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SSTC vs DRSSTC: the DC blocking capacitor value decides which

SSTC

The value of the series capacitor alone decides whether you built an SSTC or a DRSSTC. Builders usually find this out when the transistors fail.

CommonGnd10 min read

An SSTC also has a capacitor between the bridge and the primary. Its value alone decides whether you have built an SSTC or a DRSSTC. Builders usually find out by accident, when the transistors explode.

What it is and why

A bridge does not switch perfectly symmetrically, so a DC component appears at its output, and on a half bridge an imbalance in the divider adds one as well. In a Tesla coil primary that DC does not meet a magnetic circuit. The winding is air cored, so there is no core to unbalance or saturate. It meets the winding's own resistance, and against a few turns of thick wire a DC component is very nearly a short circuit. Two published coils do put ferrite in the primary's magnetic path, and what coupling actually buys lists those two separately for that reason. Every coil in the ordinary band is air cored.

The core-saturation explanation gets attached to this part anyway, and Gao, who repeats it, says where it came from. He writes that the DC blocking cap "comes from switch-mode power supply designs, where saturating a transformer can destroy the transistors" (SSTC 2). An SMPS transformer has a core. A coil's primary does not. The two failures Gao names for a coil are the ones to remember: a DC bias current in the primary, and a device held on long enough to discharge the bus capacitors through itself and the winding.

The fix is a capacitor in series with the primary to keep DC out of it. On a full bridge it goes in series. On a half bridge it goes in series, or as two capacitors across the bus rails with the primary running from the bridge output to their midpoint. Gao describes that second arrangement and uses it on his own half bridge.

It is also the arrangement on a schematic that a great many first SSTCs have been copied from:

Schematic titled Mini SSTC, Steve's High Voltage, Steve Ward, rev 1.4, 4/30/2007. On the right, a half bridge of two IRFP260 MOSFETs with MUR860 diodes runs off rectified 120 VAC with a 420 microfarad reservoir. Two capacitors, C7 and C8, each 0.68 microfarad, sit in series across the bus rails, and the primary L1, four turns of 16 AWG over the bottom of the secondary, runs from the bridge output to the point between them. A note under the drawing reads C7 and C8 equals polypropylene. On the left, an antenna input into a 74HC14, UCC37321 and UCC37322 drivers, a 16 turn trifilar gate drive transformer, and a 555 interrupter.
Steve Ward's Mini SSTC: the primary returns to the midpoint of C7 and C8, two 0.68 µF polypropylene parts across the bus, which is the half bridge version of the blocking capacitor. Schematic by Steve Ward, from his Mini SSTC page, used with his permission.

What you decide

There is one decision to make.

Whether it resonates with the primary

A capacitor in series with a coil is a resonant circuit. If its resonance comes anywhere near the working frequency you have built a DRSSTC by accident, and a DRSSTC without overcurrent protection destroys itself.

This is often stated as two rules: a reactance small enough to ignore, and a resonance far below the working frequency. They are the same rule:

X_C / X_L = 1 / ((2*pi*f)^2 * L * C) = (f_res / f)^2

A margin of three in frequency cancels a ninth of the primary's reactance, and a margin of ten cancels a hundredth. Measure whichever end you can and the other follows.

The published record is two builds by one person. Gao's figures, from his own pages:

Ramped SSTC 3   primary 2.12 uH at 452 kHz    X_L 6.02 ohm    X_C 1.035 ohm
SSTC 2          primary 7.412 uH at 252 kHz   X_L 11.74 ohm   X_C 0.134 ohm

The inductances, the frequencies, both X_L and the 1.035 Ω are his. The 0.134 Ω is ours, from his 4.7 µF at his 252 kHz. As a ratio, R-SSTC 3 cancels 17 per cent of its primary's reactance and SSTC 2 1.1 per cent, which are frequency margins of 2.4 and 9.3 [derived]. Both coils work. That is all the published evidence we have for the criterion.

The other end of the scale is on the same page. Gao says that bringing the total tank capacitance on R-SSTC 3 to 58 nF puts the primary in resonance and turns the coil into a DRSSTC, and that a 68 nF capacitor in series does it. On that machine the same slot holds about 340 nF for an SSTC and about 58 nF for a DRSSTC, a factor of six in one part [derived].

You will see other thresholds quoted, such as an absolute reactance in ohms, or a fivefold frequency margin attributed to an unnamed author. Neither has a source, and a published SSTC running happily at 2.4 contradicts the second. What is established is the identity, those two data points, and that the DRSSTC condition is a margin of one. Anyone who wants a number in between has to say whose it is.

A half bridge's pair can be read two ways. R-SSTC 3 carries two 680 nF films, and Gao's own sum treats them as a 340 nF equivalent, the two in series. His route to resonance agrees with that reading, since 68 nF in series with 340 nF is 56.7 nF [derived] against the 58 nF he quotes. Seen from the primary with a stiff bus the pair is in parallel instead, 1.36 µF, and then X_C is 0.259 Ω, the margin 4.8 and the cancellation 4.3 per cent [derived]. His page does not say which reading fits his bus, and the criterion passes on both, so nothing above depends on it. Your own coil may be closer to the line. The two readings differ by a factor of four in capacitance and two in margin, so decide which applies before you do the sum.

The same fact shows up in the phase. Because the capacitor subtracts from the primary's reactance, the current lags the bridge voltage a little less than the primary alone would make it, on a full bridge in series and on a half bridge's divider alike. This is the margin sum seen from the phase side, not a second effect, and it is not automatically a gain. Switching at zero is the failure case explains why the residual current at turn off matters, and why an SSTC couples tight keeps the two questions apart. Moving the current towards the zero helps until you get there.

What sort of part

It needs very low ESR, so use the film parts that go into a tank bank, not whatever is in the drawer. This capacitor carries the full primary current. Gao uses a 4.7 µF MKP polypropylene on SSTC 2 and a pair of 680 nF films on R-SSTC 3, and gives a range of 1 to 6.8 µF, film.

The numbers

  • The criterion: X_C / X_L = (f_res / f)^2. One sum, either way round.
  • Two published SSTCs: 17 per cent of the primary's reactance cancelled at a margin of 2.4 on R-SSTC 3, 1.1 per cent at 9.3 on SSTC 2 [derived from Gao's figures].
  • The DRSSTC condition: a margin of one. On R-SSTC 3 that is 58 nF against the 340 nF fitted, both his figures.
  • Typical values: 1 to 6.8 µF, film, which is Gao's range and not a rating. The right value for your coil comes from the sum.
  • Not established: any threshold between the margin of one that makes a DRSSTC and the margin of 2.4 that is running in public.

What will get you

Twelve white cylindrical film capacitors in two rows of six, bolted between copper strips on a white plastic board and held up in a hand on a workbench.
The same slot on a DRSSTC, filled the other way: the tank capacitor bank Steve Ward built for DRSSTC-1, which his file names as 450 nF, chosen to resonate with the primary rather than to stay clear of it. Photo by Steve Ward, from his DRSSTC-1 page, used with his permission.

Do one sum before the first power up, then decide.

  1. Take the resonance of your primary inductance with the blocking capacitor, and divide your working frequency by it. Square that ratio: it is the fraction of the primary's reactance the capacitor is cancelling, and the current is what the primary alone would pass divided by one minus that fraction.
  2. If the ratio is near one you are building a DRSSTC whether you meant to or not, and the part that keeps it alive is current protection. If it is comfortably clear of one, take the largest film capacitor the budget allows and run the sum again on the value you actually fitted.

Gao also offers a counterweight. His position is that plenty of coils have been built with no blocking capacitor at all, that it is not necessary on small ones, and that it is cheap enough to fit anyway because it can save the day. Treat the part as insurance, not a rule. What you cannot skip is knowing where its value puts you on the margin. The values that turn a coil into a DRSSTC are the small ones, and a small film capacitor is what a drawer is most likely to turn up.

What goes wrong

  • Transistors explode on a coil that was designed as an SSTC. The blocking capacitor resonates with the primary somewhere near the working frequency. Do the sum, and if it comes out near one, change the value or fit current protection.
  • The current is higher than the primary's reactance alone allows. Same cause, caught earlier. The capacitor cancels part of that reactance, and the published case is 17 per cent cancelled for 21 per cent more current [derived].
  • The bridge dies on a coil that has no blocking capacitor at all. There are two routes, and Gao names both: a DC bias current in the primary, or a device held on long enough to discharge the bus capacitors through itself and the winding. Neither needs a core, and the coil does not have one.
  • The capacitor gets hot. ESR. It carries the primary current, so it needs a film part, not whatever was in the drawer.

Where next

Attribution: every figure here not marked as ours comes from Gao Guangyan's two build pages, SSTC 2 and Ramped SSTC 3.

The SSTC department diagram draws the tank across the middle of the bridge with no capacitor in it. That is the difference between that department and the next. On the QCW page you can take the capacitor out yourself and watch what happens to the arc.

DC blockingSSTCresonance

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