoamingD]

Hello, this is their datasheet that I found on the manufacturers website, I chose to buy this capacitor for its peak rating of 1500 amps and fast rise voltage 3500 v/us I think

[Mads Barnkob]

C4/C7 in a half-bridge is acting as DC blocking capacitors and are needed to maintain a zero-average-voltage across the transformer. Without them, you risk destroying your transformer from growing magnetic flux that will force the primary into saturation.

I wrote a bit about calculating DC blocking capacitors here: https://kaizerpowerelectronics.dk/tesla-coils/sstc-design-guide/