The driver board holds all the logic: the feedback, the phase lead, the interrupter, the current protection and the output stage that reaches the gates. Without it the power section is a bridge and nothing else.
What it is and why
feedback CT -> burden -> COMPARATOR -> logic -> output -> GDT -> gates
^
protection CT -> burden -> OCD comparator interrupter -> ENABLEFour jobs on one board. It finds the coil's frequency, because that frequency is not a number you can set once. It advances the switching instant, because by the time a signal has crossed the board the current has moved on. It decides when to stop, on current or on the interrupter. And it delivers enough charge into the gates to make the transitions fast.
Everything below is one of those four going wrong.
What you decide
A comparator, not a Schmitt trigger
Criterion: whether the threshold has to be predictable. For feedback and for current protection it does.
Do not use a 74HC14. A logic trigger's thresholds move with temperature and supply voltage, and both jobs need a threshold you can predict. Use a real comparator, and put its propagation delay into the lead budget alongside the driver's and the GDT's.
How much phase lead, and how to make it
The switching has to happen before the current zero rather than on it, and the classic implementation is a series L + R in place of a bare burden resistor:
phi = arctan(w·L / R)There is a mismatch built into the idea. The network holds a constant angle while the hardware needs a constant time: the driver's delays are fixed nanoseconds, and the same angle at different frequencies is a different number of them. That is the limit of the technique across a wide sweep, and it is exactly what a QCW does to itself as the arc grows.
What the lead is made of, and how much of it is the ZVS floor rather than the delay chain, is its own piece. Winding the inductor is another.
Why the feedback comes off the primary at all
Worth putting first, because it is usually presented as a design choice and it was a fix for a specific failure that was destroying hardware. Steve Ward, in his DRSSTC log book:
When a ground strike occurs, the secondary appears to jump to another harmonic, usually 3X or 5X the Fres of the coil… This causes all sorts of bad switching transitions… I believe this is why I've lost too many IGBTs in the past. So the idea now (partially inspired by Steve Conner and Terry Fritz) is to use primary current feedback to derive the drive signals.
A secondary that has jumped to three or five times its resonance is no longer telling the driver anything about when to switch, and a driver following it switches at the wrong moments into a full bus. Taking the feedback off the primary current makes the loop deaf to what the secondary does under a strike. The whole UD line rests on that conclusion, and the reason for it is rarely stated.
The overcurrent detector arrived in the same stroke and for the same adversary, which is why the two are usually found together. The strike itself does three more things besides, and they are on one page.
How strong the feedback is
Criterion: volts per amp of primary current, not how many turns the transformer has.
feedback strength = |Z| / N, clamping at I = V_f · N / |Z|Working implementations sit around 0.02 to 0.04 V/A. Three or four times that and the signal reaches the clamping diodes almost immediately, which is the failure in the next section. The transformer end of this is two rings, not one.
What the protection does when it fires
Two behaviours, and they suit different machines. OCD ends the bang and waits for the interrupter. Pulse skip drops one cycle and lets the bang continue, which is what a long bang wants.
Either way it needs its own current transformer. A shared one, loaded by the phase lead network, saturates its core and the protection weakens exactly when it is needed. It can also be slow, a few cycles, so the threshold sits below the destructive current rather than at it. Overcurrent is not a setting is the whole argument.
What holds the gate down when the drive is not there
Three cases, and they need three different answers, which is why they get confused with each other.
- The driver is putting out nothing at all — during start-up, if its supply fails, or if a wire comes off the transformer. A pull-down from gate to emitter, tens of kilohms, holds the device shut. Cheap, and the failure it prevents is the expensive kind.
- The driver is putting out rubbish rather than silence, which is the sagging supply case below. A pull-down cannot help here, because the drive is real and it is wrong. This is UVLO's job and nothing else's.
- The driver is doing exactly what it should, and the gate is being lifted by
dV/dtfrom the other device throughC_gc. Neither of the above touches this one.
A gate-to-emitter TVS is secondary insurance rather than the answer. It belongs directly on the gate pin, bypassing the gate resistor, and the common choice is wrong: a P6KE27CA stands off about 23 V, so it only conducts above the gate's own ±20 V limit and protects nothing. An SMBJ16CA is the better part — 16 V stand-off, breaking down between 17.8 and 19.7, which is inside the limit — but read the rest of its line before believing in it. The clamping voltage at full rated surge is 26 V, well above the gate's 20. So it holds the small stuff under the limit and merely limits the big stuff to something survivable. Insurance, not protection, and that distinction is the whole reason it does not replace a short gate loop.
What will get you
And the three grounds are three different things: the high side's floating ground, the low side's fixed ground, and logic ground. Logic joins power at one star point, so that power currents have no path through the logic.
The numbers
- Feedback strength on working boards: 0.02 to 0.04 V/A.
- Burden resistors on a UD board: feedback 51 Ω 2 W, overcurrent 5.1 Ω 0.5 W. The wattage tells you what current the author was designing for.
- Delay chain on the reference bridge: about 184 ns, of which 115 is the switch itself. A datasheet sum, not a measurement.
- Where lead angles stop paying: 45 to 60 degrees. Diminishing returns rather than a physical limit, because the amplitude across
L+Rgrows as|Z|/Rwhile the useful in-phase part goes ascos φ, and 90 would take infinite inductance.
What goes wrong
- The frequency wanders, or the coil will not start. Feedback. Check the winding direction of the transformer first: turning the ring over is the test.
- It runs beautifully at low power and badly at high. Clamping. The signal is clipped above some current and the driver is no longer tracking anything.
- The protection never fires, or fires on every bang. The threshold, not the knob, and possibly a shared transformer saturating.
- Both devices in a leg die when the supply is marginal. No UVLO, or UVLO without hysteresis.
- A board that worked stops the moment a probe touches it. The probe ground went somewhere that is not the driver's ground.
Where next
- The UD boards, which most amateur DRSSTCs are built on, and what changed between revisions.
- What a phase lead actually buys.
- Overcurrent is not a setting.
The driver is a box on all three department diagrams, with the gate transformer above it and the current transformers feeding it from the right.