If you have read about primaries on a doubly resonant coil, most of it does not apply here. An SSTC's primary is wound differently, coupled differently, and sized by a different rule.
What it is and why
A DRSSTC's primary is half of a tuned circuit. An SSTC's primary is not tuned at all: it is a transformer winding, and it is trying to be a good one.
There is a capacitor in the same place in both machines. On a DRSSTC it is chosen to resonate with the primary. On an SSTC it is chosen so that it cannot, which is the whole difference between the two.
The consequence runs through everything below: without a tank there is no resonant multiplication of the current, so the coupling has to do the work instead.
What you decide
How tightly to couple, and where to put the winding
Criterion: energy transfer, and there is no resonance to help.
A DRSSTC's primary sits loose around the base of the secondary, a few turns at k = 0.13 to 0.17. An SSTC's is wound directly onto the secondary's form, through two to ten millimetres of insulation, spread over 20 to 50 per cent of the secondary's height. Coupling of 0.5 is normal.
Kaizer puts it as needing a primary geometry that gives high coupling to get good energy transfer, precisely because the primary circuit current is low.
How many turns
Burnett's rule, and it is a balance between two currents that both flow in the primary:
Lpshould be made only sufficiently large to obtain an acceptable magnetising current, thenkshould be maximised as far as possible.
Too few turns and the magnetising current is high, which heats the switches for nothing. Too many and there is little step-up, so the bridge cannot get power in.
What the current will be, before you choose devices
It falls out of the reactance rather than being set:
L = 10.16 uH (8 turns, 115 mm helix, 1.78 mm wire at 2 mm pitch)
X = 2·pi × 250000 × L = 16 ohm
I = 320 V / 16 ohm = 20 AIgnoring the primary's resistance and the blocking capacitor's reactance, which are small beside 16 Ω. Twenty amps, not the hundreds a DRSSTC sees, and that is the other reason an SSTC takes MOSFETs rather than IGBTs.
What will get you
And the things that do carry over from a DRSSTC primary. It still melts if you run it hard, it still must not sit on a metal frame that becomes a shorted turn, and skin effect still means the middle of a round conductor is not carrying anything. Those parts of what the primary is for are unchanged.
The numbers
- Coupling: about 0.5 is normal, against 0.13 to 0.17 on a DRSSTC.
- Insulation between the windings: 2 to 10 mm, extending past both ends.
- Coverage: 20 to 50 per cent of the secondary's height.
- Burnett's own coil, for scale: 18 turns, 20 µH, ±340 V at 350 kHz,
kabout 0.55, roughly 45 cm of arc. - Primary current, worked: about 20 A at 320 V into 16 Ω, against hundreds on a DRSSTC.
What goes wrong
- A flashover along the outside of the form, at the end of the primary. The insulation stopped where the winding stopped.
- The switches get hot with a short arc to show for it. Magnetising current: too few turns.
- The bridge cannot get power in however hard it is driven. Too many turns, so there is no step-up.
- A flashover in the middle of the secondary rather than the top. Not this part at all. That is the second mode, and the handle for it is the primary's height, not its turns.
Where next
- Why an SSTC couples tight, where the tight coupling turns out to decide whether the bridge switches softly at all.
- The capacitor that decides what you built.
- What the primary is for, the parts that are common to both machines.
The SSTC department diagram draws its tank across the middle of the bridge with no capacitor in it, which is the whole difference between that department and the next.