[ §1 · how it works ]

An SSTC primary is a different part

Most of what you have read about primaries does not apply here. This one is not tuned at all: it is a transformer winding trying to be a good one.

CommonGnd4 August 202612 min read

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 = 0.12 to 0.2. An SSTC's is wound directly onto the secondary's form. Mads Barnkob's SSTC design guide sets out his own practice: ordinary machine tool wire wound directly around the base of the secondary, with nothing more than 2 to 10 mm of insulating material in between, and left free enough to slide up and down so that the coupling can be adjusted afterwards. His reason is the one this whole page runs on. The primary circuit current is low, so the geometry has to supply the coupling instead of the current supplying the energy.

How high is high enough is the harder question. Richie Burnett puts a floor under it on his solid state pages: very tight coupling, above 0.35, is what the primary feed method needs for good power transfer, and he names that as the method's significant disadvantage, because it is what makes the insulation hard. His own coils are given at about 0.40 on that page and at about 0.55 on the design page.

Move the primary and you have detuned the coil. Burnett's reflected model says why: raising k couples more of the secondary to the primary and leaves less of it free to resonate, so the resonant frequency seen from the primary rises as the coupling is tightened. He is explicit that a solid state coil has to be re-tuned after any adjustment of the coupling. On a machine where the winding is deliberately slid up and down the form to set k, that is not a footnote. Which driver you have decides whether it costs you anything: Burnett's free running TL494 oscillator has to be moved by hand, while Barnkob's coil takes its frequency off an antenna and finds the new one by itself.

Coverage, meaning how much of the secondary's height the winding spans, is the one thing on this page with no rule behind it at all. The two builds on file are a long way apart. Barnkob's Kaizer SSTC 2 is eight turns of 1.78 mm wire with 2 mm between turns, which is 30.24 mm of winding [derived, 8 × (1.78 + 2), his calculator's own definition of coil length] on a secondary 275 mm long, or about 11 per cent [derived]. Burnett's is link coupled over the bottom third of his resonator, about 33 per cent. So a tenth to a third across the two coils here, and that is a spread rather than a specification.

How many turns

Burnett's rule, and it is a balance between two currents that both flow in the primary:

Lp should be made only sufficiently large to obtain an acceptable magnetising current, then k should be maximised as far as possible.

That is from his driver design page, where he also sets out the three ways it goes wrong. Too little Lp and the magnetising current becomes unacceptably high: it draws no supply current, so it does no work, but it heats the switches anyway and it puts ripple into the reservoir capacitors. Too little k and the load current is mostly magnetising current, with little of what the driver supplies reaching the secondary. Too much k and the two windings are close enough to flash over.

The lever is steeper than it looks: he notes that halving the primary turns takes Lp to roughly a quarter, and that in an ideal world the impedance the driver sees falls by the same factor, with the current rising to match. What makes it the right lever is that it moves the load current without moving the resonant frequency, which is the one thing the coupling adjustment does not leave alone.

What the current will be, before you choose devices

It falls out of the reactance rather than being set. Barnkob works it through in the design guide on his own Kaizer SSTC 2, a full bridge of IRFP460 self tuning at about 250 kHz:

L = 10.16 uH   8 turns, 115 mm diameter, 1.78 mm wire, 2 mm between turns
X = 2·pi × 250 kHz × 10.16 uH = 15.95 ohm
I = 320 V / 15.95 ohm = 20 A peak

All three lines are his. The primary's resistance and the blocking capacitor's reactance are left out, which is his own stated simplification, and the 320 V is full wave rectified 230 V mains.

The inductance is worth one line of arithmetic, because it reads wrong until you know what spacing means on his page. It is the gap between turns, not the pitch: his own helical calculator defines coil length as the turn count times the wire diameter plus the spacing, so eight turns here occupy 30.24 mm, not 16. Wheeler on 115 mm and 30.24 mm returns 10.16 µH [derived, 115² × 64 / (457 × 115 + 1016 × 30.24)], which is his figure exactly, and a filament sum over the eight turns gives 10.2 to 10.4 µH [derived] depending on whether the wire is treated as carrying current through its middle. Read spacing as pitch instead and you get 12.30 µH, and everything downstream moves 21 per cent. The tank sum on the blocking capacitor page is the check: its primary resonance of about 43 kHz on two 0.68 µF films comes out of 10.16 µH and would be 39 kHz on 12.30.

And then the argument in the comments under the design guide, which is worth more than the figure. A reader posting as Zbig objects that a reactance is a sine wave quantity while a bridge makes a square wave, and that across a bare inductor a square wave makes a triangular current: on a rounded 10 µH he gets a peak of 32 A rather than 20. Barnkob's reply is that a resonator is not an ordinary inverter load, because the secondary's series RLC is reflected onto the primary through the coupling, which is why he calculates for a sine.

Both halves are right about different components of the same current, and Burnett's model is what separates them. The primary carries a triangular magnetising current, ninety degrees behind the voltage, which flows whether or not anything is coupled to the winding at all; and on top of it the resonant current reflected back from the secondary, in phase, which is the part doing work. Zbig's route gives the first of those. Barnkob's reactance route gives the same first quantity with the square wave read as a sine.

Three routes, same three inputs:

20 A     320 / 15.95                      Barnkob's own, bus read as a sine amplitude
25.5 A   (4/pi) × 320 / 15.95             [derived] the square wave's fundamental
31.5 A   320 V × 2 us / 10.16 uH, halved  [derived] triangular peak, Zbig's route

The 2 µs is the half period at 250 kHz, over which 320 V across 10.16 µH moves the current 63.0 A from end to end [derived].

Tens of amps on every route, then, against the hundred amps and up that a DRSSTC's tank carries, which is the impedance page's subject and where those figures are sourced. 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 a DRSSTC primary is for are unchanged.

The numbers

Coupling: Burnett's floor is above 0.35, and his own coils are given at about 0.40 and about 0.55, neither with a method stated. Against 0.12 to 0.2 on a DRSSTC.
Insulation between the windings: 2 to 10 mm on Barnkob's builds, extending past both ends. Under half an inch on Burnett's.
Coverage: not a rule. About 11 per cent of the secondary's length on Kaizer SSTC 2 [derived, 30.24 mm on 275 mm], the bottom third on Burnett's.
Burnett's own coil, for scale: 18 turns, 20 µH, ±340 V at 350 kHz, k about 0.55, with 18 inch sparks common, which is 457 mm [derived, 25.4 mm/in].
Barnkob's worked chain: 10.16 µH at 250 kHz is 15.95 Ω, and 320 V across that is 20 A peak. Two other readings of the same square wave give 25.5 and 31.5 A [derived], and the reflected resonant current is on top of all three.
The only measured primary current here: 22 A peak, Burnett's H bridge coil.

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. It draws nothing from the supply and heats the bridge anyway.
The bridge cannot get power in however hard it is driven. Too many turns, so there is no step-up, or too little coupling, so the load current is magnetising current and little of it reaches the secondary.
A device chosen on a calculated 20 A that dies at full power. The calculation gives one component of the current. Put a current transformer on the primary before believing any of the three routes above.
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.
The primary is an impedance, the DRSSTC side of the current question and where the hundreds of amps are.
What a DRSSTC 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.

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