A QCW's primary is the same part as a DRSSTC's, sized by the same arithmetic, and asked to hold the current for fifty times longer while the frequency walks out from under it. That is most of the difference. The rest is what builders do about it, and on the two decisions that matter they do not agree.
What it is and why
The impedance rule does not change here. Primary design is where it is worked, and none of it is superseded by anything below.
What changes is the ontime. A DRSSTC's primary carries its current for hundreds of microseconds; a QCW's carries it for milliseconds, ramp after ramp, so the copper and the joints are a thermal problem before they are an electrical one. And the frequency the tank is tuned against is not one frequency: it slides across the bang, which is the ramp's own page.
What you decide
How much tank, and why the band is so much smaller
Single digit nanofarads to about fifteen, against the tens a DRSSTC runs. This site's own coil is at 12.2 nF and Gao's QCW 1 and 1.5 are both at a nominal 12.8.
It is not a preference. A low impedance tank accelerates the current too fast at the start of the bang, so the overcurrent detector fires before the ramp has finished and instead of a sword you get a pop. On a DRSSTC the same tank is simply a harder-working one, because the bang is over before the detector's threshold means anything about the shape of the arc.
Which way to detune, on which two published machines disagree
Both of the ferrite QCWs on the record hold the upper pole, and they get there from opposite sides.
David Knierim tunes the primary far below the secondary, with many times the resonant capacitance. Jan (dr. kilovolt) tunes his above it: 279 kHz against the secondary's 251, running at 400 kHz at the start of the ramp and 350 at the end.
And he says himself that it is the wrong way round. In his own words the primary "is not designed optimally, it would be better to use a lower inductance and higher capacitance and tune the primary lower". What makes that worth reading rather than filing is the next sentence of it, which is the warning below.
Whether to put ferrite in it
The two sections above assume copper in air. Put ferrite inside the winding and the primary stops being a coil you tap and becomes a transformer core, which is a different machine and not a better version of the one the impedance page describes.
David Knierim's replaceable ferrite-core primary is the worked case, itself following Jan's potted ferrite QCW on HVF 1073. The idea he states is "to pot the inside of the secondary with a cavity for the primary", so the core assembly slides out and can be changed. The stack is 44 E-55 halves in 3C92 with four flat E-64-50-10 cores for spacing and 3F4 top plates at 64 by 51 by 5 mm, and he prices it: "$158 for all the E-cores". The cavity is lined with "a Faraday cage with no closed loops. All the wires connect at only the top", which is the only way to put metal that close to a secondary without shorting a turn.
What it buys is coupling, and the size of the change is the point. He measures k = 0.909 with the ferrite in. Without it the thread gives two figures rather than one, 0.51 on a mock-up and an estimate of "around 0.7 or just under", so read the air-cored number as a range and the ferrite one as the measurement. His upper pole lands at 124 kHz with no topload and 106 with a 620 mm one.
What will get you
The numbers
Two machines, both on the record, quoted from their own threads.
Jan (dr. kilovolt), SiC phase-shift QCW
primary 279 kHz secondary 251 kHz
running 400 kHz start -> 350 at the end
k 0.55, "very high, but ... not utilized fully"
core ferrite rod, I-cores of 3C97, about half the
secondary's height
potting polyurethane, against flashover
arc "over 2 meters", "approximately 2-2.5m"
David Knierim, replaceable ferrite-core QCW
k 0.909 with ferrite; 0.51 on an air-cored mock-up
upper pole 124 kHz bare, 106 kHz with a 620 mm topload
core 44 E-55 halves in 3C92, $158 for the E-coresWhat goes wrong
- The detector ends the bang before the ramp does. Tank impedance too low for the start of the ramp. It reads as a pop rather than as a protection event, which is why it is usually blamed on the modulator.
- The coil runs but will not tune. If it is potted, that is the end of the conversation, and it is the failure Jan documents on himself.
- The copper is warm after a run and was not on the DRSSTC it came from. Fifty times the ontime at the same current is fifty times the heat.
Where next
- Primary design, for the impedance rule, the two regimes and the detuning arithmetic this page assumes.
- What the primary is for, if none of the above sounded like an answer to what the part does.
- How the frequency slides down the ramp, because the tank here is tuned against a moving target.