The coil runs, the bridge is warm, and nothing comes off the breakout. Or something does and it is a third of what it should be. This is the order to look in, and the order matters, because the cheap checks are also the ones that are usually wrong.
The three classic causes
Mads' list, and it accounts for most of them:
- The feedback phase is inverted, 180 degrees out. Almost no energy is transferred. Check by turning the CT over.
- The secondary's ground is bad.
- The feedback signal is weak, usually the wrong core or the wrong material in the current transformer.
None of those is a tuning problem, and all three get misdiagnosed as one.
Then the ones that look like a small coil
Lengthen the breakout rod, to three times what it is now or more. A longer rod breaks down at a lower voltage and makes everything easier to tune.
Move the coil away from metal. A chassis or a case near the secondary kills the Q and steals energy into induction heating. Half the secondary's diameter is the minimum clearance, which is 100 to 200 mm on an ordinary coil.
Test in the dark. Even a four or five kilowatt DRSSTC is pale at dusk, and in daylight you will undersell your own machine and go looking for a fault.
The primary current flattens too early
If the current stops rising partway through the bang, energy is being lost before it reaches the arc. Three places to look:
- The primary leads are too thin. Several 16 AWG in parallel, which is a home-made litz, beats a single conductor of the same area, because at these frequencies most of a round conductor is not carrying anything.
- Excessive collector-emitter drop.
- The ESR of the bus capacitors.
The bang is too short
A DRSSTC is not a spark gap coil. Cut the bang off early and most of the primary's energy goes back into the bus capacitors instead of into the arc. Something over 120 µs for a mid-sized coil. Longer ontime, longer spark, especially at modest peak current.
Tuning, in the order that works
Measure the secondary's resonance first, with the topload and the breakout point in place. Three ways: a signal generator through a resistor, 1k then 10k for a sharper dip; a ring-down with a nanofarad into the scope; or an antenna near the topload looking for the field peak.
Tune the primary slightly below the secondary. The arc adds capacitance, so the secondary slides down as the spark grows, and the detuning should increase with it. Adding primary turns lowers its frequency. The useful range is wide: one builder went from 170 kHz to 100 in the course of finding it.
JavaTC is a good starting estimate and no more. Nearby metal and walls throw it off, so try taps by hand from there.
Uspring's method: tune for the maximum ratio of secondary current to primary current at low power. The ratio does not depend on the input power, which is what makes it usable. Note that the maximum ratio is not the optimum for long arcs; for power you detune further.
Coupling at 0.13 to 0.17 for an ordinary DRSSTC. Higher transfers more and is less sensitive to detuning, until racing sparks arrive. Lowering the secondary raises it: one case went from 0.134 to 0.172 by dropping it two centimetres.
Where the current transformer goes
And the things that kill the bridge while you are looking
Two traps that present as intermittent death rather than as no spark:
The bootstrap trap. If the driver stops switching in the high state, the bootstrap capacitor never recharges, the high side desaturates, the shock heating kills it quietly, and then the low side goes in shoot-through with a bang. Keep the low side on during pauses, or use UVLO, or a floating gate supply, or an external Schottky with a low bootstrap resistor of about an ohm and at least five RC per half cycle.
Bus resonance. The film capacitors and the layout inductance resonate, and an H bridge acts as a reverse rectifier, so the ripple appears at twice the primary frequency. Land on the resonance and the bus swings hard. One diagnosis: four 3.9 µF and a 5.83 µs period gives 55 nH of standoffs and busbar. The fixes are to block the magnetic loop with copper foil over the capacitors, to detune deliberately towards the square root of two times the primary frequency, or to add series resistance to some of the capacitors to drop the Q.
The figures and cases here are from published threads and from other people's benches. The DRSSTC diagram has the current transformers on the return lead, at the low potential end, for the reason in the warning above.